Quantitative Precipitation Forecast
Issued by NWS

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FOUS11 KWBC 280811
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
310 AM EST WED JAN 28 2015

VALID 12Z WED JAN 28 2015 - 12Z SAT JAN 31 2015


DAYS 1 THROUGH 3...


...GREAT LAKES/OH VALLEY/NORTHEAST...

A SHORT WAVE CROSSING THE GREAT LAKES LATE ON DAY 1 AND EARLY DAY
2 PRODUCES SNOWFALL AND ICE ACROSS THE GREAT LAKES ON DAY 1. THE
SHORT WAVE EVOLVES INTO A LONG WAVE TROUGH ACROSS ACROSS EASTERN
LOWER GREAT LAKES AND OH VALLEY DURING DAY 2...PRODUCING LIFT FOR
SNOW AND ICE ACROSS THE LOWER GREAT LAKES...WESTERN PA INTO
WESTERN AND CENTRAL NY STATE. AS THE NEGATIVELY TILTED TROUGH
APPROACHES NEW ENGLAND ON DAY 3...LOW PRESSURE DEVELOPS OVER THE
WESTERN GULF OF ME...THREATENING PORTIONS OF CENTRAL AND EASTERN
ME WITH HEAVY SNOW.

ON DAYS 1 AND 2...THERE WAS DECENT MODEL AGREEMENT ACROSS THE
GREAT LAKES INTO THE OH VALLEY...SO THE THERMAL PROFILES WERE
BASED ON A BLEND OF THE 00Z GFS/21Z SREF MEAN. MODEL SPREAD
INCREASES DURING DAY 3...MAINLY CONCERNING WHERE LOW PRESSURE
DEVELOPS OVER THE GULF OF ME. THE 00Z GFS/UKMET AND 21Z SREF
SUPPORT LOW PRESSURE CLOSER TO THE DOWNEAST ME COAST...WHICH
BETTER FITS A POSITION ASSOCIATED WITH A NEGATIVELY TILTED TROUGH.
OTHER MODEL SOLUTIONS ARE CLOSER TO NOVA SCOTIA...AND WERE NOT
INCLUDED IN THE THERMAL PORTION OF THE FORECAST. THE POSITION OF
THE SURFACE LOW IS IMPORTANT...SINCE IT WILL DETERMINE THE AMOUNT
OF BOUNDARY LAYER WARMING ALONG THE MID COAST AND DOWNEAST ME
COASTS. THE QPF PORTION OF THE FORECAST WAS BASED PRIMARILY ON THE
LATEST WPC QPF.

DAY 1...

A SHORT WAVE TRACKING FROM MANITOBA CROSSES THE UPPER GREAT LAKES
DURING THE SECOND HALF OF DAY 1. AHEAD OF THE SHORT WAVE (AND ITS
ASSOCIATED SURFACE LOW)...A 35 TO 45 KNOT SOUTHWEST FLOW UP ALONG
THE 295 K ISENTROPIC SURFACE TRANSPORTS MOISTURE INTO THE COLUMN
ACROSS THE GREAT LAKES. WHILE MOST OF THE SIGNIFICANT SYNOPTIC
SCALE LIFT AND WARM AIR ADVECTION REMAINS NORTH OF THE BORDER
ACROSS SOUTHERN QUEBEC...ENOUGH BLEEDS SOUTH ACROSS THE UPPER
GREAT LAKES FOR AN INCH OR TWO OF SNOWFALL ACROSS THE UP OF MI.

FURTHER SOUTH...LIFT IN THE COLUMN IS SUFFICIENT FOR
PRECIPITATION...BUT MODEL SOUNDINGS SHOW NO MOISTURE IN THE
DENDRITIC GROWTH ZONE. THIS SHOULD RESULT IN LIGHT ICING ACROSS
PORTIONS OF SOUTHWEST MI...WITH ICE AMOUNTS REMAINING BELOW 0.10
INCHES.

DAY 2...

