Tennessee Weather Forum

Weather Forecasting and Discussion => Winter Weather => Topic started by: bigalpha on December 11, 2009, 10:05:48 AM

Title: Winter WX Basics
Post by: bigalpha on December 11, 2009, 10:05:48 AM
The goal of this post (and thread) is to give some basic information on winter weather forecasting.  There are always lots of questions, such as "Where do I find GFS maps" or "What's a Miller A event"?

Hopefully, this topic will get expanded upon to be a great resource for the beginners or people who are not "in the know".

Index
1.  Map Sources and Background
2.  Map Analyses
3.  Model Run Times
4.  Learning and Education
5.  Winter Teleconnection
6.  Types of Southeastern winter storms and snow data
Title: Map Sources and Background
Post by: bigalpha on December 11, 2009, 10:06:10 AM
WINTER WX FORECAST MAPS

- Weather Underground  (http://www.wunderground.com/modelmaps/maps.asp?model=NAM&domain=US)
- Air Resources Laboratory (ARL) (http://ready.arl.noaa.gov/READYcmet.php)
- NCEP Model Analysis (http://www.nco.ncep.noaa.gov/pmb/nwprod/analysis/namer/)
- Twister Data (http://www.twisterdata.com/index.php?prog=forecast&model=GFS&grid=3&model_yyyy=2009&model_mm=12&model_dd=11&model_init_hh=12&fhour=00&parameter=THCK&level=1000_850&unit=MB&maximize=n&mode=singlemap&sounding=n&output=image&archive=false)
- WX Maps (http://wxmaps.org/pix/forecasts.html)
- WX Caster (http://www.wxcaster.com/weather.php3)

MODELS
GFS
"The Global Forecast System (GFS) is a global numerical weather prediction computer model run by NOAA. This mathematical model is run four times a day and produces forecasts up to 16 days in advance, but with decreasing spatial and temporal resolution over time it is widely accepted that beyond 7 days the forecast is very general and not very accurate.

The model is run in two parts: the first part has a higher resolution and goes out to 180 hours (7 days) in the future, the second part runs from 180 to 384 hours (16 days) at a lower resolution. The resolution of the model varies in each part of the model: horizontally, it divides the surface of the earth into 35 or 70 kilometre grid squares; vertically, it divides the atmosphere into 64 layers and temporally, it produces a forecast for every 3rd hour for the first 180 hours, after that they are produced for every 12th hour.
NAM is another model (I think it stands for "North American Model").  It's a shorter range model than the GFS.  Where the GFS goes out to 384 hours (over two weeks away), the NAM only goes out to 84 hours (only 3.5 days away)." (Wikipedia)

NAM
"The North American Mesoscale Model (NAM), refers to the numerical weather prediction model run by National Centers for Environmental Prediction for short-term weather forecasting. Currently, the Weather Research and Forecasting Non-hydrostatic Mesoscale Model (WRF-NMM) model is run as the NAM, thus, three names (NAM, WRF, or NMM) typically refer to the same model output. The WRF replaced the Eta model on June 13, 2006.[1] The model is run four times a day (00, 06, 12, 18 UTC) out to 84 hours. It is currently run with 12 km horizontal resolution and with 1 hour temporal resolution, providing finer detail than other operational forecast models." (Wikipedia)

MODEL RUN TIMES
0Z - 9:30 PM CST
6Z - 3:30 AM CST
12Z - 9:30 AM CST
18Z - 3:30 PM CST
Title: Map Analyses
Post by: bigalpha on December 11, 2009, 10:06:38 AM
MAP CHARACTERISTICS
500mb Geopotential Heights, Height Change and Vorticity
(Source (http://wxmaps.org/pix/fcstkey.html))

    *  Black contours indicate the geopotential height of the 500 millibar surface, in tens of meters.
          o Low geopotential height (compared to other locations at the same latitude) indicates the
             presence of a storm or trough at mid-troposphere levels.
          o Relatively high geopotential height indicates a ridge, and quiescent weather.
    * In the forecast panels, the colored contours indicate the change in geopotential height during
       the 12 hours leading up to the valid time.
          o Decreasing geopotential height usually indicates an approaching or intensifying storm.
          o Increasing heights usually indicate clearing weather for the period.
    * The color shading indicates vorticity at 500 millibars: Red for positive vorticity, blue for negative.
          o Positive vorticity indicates counterclockwise rotation of the winds, and/or lateral shear of
             the wind with stronger flow to the right of the direction of flow.
          o Negative vorticity indicates clockwise rotation of the winds, and/or lateral shear of the wind
             with stronger flow to the left of the direction of flow.
          o Positive (or negative in the Southern Hemisphere) vorticity at 500 millibars is associated
             with cyclones or storms at upper levels, and will tend to coincide with troughs in the
             geopotential height field.
          o Negative (positive in SH) vorticity is associated with calm weather, and will tend to coincide
             with ridges in the geopotential height field.

