Quantitative Precipitation Forecast
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000
FOUS11 KWBC 292041
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
440 PM EDT WED MAR 29 2017

VALID 00Z THU MAR 30 2017 - 00Z SUN APR 02 2017


DAYS 1 THROUGH 3...


...PACIFIC NORTHWEST/NORTHERN ROCKIES...

A LONG WAVE TROUGH COMES ASHORE ACROSS THE PACIFIC NORTHWEST LATE
ON DAY 1...REACHING THE NORTHERN ROCKIES DURING DAY 2. THE
COMBINATION OF UPSLOPE FLOW AND SYNOPTIC SCALE ASCENT IS EXPECTED
TO RESULT IN HEAVY SNOWFALL ACROSS THE NORTHERN ROCKIES DURING
BOTH DAYS. THERE WAS GENERALLY GOOD MODEL AGREEMENT WITH THE
TIMING OF THE LONG WAVE TROUGH...SO THE THERMAL PORTION OF THE
FORECAST WAS BASED ON A MULTI MODEL BLEND. THE QPF PORTION OF THE
FORECAST WAS BASED ON THE MOST RECENT WPC QPF.

DAY 1
AS A LONG WAVE TROUGH CROSSES THE PACIFIC NORTHWEST EARLY ON DAY
1...THE LOW LEVEL FLOW FOCUSES PACIFIC MOISTURE ACROSS THE OR/WA
CASCADES. SNOW LEVELS HOVER BETWEEN 4500 AND 5500 FEET ACROSS THE
RANGE...WHICH SHOULD ALLOW FOR STRIPES OF 4 TO 8 INCHES OF
SNOWFALL ACROSS THE HIGHER TERRAIN...WITH THE HIGHEST AMOUNTS
ACROSS THE NORTHERN WA CASCADES.

FURTHER EAST...THE SYNOPTIC SCALE ASCENT ASSOCIATED WITH THE LONG
WAVE TROUGH IS AUGMENTED BY STRENGTHENING UPSLOPE FLOW ACROSS THE
SALMON RIVER MOUNTAINS IN ID...AS WELL AS THE SOUTHERN EXTENT OF
THE ROCKY MOUNTAIN FRONT RANGE AND NORTHERN ABSAROKA RANGE IN MT.
THE BEST UPSLOPE OCCURS OVER THE SALMON RIVER MOUNTAINS...WHERE
THERE IS A MULTI MODEL SIGNAL FOR 10 TO 18 INCHES OF
SNOWFALL...AND THESE NUMBERS ARE SUPPORTED BY MEMBERS OF THE
LATEST ECMWF ENSEMBLE OUTPUT SHOWING 12+ INCHES OF SNOWFALL HERE.
LESSER AMOUNTS (GENERALLY BETWEEN 6 AND 10 INCHES OF SNOWFALL) WAS
PLACED ACROSS THE NORTHERN ABSAROKA MOUNTAINS IN MT. THESE
SNOWFALL AMOUNTS SUPPORT A HIGH PROBABILITY OF 12+ INCHES OF
SNOWFALL HERE.

DAY 2...
THE STRONG UPSLOPE FLOW BECOMES FOCUSED ON THE WIND RIVER
MOUNTAINS IN WY...WHERE THE COMBINATION OF UPSLOPE FLOW AND MID
LEVEL LIFT IS EXPECTED TO PRODUCE 12 TO 18 INCHES OF SNOWFALL
ACROSS THIS RANGE. THESE SNOWFALL AMOUNTS WERE WELL SUPPORTED BY
MANY MEMBERS OF THE LATEST ECMWF ENSEMBLE OUTPUT SHOWING 12+
INCHES OF SNOWFALL...AND REPRESENT A HIGH PROBABILITY OF 12+
INCHES OF SNOWFALL IN THIS RANGE. LESSER AMOUNTS...GENERALLY
BETWEEN 4 AND 8 INCHES...WERE PLACED OVER THE BIG HORN RANGE.


...GREAT BASIN/SOUTHERN ROCKIES...

