*FEBUARY 19th/20th 2012 SNOW THREAT*

DISCUSSION...

SURFACE HIGH PRESSURE OVER THE MID SOUTH THIS MORNING WILL BEGIN
TO RETREAT EASTWARD AS MID AND UPPER LEVEL MOISTURE CONTINUES TO
STREAM ACROSS THE AREA IN SOUTHWEST FLOW ALOFT. PARTLY CLOUDY
SKIES AND SEASONABLY MILD TEMPERATURES CAN BE EXPECTED TODAY.

LOW LEVEL MOISTURE WILL BEGIN TO INCREASE FROM THE SOUTH LATER
TONIGHT AS AN UPPER LEVEL LOW PRESSURE NOW OVER NORTHERN MEXICO
BEGINS TO LIFT NORTHEASTWARD INTO THE SOUTHERN PLAINS. ABUNDANT
DEEP LAYER MOISTURE WILL ADVECT NORTHWARD INTO THE MID SOUTH ON
SATURDAY AS THE UPPER LOW GETS CLOSER TO THE REGION. THIS SOUTHERN
STREAM SHORTWAVE WILL ATTEMPT TO PHASE WITH A NORTHERN STREAM
SHORTWAVE DROPPING SOUTHEAST FROM THE UPPER MISSISSIPPI VALLEY. AS
THIS PHASING BEGINS TO OCCUR LATER ON SATURDAY...EXPECT SURFACE
LOW PRESSURE ORGANIZING ALONG THE NORTHWEST GULF COAST ON SATURDAY
MORNING TO LIFT NORTHEAST AND INTENSIFY ACROSS THE SOUTHEASTERN
STATES BY SATURDAY EVENING. WIDESPREAD RAIN CAN BE EXPECTED ACROSS
MUCH OF THE REGION...WITH THE EXCEPTION BEING AREAS NEAR THE
MISSOURI/KENTUCKY BORDER WHERE SLIGHTLY DRIER LOW LEVEL
AIR...ADVECTED SOUTHWARD BY NORTHEAST SURFACE WINDS...KEEPS RAIN
CHANCES LOWER. THERE WILL LIKELY EVEN BE ENOUGH ELEVATED
INSTABILITY ACROSS PORTIONS OF NORTH MISSISSIPPI BY SATURDAY
AFTERNOON AND EVENING TO RESULT IN A FEW THUNDERSTORMS.

AS THE UPPER LEVEL SYSTEMS BECOME COMPLETELY PHASED OVER THE MID
SOUTH LATER SATURDAY NIGHT AND AS COLDER AIR WRAPS AROUND THE
NORTHWEST SIDE OF SURFACE LOW PRESSURE OVER ALABAMA...THERE WILL
BE THE POTENTIAL FOR RAIN TO CHANGE TO SNOW OVER AREAS PRIMARILY
ALONG AND NORTH OF INTERSTATE 40 AFTER MIDNIGHT. LATEST MODEL
GUIDANCE INDICATES THE DEVELOPMENT OF A DEFORMATION ZONE TO THE
NORTHWEST OF THE SURFACE LOW. THIS DEFORMATION BAND WILL LIKELY
SET UP ACROSS THE MID SOUTH...PRIMARILY ALONG AND NORTH OF THE
INTERSTATE 40 CORRIDOR. STRONG LIFT WITHIN THIS BAND WILL LIKELY
LEAD TO PERIODS OF HEAVIER PRECIPITATION AND MAY ALSO RESULT IN
DYNAMIC COOLING AS THE LOW/MID LEVEL AIRMASS IS RAPIDLY LIFTED AND
COOLED. THIS COULD RESULT IN RAIN CHANGING TO SNOW...WITH THE
POTENTIAL FOR SEVERAL HOURS OF MODERATE TO POTENTIALLY EVEN HEAVY
SNOW. THERE IS STILL UNCERTAINTY WITH RESPECT TO BOUNDARY LAYER
TEMPERATURES...BUT SUFFICE TO SAY THE MODELS ALL CONTINUE TO HINT
AT THIS POTENTIAL. CONSIDERING THE STRENGTH OF THE SURFACE LOW AND
UPPER LEVEL SYSTEM...CHANCES FOR THIS TYPE OF MESOSCALE BANDING
AND DYNAMIC COOLING WITHIN THE BAND APPEARS TO BE INCREASING.
THEREFORE...HAVE MAINTAINED RAIN OR SNOW ACROSS THE NORTHERN HALF
OF THE REGION AFTER MIDNIGHT SATURDAY THROUGH SUNDAY MORNING. PLAN
TO ENHANCE THE WORDING IN THE HWO AND MENTION THE POTENTIAL FOR
ACCUMULATING SNOWFALL ACROSS EASTERN/NORTHEASTERN ARKANSAS...THE
MISSOURI BOOTHEEL...AND MUCH OF WESTERN TENNESSEE. THIS SYSTEM HAS
THE POTENTIAL TO PRODUCE SIGNIFICANT SNOWFALL IN AREAS BENEATH THE
DEFORMATION BAND. ONCE AGAIN...THIS IS HIGHLY DEPENDENT UPON
BOUNDARY LAYER TEMPERATURES. HOPEFULLY THESE DETAILS WILL BE
WORKED OUT OVER THE NEXT COUPLE OF FORECAST CYCLES AND DETAILED
PRECIPITATION TYPES WILL BE BETTER DEFINED.