Sea Level Pressure and 1000-500mb Thickness
(Source (http://wxmaps.org/pix/fcstkey.html))

    *  The colored contours indicate sea level pressure in millibars. High pressure is red, low pressure
        in green or blue. Only the last 2 digits shown -- sea level pressure is usually around 1000
        millibars, so add 1000 to values in the range of 00-50, and add 900 to values in the range of
        50-98.
          o Low sea level pressure indicates cyclones or storms near the surface of the earth. High sea
             level pressure indicates calm weather.
    *  The black contours indicate the vertical distance, or thickness, between the 1000 millibar
        surface and the 500 millibar surface, measured in tens of meters.
           o Since air behaves nearly as an ideal gas, and vertical distance is proportional to volume
              over a specified surface area, the thickness between two pressure levels is proportional to
              the mean temperature of the air between those levels. Thus, low values of thickness mean
              relatively cold air.
           o The 540 line is highlighted, since this line is often used as a rule of thumb to indicate the
              division between rain and snow for low terrain. When there is precipitation where the
              thickness is below 540dam, it is generally snow. If the thickness is above 540dam, it is
              usually rain (or sleet if the air next to the surface is below freezing).
 
Vertical Velocity and Precipitation
(Source (http://wxmaps.org/pix/fcstkey.html))

    *  The colored contours indicate vertical velocity of the wind at the 700 millibar level, in millibars
        per hour (since pressure decreases with height, negative values indicate ascending air, and
        positive values denote sinking).
          o Ascending motion is associated with cloudiness and rain. Large negative values of vertical
             velocity correspond to areas of heavy rainfall if moisture is available (see description of
             panel 4). These areas tend to correspond with the storms in the first two panels.
    *  The green shading in the forecasts indicate 12 or 24 hour accumulated precipitation, measured
        in millimeters.
           o The total is the amount of rainfall forecast during the 12 or 24 hours immediately preceding
              the verification time in the lower lefthand corner of the map.
           o Comparison with the 540 thickness line in panel 2, and the 0ºC isotherm in panel 4 can give
              a good indication of the dividing line between snow and rain.

850mb Temperature, Humidity and Winds
(Source (http://wxmaps.org/pix/fcstkey.html))

    *  Colored contours indicate the air temperature at the 850 millibar level, in degrees Celsius. The
        0ºC contour is highlighted, as this is also often used as a divider between rain and snow.
    *  The green shading indicates the relative humidity percentage at the 850 millibar level. High
        values indicate the availability of moisture. When large rates of ascent (in panel 3) are located
        with high moisture availability, heavy rainfall will likely occur.
    *  The barbs indicate the direction and speed of the wind, in meters per second. Each full barb
        indicates 10 m/s, and each half barb 5 m/s. The direction of the wind is parallel to the shaft
        with the barbs pointing into the wind.
           o Advection of moisture by the wind can be inferred by noticing the direction and rate at
              which moist areas appear to be blown. Similarly, temperature advection can be inferred by
              noticing whether the wind is blowing cold air toward a warm region, or warm air toward a
              cold region.

200mb Winds
(Source (http://wxmaps.org/pix/fcstkey.html))

    *  Purple shading indicates the speed of the winds at the 200 millibar level, in meters per second.
        This altitude is near the level of the core of the jet stream. So the tracks of the jet streams
        can be seen very clearly.
    *  The streamlines indicate the direction of flow of the wind, which is generally from west to east
        throughout most of the subtropics, mid- and high-latitudes.
    *  The color of the streamlines indicates a relative measure of divergence of the flow in the upper
        troposphere. Orange and red indicates strong divergence at upper levels, usually associated with
        strong vertical velocities in the middle troposphere, and severe weather/heavy rainfall.

Precipitable Water and Convective Available Potential Energy (CAPE) or Total-Totals Index (TTI)
(Source (http://wxmaps.org/pix/fcstkey.html))

    *  The colored contours indicate total precipitable water in the atmosphere. Precipitable water is
        the total depth of liquid water that would result if all water vapor contained in a vertical column
        of air could be "wrung out", leaving the air completely dry. It indicates the total humidity of the
        air above a location, and is a good indicator of the amount of moisture potentially available to
        supply rainfall.
    * In the analysis and forecasts for the ETA model, the yellow-brown shading indicates the amount
       of CAPE in the atmosphere, which is a good indicator of the potential for strong thunderstorms
       and severe weather. High values of CAPE indicate that most (but not necessarily all) conditions
       exist for strong thunderstorms.
    * For the other models the yellow-brown shading indicates the value of the TTI, which is a
       measure of the vertical stability of the atmosphere, and over central and eastern North America
       is also a good empirical indicator of the potential for severe weather.
          o TTI = TD[850]-T[500] + (T[850]-T[500])
            where T is temperature, TD is dew point (both in Celsius), and the [number] is the pressure
            level.
          o Values of TTI of around 40-45 indicate the potential for thunderstorms. Around 50, severe
             thunderstorms are possible. Around 55, storms producing tornados are possible. This
             rule-of-thumb does not hold over western North America and other areas where there is
             a lot of high terrain.

Title: Model Run Times
Post by: bigalpha on December 11, 2009, 10:06:54 AM
MODEL RUN TIMES
(Source) (http://www.easternuswx.com/bb/index.php?showtopic=222197)

Here is a brief guide for all you weather weenies just starting out to when you can find your models....

What time is it?
Weather models are run in UTC, which is Coordinated Universal Time, this can also be referred to as Zulu time (hence the “Z”) which is military time, or GMT, Greenwich Mean Time

“UTC” Coordinated Universal Time = “Z” Zulu = “GMT” Greenwich Mean Time

Weather observations are also reported in this style.

