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Wintry February 19-21, 2020 Winter Storm

Just woke up and took a peek at things currently. NAM falling in place and I still see the changeover taking place in the upstate around 10am-1pm. 2-4” with potentially 2-5” depending on location and if we can get that changeover a little earlier like the HRRR indicates with banding and the face that the NAM is initializing too warm with DP and temps here as well. It’s 42/30 with a NNE wind of 3-6mph so here we go.
 
38/22 currently. The most recent NAMs backed off on qpf and delayed the timing in our location. On the other hand, the RAP and HRRR are beginning to speed up the timing of the arrival of the precip and are increasing qpf. I feel good about thermals here, but will we be able to overcome the dry column and will we be able to get a decent period of heavier rates?
 
GSP Area Forecast Discussion

Classic cold chasing moisture and warm boundary layer and soil temp issues
NEAR TERM /THROUGH TONIGHT/

AS OF 300 AM: THE LATEST WATER VAPOR IMAGERY INDICATES A SPEED MAX
DIGGING ACROSS THE CORN BELT, WITH A BAROCLINIC LEAF STRUCTURE
EXTENDING FROM THE ARKLATX THROUGH THE MID-SOUTH. THUS, CYCLOGENESIS
IS WELL UNDERWAY IN THE VICINITY OF THE WESTERN GULF COAST, AND
RESULTANT INCREASE IN FRONTOGENESIS AND QG FORCING WILL ALLOW THE
PRECIP SHIELD...CURRENTLY AS FAR EAST AS CENTRAL AL AND WEST-CTRL
GA...TO CONTINUE PROGRESSING NORTHEAST, LIKELY REACHING FAR WESTERN
PORTIONS OF THE FORECAST AREA BY 12Z. IN A PERFECT WORLD, THE COLD
AIR WOULD ALREADY BE IN PLACE, AND THIS WOULD BE A BIT OF A NO-
BRAINER. ALAS, THIS IS NOT THE CASE...AS PER USUAL. AS OF 08Z,
SURFACE WET BULB TEMPS <= 32 ARE CONFINED TO ELEVATIONS ABOVE ABOUT
3000 FT ACROSS THE CTRL AND NRN NC MTNS. HOWEVER, NE FLOW RESPONDING
TO AN EXPANSIVE SURFACE RIDGE EXTENDING EASTWARD FROM AN ALMOST 1050
MB HIGH OVER THE CORN BELT WILL CONTINUE TO PUSH COOLER AND DRIER
LOW LEVEL AIR INTO THE FORECAST AREA THROUGHOUT THE DAY. HOWEVER,
THE ATMOSPHERE IS NOT EXACTLY GOING TO BE IN ANY HURRY TO PUSH THIS
COLD AIR SOUTH AND WEST...AT LEAST NOT UNTIL THE LOW LEVEL FLOW
INTENSIFIES IN RESPONSE TO PRESSURE FALLS OFF THE SOUTHEAST COAST
LATER TODAY. THIS SETS US UP FOR THE CLASSIC CAROLINAS COLD AIR VS.
FORCING/MOISTURE RACE.

BASED UPON OBSERVATIONAL TRENDS, SURFACE WET BULB TEMPS SHOULD BE
CONDUCIVE TO WINTRY WEATHER ACROSS THE NORTHERN TIER OF OUR NC
PIEDMONT/FOOTHILLS ZONES...AND ACROSS MUCH OF THE NRN/CTRL NC MTNS
AND HIGH ELEVATIONS OF THE SOUTHWEST NC MTNS BY LATE MORNING. THUS,
WHILE THERE MAY BE AN INITIAL BOUT OF RAIN OR A RA/SN MIX AS THE
PRECIP OVERSPREADS THOSE AREAS TODAY, THERE SHOULD BE A FAIRLY QUICK
TRANSITION TO SNOW. AS ONE MOVES SOUTH FROM THERE, PRECIP WILL
LIKELY UNCERTAINTY REGARDING THE TIMING OF ANY TRANSITION INCREASES
BEGIN AS RAIN, AND ANY TRANSITION TO WINTRY WEATHER INCREASES
EXPONENTIALLY WITH INCREASING DISTANCE SOUTH OF THE I-40 CORRIDOR.
FURTHER COMPLICATING MATTERS IS THE NAM'S DEPICTION OF AN
INCREASINGLY PROMINENT WARM NOSE BETWEEN ~4 AND 7 KFT IN THE
VICINITY OF THE I-85 CORRIDOR AND POINTS SOUTH. THE MODEL BEGAN
DEPICTING THIS A COUPLE OF CYCLES AGO, AND IT HAS A RATHER LENGTHY
HISTORY OF ACCURATELY PICKING UP ON SUCH FEATURES BEFORE OTHER
MODELS CATCH ON. IF THIS DOES INDEED OCCUR, IT WILL A) DELAY ANY
TRANSITION AWAY FROM RAIN, AS SURFACE TEMPS ARE UNLIKELY TO DIP
BELOW THE MID-30S ALONG THE I-85 CORRIDOR, AND B) INCREASE THE
LIKELIHOOD OF SLEET, WHICH WOULD UNDERCUT THE ACCUM POTENTIAL
SUBSTANTIALLY.

