Quantitative Precipitation Forecast
Issued by NWS

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000
FOUS11 KWBC 160725
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
324 AM EDT SAT MAY 16 2015

VALID 12Z SAT MAY 16 2015 - 12Z TUE MAY 19 2015

DAYS 1 TO 3...

...RANGES OF CA/GREAT BASIN/ROCKIES/NORTHERN PLAINS...

A 700 MB CIRCULATION AND ASSOCIATED FRONT DRIFTS SLOWLY NORTH FROM
NEAR THE MT/WY BORDER TODAY.  UPPER DIVERGENCE/LOWER CONVERGENCE
MAXIMA COMBINE WITH UPSLOPE FLOW TO PRODUCE RISING MOTION AND
RESULTANT SNOW ACROSS THE RANGES OF NORTHWESTERN WY/ADJACENT MT.

THE CLOSED UPPER LOW IN CA CROSSES NV AND UT INTO CO...WITH THE
ASSOCIATED FRONTAL CIRCULATION HELPING TO PRODUCE ASCENT ALONG THE
PATH OF THE LOW.  SEVERAL INCHES ARE EXPECTED IN THE HIGHER
ELEVATIONS OF UT INTO WESTERN CO...  INCLUDING THE SAN JUANS.

THE INTENSIFYING SHORT WAVE IN THE BASE OF THE LARGE CLOSED LOW
WILL EJECT OUT OF THE FOUR CORNERS REGION SAT NIGHT AND DEEPEN ON
SUN WHILE INDUCING A SURFACE CIRCULATION OVER SD. THIS LOW WILL
TRACK TO ERN ND/NWRN MN.
COLD ADVECTION ON THE NW QUAD OF THE LOW COULD CHANGE RAIN OVER TO
SNOW...WITH THE 12Z-00Z ECMWF SHOWING HALF TO ONE INCH LIQUID
EQUIVALENT IN THE FORM OF SNOW...SUGGESTING POTENTIAL FOR A BAND
OF HEAVY SNOW IN NORTH CENTRAL ND.  THE NAM/GFS/21Z SREF MEAN SNOW
AMOUNTS ARE LOWER.  THE DURATION OF SNOW AND ABILITY TO ACCUMULATE
ON ABOVE FREEZING SURFACES REMAIN THE UNCERTAINTIES SO A LOW PROB
OF 4 INCHES OF SNOW REMAINS IN PLACE ACROSS NRN ND.

THEN ON MON... THE NEXT 700 MB TROUGH AND FRONT SUPPORT MORE SNOW
FOR THE CENTRAL CA SIERRA AS 700 MB CONVERGENCE MAXIMA PERSIST SUN
NIGHT INTO EARLY MON.  SNOW ALSO DEVELOPS DOWNSTREAM IN THE HIGHER
ELEVATIONS OF THE RANGES OF SOUTHERN UT AND WESTERN CO.
DOWNSTREAM FROM THE MAIN CIRCULATION...LOW-MID LEVEL WARM
ADVECTION OCCURS WITH DEEP MOISTURE RESULTING FROM THE SOUTHERLY
COMPONENT OF FLOW.  THE WARM ADVECTION AND 700 MB CONVERGENCE
MAXIMA TRIGGER LIFT...AIDED BY UPSLOPE FLOW.

MANUAL PROGS EQUALLY WEIGHTED THE GFS/ECMWF/SREF QPF AND
TEMPERATURE PROFILES FOR THE SNOW FORECASTS.

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

PETERSEN


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