690
FOUS11 KWBC 171936
QPFHSD
PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
236 PM EST MON FEB 17 2025
VALID 00Z TUE FEB 18 2025 - 00Z FRI FEB 21 2025
..GREAT LAKES
DAYS 1-2...
IMPRESSIVE WINTER STORM EXITING INTO THE CANADIAN MARITIMES TODAY
WILL LEAVE CYCLONIC FLOW IN ITS WAKE AND ACROSS THE GREAT LAKES.
THIS WILL PROMOTE CONTINUED CAA THROUGH D1, WITH 850MB TEMPS
PLUMMETING TO AS LOW AS -25C (LAKE SUPERIOR) AND -20C OVER LAKE
ONTARIO. THIS WILL SUPPORT BANDS OF LAKE EFFECT SNOW (LES)
ESPECIALLY D1 DURING THE MOST INTENSE CAA BEFORE THE MID-LEVEL FLOW
PATTERN RELAXES DOWNSTREAM OF A DEEPENING TROUGH CENTERED OVER THE
NORTHERN PLAINS/UPPER MIDWEST LATER WEDNESDAY. THE GENERALLY W/NW
FLOW WILL SUPPORT HEAVY LES BANDS, LIKELY IN THE FORM OF SINGLE
BANDS WITHIN UNIDIRECTIONAL SHEAR, AND STRONG ASCENT FOCUSED INTO
THE LOW-LEVEL BUT DEEP DGZ WILL SUPPORT HEAVY RATES THAT COULD
REACH 2-3"/HR AT TIMES D1, HEAVIEST EAST OF LAKE ONTARIO WHERE AN
UPSTREAM MOISTURE CONNECTION TO LAKE HURON WILL EXIST. THESE BANDS
SHOULD BE PROLONGED AND INTENSE D1, LEADING TO WPC PROBABILITIES
THAT ARE HIGH (>70%) FOR MORE THAN 6 INCHES EAST OF LAKE ONTARIO
AND THE FAR EASTERN U.P. ALTHOUGH LES WILL WANE DURING D2,
ADDITIONAL SNOWFALL EXCEEDING 4 INCHES IS POSSIBLE (30%) IN THESE
SAME AREAS. DOWNSTREAM OF LAKE ONTARIO, TOTAL SNOWFALL OF 1-2
FEET IS POSSIBLE BEFORE LES WINDS DOWN.
..PACIFIC NORTHWEST THROUGH THE CENTRAL ROCKIES
DAYS 1-3...
TWO ROUNDS OF MOISTURE SPREADING INLAND THROUGH ENHANCED IVT WILL
SPREAD WINTRY PRECIPITATION ACROSS MUCH OF THE COASTAL AND INTERIOR
WEST.
THE PERIOD BEGINS WITH A LONGWAVE POSITIVELY TILTED TROUGH
EXTENDING FROM THE INTERIOR NORTHWEST THROUGH THE NORTHERN PLAINS.
THIS TROUGH WILL DEEPEN WITH TIME, DIVING INTO THE GREAT BASIN
TUESDAY MORNING AND THEN PIVOTING INTO THE FOUR CORNERS BY THE END
OF D1 AS SHORTWAVE ENERGY TRAVERSES THROUGH IT. AT THE SAME TIME,
A STATIONARY FRONT WILL BE ALIGNED FROM INLAND WA STATE SOUTHEAST
INTO THE CO ROCKIES, AS ONSHORE FLOW FROM THE PACIFIC SPREADS
INLAND AND LIFTS ISENTROPICALLY PRODUCING SOME MODEST ENHANCEMENT
OF LIFT. SHORTWAVE RIDGING EXPANDING ALONG THE PACIFIC COAST LATE
D1 WILL CUT OFF PACIFIC MOISTURE BY WEDNESDAY MORNING, BUT A AXIS
OF MODERATE TO HEAVY SNOW IS LIKELY BEFORE THIS OCCURS, PRIMARILY
ALONG THE STATIONARY FRONT AND INTO DOWNSTREAM TERRAIN. WPC
PROBABILITIES ARE HIGH (70-90%) FOR MORE THAN 6 INCHES IN THE
TERRAIN ABOVE GENERALLY 3000 FT FROM THE SAWTOOTH INTO THE TETONS
AND AS FAR SOUTHEAST AS THE PARK RANGE OF CO.
