Quantitative Precipitation Forecast
Issued by NWS

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FOUS11 KWBC 210829
QPFHSD

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

VALID 12Z SAT OCT 21 2017 - 12Z TUE OCT 24 2017


...PACIFIC NORTHWEST/NORTHERN ROCKIES...

DAY 1...

RADAR/SATELLITE SHOW A SURGE OF MOISTURE/PRECIPITATION COMING
TOWARDS OR/WA FROM THE EASTERN PACIFIC.
STRONG LOW TO MID LEVEL WARM AIR ADVECTION ADVECTS MOISTURE
ONSHORE AND ACROSS THE WA CASCADES AND INLAND ACROSS
THE BLUE MNTNS AND SALMON RIVER MNTNS OF ID.  THE ISENTROPIC LIFT
IS AUGMENTED BY DYNAMIC FORCING AS STRONG CONVERGENCE OCCURS ON
THE NOSE OF A 50 KT 700 MB JET STREAMING IN FROM THE EASTERN
PACIFIC ACROSS WA TODAY.
THE MODELS SHOW LIQUID EQUIVALENT PRECIPITATION AMOUNTS OF A
COUPLE OF INCHES IN THE CASCADES.
AN AREA OF A FOOT OF SNOW IS POSSIBLE IN THE NORTHERN WA CASCADES
BEFORE THE WARMING CHANGES SNOW TO RAIN.

THE GFS SOUNDINGS ALSO SHOWED MOISTURE AND LIFT IN THE DENDRITIC
GROWTH ZONE...RESULTING IN ENHANCED SNOW TO LIQUID RATIOS OVER THE
NORTHERN BITTERROOTS IN ID...AS WELL AS THE WESTERN ID SALMON
RIVER AND SAWTOOTH RANGES. 4 TO 8 INCHES SNOWFALL AMOUNTS ARE
EXPECTED HERE...AND OVER THE FRONT RANGE OF THE NORTHERN ROCKIES
IN MT AND TETONS IN WY.

DAY 2...

IN THE WA CASCADES EAST ACROSS THE NORTHERN ROCKIES...LOW TO MID
LEVEL CONFLUENT FLOW RESULTS IN CONTINUED WARM CONDITIONS AND
LOWER ELEVATION RAIN WITH POTENTIAL FOR SNOW TO CONTINUE AT HIGHER
ELEVATIONS.  THE LATTER HALF OF THE PERIOD SEES DRYING ALOFT
COMING ASHORE FORM THE EASTER PACIFIC AS A MID LEVEL RIDGE STARTS
TO BUILD IN.
GIVEN DRYING ALOFT...SNOW SHOWER COVERAGE/INTENSITY BEGINS TO
DECLINE...AND ALONG WITH IT THE THREAT FOR HEAVY SNOW.
SEVERAL INCHES ARE EXPECTED IN THE NORTHERN WA CASCADES BEFORE THE
THREAT TAPERS.

DAY 3...

THE DOMINANT NORTHWEST US LOW-MID LEVEL RIDGE LEADS TO MINIMAL
SNOWFALL TOTALS.  THE PROBABILITY IF HEAVY SNOW IS LESS THAN 10
PERCENT.


THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT DAYS
1-3.

PETERSEN

$$





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