Quantitative Precipitation Forecast
Issued by NWS

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FOUS11 KWBC 180819
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
318 AM EST SAT FEB 18 2017

VALID 12Z SAT FEB 18 2017 - 12Z TUE FEB 21 2017

DAYS 1 TO 3...

...RANGES OF THE PACIFIC NORTHWEST/CALIFORNIA/GREAT
BASIN/ROCKIES...

...WIDESPREAD SNOW EXPECTED OVER THE HIGHER ELEVATIONS OF THE
PACIFIC NORTHWEST...CALIFORNIA...GREAT BASIN...AND ROCKIES.

ON DAY 1/SATURDAY/ AS THE DEEP LAYER TROUGH CAUSING THE HEAVY CA
MOUNTAIN SNOW DRIFTS INLAND...
SNOWS ARE EXPECTED WHERE THE MOIST LOW LEVEL FLOW GETS LIFTED FROM
UPPER DIVERGENCE/LOWER CONVERGENCE COUPLETS COMBINED WITH UPSLOPE
CONDITIONS...ACROSS SOUTH CENTRAL NV AND SOUTHERN UT...THE
MOGOLLON RIM OF AZ/ADJACENT SOUTHWEST NM...AND FURTHER NORTH IN
THE RANGES OF SOUTHERN ID.

ON DAY 2/SUNDAY/ THE UPPER TROUGH CONTINUES A SLOW EAST
DRIFT...WITH PRE-FRONTAL CONVERGENCE FOCUSING ON SUPPORTING ASCENT
INTO THE SAN JUAN MOUNTAINS OF SOUTHWEST CO/NORTHERN NM AND THE
MOUNTAINS OF SOUTHEAST ID TO ADJACENT SOUTH CENTRAL MT/WESTERN WY.
AFTER A RESPITE...CA BECOMES ACTIVE AGAIN AS THE GFS FORECASTS A
RESURGENT 70 BM JET TO PROVIDE STRONG WARM/MOIST ADVECTION WITH
CONVERGENCE AGAIN COMBINING WITH TOPOGRAPHIC LIFT TO
SUPPORT HEAVY SNOW REDEVELOPING OVER HIGHER ELEVATIONS OF THE
SHASTA/SISKIYOU RANGES IN NORTHERN CALIFORNIA...EXTENDING INTO THE
NORTHERN CA SIERRA NEVADA RANGE ON PERSISTENT MOIST SOUTHWEST
FLOW.

A SEPARATE MAX IS EXPECTED OVER THE WA OLYMPICS AND NORTHERN
CASCADES AS A PERSISTENT CIRCULATION OFFSHORE TRIGGERS 700 MB
VERTICAL VELOCITY MAXIMA THAT ROTATE AROUND THE CIRCULATION AND
ONSHORE INTO WA...SUPPORTING PERIODS OF SNOW WHEN THOSE MAXIMA
CROSS THE MOUNTAINS.

ON DAY 3/MONDAY/ THE PERSISTENT CONFLUENT FLOW OVER THE EASTERN
PACIFIC LEADS TO A STRONG 700 MB JET FOCUSING STRONG MOISTURE
FLUXES AND AREAS OF CONVERGENCE AND ASCENT INTO THE NORTHERN AND
CENTRAL CA SIERRA NEVADA RANGE.  THE UPPER JET MAXIMA OVER
NORTHERN CA IS SLOW TO MOVE...WITH THE GFS/ECMWF INDICATING
130-150 KT MAXIMA IN NORTHERN CA...PLUS RESULTANT LIQUID
EQUIVALENT AMOUNTS OF 4-5 INCHES IN THE NORTHERN PORTIONS OF THE
SIERRA NEVADA RANGE (ALSO INDICATED IN THE NAM/21Z SREF MEAN/00Z
UKMET).
THE HIGH AVAILABLE MOISTURE AND EXTENDED PERIOD OF ASCENT LEADS TO
A THREAT FOR HEAVY SNOW AGAIN AT HIGHER ELEVATIONS...WITH
POTENTIAL FOR 2-3 ADDITIONAL FEET.

SECONDARY MAXIMA ARE EXPECTED AS THE MOIST SOUTHWEST FLOW
INTERSECTS THE WA/OR CASCADES AND AGAIN FURTHER INLAND IN THE
RANGES OF SOUTHERN ID...WITH ASCENT PERSISTING FOR EXTENDED
PERIODS FROM 850-500 MB.  THE UNCERTAINTY IS IN THE TIMING OF
INDIVIDUAL WAVES MOVING STEADILY NORTHEAST IN CONFLUENT LOW-MID
LEVEL FLOW.  THREE DAY TOTALS OF 18-24 INCHES ARE EXPECTED IN THE
MOUNTAINS OF SOUTHERN ID...WITH LOCALLY HIGHER TOTALS.

THE PROBABILITY OF 0.25 INCHES OF ICE IS LESS THAN 10 PERCENT DAYS
1-3.

PETERSEN


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