0000z is midnight (@ the Greenwich Meridian)
1200z is noon (@ the Greenwich Meridian)

That’s great, but what time is it here?

Eastern Standard Time (winter) is 5 hours behind UTC
Ex 12z = 7am
00z= 7pm (Remembering that 00z is actually midnight)

During Eastern Daylight Savings (summer) is 4 hours behind the UTC
Ex 12z = 8am
00z = 8pm

LOCAL EDT EST
Midnight 400 500
1 a.m. 500 600
2 a.m. 600 700
3 a.m. 700 800
4 a.m. 800 900
5 a.m. 900 1000
6 a.m. 1000 1100
7 a.m. 1100 1200
8 a.m. 1200 1300
9 a.m. 1300 1400
10 a.m. 1400 1500
11 a.m. 1500 1600
NOON 1600 1700
1 p.m. 1700 1800
2 p.m. 1800 1900
3 p.m. 1900 2000
4 p.m. 2000 2100
5 p.m. 2100 2200
6 p.m. 2200 2300
7 p.m. 2300 2400
8 p.m. 2400 100
9 p.m. 100 200
10 p.m. 200 300
11 p.m. 300 400
LOCAL EDT EST


Ok, now that’s settled, what time can I see each model run?
This depends on the model. It is important to remember with model run timing that the time indicated is actually when the model begins its run for instance 12z starts to run at (Noon Greenwhich mean time) 7 EST. Some models are faster than others.

Here I will outline the NAM, GFS, Euro, UKMET, GGEM and SREF for you.

The following times are for EST.
1.) NAM( North American Mesoscale Model) Model forecasts are produced every six hours at 00, 06, 12 and 18 UTC. Data is available in 6 hour incriments out to 84 hours.
NAM
0z: 8:45pm
6z: 2:45am
12z: 8:45am
18z: 2:45pm

2.) GFS (Global Forecast System) Model Forecasts are also produced every 6 hours. (00, 06, 12, & 18). Like the NAM they are available in 6 hour increments. Unline the NAM, the GFS is a longer range model and goes out to 384 hours.

GFS
0z: 10:30pm
6z: 4:30am
12z: 10:30am
18z: 4:30pm

3.) ECWMF( The Euro) forecasts are a bit more complicated. Some of the data from the Euro is subscription only and will change the times that you can access it for each run. Some sites allow you to access it faster than others. Many people on the board use the WSI subscription Euro, so that’s what we will outline here. The Euro has 2 runs for us here , 0z and 12z. Depending on where you access the ECWMF, will depend whether you can access 6 or 12 hour increments.

Euro (subscription services)
0z: 1:00am
12z: 1:00pm

Euro (Free services)
0z: 1:30am
12z: 1:30pm

4.) UKMET (United Kindgom Meteorological Forecast Model) is issued at 00z, 12z On most sites it is a 72 hour model that is in 6 hour increments through 48 hours, and then 12 hour increments through 72 hours.(you can also get the Ukie out to 144 in 24 hour increments) It is also run at 06z and 18z but only goes out to 60 hours.
UKMET
0z: 11:30pm
12z: 11:30am

5.) GGEM (Global Environmental Multiscale Model) Forecasts are another out of country model. This one we get twice a day 12z and 00z.
GGEM
0z: 11:45pm
12z : 11:45am

6.) SREF(Short Range Ensemble Forecasts) are a set of ensembles that are issued 4 times a day at “off” hours. These are run at 03z, 09z 15z, and 21z and run though hour 87.

09z: 8:40am
15z: 2:40pm
21Z: 8:40pm
03z: 2:40am
Title: Learning and Education
Post by: bigalpha on December 11, 2009, 10:07:09 AM
Learning and Education

This section is for websites and resources that are good for learning not only how to read winter wx, but learning about weather in general.

-  The Weather Prediction  (http://www.theweatherprediction.com/)
- The Comet Program (http://www.comet.ucar.edu/)
Title: Winter Teleconnection
Post by: bigalpha on December 11, 2009, 10:07:24 AM
Winter Teleconnections
(Source (http://www.nc-climate.ncsu.edu/))

ENSO
The El Niño-Southern Oscillation (ENSO) is a naturally occurring phenomenon that involves fluctuating ocean temperatures in the equatorial Pacific. The warmer waters essentially slosh, or oscillate, back and forth across the Pacific, much like water in a bath tub. For North America and much of the globe, the phenomenon is known as a dominant force causing variations in regional climate patterns. The pattern generally fluctuates between two states: warmer than normal central and eastern equatorial Pacific SSTs (El Niño) and cooler than normal central and eastern equatorial Pacific SSTs (La Niña).


Often, sea surface temperatures (SSTs) are used to identify this oscillation, but it is important to understand that changes in sub-surface ocean temperatures are the first to respond to an oncoming change in the ENSO phase. For instance, when ENSO is transitioning into a warm phase the sub-surface temperatures begin to warm above average, while a shallow layer of near average temperature remains at the surface. Eventually, the surface ocean temperatures will respond to the warming of the sub-surface temperatures, and a warm phase of the ENSO cycle ensues. The same cycle occurs, only opposite, for the cool phase of ENSO. When temperatures in the ENSO region of the Pacific are near average it is known as ENSO neutral, meaning that the oscillation is neither in a warm nor cool phase. Typically, atmospheric patterns during ENSO neutral are controlled more by other climate patterns (NAO, PNA) that vary on shorter timescales; these are examined on the following pages.