ALL PRECIPITATION SHOULD END BY EARLY SUNDAY AFTERNOON ACROSS
EASTERN SECTIONS AS LOW PRESSURE PULLS EAST AND HIGH PRESSURE
QUICKLY BUILDS INTO THE AREA. SINCE MUCH OF THE SNOW POTENTIAL AND
COOLER TEMPERATURES WOULD BE DYNAMICALLY PRODUCED...THE AIRMASS
FOLLOWING THIS WEEKEND SYSTEM SHOULD BE QUITE MILD. TEMPERATURES
SHOULD REBOUND QUICKLY TO NEAR OR ABOVE SEASONAL AVERAGES BY
MONDAY. ANOTHER SERIES OF SHORTWAVE DISTURBANCES WILL MOVE ACROSS
THE MID SOUTH THROUGH THE MIDDLE OF NEXT WEEK WITH CHANCES FOR
SHOWERS. TEMPERATURES WILL REMAIN SEVERAL DEGREES ABOVE AVERAGE
THROUGH THE EXTENDED FORECAST PERIOD.

Adam2014 wrote:Okay, I have looked at the models. For this storm I am going to try and make my first storm call map. Just middle tennessee only, wish me luck!

Adam2014 wrote:Thank you Tom, unfortunately I think I am going to have to go with 1-3 inches south of I-40.

snowdog wrote:In my opinion the Euro snow maps on Wunderground are waaaay overdone. This is going to be a touch and go situation for Nashville. Relying on dynamic cooling/evap cooling/etc. I really think those south of I-40, who aren't in good elevation areas, are really going to have a hard time turning over. Heck even Nashville may not turn over.

I just don't want any weenie suicides due to those Wunderground maps. They were pretty bad last event.

Stovepipe wrote:You may be right snowdog, but it's not an issue with snow algorithm on the 0z Euro maps. The text data tells the story quite well. Verbatim much of TN gets rocked. Knoxville will get 3.5 if that verifies and that would be HUGE for us.

THE MAIN FORECAST CHALLENGE FOR THE DAY REMAINS THE WINTER STORM THAT
WILL BE AFFECTING THE REGION THIS WEEKEND. THE 12Z NAM CONTINUES TO
SUPPORT EARLIER THINKING OF MAINLY RAIN FALLING ACROSS THE CWA ON SAT
NIGHT AND THEN A RATHER RAPID CHANGEOVER TO SNOW NEAR DAWN ON SUN
MORNING. THE 12Z NAM QPF ALSO IS RATHER SIMILAR TO THE 6Z GFS RUN
WITH THE AXIS OF THE HIGHEST QPF A BIT FURTHER SOUTH. HOW RAPIDLY
COOLING OCCURS AND WHERE AS WELL AS HOW INTENSE THE SNOW FALLS WILL
ULTIMATELY DETERMINE SNOWFALL AMOUNTS DURING THE DAY ON SUN AS IT
WILL BE VERY MILD GOING INTO THE EVENT AND THUS GROUND TEMPS WILL BE
WARM TOO.

Non-advisory #snow is possible from AR into west TN/KY. Then it gets interesting from KY/neastTN across WV,VA. DC could be watch by PM.

AndyP wrote:I thought I would share a tweet from Jim Cantore's twitter:

Non-advisory #snow is possible from AR into west TN/KY. Then it gets interesting from KY/neastTN across WV,VA. DC could be watch by PM.

Tom wrote:2-4, eh Stove?? Have you ingested any of the 12z models in that call yet?? That seems like a brave call. Bravo.

Stovepipe wrote:

Tom wrote:2-4, eh Stove?? Have you ingested any of the 12z models in that call yet?? That seems like a brave call. Bravo.

I have some indigestion from the Mexican food I ate last night so that might have factored in.

snowdog wrote:The bad thing for West TN (memphis to Jackson) is that the low looks to go from roughly Nawlins to south of Huntsville and really gets cranking somewhere in there. So the dynamic cooling will probably get cranking somwhere around Jackson and start changing over the deform band somewhere between Jackson and Nashville. you want to be close to the low (northwest quad)...but not too close.

Stovepipe wrote:American has devolved into a ground temps debate.