Quantitative Precipitation Forecast
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532
FOUS11 KWBC 201935
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
334 PM EDT FRI OCT 20 2017

VALID 00Z SAT OCT 21 2017 - 00Z TUE OCT 24 2017

DAYS 1 THROUGH 3...


...PACIFIC NORTHWEST/NORTHERN AND CENTRAL ROCKIES...

A LONG WAVE TROUGH CROSSES THE PACIFIC NORTHWEST DURING DAY 1 AND
EARLY DAY 2...FOLLOWED BY A FAST ZONAL MID LEVEL FLOW LATER ON DAY
2. BY DAY 3...MID LEVEL RIDGING BUILDS OVER THE WESTERN UNITED
STATES. FOR THE MOST PART...THERE WAS GOOD MODEL AGREEMENT WITH
THE OVERALL SYNOPTIC EVOLUTION...SO THE THERMAL PORTION OF THE
FORECAST WAS BASED ON A MULTI MODEL BLEND...LED BY THE 12Z
NAM/GFS/00Z ECMWF. THE QPF PORTION OF THE FORECAST WAS BASED ON
THE MOST RECENT WPC QPF.

DAY 1...
AS THE LAST SHORT WAVE ENERGY ASSOCIATED WITH THE LONG WAVE TROUGH
CROSSING THE WEST COAST AND ROCKIES COMES ASHORE DURING THE SECOND
PART OF DAY 1...STRONG LOW TO MID LEVEL WARM AIR ADVECTION
MOISTENS THE COLUMN ACROSS THE PACIFIC NORTHWEST. 12Z GFS MODEL
SOUNDINGS ACROSS THE NORTHERN WA CASCADES SHOWED THE COLUMN
BECOMING SATURATED ALONG THE ZERO CELSIUS ISOTHERM... WITH
MOISTURE AND LIFT IN THE DENDRITIC GROWTH ZONE BETWEEN 21/18Z AND
22/00Z. THERE WAS A DECENT MODEL SIGNAL FOR 8 TO 12 INCHES OF
SNOWFALL ACROSS THE NORTHERN WA CASCADES...AND THESE AMOUNTS ARE
SUPPORTED BY MEMBERS OF THE MOST RECENT ECMWF ENSEMBLE OUTPUT
SHOWING 8+ INCHES OF SNOWFALL. THESE SNOWFALL AMOUNTS SUPPORT A
SLIGHT PROBABILITY OF 12+ INCHES OF SNOWFALL HERE.

FURTHER SOUTH ALONG THE SOUTHERN WA CASCADES INTO THE OR
CASCADES...SNOW LEVELS RISE MORE QUICKLY IN THE STRONG WARM AIR
ADVECTION AHEAD OF THE SHORT WAVE...EVENTUALLY ROBBING THE COLUMN
OF ITS ABILITY TO MAKE SNOWFLAKES. THERE IS SUFFICIENT MOISTURE
AND UPSLOPE FLOW TO PRODUCE AXES OF 4 TO 8 INCHES OF SNOWFALL IN
THESE LOCATIONS....AND THE MOST RECENT ECMWF ENSEMBLE OUTPUT
SHOWED MEMBERS SUPPORTING 8+ INCHES OF SNOWFALL ACROSS THE OR
CASCADES. THESE VALUES LOOKED TOO HIGH...AS THE WARM AIR ADVECTION
SHOULD RESULT IN A PHASE CHANGE BEFORE 8+ INCH SNOWFALL AMOUNTS
CAN BE REALIZED THIS FAR SOUTH ON DAY 1.

