12/11/08 Snow Event

[Coach B]

We need to keep our eyes open for the ULL coming out on Thursday.  New thread may be needed or change this one to 9th-11th.

Per James Spann this morning:
A cold core upper low will be involved with the Thursday system, and we all know that a cold core upper low is a weatherman’s woe. Those things can produce dynamic cooling that can bring surprise snow flakes; it is happened before, and will happen again. While temperatures in the lower 5,000 feet seemingly won’t support snow, cooling from above can change things quickly.

Understand, I am sure not going there now, and we are not mentioning snow flakes in the forecast. Let’s get tonight’s event out of the way, then we can focus on Thursday. By tomorrow we will have several more model runs under our belt and we can be more comfortable with the forecast. Just understand that the idea of snow flakes somewhere in Alabama Thursday or Thursday night is back on the table to some degree.

[Woodvegas]

0Z Euro and 6Z GFS say we are really close.

[Thundersnow]

From JAN (Jackson, MS NWS office):

THE BIGGEST CONCERN IS WED NIGHT AND THU. THIS IS GETTING A LITTLE
RIDICULOUS AS THE MDLS CONTINUE TO BOUNCE BACK AND FORTH WITH THE
POSSIBILITY OF SNOW AND THIS TREND CONTINUED TONIGHT. THE LATEST GFS
AND ECMWF BOTH SHOW THE POTENTIAL FOR SNOW...AND POSSIBLY A PRETTY
SIGNIFICANT AMOUNT IF THINGS LINE UP RIGHT.

FIRST OFF LET ME SAY THAT CONFIDENCE IS LOW AND I AM HAVING A HARD
TIME BELIEVING THAT WE WILL GET MUCH SNOW BUT THE TRACK OF THE UPPER
LOW IS FAVORABLE. FIRST IT WOULD LEAD TO DYNAMIC COOLING AND BOTH
MDLS ARE SHOWING QUITE A BIT OF THIS. ALSO IT PUTS THE CWA UNDER THE
DEFORMATION ZONE. COMBINE THAT WITH A DEVELOPING TROWAL...WHICH IS
ADVERTISED BY THE GFS...AND YOU HAVE THE INGREDIENTS FOR MODERATE TO
HEAVY SNOW. ALSO FCST SNDGS BY BOTH THE GFS AND ECMWF SHOW THE COLUMN
AT SOME POINT COMPLETELY BELOW FREEZING WITH THE GFS INDICATING QUITE
A BIT OF LIFT IN THE DENDRITIC LAYER. RIGHT NOW I AM GOING TO TAKE A
BLEND OF THE MDLS AND JUST ADD LIGHT SNOW MIXING IN WITH RAIN DURING
THE OVERNIGHT HRS WED AND ON THU BUT WE MAY HAVE TO MAKE THAT
MODERATE SNOW IF THE MODELS REMAIN CONSISTENT. THE OPS RUN OF THE GFS
IS NOT OUT OF LINE WITH THE GFS ENSEMBLES AND THIS IS GIVING A LITTLE
MORE CREDIBILITY TO THE GFS. IF THE GFS ACTUALLY VERIFIES THEN THERE
WILL BE A STRIPE OF SIGNIFICANT SNOW ACROSS OUR CWA...BUT IF THAT
OCCURS THE LOCATION IS JUST TOO DIFFICULT TO PREDICT AT THIS TIME.
ALL IT TAKES IS A LITTLE DEVIATION IN TRACK OR STRENGTH IN THE UPPER
LOW AND THAT THIN STRIPE COULD BE DISPLACED 50 MILES OR SO. THAT LITTLE
CHANGE COULD ALSO BE THE DIFFERENCE BETWEEN GETTING SNOW OR KEEPING
EVERYTHING COMPLETELY LIQUID. SECOND IF THE DYNAMIC COOLING IS NOT AS
IMPRESSIVE AS THE MDLS ARE INDICATING THEN WE WILL LIKELY STAY
COMPLETLY LIQUID. ONE MORE NEGATIVE ASPECT...IT IS STILL EARLY
DECEMBER AND CLIMATOLOGICALLY SPEAKING THIS IS NOT THE BEST TIME FOR
US TO GET SNOW STORMS. NOW IF THIS WAS JAN OR FEB THEN WE WOULD
LIKELY BE TALKING ABOUT A SIGNIFICANT WINTER STORM.

PRECIP SHOULD COME TO AN END THU EVENING/OVERNIGHT AND SKIES WILL
START TO CLEAR OUT. TEMPS ARE A DIFFICULT PROPOSITION RIGHT NOW B/C
A LOT WILL DEPEND IF WE HAVE SNOW ON THE GROUND. IF THERE IS THEN
LOWS FRI MORNING WILL BE COLDER THAN EXPECTED. DUE TO THE
UNCERTAINTY I WILL JUST STICK WITH THE MEX GUI FOR NOW. HIGHS WILL
ALSO BE BELOW NORMAL WITH SOME LOCATIONS POSSIBLY QUITE COOL IF(BIG
IF THERE) IS SNOW ON THE GROUND.

[lyngo]

0Z Euro and 6Z GFS say we are really close.

12Z NAM shows precip inching a little closer as well.....

[lyngo]

12z GFS looks like it may be coming in further west with the gulf low.  Precip shield inching even closer to the mid-state...

[Thundersnow]

12Z NAM colder than the GFS but with precip further south.  12Z GFS is warmer with impressive precip making it into Middle TN.

Happy medium? 

[Cameron K.]

So know we rely on just a circle of cold air.  

http://www.nco.ncep.noaa.gov/pmb/nwprod/analysis/namer/gfs/12/images/gfs_pcp_060s.gif

Now we just got to hope it will cold enough. I mean warm enough; don't forget about reverse psychology.  

[servocrow]

HPC Low track forecast


http://www.hpc.ncep.noaa.gov/wwd/lowtrack_circles.gif

Yowza!!  That was huge!

[Thundersnow]

Actually, this is looking pretty doggone interesting:



 

[lyngo]

Yep, the ole 12z has piqued my interest! 

[Cameron K.]

Yep, the ole 12z has piqued my interest! 

 We can only hope this won't happen. 

[Eric]

Actually, this is looking pretty doggone interesting:



 

Here is the same run...but of the 2m surface temps...



Either the 2m run is an aberration or the 850mb run is the aberration.  I still don't think the GFS knows WTH it's doing.  I can't believe that there is such a disparity between temps like that...unless of course, the low is a cold-core low...that could explain why the temps are so cold behind it, but not at the surface...

[Coach B]

Was it 2004 when a ULL laid down an impressive, but narrow, strip of snow over southern mid Tenn?  We had 6 or 7 inches here while my folks 40 miles north in Williamson County had nothing, and folks 20 miles south had nothing.  

[Thundersnow]

Here is the same run...but of the 2m surface temps...

Keep in mind the possibility of dynamic cooling in the column if that low precipitates enough.  The 2M temps may not be handling that scenario well.

Besides... heavy wet snow can fall with 35-36 degree temps, in which case we would be south of the 2M temp line.

[Thundersnow]

Was it 2004 when a ULL laid down an impressive, but narrow, strip of snow over southern mid Tenn?  We had 6 or 7 inches here while my folks 40 miles north in Williamson County had nothing, and folks 20 miles south had nothing.  

Yes, and up here in Nashville, it was a mixture of light rain, sleet, and snow during that situation.  We were NORTH of a band of heavy snow but were only getting mixed precip... and may have even been above freezing.  That shows the power of dynamic cooling.