Quantitative Precipitation Forecast
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FOUS11 KWBC 082041
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
340 PM EST MON FEB 08 2016

VALID 00Z TUE FEB 09 2016 - 00Z FRI FEB 12 2016


DAYS 1 THROUGH 3...


...NEW ENGLAND...

A SERIES OF SURFACE LOWS WILL AFFECT NEW ENGLAND DURING THE NEXT
THREE DAYS. THE FIRST SYSTEM AFFECTS THE REGION DURING DAY 1...AND
THE SECOND SYSTEM BRUSHES SOUTHERN NEW ENGLAND DURING DAY 2. A
THIRD SYSTEM COULD GRAZE THE CENTRAL AND NORTHERN NEW ENGLAND
COAST DURING DAY 3.  THERE HAS BEEN SOME SPREAD ON THE TRACK OF
THE SURFACE LOWS ON DAYS 2 AND 3...SO THE THERMAL PORTION OF THE
FORECAST WAS BASED ON A MULTI MODEL BLEND. THE WPC QPF WAS BASED
ON THE MOST RECENT WPC QPF.

DAY 1...
THE FIRST SYSTEM WILL BE IN THE PROCESS OF WRAPPING UP ACROSS
EASTERN NEW ENGLAND DURING DAY 1...WITH THE WESTERNMOST
DEFORMATION BANDS CONTINUING OVER CENTRAL AND EASTERN ME THROUGH
AT LEAST 09/12Z. THESE BANDS COULD PRODUCE AN ADDITIONAL 4 TO
LOCALLY 8 INCHES OF SNOWFALL THERE...AND THESE
AMOUNTS ARE SUPPORTED BY BOTH THE LATEST ECMWF ENSEMBLE
OUTPUT...AS WELL AS OUTPUT FROM THE 09Z SREF. FURTHER WEST...A
SHORT WAVE CROSSING NORTHERN NEW ENGLAND COULD PRODUCE ENOUGH LIFT
ON THE WESTERN EDGE OF THE DEEPER MOISTURE TO PRODUCE AREAS OF 1
TO 3 INCHES OF SNOWFALL.

DAY 2...
THE NEXT SURFACE LOW TRACKS ALONG A LOW LEVEL BAROCLINIC ZONE
EXTENDING OFF THE NORTHERN MID ATLANTIC COAST. THE 12Z NAM IS THE
FURTHEST NORTH AND MOST AGGRESSIVE WITH THE SURFACE LOW...BUT
THESE SEEMS TOO FAR ENOUGH NORTH CONSIDERING THE POSITION OF THE
LOW LEVEL THERMAL RIBBON. THE AXIS OF HIGHEST QPF (AND SNOWFALL)
SHOULD BE LIMITED TO PORTIONS OF LONG ISLAND NEW YORK AND COASTAL
SOUTHERN NEW ENGLAND. IN THESE LOCATIONS...SNOWFALL AMOUNTS UP TO
2 INCHES CAN BE EXPECTED.

DAY 3...
AS THE LAST SURFACE LOW PULLS AWAY SOUTH OF NOVA SCOTIA AT THE
BEGINNING OF DAY 3...ANOTHER SHORT WAVE DIVES INTO THE BOTTOM OF A
LONG WAVE TROUGH SETTING UP OVER THE EASTERN UNITED STATES. WHILE
MID LEVEL LAPSE RATES WILL STEEPEN WITH THIS SYSTEM...THE AIRMASS
IS VERY DRY...AND SNOWFALL ACCUMULATIONS ACROSS THE HIGHER TERRAIN
COULD TOP OUT NEAR 2 INCHES. ELSEWHERE...SNOWFALL ACCUMULATIONS
SHOULD AVERAGE LESS THAN AN INCH.


...MID ATLANTIC/OH VALLEY/SOUTHERN APPALACHIANS...

