What's a Dam?

[WFayetteTN]

This is a section of today KMEM AFD.

LONG TERM...SATURDAY THROUGH TUESDAY.

COLD CORE UPPER LOW OVER THE MID MISSOURI VALLEY EASTWARD ACROSS
THE LOWER GREAT LAKES WILL SINK SOUTH FOR THE WEEKEND AND EARLY
NEXT WEEK. ON AND OFF CHANCES FOR SHOWERS AND A FEW STORMS
CONTINUE WITH GENERALLY LIGHT AMOUNTS EXPECTED. THE CORE WILL PUSH
ACROSS THE MIDSOUTH MONDAY BRINGING A PRETTY GOOD CHANCE FOR
SHOWERS. THICKNESSES OF 546 DAM...CLOUD COVER...AND A BRISK
NORTHERLY FLOW WILL PROBABLY KEEP HIGHS IN THE UPPER 50S TO MID
60S.

A WARMING TREND IS FORECAST TOWARD THE MIDDLE OF NEXT WEEK...BUT
THE NORTHEAST FLOW WILL CONTINUE SO TEMPS SHOULD STILL REMAIN
BELOW AVERAGE.

NOAA's dictionary says:

Dam
    In hydrologic terms, any artificial barrier which impounds or diverts water. The dam is generally hydrologically significant if it is:
    1. 25 feet or more in height from the natural bed of the stream and has a storage of at least 15 acre-feet.
    2. Or has an impounding capacity of 50 acre-feet or more and is at least six feet above the natural bed of the stream.

Can someone provide a bit more detail, less technical, plain english?

[toastido]

This is a section of today KMEM AFD.

NOAA's dictionary says:
Can someone provide a bit more detail, less technical, plain english?

I'm almost sure that they are not referring to an actual river/stream dam, but more likely an airmass dam...especially since they mentioned the thickness.

CAD
Cold Air Damming. The phenomenon in which a low-level cold air mass is trapped topographically. Often, this cold air is entrenched on the east side of mountainous terrain. Cold Air Damming often implies that the trapped cold air mass is influencing the dynamics of the overlying air mass, e.g. in an overrunning scenario. Effects on the weather may include cold temperatures, freezing precipitation, and extensive cloud cover

Here's some additional information on air dams and thickness:  http://weatherfaqs.org.uk/node/152

[Kevin]

Its not hydrologic related...in this sense. Dam is an abbreviation for the unit of measure "decameters"...which is equal to 10 meters. Its used for measuring atmospheric thickness...which can give a very broad sense to atmospheric temperature.

If you've heard about the "540" line which many use as a rain/snow line in the winter (though its not very accurate most of the time)...its indicating an atmospheric thickness of 540 decameters (dam or dm)...which is 5,400 meters...and can correlate to surface temperatures near freezing.

In this case...the AFD is talking about 546 dam thickness...which is one reason why cool temperatures can be expected for that period.

I'm almost sure that they are not referring to an actual river/stream dam, but more likely an airmass dam...especially since they mentioned the thickness.

Here's some additional information on air dams and thickness:  http://weatherfaqs.org.uk/node/152

You're right on with it having to do with thickness...but in this particular case it doesn't have anything to do with cold air damming/CAD either. That's a phenomenon that's generally limited to the Appalachian region.

[beneficii]

Its not hydrologic related...in this sense. Dam is an abbreviation for the unit of measure "decameters"...which is equal to 10 meters. Its used for measuring atmospheric thickness...which can give a very broad sense to atmospheric temperature.

If you've heard about the "540" line which many use as a rain/snow line in the winter (though its not very accurate most of the time)...its indicating an atmospheric thickness of 540 decameters (dam or dm)...which is 5,400 meters...and can correlate to surface temperatures near freezing.

In this case...the AFD is talking about 546 dam thickness...which is one reason why cool temperatures can be expected for that period.

You're right on with it having to do with thickness...but in this particular case it doesn't have anything to do with cold air damming/CAD either. That's a phenomenon that's generally limited to the Appalachian region.

And that would be when you're measuring the thickness between 1000 hPa and 500 hPa.  (hPa = hectopascals.)

[Kevin]

And that would be when you're measuring the thickness between 1000 hPa and 500 hPa.  (hPa = hectopascals.)

Yep...although you can do other thickness measurements between other levels...1000-500 is most commonly used...and where the 540 and 546 in this specific case...are originating from.

[WFayetteTN]

Thanks guys. Your collective knowledge is amazing.