Quantitative Precipitation Forecast
Issued by NWS

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FOUS11 KWBC 180944
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
443 AM EST WED JAN 18 2017

VALID 12Z WED JAN 18 2017 - 12Z SAT JAN 21 2017

INTERIOR OF WASHINGTON/OREGON/IDAHO...

DUE LARGELY TO CONTRIBUTIONS FROM THE WPC MODE AND SOME HIGH
RESOLUTION AND GLOBAL DETERMINISTIC SOLUTIONS...SOME WHICH SUPPORT
ANOTHER 0.25 TO 0.75 OF ICE TODAY AND TONIGHT...THE 24-HOUR
PROBABILITIES OF 0.25 OR MORE OF ICE ARE GREATER THAN 70 PERCENT
ACROSS PARTS OF THE COLUMBIA RIVER GORGE...SNOQUALMIE PASS...AND
THE SNAKE RIVER VALLEY OF SOUTHWEST IDAHO THROUGH TONIGHT. OF THE
ENSEMBLE MEMBERSHIP...ONLY ABOUT 5 OF THE SREF NMMB AND NONE OF
THE SREF ARW MEMBERS (TOTAL 5 OUT OF 26) SUPPORT 0.25 OF ICE
ACROSS THE INTERIOR OR WASHINGTON/OREGON...ALONG WITH ONLY 1 GEFS
MEMBER. THIS IS LIKELY A RESULT OF INSUFFICIENT RESOLUTION OF THE
LOWEST LEVEL TEMPERATURES WHICH ARE SUPPORTING THE ONGOING OR
ANTICIPATED FREEZING RAIN IN THESE AREAS. IN ACTUALITY...THE
PROBABILITIES OF 0.25 OR MORE OF ICE ARE HIGHER THAN SHOWN BY THE
AUTOMATED PROBABILITIES...WITH NUMEROUS REPORTS OF 0.5 TO 1 INCH
OF ICE NEAR AND EAST OR PORTLAND TOWARD THE HOOD VALLEY AREA
ALREADY RECEIVED. REFER TO THE LOCAL WFO ICE STORM AND WINTER
STORM WARNINGS FOR MORE SPECIFIC AND DETAILED INFORMATION ABOUT
THIS VERY SIGNIFICANT ICE STORM.

REST OF THE PACIFIC NORTHWEST/CALIFORNIA/INTERMOUNTAIN
WEST/SOUTHERN ROCKIES...

PERIODIC HEAVY MOUNTAIN SNOW WILL OVERSPREAD THE REGION THE NEXT
SEVERAL DAYS AS THE LEADING EDGE OF THE UPPER TROUGH CURRENTLY
MOVING ASHORE SPLITS ACROSS THE INTERMOUNTAIN WEST THURSDAY
NIGHT...WITH ANOTHER SIGNIFICANT TROUGH REACHING THE WEST COAST
FRIDAY NIGHT...RESULTING IN 2 MAIN SURGES OF SNOW. THROUGH
THURSDAY NIGHT...MODERATE TO HIGH PROBABILITIES OF 8 AND 12 INCHES
OF SNOW WILL EXTEND FROM THE CASCADES AND BITTERROOTS SOUTHWARD TO
THE SIERRAS...WITH SNOW LEVELS RANGING FROM 2 TO 5 THOUSAND FEET.
THE MODEL SPREAD IS FAIRLY SMALL THROUGH THIS PERIOD...WITH
RELATIVELY GOOD DETERMINISTIC AND ENSEMBLE MEMBER AGREEMENT. THE
LARGEST ENSEMBLE DIFFERENCE IS EVIDENT FOR THURSDAY AND THURSDAY
NIGHT...WITH THE GEFS MEMBERS MORE SUPPORTIVE OF HEAVY SNOW ALONG
THE NEVADA/UTAH BORDER...VERSUS THE SREF AND ECMWF MEMBERS WHICH
EMPHASIZE THE SIERRAS AND WASATCH. THE GEFS MEMBERS ARE GENERALLY
MORE AMPLIFIED WITH THE SPLITTING UPPER TROUGH COMPARED WITH OTHER
SOLUTIONS...WHICH HELPS TO ACCOUNT FOR THE UNCERTAINTY WITH THE
TROUGH`S EVOLUTION...THUS AT THIS TIME ITS HEAVIER SOLUTION IS
GIVEN EQUAL WEIGHTING. FOR FRIDAY AND FRIDAY NIGHT...THE HIGHEST
PROBABILITIES FOR 8 AND 12 INCHES OF SNOW RETURN TO THE SIERRAS
AND EXTEND ACROSS THE SAN BERNARDINO/SAN GABRIELS/MOGOLLON
RIM...WITH SNOW LEVELS RANGING FROM 3 TO 5 THOUSAND FEET. BY THIS
TIME...THE ENSEMBLE MEMBERSHIP IS EVENLY DISTRIBUTED WITH THE
SPREAD NEAR OR BELOW AVERAGE...SUGGESTING ABOVE NORMAL CONFIDENCE
IN THE AUTOMATED PROBABILITIES.

JAMES

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