Quantitative Precipitation Forecast
Issued by NWS

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FOUS11 KWBC 211943
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
342 PM EDT SAT OCT 21 2017

VALID 00Z SUN OCT 22 2017 - 00Z WED OCT 25 2017


DAYS 1 THROUGH 3...


...PACIFIC NORTHWEST/NORTHERN ROCKIES...

THE LAST SHORT WAVE ASSOCIATED WITH A LONG WAVE TROUGH CROSSING
THE ROCKIES INTO PLAINS ON DAY 1 AFFECTS THE PACIFIC NORTHWEST AND
NORTHERN ROCKIES. BEHIND THE SHORT WAVE...MID LEVEL RIDGING BUILDS
OVER THE WESTERN UNITED STATES ON DAY 2...AND UPSLOPE FLOW COULD
PRODUCE ACCUMULATING SNOWFALL ACROSS THE HIGHER TERRAIN OF THE
NORTHERN ROCKIES. FOR THE MOST PART...THERE WAS GOOD MODEL
AGREEMENT WITH THE SYNOPTIC SCALE FEATURES ACROSS THE REGION...SO
THE THERMAL PORTION OF THE FORECAST WAS BASED ON A MULTI MODEL
BLEND...LED BY THE 12Z GFS/00Z ECMWF. THE QPF PORTION OF THE
FORECAST WAS BASED ON THE MOST RECENT WPC QPF.

DAY 1...
THE BEST LIFT AHEAD OF THE SHORT WAVE COMING ASHORE OVER THE
PACIFIC NORTHWEST DURING THE FIRST PART OF DAY 1 OCCURS IN THE
22/06Z TO 22/12Z TIME FRAME. AHEAD OF THE SHORT WAVE...STRONG
DIFFERENTIAL WARM AIR ADVECTION MOISTENS THE COLUMN OVER THE
NORTHERN WA CASCADES...WHERE SNOW LEVELS BEGIN THE PERIOD BETWEEN
5500 AND 6500 FEET. GFS MODEL SOUNDINGS HERE SHOWED MOISTURE AND
LIFT IN THE DENDRITIC GROWTH ZONE IN THE ABOVEMENTIONED TIME
PERIOD...RESULTING IN HIGHER SNOW-TO-LIQUID (SLR) VALUES BEFORE
THE WARM AIR OVERWHELMS THE COLUMN. THERE IS A MULTI MODEL SIGNAL
FOR 8 TO 12 INCHES OF SNOWFALL ACROSS THE NORTHERN WA
CASCADES...AND THE MOST RECENT ECMWF ENSEMBLE OUTPUT SHOWED
MEMBERS WITH 12+ INCH SNOWFALL POTENTIAL. THESE SNOWFALL AMOUNTS
SUPPORT A SLIGHT TO MODERATE PROBABILITY OF 12+ INCHES OF SNOW
ACROSS THE HIGHER TERRAIN OF THE NORTHERN WA CASCADES.

THE BEST LIFT WITH THE SHORT WAVE CROSSES THE NORTHERN ROCKIES
BETWEEN 22/12Z AND 22/18Z IN THE FAST MID LEVEL FLOW. THE MOISTURE
PLUME IS NOT AS ROBUST AS IT IS EXPECTED TO BE OVER THE PACIFIC
NORTHWEST...THE COMBINATION OF SYNOPTIC SCALE LIFT (AUGMENTED BY
MARGINAL INSTABILITY...DUE AT LEAST IN PART TO STEEPENING LAPSE
RATES) AND UPSLOPE FLOW BEHIND A PACIFIC FRONTAL BOUNDARY WILL
MAKE THE MOST OF THE MOISTURE IN THE COLUMN. SNOW LEVELS RISE IN
THE STRONG WARM AIR ADVECTION TO ABOVE 6500/7500 FEET...BUT BEFORE
THAT OCCURS MODEL SOUNDINGS SHOWED THE POTENTIAL FOR HIGHER SLR
VALUES OVER THE ROCKY MOUNTAIN FRONT RANGE IN MT...AS WELL AS THE
GRAND TETONS. IN THESE LOCATIONS... THERE WAS A GOOD MODEL SIGNAL
FOR 4 TO 8 INCHES OF SNOWFALL...AND THESE AMOUNTS ARE SUPPORTED
MEMBERS OF THE LATEST SREF AND ECMWF ENSEMBLE OUTPUT SHOWING 4+
INCHES OF SNOWFALL. THE ECMWF ENSEMBLE OUTPUT ALSO SHOWED MEMBERS
WITH 8+ INCHES OF SNOWFALL...AND THESE AMOUNTS SUPPORT A SLIGHT
PROBABILITY OF 8+ INCHES OF SNOWFALL...CENTERED ON THE ROCKY
MOUNTAIN FRONT RANGE IN MT.

DAY 2...
AS THE LAST SHORT WAVE EXITS THE MID LEVEL FLOW...A MID LEVEL
RIDGE BUILDING OVER THE WEST COAST PUSHES THE FAST MID LEVEL FLOW
INTO SOUTHWEST CANADA DURING DAY 3. THE LOW TO MID LEVEL FLOW
BECOMES WEST TO NORTHWEST OVER THE NORTHERN ROCKIES...AND THE
UPSLOPE FLOW BECOMES FOCUSED ON THE HIGHER TERRAIN OVER ID/MT/WY.
THE COLUMN DRIES OUT IN THE WEST NORTHWEST FLOW...AND MOISTURE
BECOMES THE LIMITING FACTOR FOR SIGNIFICANT SNOWFALL. THE UPSLOPE
FLOW MAKES THE MOST OF THE REMAINING MOISTURE IN THE COLUMN...AND
LOCAL 2 TO 4 INCH SNOWFALL AMOUNTS WERE PLACED OVER THE ROCKY
MOUNTAIN FRONT RANGE IN MT...THE BITTERROOTS IN ID...AND THE
ABSAROKA RANGE IN WY/MT. THE MOST RECENT ECMWF ENSEMBLE OUTPUT
SHOWED A FEW MEMBERS WITH 4+ INCHES OF SNOWFALL OVER THE SOUTHERN
ABSAROKA RANGE IN WY...BUT MOISTURE PROBABLY LIMITS THAT
POSSIBILITY.


THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.


HAYES

$$




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