Quantitative Precipitation Forecast
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QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
441 PM EDT TUE MAR 28 2017

VALID 00Z WED MAR 29 2017 - 00Z SAT APR 01 2017


DAYS 1 THROUGH 3...


...PACIFIC NORTHWEST/NORTHERN ROCKIES...

A LONG WAVE TROUGH APPROACHES THE PACIFIC NORTHWEST DURING DAY 1.
THE LONG WAVE TROUGH CROSSES THE PACIFIC NORTHWEST DURING DAY
2...BEFORE CLOSING OFF IN THE MID LEVELS OVER THE GREAT BASIN ON
DAY 3.  WHILE THERE ARE SOME TIMING DIFFERENCES WITH THE PLACEMENT
OF THE LONG WAVE TROUGH...ESPECIALLY DURING DAYS 2 AND 3...THERE
WAS ENOUGH CONSENSUS TO USE A MULTI MODEL BLEND OF THE THERMAL
PORTION OF THE FORECAST. THE QPF PORTION OF THE FORECAST WAS BASED
ON THE MOST RECENT WPC QPF.

DAY 1...
AS A LONG WAVE TROUGH APPROACHES THE PACIFIC NORTHWEST DURING DAY
1...A LOW LEVEL SOUTHWEST FLOW TAPS DEEPENING PACIFIC MOISTURE AND
FOCUSES IT ON THE HIGHER TERRAIN OF WA. SNOW LEVELS RISE TO
BETWEEN 4500 AND 5500 FEET...AND THERE SHOULD BE ENOUGH QPF TO
SUPPORT 6 TO 10 INCHES OF SNOW ACROSS THE HIGHER TERRAIN OF
NORTHERN WA CASCADES. THESE AMOUNTS ARE SUPPORTED BY THE MEMBERS
OF THE LATEST ECMWF ENSEMBLE OUTPUT...AND SUPPORTED A SLIGHT TO
MODERATE PROBABILITY FOR 8+ INCHES OF SNOWFALL. FURTHER EAST...THE
MOISTURE PLUME IS NOT QUITE AS ROBUST IT WILL BE ACROSS THE
PACIFIC NORTHWEST...BUT THE COMBINATION OF UPSLOPE FLOW AND
SYNOPTIC SCALE ASCENT WILL RESULT IN AN AREA OF 4 TO 8 INCHES OVER
THE NORTHERN BITTERROOT RANGE IN ID.

DAY 2...
AS THE LONG WAVE TROUGH COMES ASHORE DURING DAY 2...THE BEST LOW
LEVEL MOISTURE BECOMES FOCUSED ON THE OR CASCADES. SNOW LEVELS
FALL TO BETWEEN 4500 AND 5500 FEET. THE FALLING SNOW LEVELS CAN
SUPPORT AN AXIS OF 6 TO 10 INCHES OF SNOWFALL ACROSS THIS RANGE.
FURTHER NORTH...THE MOISTURE AND LIFT ARE NOT AS IMPRESSIVE...BUT
LOWERING SNOW LEVELS SUPPORT 3 TO 6 INCHES OF SNOWFALL OVER THE
NORTHERN WA CASCADES.

FURTHER EAST...THE BEST LIFT ASSOCIATED WITH THE LONG WAVE TROUGH
CROSSES THE NORTHERN ROCKIES DURING THE SECOND HALF OF THE PERIOD.
UPSLOPE FLOW AUGMENTS THE SYNOPTIC SCALE LIFT TO PRODUCE A BROAD
AREA OF 6 TO 10 INCHES OF SNOW ACROSS THE SALMON RIVER AND
SOUTHERN BITTERROOT MOUNTAINS IN ID. THE LATEST ECMWF ENSEMBLE
OUTPUT SUPPORTS
8+ INCHES OF SNOWFALL OVER THE SALMON RIVER MOUNTAINS...AND THE
LIMITING FACTOR FOR HIGHER AMOUNTS ARE RISING SNOW LEVELS...WHICH
REACH ABOVE 6000 FEET.

DAY 3...
AS SURFACE LOW PRESSURE FORMS OVER THE GREAT BASIN IN RESPONSE TO
A CLOSING MID LEVEL LOW...THE UPSLOPE FLOW AHEAD OF THE SURFACE
LOW WILL FOCUS MOISTURE OVER THE WIND RIVER MOUNTAINS IN WY. THE
UPSLOPE FLOW SHOULD RESULT IN AN AXIS OF 8 TO 14 INCHES OF SNOW
OVER THE UPSLOPE REGIONS OF THE RANGE...AND THESE SNOWFALL AMOUNTS
ARE BOLSTERED BY SUPPORT FROM MANY MEMBERS OF THE LATEST ECMWF
ENSEMBLE OUTPUT SHOWING THE POTENTIAL FOR 12+ INCHES OF SNOWFALL.


