Quantitative Precipitation Forecast
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FOUS11 KWBC 291126
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION...CORRECTED
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
303 AM EST THU JAN 29 2015

VALID 12Z THU JAN 29 2015 - 12Z SUN FEB 01 2015


DAYS 1 THROUGH 3...


...GREAT LAKES/OH VALLEY/MID ATLANTIC/NORTHEAST...

A DEVELOPING MID-LEVEL LONGWAVE TROUGH GOES NEGATIVELY TILTED
ACROSS THE GREAT LAKES AND OH VALLEY DURING DAY 1.  A CLOSED LOW
SPINS UP AT THE BASE OF THE LONG WAVE TROUGH ON DAY 2...WHICH
INDUCES CYCLOGENESIS OVER THE GULF OF ME. THE LOW THEN TRACKS
NORTHEAST ON DAY 3...PASSING EAST OF NOVA SCOTIA. THERE IS FAIRLY
GOOD MODEL AGREEMENT ON DAY 1 WITH THE EVOLUTION OF THE LONG WAVE
TROUGH...SO THE THERMAL FIELDS WERE BASED ON A BLEND OF THE 00Z
GFS/00Z NAM/21Z SREF MEAN. BY DAY 2...MODEL SPREAD
INCREASES...PARTICULARLY WITH RESPECT TO THE PLACEMENT OF THE
SURFACE LOW THAT DEVELOPS OVER THE GULF OF ME. THE 00Z GFS APPEARS
TO BE TOO CLOSE TO THE ME COAST WITH THE DEVELOPMENT...AND
SUBSEQUENTLY TOO WARM IN THE BOUNDARY LAYER. BECAUSE OF THIS...THE
THERMAL PROFILES WERE BASED ON A BLEND OF THE 00Z NAM/12Z ECMWF
FOR DAYS 2 AND 3. THE QPF PORTION OF THE FORECAST THROUGHOUT WAS
BASED ON THE MOST RECENT WPC QPF.

DAY 1...

AS THE DEVELOPING LONG WAVE TROUGH TAKES ON A NEGATIVE TILT ACROSS
THE GREAT LAKES AND OH VALLEY...STRONG WARM AIR ADVECTION AHEAD OF
IT (35 TO 45 KNOT SOUTHWEST FLOW UP THE 290 K ISENTROPIC SURFACE)
INCREASES MOISTURE IN THE COLUMN. WARM AIR ADVECTION RESULTS IN
PRECIPITATION PHASE ISSUES ACROSS PORTIONS OF EASTERN IN/WESTERN
OH...AS SNOW LEVELS RISE ENOUGH FOR RAIN IN THIS SECTOR (AHEAD OF
A SURFACE WARM FRONT). FURTHER EAST...MODEL SOUNDINGS SHOWED THAT
THE COLUMN REMAINED COLD ENOUGH FOR SNOW ACROSS MUCH OF EASTERN
OH...WESTERN PA/WV/WESTERN NY STATE. THIS TRANSLATES TO AN AREA OF
3-6" ACROSS THIS REGION. AS THE WARM AIR ADVECTION ENDS...DRYING
ABOVE THE BOUNDARY LAYER ROBS THE COLUMN OF ITS ABILITY TO MAKE
SNOWFLAKES...AND LIGHT ICING IS POSSIBLE AT THE END OF THE
STEADIER PRECIPITATION.

FURTHER NORTHEAST...COLD MORNING CONDITIONS (WITH SURFACE
TEMPERATURES BELOW FREEZING THROUGH THE MID ATLANTIC) COULD RESULT
IN FREEZING RAIN ISSUES ACROSS PORTIONS OF NORTHEAST MD AS THE
COLUMN MOISTENS THROUGH ABOUT 29/15Z. ACROSS PORTIONS OF SOUTHEAST
PA INTO CENTRAL NJ...EVAPORATIVE COOLING WITH THE ONSET OF
PRECIPITATION COULD RESULT IN A PERIOD OF SNOW/SLEET, THOUGH
ACCUMULATIONS OF LESS THAN AN INCH ARE EXPECTED.