AS THE SHORT WAVE TRACKS OVER THE UPPER GREAT LAKES EARLY ON DAY
2...IT BEGINS EVOLVING INTO A LONG WAVE TROUGH...WHICH CROSSES THE
GREAT LAKES AND OH VALLEY THROUGH 30/00Z. THE LONG WAVE TROUGH
STARTS TAKING ON A NEGATIVE TILT AS IT APPROACHES WESTERN NEW
ENGLAND LATE IN THE PERIOD. STRONG WARM AIR ADVECTION (WITH A 35
TO 45 KNOT SOUTHWEST FLOW UP THE 290 K ISENTROPIC SURFACE) PUMPS
0.50 INCH PRECIPITABLE WATER VALUES INTO THE AIRMASS. MOISTURE IN
THE PRESENCE OF STRONG SYNOPTIC SCALE ASCENT PRODUCES A LARGE AREA
OF 4 TO 6 INCHES OF SNOW ACROSS EASTERN OH...WESTERN PA AND
NORTHERN WV. BEHIND THE LONG WAVE TROUGH...LAKE ENHANCED
CONDITIONS DEVELOP...ESPECIALLY ACROSS THE LOWER GREAT LAKES.
MODEL SOUNDINGS EAST OF LAKES ERIE AND ONTARIO SHOWED A 6 TO 9
HOUR PERIOD OF CAPPING INVERSION NEAR 775 MB...VERTICAL MOTION IN
THE LOWER LAYERS...AND BOUNDARY LAYER WINDS NEAR THE LONG AXIS OF
THE LAKES. THE ADDITIONAL LAKE ENHANCEMENT SHOULD RESULT IN 4 TO 8
INCHES OF SNOWFALL EAST OF EACH OF THE LAKES.

DAY 3...

MODEL SPREAD BEGINS TO INCREASE DURING DAY 3. AS THE LONG WAVE
TROUGH BECOMES NEGATIVE...IT INDUCES CYCLOGENESIS ON A LOW LEVEL
BOUNDARY OFF THE CENTRAL NEW ENGLAND COAST. THERE IS A MODEL
SIGNAL THAT THE MID LEVEL SYSTEM TRIES CLOSING OFF OVER OR EAST OF
THE GULF OF ME. THERE IS SOME MODEL SPREAD ON THE LOCATION OF THE
SURFACE DEVELOPMENT...AND THAT HAS AN IMPACT ON THE BOUNDARY LAYER
TEMPERATURES EARLY...ESPECIALLY ACROSS CENTRAL AND DOWNEAST ME. AS
MENTIONED EARLIER...A SOLUTION CLOSER TO THE DOWNEAST ME COAST
THAN NOVA SCOTIA IS PREFERRED...GIVEN EXPECTED POSITION OF THE MID
LEVEL CLOSED SYSTEM. THIS BEING THE CASE...THE INITIAL SHOT OF
PRECIPITATION HERE COULD COME IN THE FORM OF RAIN.

HOWEVER...AS COLDER AIR IS DRAWN INTO THE DEVELOPING LOW LEVEL
CENTER...THE COLUMN BECOMES COLD ENOUGH TO SUPPORT SNOWFALL FOR
ALL OF NH AND ME. THE BEST MID LEVEL LIFT AND MOISTURE BECOME
CENTERED ACROSS INTERIOR SECTIONS OF CENTRAL ME. MODEL SOUNDINGS
SHOWED MOISTURE AND LIFT IN THE DENDRITIC ZONE...SUPPORTING THE
THREAT FOR HEAVY SNOWFALL ACROSS ME. QPF SUPPORTS AN AREA OF 8 TO
14 INCHES OF SNOW ACROSS MUCH OF CENTRAL AND SOUTHERN ME. BOTH THE
LATEST ECMWF AND 21Z SREF ENSEMBLE OUTPUT SUPPORTS 8 TO 12 INCHES
OF SNOWFALL ACROSS CENTRAL AND SOUTHERN ME...THOUGH THERE ARE SOME
POSITIONAL DIFFERENCES WITH RESPECT TO THE MAXIMUM.

BECAUSE THERE IS SOME MODEL SPREAD IN THE POSITION OF THE SURFACE
LOW (AND BOUNDARY LAYER WARMING)...THERE PRECISE LOCATION OF THE
HEAVIEST SNOWFALL IS STILL IN DOUBT. THE THREAT FOR HEAVY SNOW
EXISTS...BUT THE LOCATION MAY CHANGE DURING THE NEXT FEW FORECAST
CYCLES.


...SOUTHWEST/SOUTHERN ROCKIES/SOUTHERN PLAINS...

A CLOSED LOW OVER SOUTHERN ON DAY 2 TRUNDLES EASTWARD INTO WESTERN
AZ ON DAY 3. AHEAD OF THE MID LEVEL SYSTEM...A MOIST LOW LEVEL
FLOW TRANSPORTS MOISTURE INTO THE SOUTHWEST AND SOUTHERN PLAINS
DURING THE PERIOD. THE COMBINATION OF LIFT AND MOISTURE POSES THE
THREAT FOR HEAVY SNOWFALL...ESPECIALLY ACROSS NORTHERN
NM...SOUTHWEST CO AND SOUTHERN UT ON DAY 3. FOR THE MOST
PART...THERE WAS DECENT MODEL AGREEMENT WITH THE OVERALL SYNOPTIC
SETUP...SO THE THERMAL PORTION OF THE FORECAST WAS BASED ON A 00Z
GFS/21Z SREF MEAN/12Z ECMWF BLEND. THE QPF PORTION OF THE FORECAST
WAS BASED PRIMARILY ON THE LATEST WPC QPF.