El Niño (Warm Phase)
The warm phase of the ENSO cycle features warmer than normal SSTs across the central and eastern equatorial Pacific
along with:

Weaker low-level atmospheric winds along the equator
Enhanced convection across the entire equatorial Pacific
Effects are strongest during northern hemisphere winter due to the fact that ocean temperatures worldwide are at their warmest. This increased ocean warmth enhances convection, which then alters the jet stream such that it becomes more active over parts of the U.S. during El Niño winters. This results in enhanced precipitation across the southern U.S., including NC
In the southeast, winter temperatures are often cooler than normal
During hurricane season (June to November), the jet stream is aligned in such a way that the vertical wind shear is increased over the Caribbean and Atlantic. The increased wind shear helps to prevent tropical disturbances from developing into hurricanes

La Niña (Cool Phase)
This phase of the ENSO cycle features cooler than normal SSTs across the central and eastern equatorial Pacific along with:

Stronger low-level atmospheric winds along the equator
Decreased convection across the entire equatorial Pacific results in a more suppressed southern jet stream. Consequently, the southern U.S., including NC, sees less precipitation
In the U.S., winter temperatures are often warmer than normal in the southeast, and cooler than normal in the Northwest
During hurricane season (June to November), upper level winds are much lighter, and therefore more favorable for hurricane development in the Caribbean and Atlantic

AO/NAO
Arctic Oscillation (AO)
The Arctic Oscillation (AO) is a climate index of the state of the atmospheric circulation over the Arctic. It consists of a negative phase, featuring below average geopotential heights , which are also referred to as negative geopotential height anomalies , and a positive phase in which the opposite is true. In the negative phase, the polar low pressure system (also known as the polar vortex) over the Arctic is weaker, which results in weaker upper level winds (the westerlies). The result of the weaker westerlies is that cold, Arctic air is able to push farther south into the U.S., while the storm track also remains farther south. The opposite is true when the AO is positive: the polar circulation is stronger which forces cold air and storms to remain farther north. The Arctic Oscillation often shares phase with the North Atlantic Oscillation (NAO) (discussed below), and its phases directly correlate with the phases of the NAO concerning implications on weather across the U.S.
North Atlantic Oscillation (NAO)

The North Atlantic Oscillation (NAO) consists of two pressure centers in the North Atlantic: one is an area of low pressure typically located near Iceland, and the other an area of high pressure over the Azores (an island chain located in the eastern Atlantic Ocean). It is important to note that these two locations are most commonly used to measure the NAO, but studies have found that the pressure centers move around on a seasonal basis, and other locations have also been used for measuring this index. Fluctuations in the strength of these features significantly alters the alignment of the jet stream, especially over the eastern U.S., and ultimately affects temperature and precipitation distributions in this area. It is also important to note that the AO and NAO are two separate indices that are ultimately describing the same phenomenon of varying pressure gradients in the northern latitudes and the resultant effects on temperature and storm tracks across the continent

Positive NAO
During a positive NAO there is a strengthening of the Icelandic low and Azores high. This strengthening results in an increased pressure gradient over the North Atlantic, which cause the westerlies to increase in strength. The increased westerlies allow cold air to drain off the North American continent rather than letting it build up and move south.
Above average geopotential heights  are observed over the eastern U.S., which correlates to above average temperatures
The eastern U.S. often sees a wetter pattern with stronger storms during the winter season in this phase due to increased upper level winds
Recent studies at the SCO indicate a decreased potential for wintry weather in NC due to the lack of cold air availability and above average temperatures associated with a positive NAO in this region

Negative NAO
A negative NAO indicates weakening of both the Icelandic low and Azores high, which decreases the pressure gradient across the North Atlantic. This decreased pressure gradient results in a slackening of the westerlies. The decrease in the westerlies allows cold air to build up over Canada, and this combined with below average heights (troughing) over the eastern U.S. gives the cold air a greater chance to move south and affect the eastern United States.
Below average geopotential heights  are often observed over the eastern U.S. during the negative phase of the NAO, which correlates to below average temperatures
The eastern U.S. typically receives colder, drier air masses during the winter season in this phase
Recent studies at the SCO indicate an increased potential for wintry weather in NC due to the position and availability of cold air, and a more favorable upper level pattern conducive to coastal storm tracks

PNA
The Pacific/North American teleconnection pattern (PNA) is one of the most recognized, influential climate patterns in the Northern Hemisphere mid-latitudes beyond the tropics. It consists of anomalies in the geopotential height  fields (typically at 700 or 500mb) observed over the western and eastern United States. It is important to note that the PNA has been found to be strongly influenced by the El Niño-Southern Oscillation (ENSO) phenomenon. The positive phase of the PNA pattern tends to be associated with Pacific warm episodes (El Niño), and the negative phase tends to be associated with Pacific cold episodes (La Niña).

