Quantitative Precipitation Forecast
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FOUS11 KWBC 200754
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
254 AM EST MON FEB 20 2017

VALID 12Z MON FEB 20 2017 - 12Z THU FEB 23 2017

DAYS 1 TO 3...

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

AN ACTIVE PERIOD WILL CONTINUE ACROSS THE WEST INTO MID WEEK AS
THE STRONG MID-UPPER JET IN ADVANCE OF THE EAST PACIFIC TROUGH
PERSISTS ACROSS CA AND INLAND INTO SOUTHERN ID AND WY ON DAY
1/MON/.
AN IMPRESSIVE ATMOSPHERIC RIVER WILL BE FOCUSED INTO CENTRAL
CA...BRINGING A LONG DURATION HEAVY SNOWFALL TO THE HIGHER
ELEVATIONS OF THE CENTRAL AND NORTHERN SIERRA NEVADA MONDAY INTO
THE DAY TUESDAY...WITH THE HEAVIEST RATES TODAY THROUGH MONDAY
NIGHT. SEVERAL FEET OF SNOW ARE LIKELY THROUGH TUESDAY. A COUPLE
FEET OF SNOW IS ALSO LIKELY ACROSS THE NORTHERN CA MOUNTAINS IN
THE MT SHASTA VICINITY.

MULTIPLE SHORTWAVES MOVING NORTH OF THE ATMOSPHERIC RIVER WILL
BRING SEVERAL PERIODS OF SNOWFALL TO THE WA AND OR CASCADES INTO
TUESDAY AS WELL...LOOKING AT TOTALS OF 10-20 INCHES (LOCALLY
HIGHER) IN THE FAVORED TERRAIN.

SIGNIFICANT SNOWFALL IS ALSO EXPECTED FURTHER INLAND ON MON AND
TUE AS BROAD SOUTHWEST FLOW AND ANOMALOUS MOISTURE FLUX MOVES INTO
THE NORTHERN GREAT BASIN AND NORTHERN ROCKIES.
STRONG AND PERSISTENT 700 MB ASCENT IN THE PRESENCE OF THE HIGH
MOISTURE CONTENT LEADS TO HEAVY SNOW...WITH ONE TO TWO FEET PLUS
OF SNOWFALL EXPECTED ACROSS THE SAWTOOTH RANGE OF CENTRAL ID AND
THE RANGES OF NORTHWEST WY.  SNOW IS FOCUSED ON MON
NIGHT-TUE...WHEN PWAT AND MOISTURE FLUX ANOMALIES ARE GREATEST
INTO THE FAVORED TERRAIN.

ON DAY 3/ WED/ ...THE 12Z ECMWF AND 18Z GFS INDICATE A 700 MB WAVE
MOVING EAST FROM SOUTHERN CA AND NV TO MOVE INTO UT AND POSSIBLY
WESTERN CO.  AS THE WAVE SHARPENS...AN AREA OF WELL DEFINED 700 MB
CONVERGENCE DEVELOPS IN CENTRAL TO NORTHERN UT AND THEN NORTHWEST
CO AND ADJACENT SOUTHEAST WY. SEVERAL INCHES OF SNOW ARE EXPECTED
IN THE PATH OF THE WAVE AND POTENTIAL FORMING 700 MB LOW.
SNOWFALL COVERAGE AND INTENSITY START TO WANE IN THE PACIFIC
NORTHWEST WED AS THE UPPER TROUGH MOVES ONSHORE AND DRIFTS INLAND.

THE ECMWF AND GFS SHOW A SHEARED 700 MB WAVE CROSSING MT WITH A
WEST-NORTHWEST TO EAST SOUTHEAST ORIENTED BAND OF 850 MB
CONVERGENCE IN CENTRAL MT THAT EXTENDS TO THE WY BORDER...WHICH
MAY RESULT IN A PERIOD OF ACCUMULATING SNOW. THE 00Z GFS FORECASTS
HALF AN INCH LIQUID EQUIVALENT IN THE FORM OF SNOW...SO SEVERAL
INCHES ARE POSSIBLE IF THIS VERIFIES. THE 12 TO 00Z ECMWF BAND
LOCATIONS DO NOT OVERLAP WELL WITH THE GFS FORECAST BAND...SO
CONSIDERABLE UNCERTAINTY EXISTS.

...NORTHERN PLAINS TO LAKE SUPERIOR...

AN AREA OF LOW PRESSURE IS FORECAST TO EJECT EAST OUT OF MT INTO
THE NORTHERN PLAINS TUESDAY INTO WEDNESDAY.  STILL QUITE A BIT OF
SPREAD WITH THIS FEATURE...BUT DEPENDING ON THE EXACT TRACK AND
STRENGTH OF THE LOW...SOME LIGHT SNOW ACCUMULATIONS ARE POSSIBLE
ACROSS NORTHERN MT/ND/MN. MORE WEIGHTING WAS GIVEN TO THE ECMWF
AND GFS..AS THE 00Z NAM WAS A BIG OUTLIER WITH THE STRENGTH OF ITS
DEVELOPING 850 MB LOW AND RESULTANT BAND OF HEAVY SNOW IN THE
ARROWHEAD OF MN...CONTRASTED BY NO SNOW IN THE 21Z SREF MEAN.

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

PETERSEN



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