AFTER A BRIEF RESPITE, A MORE PRONOUNCED SYSTEM WILL APPROACH THE
PACIFIC COAST AS A POTENT SHORTWAVE CLOSES OFF OVER THE PACIFIC
OCEAN AND SHEDS EASTWARD. THIS FEATURE IS PROGGED TO COME ONSHORE
NEAR THE WA/OR BORDER WEDNESDAY EVENING, AND THERE IS GOOD MODEL
CONSENSUS IN THIS EVOLUTION DESPITE MODEST DIFFERENCES IN
AMPLITUDE. THE CORE OF THIS TROUGH COMING ONSHORE WA/OR WILL
MANIFEST AS IMPRESSIVE HEIGHT FALLS AS FAR SOUTH AS CENTRAL CA, AND
AS THE DOWNSTREAM RIDGE ERODES, THIS ENTIRE FEATURE WILL ELONGATED
AND PIVOT INTO THE GREAT BASIN/CENTRAL ROCKIES BY THE END OF THE
FORECAST PERIOD. MODEST JET LEVEL ASCENT THROUGH THE DEVELOPMENT OF
A JET STREAK ROTATING AROUND THE BASE OF THIS TROUGH WILL ENHANCE
LIFT, BUT IN GENERAL FORCING IS EXPECTED TO BE BROAD AND DRIVEN
PRIMARILY BY HEIGHT FALLS THROUGH D2-D3. DESPITE THAT, SOME
ENHANCED IVT PROGGED TO EXCEED THE 90TH PERCENTILE WITHIN THE CFSR
CLIMATOLOGY WILL SPREAD INLAND WITHIN WAA DOWNSTREAM OF THE TROUGH
AXIS, LEADING TO RISING SNOW LEVELS BUT ENHANCED WINTRY
PRECIPITATION PROSPECTS. DURING THIS TIME, WPC PROBABILITIES FOR
MORE THAN 6 INCHES CLIMB ABOVE 30% IN THE CASCADES D2, BUT WITH
SNOW LEVELS CLIMBING ABOVE PASS LEVELS, WITH ADDITIONAL MODERATE
CHANCES (30-50%) FOR 6+ INCHES ON D3 IN THE SAME GENERAL AREAS.
..CENTRAL PLAINS THROUGH THE MID-ATLANTIC STATES
DAYS 1-3...
A SIGNIFICANT WINTER STORM BEGINS TONIGHT ACROSS THE CENTRAL PLAINS
AND THEN SPREADS ALMOST DUE EAST TO THE MID-ATLANTIC ON WEDNESDAY.
THERE IS STILL CONSIDERABLE UNCERTAINLY AS TO HOW THIS EVOLVES OFF
THE COAST WEDNESDAY NIGHT, BUT AT LEAST MODERATE SNOW AND
SIGNIFICANT ICING IS LIKELY FOR MANY AREAS.
