Quantitative Precipitation Forecast
Issued by NWS

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FOUS11 KWBC 260908
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
407 AM EST SUN FEB 26 2017

VALID 12Z SUN FEB 26 2017 - 12Z WED MAR 01 2017

DAYS 1 THROUGH 3...

...WEST...

UNSETTLED WEATHER WILL CONTINUE ACROSS THE WEST THROUGH
TUESDAY...WITH SEVERAL SHORTWAVES EMBEDDED WITHIN THE LONGWAVE
TROUGH BRINGING PERIODS OF SNOW. MULTIPLE REGIONS OF MODERATE TO
HEAVY SNOWFALL ACCUMULATIONS ARE EXPECTED THROUGH THE PERIOD. ONE
AREA OF INTEREST WILL BE THE WASHINGTON AND OREGON
CASCADES...WHERE A PROLONGED PERIOD OF SNOWFALL IS EXPECTED. A
STRONG SHORTWAVE AND ASSOCIATED LOW...ALONG WITH UPPER LEVEL
DIFFLUENCE IN THE LEFT EXIT REGION OF AN UPPER JET...WILL BE
RESPONSIBLE FOR THE SNOWFALL TODAY. ANOTHER ROUND OF SNOW IS
LIKELY MONDAY INTO TUESDAY WITH TROUGHING AND UPSLOPE FLOW
CONTINUING. TOTAL AMOUNTS OF 1-2 FEET ARE EXPECTED...ALTHOUGH
GIVEN THE LONG DURATION...AMOUNTS OF THIS MAGNITUDE IN A COUPLE
DAY PERIOD IS NOT ALL THAT UNUSUAL FOR THE CASCADES. THE MORE
SIGNIFICANT ASPECT OF THE SYSTEM TODAY WILL BE THE SNOWFALL IN THE
HILLS OF SOUTHWEST OR AND NORTHWEST CA...WHERE IT IS NOT AS COMMON
TO SEE SIGNIFICANT SNOWFALL. SNOW LEVELS WILL DROP TO NEAR THE
COAST ACROSS THIS REGION...WITH AMOUNTS AROUND A FOOT IN THE HILLS.

ANOTHER REGION OF WIDESPREAD MODERATE TO HEAVY SNOWFALL
ACCUMULATIONS WILL BE ACROSS THE HIGHER TERRAIN OF
CENTRAL/NORTHERN ID INTO NORTHWEST MT. AGAIN EXPECTING A LONG
DURATION LIGHT TO MODERATE SNOWFALL...WITH ACCUMULATIONS OVER THE
THREE DAY PERIOD UPWARDS OF 1 TO 2 FEET. AMOUNTS APPROACHING A
FOOT ARE ALSO PROBABLE ACROSS THE HIGHER TERRAIN OF NORTHWEST WY.
THE OVERALL LACK OF IMPRESSIVE MOISTURE AND GENERAL BROAD SYNOPTIC
SUPPORT AND UPSLOPE FLOW...SUPPORTS MORE OF A LONG DURATION
SNOWFALL...WITH AMOUNTS OF THE FORECASTED MAGNITUDE NOT REALLY ALL
THAT EXTREME FOR THE AREA.

THE BIGGEST THREAT FOR SIGNIFICANT SNOWFALL IS PROBABLY ACROSS THE
CO ROCKIES INTO THE MOGOLLON RIM REGION OF AZ AND THE HIGHER
TERRAIN OF NORTHERN NM. CONFIDENCE IS INCREASING THAT WE WILL SEE
A SUBTROPICAL PLUME OF MOISTURE MOVE INTO THIS REGION AHEAD OF THE
DIGGING TROUGH MOVING DOWN THE WEST COAST. THIS ANOMALOUS MOISTURE
COMBINED WITH INCREASED FORCING AHEAD OF THE APPROACHING SHORTWAVE
SHOULD RESULT IN SIGNIFICANT SNOWFALL ACCUMULATIONS BY LATER
MONDAY INTO TUESDAY. LOOKING AT A HIGH PROBABILITY OF 6"+ ACROSS
THE MOGOLLON RIM INTO THE HIGHER TERRAIN OF NORTHWEST NM INTO THE
CO ROCKIES...WITH PROBABILITIES OF A FOOT OF SNOW INCREASING AS
WELL. RECENT RUNS OF BOTH THE GFS AND ECMWF HAVE SHOWN A MORE
CONSISTENT SIGNAL FOR THIS HEAVY SNOWFALL THREAT.


...UPPER MS VALLEY/UPPER GREAT LAKES...

LIGHT SNOWFALL ACCUMULATIONS ARE EXPECTED TODAY ACROSS PORTIONS OF
THE UP OF MI AS A SHORTWAVE MOVES ACROSS ALONG WITH SOME LAKE
ENHANCEMENT. A MORE INTERESTING SETUP MAY DEVELOP ON TUESDAY
ACROSS PORTIONS OF CENTRAL/NORTHERN MN INTO NORTHERN WI AND THE UP
OF MI. SOME SIGNAL FOR INCREASING FRONTOGENESIS NORTH OF A SHEARED
OUT SURFACE LOW AND AHEAD OF A SHORTWAVE EMBEDDED WITHIN THE
APPROACHING LONGWAVE TROUGH. MODELS HAVE NOT BEEN REAL CONSISTENT
WITH THIS FEATURE...THUS CONFIDENCE IS LOW ON THE DETAILS. THE 0Z
GFS IS THE MOST AGGRESSIVE WITH THIS REGION OF ACCUMULATING
SNOWFALL...ALTHOUGH RUN TO RUN CONSISTENCY HAS GENERALLY BEEN
LACKING. THE ECMWF IS A BIT MORE SUPPRESSED AND LIGHTER WITH THE
MAGNITUDE OF PRECIPITATION AMOUNTS. THE 0Z GEFS MEAN...12Z ECMWF
MEAN AND 0Z UKMET ARE ALL GENERALLY IN BETWEEN THE TWO EXTREMES
SHOWN BY THE DETERMINISTIC GFS/ECMWF...AND THIS MIDDLE GROUND
SOLUTION IS PREFERRED BY WPC. CURRENT PROBABILISTIC GUIDANCE SHOWS
ABOUT A 40-60% CHANCE OF 4"+ ACROSS THE AFOREMENTIONED
AREAS...WITH ABOUT A 10-30% CHANCE OF EXCEEDING 6"...AND A NON
ZERO THREAT OF 8"+. A LARGE SPREAD BETWEEN THE 10TH AND 90TH
PERCENTILE OF THE FORECAST CONFIRMS THE UNCERTAINTY THAT IS
PRESENT WITH THIS SYSTEM. THUS WILL HAVE TO CONTINUE TO MONITOR
TRENDS...WITH FORECAST CHANGES LIKELY OVER THE COMING DAYS.

CHENARD



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