Positive PNA
The positive phase consists of above normal geopotential heights over the western U.S. and below normal geopotential heights over the eastern U.S. This correlates to ridging over the western U.S., and deep troughing over the east. The net result of the height field pattern in this phase is that it forces cold air residing in Canada to plunge southeastward, which results in below normal temperatures over the eastern U.S. and above normal temperatures over the western U.S.
Research at the SCO indicates that a positive PNA, especially during an El Niño year, produces an above average number of winter weather events in NC

Negative PNA
The negative phase features troughing and below normal geopotential heights over the western U.S. and ridging with above normal geopotential heights over the eastern U.S. The result is below average temperatures for the western U.S., and above average temperatures over the eastern U.S.
Research at the SCO indicates that a negative PNA typically results in a reduced potential for winter
weather in NC

PDO
The Pacific Decadal Oscillation (PDO) is a pattern of Pacific climate variability similar to ENSO in character, but which varies over a much longer time scale. The PDO can remain in the same phase for 20 to 30 years, while ENSO cycles typically only last 6 to 18 months. The PDO, like ENSO, consists of a warm and cool phase which alters upper level atmospheric winds. Shifts in the PDO phase can have significant implications for global climate, affecting Pacific and Atlantic hurricane activity, droughts and flooding around the Pacific basin, the productivity of marine ecosystems, and global land temperature patterns. Experts also believe the PDO can intensify or diminish the impacts of ENSO according to its phase. If both ENSO and the PDO are in the same phase, it is believed that El Niño/La Nina impacts may be magnified. Conversely, if ENSO and the PDO are out of phase, it has been proposed that they may offset one another, preventing "true" ENSO impacts from occurring. Researchers have found evidence for just two full PDO cycles in the past century: cold PDO regimes prevailed from 1890-1924 and again from 1947-1976, while warm PDO regimes dominated from 1925-1946 and from 1977 through the mid-1990's.

Warm PDO
The broad area of above average water temperatures off the coast of North America from Alaska to the equator is a classic feature of the warm phase of the Pacific Decadal Oscillation (PDO). The warm waters wrap in a horseshoe shape around a core of cooler-than-average water. Impacts from the PDO depend in part on how it is aligned with the ENSO cycle; if the cycles are in opposite phases, then effects will be weakened. However, when both the PDO and ENSO are in the warm phase, meaning ENSO would be in the El Niño phase, expected impacts on the southeast include:
Below average winter temperatures
Above average winter precipitation

Cold PDO
Opposite of the warm PDO, the expansive area of below average water temperatures off the coast of North America from Alaska to the equator signals the cold phase of the PDO. The area of warmer-than-average sea surface temperatures in the central Pacific are surrounded by below average temperatures near the North American continent. Expected impacts from a cold PDO and ENSO (La Nina) phase on the southeast include:
Above average winter temperatures
Below average winter precipitation
Title: Types of Southeastern winter storms and snow data
Post by: bigalpha on December 11, 2009, 10:07:55 AM
Types of Southeastern winter storms

Lakes Cutter: Slang term given to a low pressure area that originates in the southwest and turns northeast before it gets to the deep south.  From TX, up through AR, MO towards the Great Lakes.

Apps Runner: Surface low that turns northeastward and rides near the Appalachian mountains.

Southern Slider: Surface low rides the gulf coast, or just north of the gulf. Typically goes out to sea, though can turn up the east coast.

Gulf Runner: low pressure that rides along the baroclinic zone (area where cooler temps meet warmer temps) in the Gulf of Mexico.  Usually a Gulf Runner hugs the gulf coast then turns northeastward as it hits the Atlantic.


Snow Data
Monthly snowfall at Nashville, TN (BNA): LINK (http://www.srh.noaa.gov/ohx/?n=monthlysnow)
Title: Re: Winter WX Basics
Post by: bigalpha on December 11, 2009, 10:08:07 AM
Reserved.
Title: Re: Winter WX Basics
Post by: bigalpha on December 11, 2009, 10:08:21 AM
Reserved.
Title: Re: Winter WX Basics
Post by: bigalpha on December 11, 2009, 10:08:34 AM
Reserved.
Title: Re: Winter WX Basics
Post by: bigalpha on December 11, 2009, 10:08:43 AM
Reserved.
Title: Re: Winter WX Basics
Post by: Woodvegas on February 02, 2010, 06:42:00 PM
This is from stratuslove at Easternuswx...

http://www.easternuswx.com/bb/index.php?showtopic=222197

Quote

Here is a brief guide for all you weather weenies just starting out to when you can find your models....

What time is it?
Weather models are run in UTC, which is Coordinated Universal Time, this can also be referred to as Zulu time (hence the “Z”) which is military time, or GMT, Greenwich Mean Time

“UTC” Coordinated Universal Time = “Z” Zulu = “GMT” Greenwich Mean Time

Weather observations are also reported in this style.

0000z is midnight (@ the Greenwich Meridian)
1200z is noon (@ the Greenwich Meridian)

That’s great, but what time is it here?