AFTER THE PASSAGE OF THE LONG WAVE TROUGH...CONDITIONS BECOME
FAVORABLE FOR LAKE-ENHANCED SNOWFALL EAST OF LAKES ERIE AND
ONTARIO. MODEL SOUNDINGS EAST OF THESE LAKES SHOWED A CAPPING
INVERSION RISING TO NEAR 800 MB...BOUNDARY LAYER WINDS NEAR THE
LONG AXIS OF THE LAKE...AND VEERING WINDS... RESULTING IN A SIX
HOUR PERIOD OF ENHANCEMENT. THIS COULD BUMP UP SNOW TOTALS EAST OF
LAKE ONTARIO TO 4 TO 8 INCHES.


DAY 2...

AS THE NEGATIVELY-TILTED LONG WAVE TROUGH BREAKS ACROSS NEW
ENGLAND ON DAY 2...A CLOSED MID-LEVEL SYSTEM FORMS OFF THE
SOUTHEAST NEW ENGLAND COAST...THOUGH THERE IS SOME MODEL SPREAD ON
THE POSITION. THIS INDUCES CYCLOGENESIS OVER THE WESTERN GULF OF
ME DURING THE FIRST HALF OF DAY 2. THERE IS SOME SPREAD ON WHERE
THIS OCCURS...THOUGH THE 00Z GFS APPEARS TO BE THE OUTLIER WITH
RESPECT TO THE PLACEMENT OF THE LOW AND THE BOUNDARY LAYER WARMING
THAT RESULTS AHEAD OF IT. A WESTERN GULF OF ME DEVELOPMENT PLACES
MUCH OF EASTERN MA...AS WELL AS COASTAL CENTRAL AND EASTERN ME
WITHIN SOUTHEAST BOUNDARY LAYER FLOW THROUGH 30/18Z AND 31/00Z.
THE BOUNDARY LAYER ISSUES DELAY THE CHANGE TO SNOW ACROSS THE
ABOVEMENTIONED LOCATIONS...BEFORE BOUNDARY LAYER WINDS SHIFT TO
THE NORTH. ON THE NORTHWEST FLOW ACROSS EASTERN MA...THERE MAY BE
ENOUGH MOISTURE AND LIFT FOR AN AREA OF 2-4" OF SNOWFALL.

FURTHER EAST...THE BOUNDARY LAYER ISSUES SHOULD BE WIPED OUT AFTER
31/00Z. THE BEST LIFT OCCURS BETWEEN 30/18Z AND 31/06Z ACROSS
CENTRAL AND EASTERN ME...AND MODEL SOUNDINGS SHOW LIFT IN THE
DENDRITIC GROWTH ZONE HERE. THE LIFT AND MOISTURE SUPPORTS A
PERIOD OF HEAVY SNOW CENTERED ON INTERIOR CENTRAL ME...AND 8-14"
OF SNOWFALL IS EXPECTED ACROSS MUCH OF SOUTHERN AND CENTRAL
ME...WITH HIGHEST AMOUNTS AWAY FROM THE IMMEDIATE COAST FROM
AUGUSTA/KAUG TO BANGOR/KBGR. THESE AMOUNTS ARE SUPPORTED BY
MEMBERS OF THE LATEST ECMWF ENSEMBLE OUTPUT SHOWING 12"+ SNOWFALL
POTENTIAL ACROSS CENTRAL ME...AND 21Z SREF MEMBERS SHOWING SIMILAR
POTENTIAL ACROSS INTERIOR DOWNEAST ME.


DAY 3...

AS THE SURFACE LOW TRACKS FROM OFF THE ME COAST TO A POSITION EAST
OF NOVA SCOTIA ON DAY 3...THE BEST LIFT AND MOISTURE SHIFT ACROSS
EASTERN ME INTO NEW BRUNSWICK. THE BEST GRADIENT OCCURS ACROSS
EASTERN ME...WHERE QPF SUPPORTS 3-5" OF SNOW FROM CARIBOU/KCAR
DOWN THROUGH EASTPORT ME.



