Quantitative Precipitation Forecast
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FOUS11 KWBC 240820
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
419 AM EDT THU APR 24 2014

VALID 12Z THU APR 24 2014 - 12Z SUN APR 27 2014


DAYS 1 THROUGH 3...


...MAINE...

A CLOSING MID LEVEL SYSTEM WILL SPREAD PRECIPITATION BACK INTO
MARGINALLY COLD AIR FOR SNOW OVER NORTHERN ME DURING DAY 1. THERE
IS EXCELLENT MODEL AGREEMENT WITH RESPECT TO THE OVERALL SYNOPTIC
SETUP FOR THIS SYSTEM...SO THE QPF PORTION OF THE FORECAST WAS
BASED ON A MULTI MODEL BLEND. THE THERMAL PORTION OF THE FORECAST
WAS BASED ON A BLEND OF THE 00Z NAM/GFS...WHICH ARE IN CLOSER
AGREEMENT THAN THEY HAVE BEEN THE PAST FEW MODEL CYCLES.

A SHARP NEGATIVELY TILTED MID LEVEL TROUGH OVER THE GULF OF ME IS
IN THE PROCESS OF CLOSING OFF EARLY ON DAY 1...AND MOISTURE IS
WRAPPING BACK WESTWARD INTO EASTERN ME. MODEL SOUNDINGS SUGGEST
THAT THE COLUMN IS MARGINALLY COLD FOR SNOW ACROSS THE ROOFTOP OF
ME...AND SHOULD REMAIN SO THROUGH THE FIRST HALF OF THE PERIOD.
SOLAR INSOLATION COULD RESULT IN PRECIPITATION TYPE ISSUES AFTER
14Z...AND MOST OF THE ACCUMULATION PROBABLY OCCURS IN THE HIGHER
TERRAIN. A SWATH OF 1 TO 4 INCHES OF SNOW WAS PAINTED ACROSS
NORTHEAST ME...AND THESE AMOUNTS REPRESENTED A CONSENSUS OF MODEL
OUTPUT.

ANOTHER CLOSED MID LEVEL LOW OVER THE GULF OF ME WILL TOSS
MOISTURE BACK ACROSS THE HIGHER TERRAIN OF ME ON DAY 3...WHERE THE
COLUMN MAY BE COLD ENOUGH TO SUPPORT SNOWFALL. THERE WAS MORE
MODEL SPREAD WITH THIS SYSTEM...AS THE COLD AIR IN THE COLUMN MAY
BE MORE TENUOUS. BASED ON CLIMATOLOGY...THE MOST LIKELY PLACEMENT
OF ANY SNOW ACCUMULATION WOULD BE IN THE HIGHER TERRAIN ACROSS
WESTERN ME. BASED ON THE MODEL CONSENSUS...1 OR 2 INCHES OF SNOW
IS THE MOST LIKELY OUTCOME...MAINLY ABOVE 3000 FEET. IT SHOULD BE
NOTED THAT THE 00Z NAM AND 21Z SREF MEAN SHOWED THAT HIGHER SNOW
ACCUMULATIONS ARE POSSIBLE...BUT MODEL SPREAD AT THIS POINT MAKE
THESE OUTCOMES LESS LIKELY.


...UPPER MS VALLEY/UPPER GREAT LAKES...

A SHORT WAVE OVER THE NORTHERN PLAINS EARLY ON DAY 1 WILL CLOSE
OFF...PROVIDING MOISTURE AND LIFT FOR A SIGNIFICANT LATE SEASON
SNOWFALL ACROSS PORTIONS OF NORTHEAST MN...NORTHERNMOST WI AND THE
UP OF MI. THE MODEL DIFFERENCES OF THE PAST FEW CYCLES HAVE BECOME
LESS IMPORTANT...SO THE QPF WAS BASED ON A MULTI MODEL
BLEND...WITH INCLUDED THE 00Z REGIONAL GEM. THE THERMAL
DIFFERENCES HAVE ALSO BECOME MINIMAL...SO MUCH SO THAT THE THERMAL
COMPONENT OF THE FORECAST WAS ALSO BASED ON A MULTI MODEL
BLEND...WEIGHTED MORE HEAVILY TOWARD THE 00Z GFS/NAM.