FINALLY...AS THE LONG WAVE TROUGH CROSSES THE NORTHERN AND CENTRAL
ROCKIES...LOWERING HEIGHTS RESULT IN LOWER SNOW LEVELS...DROPPING
BELOW 6000 FEET FOR A TIME. WHILE THE MOISTURE BECOMES MORE OF A
LIMITING FACTOR (AS IT DROPS OFF WITH THE TROUGH PASSAGE)...THE
COMBINATION OF UPSLOPE FLOW AND DYNAMIC LIFT (PUNCTUATED BY
MARGINAL INSTABILITY DUE TO STEEPENING LAPSE RATES) MAKES THE MOST
OF THE MOISTURE IN THE COLUMN. 12Z GFS SOUNDINGS ALSO SHOWED
MOISTURE AND LIFT IN THE DENDRITIC GROWTH ZONE...RESULTING IN
ENHANCED SLR OVER THE NORTHERN BITTERROOTS IN ID...AS WELL AS THE
WESTERN ID SALMON RIVER AND SAWTOOTH RANGES. LOCAL 4 TO 8 INCHES
SNOWFALL AMOUNTS ARE EXPECTED HERE...WITH LESSER AMOUNTS
(GENERALLY BETWEEN 3 AND 6 INCHES) EXPECTED OVER THE NORTH CENTRAL
CO ROCKIES...WHERE SNOW LEVELS DROP BELOW 6000 FEET FOR A TIME
JUST BEHIND THE LONG WAVE TROUGH PASSAGE. THE SNOWFALL AMOUNTS
OVER THE NORTHERN ROCKIES SUPPORT A SLIGHT TO MODERATE PROBABILITY
OF 8+ INCHES OF SNOWFALL OVER THE BITTERROOTS RANGE.

DAY 2...
THE MID LEVEL FLOW BECOMES FAST AND ZONAL IN THE WAKE OF THE LONG
WAVE TROUGH THAT EXITS INTO THE PLAINS ON DAY 2. SNOW LEVELS RISE
SLOWLY AS HEIGHTS RISE...BEGINNING THE DAY BETWEEN 5500 AND 6500
FEET OVER THE NORTHERN WA CASCADES. THE LAST OF THE WARM AIR
ADVECTION PEAKS EARLY IN THE PERIOD OVER THIS AREA...AND THE
UPGLIDE SUPPORTS ENOUGH QPF FOR 8 TO 12 INCHES OF SNOWFALL HERE.
ONCE AGAIN...THE MOST RECENT ECMWF ENSEMBLE OUTPUT SUPPORTS 8+
INCHES OF SNOWFALL HERE...WITH A FEW MEMBERS SUGGESTING 12+ INCHES
OF SNOWFALL IS POSSIBLE. THESE SNOWFALL AMOUNTS SUPPORT A SLIGHT
TO MODERATE PROBABILITY OF
12+ INCHES OF SNOWFALL OVER THE HIGHER TERRAIN OF THE NORTHERN WA
CASCADES.

FURTHER EAST ACROSS THE NORTHERN ROCKIES...THE RESIDUAL LIFT IN
THE WARM AIR ADVECTION AHEAD OF THE LAST SHORT WAVE IN THE FLOW
SHOULD BE ENOUGH TO SUPPORT A LARGER AREA OF SYNOPTIC SCALE
ASCENT. THE LIFT IS AUGMENTED BY UPSLOPE FLOW BEHIND A PACIFIC
FRONTAL BOUNDARY...AND THE COMBINATION OF LIFT AND UPSLOPE FLOW
SHOULD SUPPORT AREAS OF 3 TO 6 INCHES OF SNOW ACROSS THE SAWTOOTH
RANGE IN ID...AS WELL AS THE ABSAROKA AND GRAND TETON RANGES IN
WY. THESE AMOUNTS ARE WELL SUPPORTED BY MEMBERS OF THE RECENT
ECMWF ENSEMBLE OUTPUT SHOWING 4+ INCHES OF SNOWFALL. RISING SNOW
LEVELS IN THE WARM AIR ADVECTION SHOULD MITIGATE HIGHER SNOWFALL
NUMBERS HERE DURING DAY 2.

DAY 3...
THE UPSLOPE FLOW IN THE WEST NORTHWEST FLOW ACROSS THE NORTHERN
ROCKIES WILL MAKE THE MOST OF THE REMAINING MOISTURE IN THE COLUMN
DURING DAY 3. LOCAL 2 TO 4 INCH SNOWFALL AMOUNTS ARE EXPECTED OVER
THE ROCKY MOUNTAIN FRONT RANGE IN MT...AND THE SOUTHERN ABSAROKA
RANGE IN WY. WITH MID LEVEL RIDGING BUILDING NORTH ACROSS THE WEST
COAST DURING THE SECOND HALF OF THE PERIOD...SNOW LEVELS RISE
ACROSS THE REGION...LIMITING THE POTENTIAL FOR HIGHER SNOWFALL
AMOUNTS DURING DAY 3.


THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.

HAYES

$$





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