...SOUTHWEST/SOUTHERN ROCKIES/SOUTHERN AND CENTRAL PLAINS...

A LONG WAVE TROUGH OVER SOUTHERN CA ON DAY 1 CLOSES OFF...THEN
TRUNDLES SLOWLY EASTWARD INTO WESTERN AZ THROUGH DAY 2. AHEAD OF
THE CLOSED MID LEVEL SYSTEM...LOW LEVEL FLOW BRINGS INCREASING
MOISTURE INTO THE SOUTHWEST AND SOUTHERN ROCKIES ON DAY 1...THEN
TO THE SOUTHERN PLAINS DURING DAY 2. A NORTHERN STREAM SHORT WAVE
PUSHES A FRONT INTO THE CENTRAL PLAINS ON DAY 3. THROUGH DAY
2...THERE WAS DECENT MODEL AGREEMENT...WITH THE 12Z ECMWF/UKMET A
BIT SLOWER MOVING THE CLOSED MID LEVEL SYSTEM...SO THE THERMAL
FIELDS WERE BASED ON A MULTI MODEL BLEND. MODEL SPREAD INCREASES
ON DAY 3...ESPECIALLY WITH RESPECT TO THE PLACEMENT OF THE SURFACE
FRONT (AND THE THERMAL PROFILES ASSOCIATED WITH IT). IN AN ATTEMPT
TO MITIGATE SOME THE DIFFERENCES ON DAY 3...THE THERMAL FIELDS
WERE BASED ON A BLEND OF THE 21Z SREF MEAN/12Z ECMWF. THE QPF
PORTION OF THE FORECAST WAS BASED ON THE MOST RECENT WPC QPF.

DAY 1...

AS THE LONG WAVE TROUGH CLOSES OFF ACROSS SOUTHERN CA ON DAY
1...LOW LEVEL 15-20 KNOT EAST SOUTHEAST FLOW TRANSPORTS MOISTURE
INTO SOUTHERN AZ AND SOUTHERN NM. THE COMBINATION OF LIFT AHEAD OF
THE CLOSED LOW AND MOISTURE FROM THE WESTERN GULF OF MEXICO SHOULD
PRODUCE A LARGE AREA OF PRECIPITATION ACROSS AZ/NM. AS MID LEVEL
HEIGHTS RISE AHEAD OF THE CLOSED LOW...SNOW LEVELS RISE TO BETWEEN
8000 AND 9000 FEET...AND THIS LIMITS SNOWFALL ACCUMULATIONS OF
2-4" TO THE HIGHER TERRAIN OF WESTERN AND CENTRAL NM.


DAY 2...

MOISTURE AND LIFT INCREASES ACROSS THE SOUTHWEST AND SOUTHERN
ROCKIES DURING DAY 2...AS THE INFLOW BECOMES DEEPER AND THE CLOSED
LOW EDGES ACROSS WESTERN AZ. AS MID LEVEL HEIGHTS START
FALLING...SNOW LEVELS WILL START ON A DOWNWARD TREND...FALLING TO
BETWEEN 7000 AND 8000 FEET ACROSS THE REGION. FALLING SNOW LEVELS
AND INCREASING MOISTURE SET THE STAGE FOR A LARGE AREA OF 4-8" OF
SNOW TO STRETCH FROM THE SOUTHERN WASATCH RANGE IN UT ACROSS THE
SAN JUAN RANGE IN SOUTHWEST CO AND MUCH OF NORTHERN AND EASTERN
NM. MEMBERS OF BOTH THE LATEST ECMWF AND 21Z SREF ENSEMBLE OUTPUT
SHOW THE POTENTIAL FOR 8"+ OF SNOWFALL ACROSS PORTIONS OF THE
HIGHER TERRAIN IN EASTERN NM...AND A LARGE SWATH OF 4 -8" OF
SNOWFALL IN THE ABOVEMENTIONED AREAS.