A COMPLEX LOW PRESSURE SYSTEM FORMING OFF THE MID ATLANTIC COAST
WILL FOCUS MOISTURE AND LIFT ALONG A SURFACE TROUGH EXTENDING BACK
ACROSS NORTHERN MID ATLANTIC STATES DURING DAY 1. THERE IS SOME
SPREAD IN THE GUIDANCE CONCERNING HOW THE THERMAL FIELDS SET UP
ALONG THE TROUGH...AND MODEL SOUNDINGS INDICATE THIS EVENT COULD
HAVE SOME ELEVATION DEPENDENCIES WITH IT. BASED ON THE
SPREAD...AND THE POTENTIAL FOR BOUNDARY LAYER TEMPERATURE ISSUES
AND ELEVATION ISSUES...THE THERMAL PORTION OF THE FORECAST WAS
BASED ON A 12Z NAM/GFS/09Z SREF MEAN BLEND. THE WPC QPF WAS BASED
ON THE MOST RECENT WPC QPF

DAY 1...
SURFACE LOW PRESSURE IS EXPECTED TO SPINS UP ON A LOW LEVEL
BOUNDARY EXTENDING FROM FILLING LOW PRESSURE OVER THE LOWER GREAT
LAKES TO A POSITION OFF THE SOUTHERN DELMARVA COAST BEFORE 09/12Z.
THE SURFACE LOW AND TROUGH BECOMES THE FOCUS FOR LOW LEVEL INFLOW
AHEAD OF A SHORT WAVE CROSSING THE OH AND TN VALLEY. MODEL
SOUNDINGS SHOWED STEEPENING MID LEVEL LAPSE RATES (APPROACHING 8.0
C/KM) AFTER 09/06Z...AND THE STRONG MID LEVEL LIFT IS EXPECTED TO
INITIATE A BLOOM OF PRECIPITATION WITH THE SHORT WAVE AND
DEVELOPING SURFACE WAVE.

THE BOUNDARY LAYER IS INITIALLY WARM AHEAD OF THE MID LEVEL AND
SURFACE SYSTEMS...AND A LOW LEVEL EASTERLY FLOW AHEAD OF THE
TROUGH AND SURFACE WAVE WILL CONTINUE TO KEEP THE BOUNDARY LAYER
WARM THROUGH AT LEAST 09/06Z. EVAPORATIVE COOLING OCCURS WHEN THE
COLUMN SATURATES..AND THE INITIAL AREA OF RAIN WILL BEGIN TO
TRANSITION TO SNOW BETWEEN 09/06Z AND 09/12Z. SOUTH AND EAST OF
THE FALL LINE...THE BOUNDARY LAYER MAY REMAIN WARM ENOUGH FOR THE
SNOW TO DEVELOP LATER IN THE BEST LIFT...AND SHOULD RESULT IN LESS
ACCUMULATION. CLOSER TO THE FALL LINE...THE EVENT BECOMES MORE
ELEVATION DRIVEN... AS THE INITIALLY WARM BOUNDARY IS OVERCOME BY
THE COOLING IN THE SATURATED COLUMN. THIS IS SHOWN WELL BY THE 12Z
NAM...WHICH INDICATES THAT RIMING COULD BE ENOUGH OF A FACTOR FOR
LOWER SNOW TO LIQUID RATIOS CLOSE TO THE FALL LINE.

A SHARP SNOWFALL GRADIENT IS EXPECTED ACROSS NORTHERN AND EASTERN
MD/NORTHERN VA...WHERE RIMING AND PRECIPITATION PHASE PROBLEMS ARE
EXPECTED. THE AXIS OF HIGHEST SNOWFALL SHOULD LIE FROM NORTHERN WV
ACROSS NORTHERN MD AND EASTERN VA INTO SOUTHEAST PA/NORTHERN
DE/SOUTHWEST...WHERE THE BEST COMBINATION OF MOISTURE AND LIFT IS
EXPECTED. IN THIS AXIS...4 TO 8 INCHES OF SNOWFALL IS
EXPECTED...AND THIS IS SUPPORTED BY BOTH THE ECMWF ENSEMBLES AND
09Z SREF OUTPUT. GIVEN THE STEEP MID LEVEL LAPSE RATES...THUNDER
IS POSSIBLE IN THIS BAND...AND THE CONVECTIVE ASPECT COULD RESULT
IN HIGHER SNOWFALL AMOUNTS. THE BEST LIFT EXISTS BETWEEN 09/12Z
AND 09/18Z FROM WEST TO EAST...AFTER WHICH TIME LIGHT SNOW OR
MIXED PRECIPITATION IS POSSIBLE.