THE SYSTEM BEGINS TO TAKE SHAPE TONIGHT AS A SHORTWAVE EJECTS FROM
THE FOUR CORNERS AND TRAVERSES TO THE EAST, REACHING THE CENTRAL
PLAINS TUESDAY EVENING. THIS WILL OCCUR JUST NORTH OF A SURFACE
COLD FRONT THAT WILL SURGE SOUTH INTO TEXAS, AND THE INTERACTION OF
THIS IMPULSE WITH THE ELEVATED BAROCLINIC ZONE WILL HELP INDUCE
SOME BACKING OF THE LOW-LEVEL WINDS, LEADING TO ENHANCED MOIST
ISENTROPIC ASCENT EMERGING FROM THE GULF, ESPECIALLY ALONG THE
285-290K SURFACES WHERE MIXING RATIOS WILL RISE TO 3-4 G/KG. AT
THE SAME TIME, A JET STREAK ANGLED INTO THE NORTHEAST WILL LEAVE
ITS FAVORABLE RRQ FOR ASCENT ATOP THE REGION, RESULTING IN AN
OVERLAP OF SYNOPTIC AND MESOSCALE ASCENT. WITH MOISTURE CONTINUING
TO SURGE INTO THE REGION, THIS WILL RESULT IN AN EXPANDING AXIS OF
HEAVY SNOW, ESPECIALLY FROM EASTERN KS AND OK INTO NORTHERN AR,
SOUTHERN MO, AND WESTERN TN/KY. HERE, THE OVERLAP OF ASCENT AND
MOISTURE WILL COMBINE WITH A DEEPENING DGZ (SREF PROBS FOR 100MB OF
DEPTH EXCEEDING 70% TUESDAY EVENING AND TUESDAY NIGHT, RESULTING IN
AN AXIS OF HEAVY SNOWFALL. ADDITIONALLY, A SECONDARY JET STREAK
INTENSIFYING DOWNSTREAM OF THE SHARPENING TROUGH WILL PRODUCE AN
ENVIRONMENT SUPPORTING MODESTLY COUPLED JET STREAKS, AND THE
RESULTANT AGEOSTROPHIC RESPONSE TO THE ALREADY IMPRESSIVE FGEN WILL
LIKELY (>80% CHANCE) DRIVE SNOWFALL RATES TO 1-2"/HR AS SHOWN BY
BOTH HREF PROBABILITIES AND THE WPC PROTOTYPE SNOWBAND TOOL.
THIS SYSTEM WILL BECOME INCREASINGLY PROGRESSIVE TO THE EAST, BUT
AN 18-24 HOUR PERIOD OF MODERATE TO HEAVY SNOW WITH FLUFFY SLRS IN
THE COLD ENVIRONMENT WILL SUPPORT RAPID AND SIGNIFICANT SNOWFALL
ACCUMULATIONS, ESPECIALLY TUESDAY NIGHT FROM THE CENTRAL PLAINS
INTO THE WESTERN TENNESSEE VALLEY. HERE, WPC PROBABILITIES FOR
MORE THAN 8 INCHES REACH AS HIGH AS 70%, WITH LOCALLY 12+ INCHES
POSSIBLE IN THE VICINITY OF THE MOST INTENSE BANDING. SOUTH OF THIS
HEAVY SNOW, AN AXIS OF MIXED SLEET AND FREEZING RAIN IS LIKELY,
WITH LOCALLY MORE THAN 0.1" OF ICE POSSIBLE (30%), HIGHEST ACROSS
SOUTHERN AR.
THIS SHORTWAVE BEGINS TO RACE EASTWARD TOWARDS THE MID-ATLANTIC
WEDNESDAY AS IT GETS SQUEEZED INTO MORE ZONAL FLOW DOWNSTREAM OF A
MORE IMPRESSIVE NORTHERN STREAM TROUGH (500MB HEIGHTS FALLING BELOW
THE 2.5 PERCENTILE ANOMALY ACCORDING TO NAEFS) DIGGING OUT OF THE
NORTHERN PLAINS. THIS IS WHERE THE THE FORECAST BEGINS TO GET MORE
CHALLENGING. THIS NORTHERN STREAM TROUGH WILL DIVE SOUTHEAST
TOWARDS THE OHIO VALLEY WHILE CONTINUING TO DEEPEN INTO A POTENT
CLOSED LOW, RESULTING IN DOWNSTREAM (SOUTH/EAST) JET DEVELOPMENT
WHICH WILL INTENSIFY BEYOND 150 KTS. THE OVERLAP OF THE IMPRESSIVE
HEIGHT FALLS AND LFQ OF THIS JET STREAK WILL DRIVE PRONOUNCED
ASCENT INTO THE MID-ATLANTIC STATES, CONCURRENT WITH CONTINUED
MOIST ISENTROPIC LIFT FROM THE GULF INTO THE AREA. THIS SUGGESTS
THAT AFTER A MODEST "BREAK" IN INTENSITY OF PRECIPITATION IN THE
VICINITY OF THE CENTRAL APPALACHIANS, RENEWED HEAVY PRECIP WILL
DEVELOP, ESPECIALLY AS SECONDARY SURFACE LOW PRESSURE DEVELOPS
OFFSHORE.