A CLOSED MID LEVEL LOW TRACKS ACROSS THE SOUTHWEST STATES DURING
DAY 2...SLOWING DURING DAY 3 ACROSS NM. AHEAD OF THE MID LEVEL
SYSTEM (AND ITS ATTENDANT SURFACE LOW)...STRENGTHENING UPSLOPE
FLOW AIDS IN PRODUCING VERY HEAVY SNOWFALL AMOUNTS ACROSS THE
SOUTHERN ROCKIES...MAINLY DURING DAY 3. THERE WAS GOOD MODEL
AGREEMENT WITH RESPECT TO THE TRACK AND TIMING OF THE MID LEVEL
SYSTEM...SO THE THERMAL PORTION OF THE FORECAST WAS BASED ON A
MULTI MODEL BLEND. THE QPF PORTION OF THE FORECAST WAS BASED ON
THE MOST RECENT WPC QPF.

DAY 2...
INFLOW AHEAD OF THE CLOSED MID LEVEL LOW THAT TRACKS ACROSS AZ
INTO NM DURING DAY 2 BECOMES FOCUSED ON THE HIGHER TERRAIN OF THE
WASATCH AND UINTA RANGES IN UT. SNOW LEVELS DROP TO BETWEEN 4500
AND 5500 FEET AS THE BEST MID LEVEL LIFT ASSOCIATED WITH THE
CLOSED MID LEVEL SYSTEM ACROSS THE RANGES. THERE IS A MULTI MODEL
SIGNAL FOR 12+ INCHES OF SNOWFALL ACROSS THE UINTAS...AND THESE
AMOUNTS ARE JUSTIFIED BY MANY MEMBERS OF THE LATEST ECMWF ENSEMBLE
OUTPUT SHOWING 12+ INCHES OF SNOWFALL. THE SNOWFALL AMOUNTS ALSO
SUPPORT A HIGH PROBABILITY OF 12+ INCHES OF SNOWFALL. THE LOW
LEVEL INFLOW IS NOT QUITE AS IMPRESSIVE ACROSS THE CENTRAL AND
SOUTHERN WASATCH RANGE...BUT THE MID LEVEL LIFT IS BETTER...SO AN
AXIS OF 8 TO 12 INCHES WAS STRETCHED ACROSS THE ENTIRE RANGE.

FURTHER EAST...UPSLOPE FLOW BECOMES FOCUSES ON THE SAN JUAN RANGE
IN SOUTHWEST CO...AS LOW LEVEL WINDS BACK AHEAD OF DEVELOPING
SURFACE LOW PRESSURE. AN AREA OF 4 TO 8 INCHES WAS PLACED HERE TO
ACCOUNT FOR THE ENHANCED MOISTURE TRANSPORT.

DAY 3...
THE SLOW MOVING MID LEVEL LOW MOVES ACROSS NM DURING DAY 3. THE
LOW LEVEL INFLOW BECOMES UPSLOPE OVER THE FRONT RANGE OF THE
ROCKIES...AND THE STRENGTHENING UPSLOPE FLOW ON THE NORTH SIDE OF
SURFACE LOW PRESSURE TRACKING INTO THE SOUTHERN PLAINS FOCUSES THE
MOISTURE HERE. THERE IS A MULTI MODEL SIGNAL FOR 12 TO 24 INCHES
OF SNOWFALL FROM NORTH OF KVTP THROUGH KFCS/KCOS TO WEST OF
KAPA/WEST OF KDEN...AS A MID LEVEL FRONTOGENETICALLY FORCED BAND
PIVOTS WESTWARD NORTH OF THE MID LEVEL LOW. THESE SNOWFALL AMOUNTS
ARE SUPPORTED VERY WELL BY MANY MEMBERS OF THE LATEST ECMWF
ENSEMBLE OUTPUT SHOWING 12+ INCHES OF SNOWFALL (AND SUPPORT A HIGH
PROBABILITY OF 12+ INCHES OF SNOWFALL)...AND ALL MODELS SHOW A
TRACK OF THE MID LEVEL/SURFACE SYSTEMS FOR A SIGNIFICANT WINTER
STORM. FURTHER SOUTH...WHERE THE MOISTURE AND LIFT ARE NOT NEARLY
AS IMPRESSIVE...AN AXIS OF 6 TO 12 INCHES OF SNOWFALL WAS PLACED
OVER THE NORTHERN SANGRE DE CRISTO RANGE IN NM.


...UPPER GREAT LAKES...