DAY 2...
THE NEXT SURFACE LOW THAT FORMS ON THE LOW LEVEL BAROCLINIC ZONE
AHEAD OF A SHORT AVE DURING DAY 2 SHOULD AFFECT MAINLY SOUTHEAST
PA/CENTRAL AND SOUTHERN NJ INTO A PORTIONS OF SOUTHEAST NY STATE.
THE BEST LIFT WITH THE LOW AND MID LEVEL SYSTEM IS EXPECTED TO
STRETCHED FROM NORTHEAST MARYLAND ACROSS SOUTHERN NJ...WHERE 2 TO
4 INCH SNOWFALL AMOUNTS ARE POSSIBLE. AS MENTIONED EARLIER...THE
12Z NAM STANDS ALONG WITH A SWATH OF 4 TO 6 INCHES OF SNOWFALL
ACROSS PORTIONS OF SOUTHEAST PA INTO CENTRAL NJ...BUT THIS SEEMS
TO BE TOO FAR NORTH AND WEST...GIVEN THE POSITION OF THE BEST LOW
LEVEL THERMAL RIBBON.


...GREAT LAKES...

A CLOSED MID LEVEL SYSTEM OVER THE GREAT LAKES DURING DAY 1 OPENS
INTO THE LONG WAVE TROUGH ACROSS THE REGION FOR THE REMAINDER OF
THE PERIOD. ADDITIONAL SHORT WAVE ENERGY DROPPING INTO THE LONG
WAVE TROUGH WILL REINFORCE LAKE EFFECT CIRCULATIONS DURING THE
NEXT THREE DAYS. THERE WAS GENERALLY GOOD MODEL AGREEMENT WITH THE
OVERALL SYNOPTIC SETUP...SO THE THERMAL PORTION OF THE FORECAST
WAS BASED MAINLY ON THE 12Z NAM/09Z SREF...AS THE HIGHER
RESOLUTION OF THESE MODELS BETTER DEPICT THE LAKE EFFECT
STRUCTURE. THE WPC QPF WAS BASED ON THE MOST RECENT WPC QPF.

DAY 1...
AS THE CLOSED MID LEVEL SYSTEM OPENS UP...THE BROAD CYCLONIC FLOW
SUPPORTS THE SETUP OF LAKE EFFECT CIRCULATIONS. THE UPPER GREAT
LAKES ARE FAVORED IN THE NORTH NORTHWEST BOUNDARY LAYER FLOW...AS
THIS FLOW IS CLOSER TO THE MAJOR LAKE AXES THAN IT IS ACROSS THE
LOWER GREAT LAKES. THE NORTH NORTHWEST FLOW SUPPORTS AREAS OF 4 TO
6 INCHES OF SNOWFALL OVER THE UP OF MI (ESPECIALLY THE KEWEENAW
PENINSULA)...THE NORTHERN PORTION OF THE LP OF MI (WITH FLOW OFF
NORTHERNMOST LAKE MI AND EASTERNMOST LAKE SUPERIOR)...AS WELL AS
THE EASTERNMOST PORTION OF THE LP OF MI. THESE SNOWFALL AMOUNTS
ARE WELL SUPPORTED BY MEMBERS OF THE LATEST ECMWF ENSEMBLE
OUTPUT...AS WELL MEMBERS FROM THE 09Z SREF.

THE LOW LEVEL BOUNDARY LAYER FLOW ALSO SUPPORTS AN AREA OF 4 TO 6
INCHES OF SNOWFALL ACROSS FAR SOUTHWEST MI AND NEARBY NORTH
CENTRAL IN. THE NORTH NORTHWEST FLOW NEARLY PARALLELS THE MAJOR
AXIS OF LAKE MI...AND THE FETCH SHOULD ALLOW THIS AXIS TO AFFECT
MORE OF NORTHERN IN. THERE IS SUPPORT FROM NEITHER THE ECMWF
ENSEMBLE NOR THE 09Z SREF FOR THIS...BUT THE DETERMINISTIC 12Z NAM
CAN SUPPORT THESE SNOWFALL AMOUNTS.