DAY 2...

AS THE CLOSED MID LEVEL SYSTEM SLIDES EASTWARD ON DAY 2...THE BEST
LIFT AHEAD OF IT OCCURS ACROSS THE SOUTHWEST AND SOUTHERN ROCKIES
DURING THE SECOND HALF OF THE PERIOD. AS MID LEVEL HEIGHTS
FALL...SNOW LEVELS DROP TO NEAR 8000 FEET ACROSS NORTHERN AND
CENTRAL NM IN SOUTHERN CO. AHEAD OF THE CLOSED LOW...A 15 TO 25
KNOT EAST SOUTHEAST LOW LEVEL FLOW (CENTERED NEAT 850 MB)
TRANSPORTS 0.75 INCH PRECIPITABLE WATER VALUES INTO SOUTHERN AZ
AND SOUTHERN NM. THE COMBINATION OF MOISTURE AND LIFT SHOULD
PRODUCE 1 TO 3 INCH SNOWFALL AMOUNTS ACROSS PORTIONS OF NORTHERN
AND CENTRAL NM.

DAY 3...

THE CLOSED MID LEVEL LOW CONTINUES TO DEEPEN AS IT REACHES WESTERN
AZ BY THE END OF DAY 3. THE MAIN THRUST OF MOISTURE AND LIFT AHEAD
OF THE CLOSED LOW IS ACROSS NM AND SOUTHERN CO INTO NORTH TX AND
THE REMAINDER OF THE SOUTHERN PLAINS. SNOW LEVELS DROP EVEN
FURTHER... REACHING BELOW 6000 FEET ACROSS PORTIONS OF NORTHERN NM
AND SOUTHWEST CO. STEEPENING MID LEVEL LAPSE RATES SUGGEST
MARGINAL INSTABILITY...WHICH TARGET THE HIGHER TERRAIN OF SOUTHERN
UT/SOUTHWEST CO AND NORTH CENTRAL NM FOR THE HIGHEST QPF AMOUNTS.
FALLING SNOW LEVELS AND INCREASING SYNOPTIC SCALE LIFT PRODUCES A
LARGE AREA OF 6 TO 10 INCHES OF SNOW ACROSS NORTHERN NM...WITH 4
TO 8 INCH AMOUNTS EXPECTED OVER THE SOUTHERN WASATCH RANGE IN UT
AND THE SAN JUAN RANGE IN SOUTHWEST CO. THE HIGHEST SNOWFALL
AMOUNTS ARE WELL SUPPORTED BY THE LATEST ECMWF ENSEMBLE
OUTPUT...WHICH HAS MEMBERS SHOWING 8+ INCHES OF SNOW ACROSS
NORTHERN CENTRAL NM.

THERE IS A MODEL SIGNAL FOR ICE ACCUMULATIONS ACROSS PORTIONS OF
SOUTH CENTRAL NM...WHERE MODEL SOUNDINGS SUGGEST COLD AIR TRAPPED
IN SOME OF THE VALLEYS BECOMES OVERRUN BY WARMER AIR ABOVE 850 MB.
AT THIS POINT...IT APPEARS AS THOUGH MOST ICE ACCUMULATIONS REMAIN
BELOW 0.10 INCHES.


...NORTHERN AND CENTRAL ROCKIES...

A SHORT WAVE CROSSING THE NORTHERN AND CENTRAL ROCKIES ON DAY 1
PROVIDES SUFFICIENT LIFT FOR SNOWFALL ACROSS THE HIGHER TERRAIN OF
WY...AND NORTHERN AND CENTRAL CO. THE BEST LIFT WITH THE SHORT
WAVE CROSSES SOUTHERN ID/WY AND CO BEFORE 29/00Z..AND THE LIFT
MAKES THE MOST OF THE MOISTURE IN THE COLUMN (WHICH AVERAGES 0.25
INCHES OR LESS). THE COMBINATION OF SYNOPTIC SCALE LIFT AND
UPSLOPE FLOW RESULTS IN 2 TO 4 INCH SNOWFALL AMOUNTS ACROSS THE
GRAND TETONS AND BIG HORN RANGES IN WY...AND THE ROCKIES OF
NORTHERN AND CENTRAL CO.


HAYES



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