MOISTURE TRANSPORTS NORTH OVER A WARM FRONT INTO THE UPPER GREAT
LAKES DURING DAY 1. ON THE NORTHERN EDGE PRECIPITATION SHIELD
ASSOCIATED WITH THE UPGLIDE...THE COLUMN REMAINS COLD ENOUGH FOR
WINTRY WEATHER...MAINLY DURING DAY 1 AND EARLY DAY 2. THERE WAS
GENERALLY GOOD MODEL AGREEMENT WITH THE OVER SYNOPTIC SETUP
(THOUGH THE 12Z NAM MAY BE ALLOWING THE COLD AIR TO REMAIN IN
PLACE ACROSS WI/LP OF MI TOO LONG ON DAY 1)...SO THE THERMAL
PORTION OF THE FORECAST WAS BASED ON A MULTI MODEL BLEND. THE QPF
PORTION OF THE FORECAST WAS BASED ON THE MOST RECENT WPC QPF.

DAY 1...
THE BEST LIFT IN THE UPGLIDE NORTH OF A WARM FRONT EXTENDING
ACROSS THE MID MS VALLEY CROSSES THE UPPER GREAT LAKES BETWEEN
30/12Z AND 30/18Z. MODEL SOUNDINGS OVER NORTH CENTRAL WI AND THE
NORTHERN PORTION OF THE LP OF MI SHOWED ISOTHERMAL PROFILES NEAR 0
C...BUT THE BEST LIFT APPEARS TO OCCUR JUST A BIT HIGHER THAN THE
-12/-18 C THERMAL RIBBON...THUS ENHANCED SNOWFALL RATIOS DO NOT
SEEM LIKELY DURING THE BEST LIFT. SNOWFALL AMOUNTS COULD BE
SOMEWHAT PROBLEMATIC...AS THE BEST SNOWFALL RATES OCCUR DURING THE
DAYLIGHT HOURS...WHICH COULD RESULT IN SOME COMPACTION OR MELTING.
STRIPES OF 2 TO 4 INCHES OF SNOWFALL WERE PLACED IN THE
ABOVEMENTIONED LOCATIONS.

AS THE BEST LIFT ENDS..THE COLUMN COULD LOSE ITS ABILITY TO MAKE
SNOWFLAKES. THIS MEANS THAT THE PRECIPITATION COULD END AS A
PERIOD OF LIGHT FREEZING RAIN OR FREEZING DRIZZLE. IN ANY
EVENT...ICE AMOUNTS SHOULD AVERAGE LESS THAN 0.05 INCHES.

DAY 2...
AS THE BEST LIFT EXITS EARLY DURING DAY 2...LESS THAN 2 INCHES OF
SNOWFALL IS POSSIBLE ACROSS THE NORTHERN OF LP OF MI...AND LIGHT
ICING COULD OCCUR AS THE PRECIPITATION ENDS AS A PERIOD OF LIGHT
FREEZING RAIN OR FREEZING DRIZZLE.


...NORTHEAST...

AS A SURFACE LOW OVER THE OH VALLEY WEAKENS DURING DAY 2...A
SECONDARY LOW FORMS ALONG THE MID ATLANTIC COAST EARLY ON DAY
3...WHICH TRACKS SOUTH OF LONG ISLAND NY. COLD AIR REMAINS IN
PLACE ACROSS EASTERN NY STATE AND PORTIONS OF NEW ENGLAND FOR
HEAVY SNOWFALL. THE 12Z NAM APPEARS TO BE TOO FAR NORTHWEST WITH
THE SURFACE LOW...AND SUBSEQUENTLY PRODUCES TOO MUCH QPF TOO FAR
INTO THE COLD AIR ACROSS ME. FOR THE MOST PART...THE THERMAL
PORTION OF THE FORECAST WAS BASED ON A BLEND OF THE 12Z GFS/00Z
ECMWF/00Z ECMWF ENSEMBLE MEAN...WHILE THE THE QPF PORTION OF THE
FORECAST WAS BASED ON THE MOST RECENT WPC QPF.

