Good evening… again. While we await the latest model runs, I thought it would be good to show you forecast from the HPC. The Hydrometeorological Prediction Center puts out forecasts on heavy rain or snow events across the country.

Here is their snowfall probability map for the upcoming winter storm…

Think of that as being similar to the severe weather risk maps coming from the Storm Prediction Center. Those maps show a good risk for significant snowfall for much of the state.

Here is the HPC Discussion about the upcoming winter storm:

…CENTRAL APPALACHIANS/MID ATLANTIC STATES DAYS 2 INTO EARLY DAY
3 (SUN NIGHT)…

INGREDIENTS CURRENTLY TAKING SHAPE FOR THE MUCH-ADVERTISED/LONG
AWAITED WINTER WEATHER EVENT FOR THE LOWER OH/NRN TN VLYS EAST TO
THE MID ATLC REGION. THE DEEP UPPER TROUGH OUT WEST HAS NOW
SHIFTED E OF BAJA…WITH AN ELONGATED…NARROW COLD CONVEYOR BELT
AXIS EXTENDING FROM NRN TX SW THROUGH SRN NM/AZ PER THE LATEST
STLT WV/IR IMAGERY. MEANWHILE…OUT AHEAD OF THE UPPER SYSTEM A
LENGTHY SUBTROPICAL CONNECTION CAN BE SEEN WITH THE WARM CONVEYOR
BELT…ONE THAT WILL ULTIMATELY WILL TAP THE GULF AS THE TROUGH
MOVES E.

IN TERMS OF THE LATEST MODEL SUITE…WHILE THE OPERATIONAL RUNS
WERE IN DECENT ALIGNMENT WITH RESPECT TO THE MASS FIELDS…THERE
AGAIN WERE DIFFERENCES IN S-N DISTRIBUTION OF QPF AND TO SOME
EXTEND THE LLVL THERMAL PROFILE. HPC LEANED TOWARD A GFS-UKMET
BLEND…WHICH CLUSTERED FAIRLY WELL IN TERMS OF THE QPF COMPARED
TO THE NAM (A BIT MORE ROBUST FARTHER N) AND THE ECMWF (HEAVIER
PCPN SHIFTED S). WHILE THE CONFIDENCE IN THE MODELS (MASS FIELDS
IN PARTICULAR) ARE INCREASING…SUBTLE YET SIGNIFICANT DIFFERENCES
IN THE LLVL THERMAL AND MSTR PROFILES REMAIN DISCONCERTING.
ESPECIALLY WITH A SYSTEM THAT WILL GENERATE PREDOMINANTLY RAIN ON
THE FRONT END WITH THE WAA/WCB…ALONG WITH THE ANTECEDENT WARM
GROUND WITH THE MILD CONDITIONS TODAY AND SAT. THE MAIN PLAYER
FROM A SNOWFALL STANDPOINT WILL BE THE EVOLVING CCB (TROWAL
POTENTIAL)…WHICH THE ORIENTATION…STRENGTH…AND SPEED WILL BE
BASED HEAVILY ON THE THE PROGRESSION AND SRN EXTENT OF A SECONDARY
NRN STREAM SHORTWAVE. THE MORE CONFLUENCE ALOFT…THE TIGHTER THE
N-S GRADIENT IN TERMS OF DEEP MOISTURE (FAVORING A DEEPER NRLY
ISALOBARIC COMPONENT WITH DRIER LLVL AIR PUSHED SOUTH).

MEANWHILE…INCREASED MID/UPPER LEVEL DEFORMATION WILL HAVE AN
EFFECT ON THE ORIENTATION AND N-S EXTEND OF THE
CCB/COMMA-HEAD…I.E. RESULTING IN A NARROWER…
MORE W-E VICE NW-SE ORIENTED PCPN FIELD. THE UPSHOT WITH THE
GFS/UKMET BLEND IS A COMPROMISE BETWEEN THE NAM (FARTHER N WITH
THE MSTR) AND ECMWF (FARTHER S). THE NAM ALSO CONTINUES TO HAVE
ISSUES WITH THE DEGREE OF BNDRY LAYER COOLING OVER THE SRN
PERIPHERY OF THE CCB (CLOSER TO THE 850 MB LOW TRACK). IN ORDER TO
CAPITALIZE ON THE DYNAMICAL COOLING…PCPN RATES NEED TO BE
SUFFICIENTLY HIGH…WHICH IS IN QUESTION GIVEN WHAT THE MODELS ARE
SHOWING IN TERMS OF A VERY SLOW CYCLOGENIC PROCESS UNTIL THE LOW
REACHES THE COAST. THEREFORE…A DEPICTION OF A MORE PROGRESSIVE
CCB/DEFORMATION BANDED PCPN WITH LITTLE EVIDENCE OF A PERSISTENT
NOR ELONGATED TROWAL.
AGAIN…PREFERRED A GFS/UKMET SCENARIO WHERE THE RAIN WOULD CHANGE
OVER TO SNOW ACROSS NRN TN/NC AND SRN VA…WHILE UNDERCUTTING THE
NAM ACCUMS IN THESE REGIONS.

CONVECTION WILL ALSO ADD TO THE UNCERTAINTY…I.E. IN TERMS OF THE
EFFECTS ON THE NORTHWARD MOISTURE TRANSPORT AND DEEPENING OF THE
SYSTEM (AND THUS EFFECTS ON ENHANCING THE POTENTIAL VORTICITY).
ELEVATED CONVECTION AND POTENTIAL FOR ENHANCED MESOSCALE BANDING
WILL ALSO BE A CONCERN…ESPECIALLY AT THE APEX OF THE DRY SLOT
ABOVE THE -10C ISOTHERM. ALL-IN-ALL…POTENTIAL FOR 6-12 INCHES
WILL REMAIN HIGHEST ACROSS THE MNTNS IN WRN VA…ACROSS MUCH OF
SRN WV…AND ERN KY. FCST CONFIDENCE DECREASES TOWARD I-95
INCLUDING THE MID ATLC CITIES…THOUGH BECAUSE OF THE SPREAD OF
POSSIBILITIES HPC HAS INCLUDED A MOD RISK OF AT LEAST 4″ INTO NRN
AND CEN VA…ALONG WITH A SLIGHT (1 IN 10) RISK OF 8 INCHES. STAY
TUNED.

I will have a full update later tonight and will be tweeting information coming from the new model runs filtering in. Take care.