Quantitative Precipitation Forecast
Issued by NWS

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FOUS11 KWBC 230737
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
337 AM EDT SAT SEP 23 2017

VALID 12Z SAT SEP 23 2017 - 12Z TUE SEP 26 2017

DAYS 1-3...

...GREAT BASIN AND CENTRAL TO NORTHERN ROCKIES...

A SLOW MOVING LOW LEVEL FRONTAL BNDRY COMBINES WITH AN UPPER
TROUGH TO PRODUCE
AREAS OF SNOW IN HIGHER ELEVATIONS ACROSS UT/WY/NORTHERN CO OVER
THE NEXT FEW DAYS.
THE THREAT WANES ONCE THE UPPER TROUGH WEAKENS AND MOVES NORTHEAST
EARLY NEXT WEEK.

THE INITIAL CIRCULATION DRIFTS NORTHEAST FROM UT INTO WY ON DAY 1.
  700 MB CONVERGENCE NEAR THE LOW PRODUCES ASCENT...AIDED IN THE
RANGES OF WESTERN WY LIKE THE WIND ROVER WHERE MOIST EASTERLY FLOW
TURNS UPSLOPE SAT NIGHT-SUN MORNING. ON DAY 1 SNOW FALL AMOUNTS
TIED MORE CLOSELY TO THE CANADIAN REGIONAL GEM AND THE MEAN OF THE
HIGH RESOLUTION WINDOW FORECASTS.

ON DAY 2...THE INITIAL CIRCULATION IS FORECAST TO DRIFT NORTHEAST
FROM THE UT UINTA MOUNTAINS ACROSS WY AND GRADUALLY WEAKEN.
PERSISTING 700 MB CONVERGENCE LEADS TO CONTINUING SNOW IN THE UT
UINTAS/SOUTHWEST WY THAT SPREAD ACROSS THE BIGHORNS AND MEDICINE
BOW/LARAMIE MNTS.

THE MODELS SPLIT ON MONDAY/TUE THE PHASING OF THE UPPER TROUGH
ONCE THE INITIAL LEAD WAVE EXITS THROUGH THE NORTHERN PLAINS.  THE
00Z NAM DEVELOPS AN ANOMALOUS 700 MB CIRCULATION IN CO...LEADING
TO HEAVIER SNOW TOTALS THAN THE OTHER MODELS IN THE CO RANGES.
LESS WEIGHTING WAS GIVEN TO THE NAM UNTIL OTHER MODELS JOIN ITS
SOLUTION.  THE ECMWF/GFS FORECASTS HAVE RELATIVELY LIGHT AMOUNTS
GIVEN WEAK LOW LEVEL WINDS AND RESULTANT MODEST CONVERGENCE
SIGNALS.  THE MAJORITY CLUSTER OF SOLUTIONS WAS PREFERRED AND
GIVEN THE MOST WEIGHTING.

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.

PETERSEN

$$





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