Eastern Standard Time (winter) is 5 hours behind UTC
Ex 12z = 7am
00z= 7pm (Remembering that 00z is actually midnight)

During Eastern Daylight Savings (summer) is 4 hours behind the UTC
Ex 12z = 8am
00z = 8pm

LOCAL EDT EST
Midnight 400 500
1 a.m. 500 600
2 a.m. 600 700
3 a.m. 700 800
4 a.m. 800 900
5 a.m. 900 1000
6 a.m. 1000 1100
7 a.m. 1100 1200
8 a.m. 1200 1300
9 a.m. 1300 1400
10 a.m. 1400 1500
11 a.m. 1500 1600
NOON 1600 1700
1 p.m. 1700 1800
2 p.m. 1800 1900
3 p.m. 1900 2000
4 p.m. 2000 2100
5 p.m. 2100 2200
6 p.m. 2200 2300
7 p.m. 2300 2400
8 p.m. 2400 100
9 p.m. 100 200
10 p.m. 200 300
11 p.m. 300 400
LOCAL EDT EST


Ok, now that’s settled, what time can I see each model run?
This depends on the model. It is important to remember with model run timing that the time indicated is actually when the model begins its run for instance 12z starts to run at (Noon Greenwhich mean time) 7 EST. Some models are faster than others.

Here I will outline the NAM, GFS, Euro, UKMET, GGEM and SREF for you.

The following times are for EST.
1.)NAM( North American Mesoscale Model) Model forecasts are produced every six hours at 00, 06, 12 and 18 UTC. Data is available in 6 hour incriments out to 84 hours.
NAM
0z: 8:45pm
6z: 2:45am
12z: 8:45am
18z: 2:45pm

2.)GFS (Global Forecast System) Model Forecasts are also produced every 6 hours. (00, 06, 12, & 18). Like the NAM they are available in 6 hour increments. Unline the NAM, the GFS is a longer range model and goes out to 384 hours.

GFS
0z: 10:30pm
6z: 4:30am
12z: 10:30am
18z: 4:30pm

3.)ECWMF( The Euro) forecasts are a bit more complicated. Some of the data from the Euro is subscription only and will change the times that you can access it for each run. Some sites allow you to access it faster than others. Many people on the board use the WSI subscription Euro, so that’s what we will outline here. The Euro has 2 runs for us here , 0z and 12z. Depending on where you access the ECWMF, will depend whether you can access 6 or 12 hour increments.

Euro (subscription services)
0z: 1:00am
12z: 1:00pm

Euro (Free services)
0z: 1:30am
12z: 1:30pm

4.)UKMET (United Kindgom Meteorological Forecast Model) is issued at 00z, 12z On most sites it is a 72 hour model that is in 6 hour increments through 48 hours, and then 12 hour increments through 72 hours.(you can also get the Ukie out to 144 in 24 hour increments) It is also run at 06z and 18z but only goes out to 60 hours.
UKMET
0z: 11:30pm
12z: 11:30am

5.)GGEM (Global Environmental Multiscale Model) Forecasts are another out of country model. This one we get twice a day 12z and 00z.
GGEM
0z: 11:45pm
12z : 11:45am
6.)SREF(Short Range Ensemble Forecasts) are a set of ensembles that are issued 4 times a day at “off” hours. These are run at 03z, 09z 15z, and 21z and run though hour 87.

09z: 8:40am
15z: 2:40pm
21Z: 8:40pm
03z: 2:40am

Title: Re: Winter WX Basics
Post by: Woodvegas on February 02, 2010, 06:46:48 PM
Good link for US models...

http://www.nco.ncep.noaa.gov/pmb/nwprod/analysis/namer/


Here's a link for raw data, meteorograms, soundings, etc...

http://www.arl.noaa.gov/READYcmet.php
Title: Re: Winter WX Basics
Post by: msdawg911 on February 02, 2010, 10:17:06 PM
One site that I've favored for years is Wxcaster.com (http://www.wxcaster.com/weather.php3). The page is simple in layout, but it has a wealth of model maps and similar info.
Title: Re: Winter WX Basics
Post by: bigalpha on January 04, 2011, 10:24:09 PM
Thanks - I put that info in there, and updated some other stuff too.
Title: Re: Winter WX Basics
Post by: Chelsea in Tn on January 17, 2011, 06:45:13 PM
Thank you so much for your time and effort to put this together. I do not have time to read all of it now... but I definitely will be reading it!
Title: Re: Winter WX Basics
Post by: SKEW-TIM on February 02, 2011, 08:42:28 AM
Can someone please explain - Upper Level Lows are vs. Surface Level Lows? I know what LOW Pressures are, but, I am not that familiar with ULLs or a surface low....   
Title: Re: Winter WX Basics
Post by: justinmundie on February 02, 2011, 08:48:38 AM
Can someone please explain - Upper Level Lows are vs. Surface Level Lows? I know what LOW Pressures are, but, I am not that familiar with ULLs or a surface low....   

It's basically a difference in where the low is in the atmosphere. Surface lows are at the surface. Upper level lows are at 500mb
Title: Re: Winter WX Basics
Post by: SKEW-TIM on February 02, 2011, 08:54:49 AM
Is one stronger than the other? Or is it simple just a case of where the area of circulation is?

For example:

SURFACE LOW- Circulation and weather system is at the ground

ULL- Circulation and weather system is above 500mb- Which means the effects of the system can be felt higher up?
Title: Re: Winter WX Basics
Post by: justinmundie on February 02, 2011, 09:08:31 AM
Is one stronger than the other? Or is it simple just a case of where the area of circulation is?