THE INITIAL SURGE OF PRECIPITATION...DEVELOPING IN THE STRONG
UPGLIDE ON THE NOSE OF A 35 TO 45 KNOT LOW LEVEL SOUTHEAST
JET...IS FALLING IN THE FORM OF RAIN...AS THE COLUMN REMAINS WARM.
THERE IS NOW GOOD MODEL AGREEMENT THAT...AS THE 850 MB LOW DEEPENS
AND TRACKS ACROSS EASTERN MN INTO NORTHWEST WI...THE COLDER AIR
WILL SEEP SOUTHWARD DURING THE DAY. MODEL SOUNDINGS SHOW THAT THE
COLD AIR GETS INTO THE NORTHEAST MN AND NORTHERNMOST WI DURING THE
AFTERNOON. THE COMBINATION OF LOW LEVEL COLD AIR ADVECTION AND
EVAPORATIVE COOLING...AS A RESULT OF THE BETTER PRECIPITATION
RATES...SHOULD ALLOW A TRANSITION TO SNOW AFTER 18Z AND BEFORE
ABOUT 22Z IN THE ABOVEMENTIONED AREAS.

LIFT ASSOCIATED WITH THE CLOSING MID LEVEL SYSTEM SHOULD MAXIMIZE
BETWEEN 21Z THU AND 06Z FRI...FIRST ACROSS NORTHEAST MN...THEN
ACROSS NORTHERNMOST WI INTO THE UP OF MI. MELTING COULD SLOW
ACCUMULATION BEFORE SUNSET...BUT AFTER THAT TIME...THE COMBINATION
OF INCREASED SNOWFALL RATES AND COOLING OF THE COLUMN SHOULD ALLOW
FOR HEAVY SNOW TO FALL. THE LIFT AND MOISTURE REACH THE DENDRITIC
GROWTH DURING THIS PERIOD...ALLOWING FOR THE BULK OF THE
ACCUMULATION TO OCCUR DURING THIS TIME.

A SWATH OF 4 TO 8 INCHES OF SNOW WAS PLACED ACROSS NORTHEAST MN...
NORTHERNMOST WI AND THE UP OF MI THROUGH 12Z THU...WITH A MAXIMUM
NEAR 10 INCHES ACROSS NORTHEASTERN MOST MN. THESE SNOWFALL AMOUNTS
WERE WELL SUPPORTED BY MORE THAN HALF OF THE LATEST ECMWF ENSEMBLE
MEMBERS.
THE 00Z NAM AND 21Z SREF CONTINUES TO SHOW THE HIGHEST
ACCUMULATION HERE...IN EXCESS OF 14 INCHES BY 12Z THU....AND THERE
ARE A FEW ECMWF ENSEMBLES MEMBERS THAT SUPPORT THESE VALUES AS
WELL. AT THIS POINT...THESE HIGHER AMOUNTS SEEM LIKE A LOW
PROBABILITY DUE TO THE UNCERTAINTY IN ACCUMULATIONS DURING THE
DAYLIGHT HOURS.

THE BEST LIFT ASSOCIATED WITH THE CLOSED MID LEVEL SYSTEM TRACKS
INTO QUEBEC DURING THE FIRST HALF OF DAY 2. THE COLUMN REMAINS
COLD ENOUGH FOR SNOW ACROSS NORTHEAST MN...NORTHERNMOST WI AND THE
UP OF MI...SO AN ADDITIONAL 1 TO 3 INCHES OF SNOW IS EXPECTED
BEFORE THE BEST LIFT EXITS.


...WEST COAST/GREAT BASIN/NORTHERN AND CENTRAL ROCKIES...