DAY 2...
AS THE PASSAGE OF A SHORT WAVE IN THE LONG WAVE TROUGH PATTERN
(AND ITS SURFACE REFLECTION)...THE BOUNDARY LAYER FLOW STARTS TO
BACK TO THE NORTHWEST. THIS ALLOWS THE LOWER LAKES TO BECOME MORE
INVOLVED IN THE LAKE EFFECT CIRCULATIONS. THE NORTHWEST BOUNDARY
LAYER LAYER FLOW...COUPLED WITH CAPPING INVERSIONS DEEPENING TO
825 MB...SHOULD SUPPORT LOCAL 4 TO 6 INCHES SNOWFALL AMOUNTS
DOWNWIND OF LAKES ERIE AND ONTARIO. ACROSS THE UPPER GREAT
LAKES...THE FETCHES ARE SOMEWHAT MORE LIMITED BY THE BACKING
BOUNDARY LAYER FLOW. WHILE THIS COULD SUPPORT A LARGER AREA OF 2
TO 4 INCHES OF SNOWFALL...AT THIS POINT IT APPEARS AS THOUGH
HIGHER AMOUNTS (OUTSIDE OF THE UP OF MI) ARE PROBABLY UNLIKELY.

DAY 3...
THE WEST TO NORTHWEST BOUNDARY LAYER FLOW AGAIN SUPPORTS LAKE
EFFECT CIRCULATIONS OFF EACH LAKE ON DAY 3. THE AMOUNTS ARE NOT AS
ROBUST AS DAY 2...AS CAPPING INVERSIONS ARE NOT AS DEEP...AND
GENERALLY 2 TO 4 INCHES OF SNOWFALL IS EXPECTED DOWNWIND OF EACH
LAKE.


...NORTHERN PLAINS...

A SHORT WAVE STREAKING SOUTHEAST FROM CENTRAL CANADA WILL PROVIDE
SYNOPTIC SCALE ASCENT ACROSS A LOW LEVEL BAROCLINIC ZONE
STRETCHING FROM CENTRAL CANADA INTO EASTERN SD DURING DAY 2. THE
12Z GFS/NAM AREA CLOSER WITH THE PLACEMENT OF QPF AND SNOWFALL
WITH THIS FEATURE...SO THE THERMAL PORTION OF THE FORECAST WAS
BASED PRIMARILY ON A BLEND OF THESE SOLUTIONS. THE WPC QPF WAS
BASED ON THE MOST RECENT WPC QPF.

THE BEST LIFT ASSOCIATED WITH THE SHORT WAVE OCCURS IN THE 10/12Z
TO 10/18Z TIME FRAME ACROSS CENTRAL ND INTO EASTERN SD. THE LOW
LEVEL BAROCLINIC ZONE LIES IN THE SAME GENERAL VICINITY...AND THE
LOW LEVEL BOUNDARY SERVES AS A FOCUS FOR UPGLIDE AND MOISTURE
AHEAD OF THE SHORT WAVE. THE MOISTURE IN THE COLUMN IS FAIRLY
SCANT...BUT MODEL SOUNDINGS ACROSS THIS AREA SHOWED MOISTURE AND
LIFT IN THE DENDRITIC GROWTH ZONE. THE LIFT AND MOISTURE IN THE
DENDRITIC GROWTH ZONE SHOULD ALLOW FOR A SWATH OF 2 TO 4 INCHES OF
SNOWFALL EXTENDING FROM NORTH CENTRAL ND INTO NORTH CENTRAL SD.
NONE OF THE LATEST ECMWF ENSEMBLE MEMBERS SHOWED 4+ INCH SNOWFALL
AMOUNTS HERE...AND ONLY A FEW OF THE 09Z SREF MEMBERS SHOWED THIS
POTENTIAL. GIVEN THE RELATIVE PAUCITY OF MOISTURE AND FAST
MOVEMENT OF THE MID LEVEL SYSTEM...4+ INCH SNOWFALL AMOUNTS SEEM
UNLIKELY FOR THIS PERIOD.


THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.

HAYES

$$




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