For example:

SURFACE LOW- Circulation and weather system is at the ground

ULL- Circulation and weather system is above 500mb- Which means the effects of the system can be felt higher up?


No each system has different dynamics. A ull can spin up a surface low and they work in tandem.  But they bot h translate to weather at the surface
Title: Re: Winter WX Basics
Post by: SKEW-TIM on February 02, 2011, 09:21:17 AM
I see, So a ULL can create a Surface Low- Does that intensify the Low Pressure system? or is that what they "closing off" ? 
Title: Re: Winter WX Basics
Post by: Stovepipe on February 02, 2011, 11:26:42 AM
Seems like a good place to ask my question.  Why is it, when looking at the upper air maps, the energies (vorts, eddys, s/w's etc) are generally 100 to 200 miles behind where the precip is breaking out on the surface?  What causes this apparent lag?

I'd love to get some pointers on reading the H5 maps in general.  I mean I can get the gist of the flow of the troughs but it isn't clear to me how this corresponds to what we see on the surface maps.  I do notice that the front side of these upper level energies often lines up perfectly with the 540 thickness line shown on the surface.

Any insight on this would be appreciated.
Title: Re: Winter WX Basics
Post by: Thundersnow on February 02, 2011, 11:32:31 AM
I believe that's how systems typically work... the precip will tend to be out ahead of the center of the low, since that's where the inflow of moisture is.
Title: Re: Winter WX Basics
Post by: Nashville_Wx on September 13, 2011, 08:48:35 PM
I believe that's how systems typically work... the precip will tend to be out ahead of the center of the low, since that's where the inflow of moisture is.

Precipitation is most likely to right of shortwave axis/500mb trough axis.
Title: Re: Winter WX Basics
Post by: Cyclonicjunkie on September 13, 2011, 09:40:40 PM
Seems like a good place to ask my question.  Why is it, when looking at the upper air maps, the energies (vorts, eddys, s/w's etc) are generally 100 to 200 miles behind where the precip is breaking out on the surface?  What causes this apparent lag?

Hey Stove, I know this is a old question but I didnt see it until Nashville wx brought this thread out of the graveyard ;D


Some extratropical surface lows become so strong that they bend towards the direction they are headed. That alone is enough to displace the upper level energy/ULL hundreds of miles away to the west, SW, or NW. But usually the reason you see this... is that troffs and shortwave troffs associated with extratropical surface lows bends towards the cooler air which pushes the ULL's/ upper level vorticies/energies that accompany the surface low farther away to the west, SW, or NW from the actual low level stormcenter that u see on radar.

Now when the upper low and surface low become stacked it is due to a strongly amplified troff becoming closed off and the rest of the upper level jet is free to race off straight ahead. Then the surface low sorta loses its baroclinicity and steering mechanism to some degree allowing the ULL to fill in and become vertically stacked above the surface low..... much like a tropical systems evolution. ::rain::

But when this happens it usually becomes cut off from the jet stream ( Hence the name cutoff low) and there isnt much left to steer it and it sorta just sits there and weakens as it dumps its energy in the form of precip whether that be rain (You've ever heard the terminology 'Rain itself out") or it could be snow depending on the temps of the airmass it occupies. ::snowman::

Or some other synoptic system will grab hold of it, and steer it away to possibly restrengthen. But once an extratropical system loses alot of its baroclinicity it will lose it's source for attaining energy and if it dont regain some baroclinicity soon it won't be able to deepen or intensify.
 

Thats how I understand it anyways, if you have any more specific questions on upper air meteorology, just ask me and I will answer it if I can. :)


 

Title: Re: Winter WX Basics
Post by: Adam on September 13, 2011, 09:45:37 PM
Does anyone know if there is any sort of skill to pointing out where a heavy band of snow may form in a snow storm. Lately they have really been South of I-40 I was just wondering why.
Title: Re: Winter WX Basics
Post by: Cyclonicjunkie on September 13, 2011, 10:08:52 PM
Does anyone know if there is any sort of skill to pointing out where a heavy band of snow may form in a snow storm. Lately they have really been South of I-40 I was just wondering why.

Usually you will get the heaviest snow right on the border of the cold and warm sectors of a LPS. This is where there is ALOT of moisture available because of the warmer air and its just barely cold enough to snow so you get the best of both worlds....Warm moist air mixed with temps just barely cold enough to turn the moisture into snow. Its usually big wet silver dollar flakes in this hybrid zone.
Title: Re: Winter WX Basics
Post by: Nashville_Wx on September 14, 2011, 12:18:01 AM
Does anyone know if there is any sort of skill to pointing out where a heavy band of snow may form in a snow storm. Lately they have really been South of I-40 I was just wondering why.

The storm took a track that was favorable for that area. Different type of systems have different zones that favor heavy snow. The area where vertical lift is the strongest, strong omega zone values... Heavy snow can have many different moisture contents so the total QPF could be different. Hopefully we all can watch things pan out this year. I am ready for 7-8" + Imby...
Title: Re: Winter WX Basics
Post by: Cyclonicjunkie on September 14, 2011, 01:28:11 AM
Different type of systems have different zones that favor heavy snow.