IN TERMS OF LIQUID EQUIVALENT PRECIP FORECAST...WHICH WILL ALSO BE
CRITICAL IN DETERMINING ADVISORY VS. WARNING-LEVEL SNOWFALL SNOWFALL
AMOUNTS ACROSS THE MTNS AND THE I-40 CORRIDOR, THE NAM REMAINS THE
WETTER MODEL, THE GFS THE DRIEST, AND THE ECMWF A REASONABLE
COMPROMISE OF THE TWO. SREF PLUMES SUGGEST THE NAM IS A WET OUTLIER
AMONG THE NORTH AMERICAN ENSEMBLE SYSTEM, WITH THE ECMWF IN BETTER
AGREEMENT WITH THE SREF ENSEMBLE MEAN. CONSIDERING THAT, AND RECENT
ECMWF SUPERIORITY IN TERMS OF QPF VERIFICATION, IT AND WPC GUIDANCE
WERE LEANED ON QUITE HEAVILY FOR THE QPF...RESULTING IN A LIQ EQUIV
FORECAST OF AROUND 1/3 INCH IN THE AREAS THAT ARE LIKELY TO SEE
ALMOST ALL SNOW. WHILE THE GOOD OL' 10:1 SNOW:LIQUID RATIO WOULD
PLACE PORTIONS OF THE PIEDMONT/FOOTHILLS APPROACHING WARNING
CRITERIA, A WARM GROUND SHOULD RESULT IN A GOOD DEAL OF MELTING, AND
IT IS THUS UNLIKELY THAT SUFFICIENT QPF WILL BE REALIZED TO YIELD
WARNING CRITERIA SNOWFALL. AN EXCEPTION MAY BE THE HIGHER ELEVATIONS
OF THE BALSAMS AND SMOKIES, BUT THE CURRENT ADVISORY FOR THOSE AREAS
IS SUFFICIENT TO COVER LOW-END WARNING CRITERIA BEING REACHED ABOVE
~3500 FEET. FARTHER SOUTH, EXPECT DELAYED TRANSITION TO SNOW, A WARM
GROUND, AND PERHAPS A MIX OF SLEET TO YIELD ONLY AROUND AN INCH
ALONG THE I-85 CORRIDOR IN NC, WITH AMOUNTS DROPPING OFF TO AROUND A
HALF INCH OR LESS TOWARD THE GREENVILLE-SPARTANBURG AREA. AS COLD
AIR MAKES MORE OF A SOUTHWARD PUSH AND DEFORMATION ZONE BECOMES
ESTABLISHED ACROSS THE PIEDMONT IN RESPONSE TO DEEPENING CYCLONE OFF
THE CAROLINA COAST, AREAS SOUTH OF I-85 AND EAST OF I-77 SHOULD ALSO
SEE UP TO AN INCH LATE THIS AFTERNOON INTO THE EVENING. THIS IS ALL
AN EXTRAORDINARILY LONG-WINDED WAY OF SAYING THAT I CAN'T FIND ANY
QUARREL WITH THE CURRENT ADVISORY CONFIGURATION.

IN TERMS OF THE IMPACTS, BECAUSE OF THE WARM GROUND AND TEMPS LIKELY
HOVERING IN THE 32-35 OR SO RANGE, WE AREN'T EXPECTING WIDESPREAD
TRAVEL ISSUES THIS AFTERNOON AND EVENING EVENT, ALTHOUGH CERTAINLY
LOCALIZED PROBLEMS CAN BE EXPECTED, ESP OVER THE CTRL AND NRN NC
MTNS. THE BIGGER CONCERN IN TERMS OF IMPACT WILL ACTUALLY BE AFTER
THE PRECIP ENDS LATE TONIGHT AND EARLY FRIDAY, WHEN TEMPS ARE
EXPECTED TO BOTTOM OUT IN THE LOWER TO MID-20S ACROSS MUCH OF THE
AREA. THIS WILL CERTAINLY PRODUCE SOME BLACK ICE PROBLEMS, EVEN IN
AREAS THAT DO NOT RECEIVE MUCH SNOW TODAY
 
Given the cold column, I'm convinced we see primarily snow here with maybe a very brief period of light rain at the onset (15-30 minutes?). But it's going to tough to get solid accumulations during the mid day with surface temps hovering just above freezing.
 
HRRR initializing too warm on DP around her and the R/S line sets up from Greenville, Spartanburg, Gaffney as soon as initial precip moves in around 10am. Then drops further south by 11. So that’s interesting
 
Poimen said:
Given the cold column, I'm convinced we see primarily snow here with maybe a very brief period of light rain at the onset (15-30 minutes?). But it's going to tough to get solid accumulations during the mid day with surface temps hovering just above freezing.
Click to expand...
Agree... I've been watching this.. most of us don't actually start to get below freezing at the surface until Friday AM when most of precipitation is already gone:
1582191506760.png
 
This side of the Apps, MRX says the NAM is way too cold, 3-5 degrees, thus it's not correct in p-type/snowfall. I'm at 35 with snow on radar overhead but it's not reaching the ground yet.
 
Everyone should be looking at the wet bulb rather than dew point. WB will tell where the temp should be once you saturate the atmosphere. Last I looked, GS was at 33 for the WB. I think if heavier precip moves in, the triad should quickly get to 32 or just below.
TW
 
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