SHORT WAVES ASSOCIATED WITH A LONG WAVE TROUGH CROSSING THE WEST
COAST WILL PROVIDE LIFT FOR WINTER WEATHER ACROSS THE REGION EACH
OF THE NEXT THREE DAYS. OVERALL...THE 00Z MODEL GUIDANCE SUITE WAS
IN FAIRLY GOOD AGREEMENT WITH THE SYNOPTIC SETUP. THE QPF WAS
BASED ON A MULTI MODEL BLEND...WITH THE 00Z NAM USED TO BETTER
DELINEATE HIGHER TERRAIN QPF MAXIMA. THE THERMAL PORTION OF THE
FORECAST AS BASED ON A BLEND OF THE 00Z GFS/NAM/21Z SREF
MEAN...WITH SOME 12Z ECMWF INFLUENCE ON DAYS 2 AND 3 ACROSS THE
GREAT BASIN AND CENTRAL ROCKIES.

THE FIRST STRONG SHORT WAVE NEAR 47N 130W EARLY THIS MORNING WILL
AFFECT THE PACIFIC NORTHWEST AND NORTHERN ROCKIES DURING THE FIRST
HALF OF DAY 1. FREEZING LEVELS RANGE BETWEEN 4000 AND 6000 FEET
ACROSS THE HIGHER TERRAIN IN WA AND OR. THE HIGHER FREEZING LEVELS
WOULD SUPPORT 2 TO 4 INCHES OF SNOW ACROSS THE CASCADES OF WA AND
OR...GENERALLY ABOVE 4000 FEET. FURTHER EAST... FREEZING LEVELS
OVER THE NORTHERN ROCKIES SHOULD REMAIN BETWEEN 2500 AND 3500
FEET. WHILE MOISTURE AHEAD OF THE SHORT WAVE WILL NOT BE AS
PLENTIFUL AS IT WAS ALONG THE WA/OR COAST... UPSLOPE MAKES THE
MOST OF THE AVAILABLE MOISTURE OVER THE NORTHERN ROCKIES. THE
COMBINATION OF LIFT AND MOISTURE SHOULD SUPPORT 6 TO 10 INCHES OF
SNOW ACROSS HIGHER BITTEROOTS OF ID...WITH LESSER AMOUNTS (2 TO 4
INCHES) ACROSS THE GRAND TETONS OF WY.

THE LONG WAVE TROUGH COMES ASHORE OVER CA ON DAY 2. CRASHING MID
LEVEL HEIGHTS SHOULD RESULT IN DROPPING FREEZING LEVELS...WHICH
FALL TO NEAR VALLEY LEVELS ACROSS EAST CENTRAL CA AND THE HIGHER
TERRAIN OF NORTHEAST AND CENTRAL NV.  A STRONG SHORT WAVE CROSSES
THE CENTRAL CA COAST...AND SHOULD TARGET THE LIFT AND MOISTURE
FIRST ON THE SIERRA NEVADA RANGE. THE UPSLOPE FLOW FROM THE
PACIFIC SHOULD BECOME FOCUSED ON THE SIERRA NEVADA...WITH QPF
SUPPORTING 12 TO 18 INCHES OF SNOW ACROSS THE RANGE. THESE NUMBERS
ARE BEST SUPPORTED BY THE 00Z NAM/21Z SREF MEAN...AND UNLIKE THE
23/00Z ECMWF ENSEMBLE SOLUTION...THE LATEST RUN HAS MEMBERS WITH
12+ INCHES OF SNOW HERE. FURTHER EAST...AS THE BEST LIFT TRACKS
ACROSS NV...THE MOISTURE SOURCE IS NOT AS PLENTIFUL.
HOWEVER...UPSLOPE FLOW AND MARGINAL INSTABILITY SHOULD HELP PLACE
THE HIGHEST QPF (AND SNOWFALL) ACROSS THE TOIYABE RANGE AND THE
MONITOR MOUNTAINS...WHERE 5 TO 10 INCHES OF SNOW WAS PLACED.