You only have two diferent type of extratropical systems in general that produce snow here. You have your normal Mid lattitude extratropical cyclone (Miller a's, b's  Southern sliders, Alberta clippers, Manitoba Maulers, Apps Runners, and Lakes cutters) are all extratropical mid lattitude cyclones that only produce snow  in the cold sector of the LPS here in the SE.

Then you have a upper/mid level LPS (AKA cold core low, AKA bowling ball) which produces its on cold air dynamically and at the center of the ULL/MLL., you will find most, if not all of the snow in these at the very center of the low. 

They are really unpredictable and are usually diurnally driven due to the sun warming the ground and destabilizing the atmosphere and that causes upward VERT motion which usually only creates snow during the day.

All of this is why you usually dont know where the snow will fall with these types of LPS's that are aloft.
Title: Winter WX Basics
Post by: Thundersnow on September 14, 2011, 07:47:48 AM
(Miller a's, b's  Southern sliders, Alberta clippers, Manitoba Maulers, Apps Runners, and Lakes cutters)

And, don't forget about Saskatchewan Screamers. ;)
Title: Re: Winter WX Basics
Post by: Cyclonicjunkie on September 14, 2011, 07:58:10 AM
And, don't forget about Saskatchewan Screamers. ;)

Yes I forgot about that bad boy ;) ::snowman::
Title: Re: Winter WX Basics
Post by: cbrentv3 on February 03, 2014, 10:01:09 PM
Can anyone explain a miller b type of storm they are talking about.

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Title: Re: Winter WX Basics
Post by: justinmundie on February 03, 2014, 10:10:15 PM
Can anyone explain a miller b type of storm they are talking about.

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A miller b is a storm where the low pressure center transfers. Most of the time they are either clippers or apps runners that get stuck by cold high pressure and what was the primary low spawns a new surface low to its east.
Title: Re: Winter WX Basics
Post by: cbrentv3 on February 03, 2014, 10:13:31 PM
Thank you for the reply. I thought this was the propper place to ask. 

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Title: Re: Winter WX Basics
Post by: Eric on February 03, 2014, 10:24:21 PM
Graphical representation:

(http://www.erh.noaa.gov/rnk/Newsletter/Fall%202006/Climatology_of_Heavy_Snow_files/Miller_B.jpg)
Title: Re: Winter WX Basics
Post by: cbrentv3 on February 03, 2014, 10:30:41 PM
Thank u for the info.

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Title: Re: Winter WX Basics
Post by: cbrentv3 on February 06, 2014, 04:22:54 PM
Eric would you care to explain what gfs feed back problems that hun is saying

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Title: Re: Winter WX Basics
Post by: Eric on February 06, 2014, 04:25:58 PM
Eric would you care to explain what gfs feed back problems that hun is saying

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From the WPC:
Quote
When specific thresholds in the mass fields are met, convective scheme is triggered and then dumps a large amount of QPF over a grid point - releasing so much latent heat over the grid point that the model is forced to adjust the mass fields by producing a local vertical motion max in the mid troposphere (~ 500mb), a corresponding upper level jet max over the vertical motion max - an intense and small scale vort max in the mid levels (MCV).
The model scales up the mesoscale circulation at mid levels and holds onto it as a real feature for as long as 3 days.

The model can produce precipitation in association with the feature as it tracks along in the flow.

Basically, the model "sees" something that's not there and dumps qpf in response.
Title: Re: Winter WX Basics
Post by: cbrentv3 on February 06, 2014, 06:45:52 PM
With the Pacific moisture flowing in the west will that limit the movement in the noth pole of the bitter cold?   Will it shove the bitter cold to another part of the globe? Lastly how was spring 1986 in general.  Thank your

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Title: Re: Winter WX Basics
Post by: Eric on February 06, 2014, 07:56:03 PM
With the Pacific moisture flowing in the west will that limit the movement in the noth pole of the bitter cold?   Will it shove the bitter cold to another part of the globe? Lastly how was spring 1986 in general.  Thank your

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Honestly, I have no idea on either counts, sorry.  I was only 9 in '86...I have no idea how it was.  Maybe some of our climatology folks can chime in.  That's not my bag.
Title: Re: Winter WX Basics
Post by: collinsk on February 06, 2014, 08:10:00 PM
Spring 1986 was not above or below normal. I recall it as average. i was 22 that year. We had a very warm April and May I recall.
Title: Re: Winter WX Basics
Post by: @NashSevereWx on December 03, 2014, 12:14:19 PM
For the record, I was 11 in 1986.
Title: Re: Winter WX Basics
Post by: bugalou on December 29, 2018, 01:09:56 PM
Can anyone explain a miller b type of storm they are talking about.

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Back when I was in NJ Miller B's where the systems that constantly screwed us over with snow fall, or better siad, lack there of.        ::doh::                                                                                                                                                                                               
Title: Re: Winter WX Basics
Post by: JayCee on December 29, 2018, 05:37:51 PM
Back when I was in NJ Miller B's where the systems that constantly screwed us over with snow fall, or better siad, lack there of.        ::doh::                                                                                                                                                                                               

You love diggin' in the dusty archives, don't ya?  ;)

(https://www.timeshighereducation.com/sites/default/files/Pictures/web/y/a/l/archives.jpg)