...GREAT BASIN/CENTRAL AND SOUTHERN ROCKIES...

A STRONG CLOSED MID LEVEL LOW OVER NM EARLY ON DAY 1 TRACKS TO
WESTERN OK...SPINNING UP LOW PRESSURE THAT CROSSES THE SOUTHERN
PLAINS. THE COMBINATION OF LIFT AND UPSLOPE FLOW IS EXPECTED TO
RESULT IN HEAVY SNOWFALL ACROSS PORTIONS OF SOUTH CENTRAL CO INTO
NORTHEAST NM. ANOTHER CLOSED MID LEVEL SYSTEM DROPS INTO THE GREAT
BASIN DURING DAY 3...AS THE FOCUS FOR HEAVY SNOWFALL SHIFTS TO UT.
THERE WAS SOME SPREAD IN THE PLACEMENT OF THE PLACEMENT OF THE
CLOSED MID LEVEL LOW ON DAY 3...SO A BLEND OF THE 12Z GFS AND 00Z
ECMWF WAS USED FOR THE THERMAL PORTION OF THE FORECAST. THE QPF
PORTION OF THE FORECAST WAS BASED ON THE MOST RECENT WPC QPF.

DAY 1...
AS A CLOSED MID LEVEL LOW DROPS INTO EASTERN NM DURING DAY 1...IT
SPINS UP SURFACE LOW PRESSURE OVER NORTHEAST NM...WHICH TRACKS
INTO THE WESTERN OK PANHANDLE. STRENGTHENING UPSLOPE AHEAD OF THE
SURFACE AND MID LEVEL SYSTEMS FOCUSES MOISTURE ACROSS THE SAN JUAN
RANGE IN CO AND SANGRE DE CRISTO MOUNTAINS IN CO AND NEARBY NM.
THE COMBINATION OF SYNOPTIC SCALE ASCENT AND UPSLOPE FLOW IS
EXPECTED TO PRODUCE AN AREA OF 8 TO 14 INCHES OF SNOWFALL CENTERED
OVER THE SANGRE DE CRISTO MOUNTAINS...AND THERE WAS A MULTI MODEL
SIGNAL FOR A SNOWFALL MAXIMUM HERE. THE AMOUNTS WERE ALSO
SUPPORTED BY MANY MEMBERS OF THE LATEST ECMWF ENSEMBLE OUTPUT
SHOWING THE POTENTIAL FOR 12+ INCHES OF SNOWFALL. FURTHER
EAST...SNOW LEVELS RISE ENOUGH TO PRECLUDE THE THREAT FOR HEAVY
SNOWFALL INTO THE FRONT RANGE OF ROCKIES IN CO.

DAY 3...
A CLOSED MID LEVEL LOW FORMS AT THE BASE OF A LONG WAVE TROUGH
OVER THE FOUR CORNERS AREA ON DAY 3. AHEAD OF THE CLOSED MID LEVEL
LOW (AND A DEVELOPING SURFACE LOW OVER NV)...STRENGTHENING UPSLOPE
FLOW FOCUSES MOISTURE ACROSS THE WASATCH RANGE IN UT. THE
COMBINATION OF MOISTURE AND SYNOPTIC SCALE ASCENT AHEAD OF THE MID
LEVEL FLOW IS EXPECTED TO PRODUCE ENOUGH QPF TO SUPPORT 6 TO 10
INCHES OF SNOWFALL OVER THE SOUTHERN PORTION OF THE WASATCH
RANGE...AS SNOW LEVELS RISE TO BETWEEN 6000 AND 8000 FEET. THE
RISING SNOW LEVELS SHOULD LIMIT THE EXTENT OF THE HEAVIER
SNOWFALL...BUT THE LATEST ECMWF ENSEMBLE OUTPUT SHOWED SUPPORT FOR
8+ INCHES OF SNOWFALL ACROSS THE WASATCH...AS WELL AS THE UINTA
RANGE IN FAR NORTHEAST UT.


...UPPER GREAT LAKES INTO THE NORTHEAST...

UPGLIDE OVER A FRONTAL BOUNDARY STRETCHING ACROSS MID MS VALLEY ON
DAY 2 WILL PROVIDE SUFFICIENT LIFT FOR GENERALLY LIGHT SNOWFALL
ACROSS THE UPPER GREAT LAKES ON DAY 2. THE UPGLIDE IS INTENSIFIED
OVER THE NORTHEAST ON DAY 3...AS RETREATING COLD AIR SUPPORTS THE
POTENTIAL FOR LOCALLY HEAVY SNOWFALL FROM EASTERN NY STATE ACROSS
PORTIONS OF SOUTHERN AND CENTRAL NEW ENGLAND. THE 12Z NAM WAS
SLOWER THAN THE CONSENSUS WITH THE UPGLIDE...AND FOR THE MOST PART
DID NOT FACTOR HEAVILY INTO THE FORECAST. THE THERMAL PORTION OF
THE FORECAST WAS BASED ON A BLEND OF THE 12Z GFS/00Z ECMWF. THE
QPF PORTION OF THE FORECAST WAS BASED ON THE MOST RECENT WPC QPF.

