Quantitative Precipitation Forecast
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FOUS11 KWBC 290803
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
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 LONG WAVE TROUGH GOES NEGATIVE ACROSS THE
GREAT LAKES AND OH VALLEY DURING DAY 1. A CLOSED LOW SPINS UP AT
THE BASE ON 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 DEVELOP OVER THE GULF OF ME. THE 00Z GFS APPEARS
TO BE TOO CLOSE TO THE ME COAST WITH THE DEVELOP...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 TILE 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 RESULT 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 TO 6 INCHES ACROSS THESE AREAS. 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...BUT
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 CAPPING
INVERSION RISING TO NEAR 800 MB...BOUNDARY LAYER WINDS NEAR THE
LONG AXIS OF THE LAKE...AND VEERING WINDS... RESULTING IN A 6 HOUR
PERIOD OF ENHANCEMENT. THIS COULD BUMP UP SNOW TOTALS EAST OF LAKE
ONTARIO TO 4 TO 8 INCHES.

DAY 2...

AS THE NEGATIVE 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 RESULT
AHEAD OF IT. A WESTERN GULF OF ME DEVELOPMENT PLACES MUCH OF
EASTERN MA...AS WELL AS COASTAL CENTRAL AND EASTERN ME...IN THE
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 TO 4 INCHES OF SNOWFALL.

FURTHER EAST...THE BOUNDARY LAYER ISSUES SHOULD BE WIPED OUT AFTER
31/00Z. THE BEST LIFT OCCURS IN THE 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 TO 14
INCHES OF SNOWFALL IS EXPECTED ACROSS MUCH OF SOUTHERN AND CENTRAL
ME...WITH HIGHEST AMOUNTS AWAY FROM THE IMMEDIATE COAST FROM KAUG
TO KBGR. THESE AMOUNTS ARE SUPPORTED BY MEMBERS OF THE LATEST
ECMWF ENSEMBLE OUTPUT SHOWING 12+ INCH 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 TO 5 INCHES OF SNOW FROM 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...A 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...A
LOW LEVEL 15 TO 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
TO 4 INCHES 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 TO 8
INCHES 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+ INCHES OF SNOWFALL ACROSS
PORTIONS OF THE HIGHER TERRAIN IN EASTERN NM...AND A LARGE SWATH
OF 4 TO 8 INCHES 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 TO 0.25 INCHES OF ICE ACROSS PORTIONS SOUTHWEST
AND SOUTHEAST NM (AS BOTH THE 00Z GFS AND NAM SHOWED THIS).

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 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 TO 6 INCHES 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 TO 4
INCHES OF SNOWFALL WAS PLACED...AS SNOW LEVELS CONTINUE TO FALL
WITH DROPPING MID LEVEL HEIGHTS (REACHING BELOW 5500 FEET BY THE
END OF THE PERIOD). ICE ACCUMULATIONS OF 0.10 INCHES 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 QPF
PORTION OF THE FORECAST 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 INCHES). 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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