THERE IS A MULTI MODEL SIGNAL FOR FREEZING RAIN ACROSS VALLEY
PORTIONS OF EASTERN AZ AND MUCH OF CENTRAL AND SOUTHERN NM...AS
MOISTURE AND WARMER TEMPERATURES RIDE UP AND OVER SHALLOW COLD AIR
TRAPPED IN VALLEYS. AT THIS POINT...IT APPEARS THERE IS SOME
POTENTIAL FOR 0.10-0.25" OF ICE ACROSS PORTIONS SOUTHWEST AND
SOUTHEAST NM (PER THE 00Z GFS AND NAM SOLUTIONS).


DAY 3...

AS SHORT WAVE ENERGY IS EJECTED FROM THE CLOSED LOW OVER AZ ON DAY
3...MOISTURE GETS FUNNELED ACROSS THE SOUTHERN AND CENTRAL PLAINS
AHEAD OF A COLD FRONT APPROACHING FROM THE NORTHERN ROCKIES. THERE
IS SOME MODEL SPREAD ON THE POSITION OF THE FRONT BY THE END OF
DAY 3...AND THIS HAS BEARING ON THE PRECIPITATION PHASE STRETCHING
FROM EASTERN NE/KS THROUGH IA/MO AND MUCH OF IL. USING A CONSENSUS
APPROACH (EXCLUDING THE 00Z GFS) WITH RESPECT TO THERMAL
PROFILES...A SWATH OF 3-6" OF SNOWFALL WAS EXTENDED FROM EASTERN
NE/KS ACROSS NORTHERN MS AND SOUTHERN IA INTO NORTH CENTRAL IL.

FURTHER SOUTHWEST ACROSS CO...NM AND NORTH TX...AN AREA OF 2-4" OF
SNOWFALL WAS PLACED...AS SNOW LEVELS CONTINUE TO FALL WITH
LOWERING MID LEVEL HEIGHTS (REACHING BELOW 5500 FEET BY THE END OF
THE PERIOD). ICE ACCUMULATIONS OF 0.10" OR LESS COULD CONTINUE TO
BE A PROBLEM ACROSS PORTIONS OF SOUTHEAST NM AND A PORTION OF
NORTH TX...WHERE SHALLOW COLD AIR TRAPPED UNDERNEATH A WARM AIR
ADVECTION INDUCED INVERSION RESULTS IN FREEZING TEMPERATURES AT
THE SURFACE.


...NORTHERN ROCKIES/NORTHERN PLAINS...

A SHORT WAVE TRACKING SOUTHEAST FROM WESTERN CANADA CROSSES THE
NORTHERN AND CENTRAL PLAINS DURING DAY 3...PUSHING ALONG A COLD
FRONT AHEAD OF IT. THERE IS A FAIR AMOUNT OF MODEL SPREAD
CONCERNING THE STRENGTH OF THE SHORT WAVE AND THE SOUTHERN EXTENT
OF THE FRONTAL BOUNDARY. THE 00Z GFS APPEARS TO BE TOO AMPLIFIED
WITH THE SHORT WAVE...AND PUSHES THE FRONT TOO FAR SOUTH INTO THE
CENTRAL PLAINS AND MID MS VALLEY. FOR NOW...THE THERMAL FIELDS
WERE BASED MORE ON A 21Z SREF MEAN/12Z ECMWF BLEND...AND THE
LIQUID EQUIVALENT EXPECTED WAS BASED ON THE MOST RECENT WPC QPF.

THE SHORT WAVE PUSHES THE FRONT ACROSS THE NORTHERN ROCKIES AND
NORTHERN PLAINS...MAINLY DURING THE SECOND HALF OF THE PERIOD. THE
LOW LEVEL INFLOW AHEAD OF THE FRONT IS FAIRLY WEAK (GENERALLY LESS
THAN 15 KNOTS)...AND THE AIRMASS IN PLACE IS DRY (AS PRECIPITABLE
WATER VALUES TOP OUT AT OR BELOW 0.25"). THE COMBINATION OF LIFT
WITH THE SHORT WAVE AND LOCAL UPSLOPE FLOW PRODUCES A SWATH OF 2
TO 4 INCHES OF SNOW FROM NORTH CENTRAL MT INTO SOUTHWEST ND AND
NORTH CENTRAL SD.


HAYES
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