DAY 2...
A STRENGTHENING LOW LEVEL FLOW TRANSPORTS MOISTURE OVER A WARM
FRONT EXTENDING ACROSS THE OH VALLEY INTO PORTIONS OF EASTERN NY
STATE AND WESTERN AND CENTRAL NEW ENGLAND DURING THE FIRST PART OF
DAY 2. COLD AIR IN PLACE OVER EASTERN NY STATE (INCLUDING THE
ADIRONDACKS) AND WESTERN AND CENTRAL NEW ENGLAND (INCLUDING THE
CENTRAL AND SOUTHERN GREEN MOUNTAINS IN VT...AND THE NORTHERN AND
CENTRAL BERKSHIRES IN MA) SHOULD ALLOW THE PRECIPITATION TO FALL
IN THE FORM OF SNOW. THE BEST LIFT IN THE FOCUSED WARM AIR
ADVECTION IS EXPECTED TO PRODUCE AREAS OF 4 TO 8 INCHES OF
SNOWFALL IN THE ABOVEMENTIONED AREAS...WITH THE HIGHEST AMOUNTS
OVER THE ADIRONDACKS. AS THE WARM AIR ADVECTION CONTINUES...THE
EVENT BECOMES ELEVATION DEPENDENT...AND 12Z GFS SOUNDINGS SHOWED
THE POTENTIAL FOR SLEET ACROSS THIS AREA. MEMBERS OF THE LATEST
ECMWF ENSEMBLE OUTPUT SHOWED 12+ INCHES OF SNOWFALL POSSIBLE OVER
THE HIGHER TERRAIN IN EASTERN NY...BUT THE SLEET POTENTIAL MAKE
MAKE SNOWFALL AMOUNTS THIS HIGH DIFFICULT TO REALIZE.

FURTHER SOUTH...THERE IS A MULTI MODEL SIGNAL THAT COLD AIR
REMAINS WEDGED IN ACROSS PORTIONS OF THE SOUTHERN TIER AND LOWER
HUDSON VALLEY ON NY STATE...EXTENDING INTO PORTIONS OF NORTHEAST
PA AND POSSIBLY NEARBY NORTHWEST NJ. BOTH THE 12Z GFS AND 00Z
ECMWF INDICATED THE POTENTIAL FOR LOCAL 0.10 INCH ICE
ACCUMULATIONS ACROSS THE CATSKILLS IN NY AND THE SOUTHERN
BERKSHIRES IN MA AND NEARBY NORTHWEST CT. MODEL SOUNDINGS
SUGGESTED FREEZING RAIN PROFILES BEFORE 31/15Z IN THESE
LOCATIONS...WITH A WARM NOSE NEAR 800 MB.

DAY 3..
AS THE SURFACE LOW DEVELOPS ALONG THE MID ATLANTIC COAST EARLY ON
DAY 3...MOISTURE IS TRANSPORTED BACK ACROSS MUCH OF CENTRAL INTO
PORTIONS OF NORTHERN NEW ENGLAND. A LOW LEVEL EAST NORTHEAST FLOW
LOCKS IN COLD AIR IN THE COLUMN ACROSS THESE AREAS...WITH MAINLY
SNOW EXPECTED ACROSS THE HIGHER TERRAIN OF CENTRAL AND NORTHERN
NEW ENGLAND. THERE IS STILL A FAIR AMOUNT OF SPREAD WITH RESPECT
TO THE TRACK OF THE SURFACE LOW...AND THE PLACEMENT OF THE BEST
MID LEVEL LIFT ASSOCIATED WITH IT. FOR NOW...A SOLUTION BLENDING
THE 12Z GFS/00Z ECMWF WAS USED FOR THE FORECAST...WHICH PLACES THE
BEST LIFT ACROSS MA/SOUTHEAST NH INTO MUCH OF WESTERN ME. AS
MENTIONED EARLIER...THE COLUMN IS COLD ENOUGH FOR SNOW IN THESE
AREAS...WITH 6 TO 10 INCHES OF SNOWFALL STRETCHED OVER FAR
SOUTHERN VT ACROSS SOUTHERN NH INTO SOUTHWEST ME. THESE SNOWFALL
AMOUNTS SUPPORT A MODERATE PROBABILITY OF 8+ INCHES OF
SNOWFALL...AND A LOW RISK OF 12+ INCHES OF SNOWFALL.

GIVEN THE SPREAD IN GUIDANCE...THERE IS STILL SOME ROOM FOR
CHANGES HERE...BUT A SIGNIFICANT LATE SNOWFALL IS LOOKING MORE
LIKELY DURING DAY 3.

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.

HAYES

$$





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