AS THE BEST LIFT WITH THE SHORT WAVE REACHES THE WASATCH RANGE AT
THE BEGINNING OF DAY 3. AGAIN...THE MOISTURE WANES ACROSS THE
GREAT BASIN. HOWEVER...LOCAL UPSLOPE FLOW AND MARGINAL
INSTABILITY...IN CONJUNCTION WITH THE SYNOPTIC SCALE LIFT
ASSOCIATED WITH THE SHORT WAVE... SHOULD MAKE THE MOST OF THE
AVAILABLE MOISTURE. THE QPF SUPPORTS AN AREA OF 4 TO 8 INCHES OF
SNOW ACROSS THE RANGE...MAINLY ABOVE 7000 FEET. THESE AMOUNTS ARE
ALSO SUPPORTED BY MEMBERS OF THE LATEST ECMWF ENSEMBLE OUTPUT.


..SOUTHWEST/SOUTHERN ROCKIES...

A STRONG SHORT WAVE DIVING INTO THE BOTTOM OF A LONG WAVE TROUGH
CROSSING THE ROCKIES ON DAY 3 WILL PROVIDE THE LIFT FOR SNOW
ACROSS THE SOUTHERN ROCKIES AND THE HIGHER TERRAIN OF THE
SOUTHWEST. OVERALL...THERE WAS GOOD MODEL AGREEMENT WITH THE
TIMING THE LONG WAVE TROUGH... THOUGH THERE WERE SOME DIFFERENCES
REGARDING THE SPEED OF THE SHORT WAVE. WITH THIS IN MIND... THE
QPF WAS BASED ON A MULTI MODEL BLEND...WITH THE 00Z NAM USED TO
BETTER DELINEATE HIGHER TERRAIN QPF MAXIMA. THE THERMAL PORTION OF
THE FORECAST WAS BASED ON A BLEND OF THE 00Z GFS/00Z NAM/21Z SREF
MEAN.

LOWERING HEIGHTS AHEAD OF THE LONG WAVE TROUGH WILL ALLOW FREEZING
LEVELS TO DROP TO NEAR VALLEY LEVEL ACROSS PORTIONS OF THE
SOUTHERN ROCKIES...AND AS LOW AS 6000 FEET ACROSS PORTIONS OF
NORTHERN AZ AND NORTHERN NM. THE BEST LIFT OCCURS BETWEEN 18Z SAT
AND 06Z SUN...AHEAD OF THE SHORT WAVE THAT BOTTOMS OUT IN THE LONG
WAVE TROUGH ACROSS THE REGION...BEFORE IT TRACKS NORTHEAST. A
STRONG LOW LEVEL SOUTHWEST FLOW AHEAD OF THE SHORT WAVE BECOMES AN
UPSLOPE FLOW ACROSS THE SOUTHERN ROCKIES...MAKING THE MOST OF THE
AVAILABLE MOISTURE. IN ADDITION... STEEPENING MID LAPSE RATES
SHOULD PRODUCE MARGINAL INSTABILITY...WHICH THEN TARGETS THE
HIGHER TERRAIN FOR HEAVIER SNOWFALL.

THE QPF COULD SUPPORT 5 TO 8 INCHES OF SNOW ACROSS THE SAN JUANS
IN SOUTHWEST C0...AND 2 TO 4 INCHES OVER THE HIGHER TERRAIN OF
NORTHERN NM. THE SNOW LEVELS OVER NORTHERN AZ DROP TO AS LOW AS
6000 FEET...WHICH SUPPORT AN INCH OR TWO OF SNOW ACROSS NORTHERN
AZ...AND BETWEEN 2 AND 4 INCHES OVER THE HIGHER TERRAIN OF THE
KAIBAB PLATEAU AND THE SAN FRANCISCO PEAKS.

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.


HAYES

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