DAY 2...
A CLOSED MID LEVEL LOW OVER KS/OK EARLY ON DAY 2 OPENS UP INTO A
POSITIVELY TILTED MID LEVEL TROUGH AS IT CROSSES THE MID MS
VALLEY. AHEAD OF THE MID LEVEL SYSTEM...A BROAD SOUTH SOUTHWEST
FLOW STREAMS MOISTURE OVER A WARM FRONT EXTENDING FROM SURFACE LOW
PRESSURE OVER THE SOUTHERN PLAINS INTO THE MID MS VALLEY. ON THE
NORTHERN EDGE OF THE PRECIPITATION SHIELD ASSOCIATED WITH THE
UPGLIDE...MODEL SOUNDINGS ACROSS CENTRAL WI INTO THE NORTHERN LP
OF MI SHOWED THAT THE COLD AIR SHOULD INITIALLY BE DEEP ENOUGH TO
SUPPORT SNOWFALL HERE. EVENTUALLY...THE COLUMN BECOMES FLOODED
WITH WARMER AIR...BUT ACROSS THE ABOVEMENTIONED LINE AN AXIS OF 1
TO 3 INCHES OF SNOWFALL IS EXPECTED. IN ADDITION...AS THE WARM AIR
PEAKS BETWEEN 825 MB AND 800 MB...THERE COULD BE SOME LIGHT ICING
AS THE BEST LIFT IN THE UPGLIDE EXITS. HOWEVER...ANY ICING AMOUNTS
ARE EXPECTED TO REMAIN BELOW 0.05 INCHES.

DAY 3...
THE LONG WAVE TROUGH CROSSES INTO THE OH VALLEY DURING DAY 3.
AHEAD OF THE LONG WAVE TROUGH...A STRENGTHENING LOW LEVEL FLOW
TRANSPORTS MOISTURE OVER A WARM FRONT EXTENDING FROM LOW PRESSURE
OVER THE OH VALLEY INTO THE MID ATLANTIC. THE INCREASING WARM AIR
ADVECTION WILL SATURATE AN INITIALLY DRY AIRMASS...AS COLD SURFACE
HIGH PRESSURE EXITS NEW ENGLAND. ON THE FRONT END OF THE WARM AIR
ADVECTION LIFT...MODEL SOUNDINGS SHOWED THAT THE COLUMN SHOULD BE
COLD ENOUGH TO SUPPORT SNOWFALL ACROSS PORTIONS OF EASTERN NY
STATE INTO SOUTHERN AND CENTRAL NEW ENGLAND....WHERE PROFILES GO
ISOTHERMAL. THERE IS A MULTI MODEL SIGNAL FOR MOISTURE AND LIFT IN
THE DENDRITIC GROWTH ZONE...WHICH SHOULD RAISE SNOW TO LIQUID
RATIOS. OTHER THAN THE SLOWER 12Z NAM...THERE WAS A SOLID MODEL
SIGNAL FOR 4 TO 8 INCHES OF SNOWFALL STRETCHING FROM THE
ADIRONDACKS IN NY STATE INTO THE SOUTHERN GREEN MOUNTAINS (THOUGH
THERE MAY BE SOME PLAY IN THE POSITIONING OF THE HEAVIEST STRIPE
OF SNOW).

THE EVENT HAS THE HALLMARK OF A LATE SEASON ELEVATION
SNOWFALL...AS THE COLD AIR RETREATS IN THE FACE OF STRONG UPGLIDE.
 THE LATEST ECMWF ENSEMBLE OUTPUT SHOWED AN AREA OF 8+ INCHES OF
SNOWFALL FROM EASTERN NY STATE INTO WESTERN MA AND NORTHWEST CT
(ENCOMPASSING THE HIGHER ELEVATION OF THE BERKSHIRE RANGE). THERE
WAS SOME INDICATION OF A TRANSITION ZONE WHERE THE SURFACE REMAINS
CLOSE TO FREEZING ACROSS PORTIONS OF NORTHEAST PA AND NEARBY
SOUTHEAST NY STATE. HOWEVER...MOST OF THE SUPPORT FOR THIS COMES
FROM THE SLOWER 12Z NAM...WHICH IS SLOWER TO EVACUATE THE LOW
LEVEL COLD AIR. WHILE ICING SEEMS UNLIKELY...IF THE SIGNAL
PERSISTS...ICING MAY NEED TO BE ADDED AT SOME POINT TO THE
FORECAST IN THESE AREAS.

HAYES




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