THERE HAS BEEN A STEADY TREND IN GUIDANCE TO KEEP THIS
PRECIPITATION SUPPRESSED SOUTH ACROSS THE CAROLINAS AND VIRGINIA,
WITH THE MOST UNCERTAINTY NOW INVOLVING THE SPEED AND PLACEMENT AT
WHICH THE SECONDARY LOW DEVELOPS AND MOVES NORTHEAST UP THE COAST.
WITH THE SOUTHERN STREAM LESS AMPLIFIED AND FASTER, PHASING OF
ENERGY IS PROGGED TO OCCUR MUCH LATER WHICH SHOULD SPARE THE
NORTHERN MID-ATLANTIC AND NORTHEAST MUCH IF ANY SNOW. HOWEVER,
HEAVY SNOW AND MIXED PRECIPITATION IS STILL LIKELY FROM EASTERN
TN/KY INTO NC AND VA, WITH A SWATH OF THE HEAVIEST SNOW MOST
LIKELY ACROSS EASTERN VA. THERE IS STILL A LOT OF UNCERTAINTY,
HOWEVER, AND CLUSTERS INDICATE THE PRIMARY EOF IS DEPENDENT ON THE
AMPLITUDE OF THIS TROUGH, WITH THE GEFS MEMBERS APPEARING TO BE A
BIT UNDER-DISPERSIVE WITH A FLATTER WAVE, WHILE THE EC/CMC ARE A
BIT DEEPER IN GENERAL. HOWEVER, THESE DEEPER WAVES ARE ALSO
GENERALLY SLOWER, AGAIN SUGGESTING THE THREAT FOR A PHASED MAJOR
WINTER EVENT ARE QUITE LOW.
HOWEVER, WITH THE LOW EVENTUALLY DEEPENING RAPIDLY OFFSHORE, A
POTENT DEFORMATION AXIS COLLOCATED WITH THE GREATEST MOISTURE WILL
RESULT IN HEAVY SNOW ACCUMULATIONS, FOR WHICH WPC PROBABILITIES
INDICATE A HIGH RISK (70-90%) FOR 6+ INCHES CENTERED ALONG THE
NORFOLK/VIRGINIA TIDEWATER REGION. SURROUNDING THIS MAXIMUM, WPC
PROBABILITIES FOR 4 OR MORE INCHES ARE 10-30% FROM EASTERN KY
THROUGH WV AND EAST TOWARDS COASTAL DELAWARE AND FAR NORTHERN NC.
SOUTHEAST OF THE SNOW, A MIX OF SLEET AND FREEZING RAIN IS
EXPECTED, AND FREEZING RAIN POTENTIAL HAS INCREASED BENEATH THE
WARM NOSE LIFTING NORTHWARD. WPC PROBABILITIES FOR 0.1" PEAK ABOVE
70% D2 INTO D3 IN PARTS OF EASTERN AND SOUTHEASTERN NC, WITH
LOCALLY 0.25-0.35 INCHES OF ICE POSSIBLE.
WEISS
...WINTER STORM KEY MESSAGES ARE IN EFFECT. PLEASE SEE CURRENT
KEY MESSAGES BELOW...
HTTPS://WWW.WPC.NCEP.NOAA.GOV/KEY_MESSAGES/LATESTKEYMESSAGE_2.PNG
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