Quantitative Precipitation Forecast
Issued by NWS

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FOUS11 KWBC 182106
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
405 PM EST SUN JAN 18 2015

VALID 00Z MON JAN 19 2015 - 00Z THU JAN 22 2015

...DAYS 1 TO 3...


...PACIFIC NORTHWEST/ROCKIES...

A PERSISTENT DEEP LAYER WESTERLY FLOW TONIGHT WITH STRONG ASCENT
850-700 MB...UPPER LEVEL DIVERGENCE MAXIMA... AND UPSLOPE
CONDITIONS WILL BRING  HEAVY SNOW ACROSS THE WA CASCADE RANGE WITH
LIGHT TO MODERATE AMOUNTS FURTHER INLAND INTO THE BITTERROOTS AND
MOUNTAINS OF CENTRAL IDAHO AS WELL AS THE MOUNTAINS OF NORTHWEST
MONTANA AND NORTHWESTERN WYOMING. ACCUMULATIONS OF A FOOT OR MORE
ARE LIKELY IN PORTIONS OF THE WASHINGTON CASCADES.

AS AN UPPER LEVEL RIDGE BUILDS UPSTREAM OVER THE NORTHEAST PACIFIC
MON AFTERNOON/EVENING THE SNOW THREAT DIMINISHES AS DRY AIR ALOFT
ADVECTS INTO THE PACIFIC NORTHWEST AND NORTHERN ROCKIES.
WEAK LOW LEVEL CONVERGENCE/UPPER DIVERGENCE SIGNATURES LEAD TO
LIGHT SNOW IN THE NORTHERN AND EVEN CENTRAL ROCKIES WITH LOW
CHANCES OF GREATER THAN 4 INCHES OF SNOW.

ON TUE NIGHT/WED...THE MODELS SHOW AN UPPER LEVEL TROUGH
DEVELOPING OVER THE SOUTHWEST AND CONFLUENT FLOW CROSSING THE
SOUTHWEST TO SOUTHERN PLAINS. MODEST 700 MB CONVERGENCE AN 300 MB
DIVERGENCE MAXIMA LEADS TO
LIGHT SNOW DEVELOPING IN THE RANGES OF SOUTHERN UT AND CO DOWN
INTO NORTHERN AZ AND NM.  THE 00-12Z ECMWF SHOW BANDS OF 2-4
INCHES OF SNOW IN THESE AREAS...WITH A LITTLE LESS COVERAGE IN THE
GFS AND 09Z SREF MEAN/12Z NAM SNOW FORECASTS.


...NORTHEAST DAY 1...

A BAND OF HEAVY SNOW IS FORECAST FOR SUNDAY NIGHT INTO MONDAY
ACROSS NORTHERN VERMONT AND ADJACENT NORTHEAST NEW YORK.  THE
INTENSIFICATION OF THE LOW LEVEL CYCLONE SETS UP BOTH A
STRENGTHENING LOW LEVEL JET PLUS A WELL DEFINED MID LEVEL
DEFORMATION AND FRONTOGENESIS ZONE WEST OF THE LOW LEVEL CYCLONE
TRACK.
UNCERTAINTIES CONTINUE REGARDING PRECIP TYPE AS INITIAL PRECIP HAS
BEGUN AS MIXED OR EVEN RAIN IN A FEW WARMER SPOTS.  THE DURATION
OF SNOW WILL COMBINE WITH QPF WEST OF THE LOW LEVEL CIRCULATION TO
DETERMINE SNOW AMOUNTS.

FOR SNOW TOTALS...GAVE GREATER WEIGHTING TO THE 12Z GEFS MEAN AND
12Z ECMWF/12Z NAM WITH HIGHER AMOUNTS THAN THE 12Z GFS AND 09Z
SREF MEAN.  THE PREFERRED CYCLONE TRACK WOULD BRING THE 850 MB
FREEZING ISOTHERM ALL THE WAY INLAND INTO VT THIS EVENING AND THEN
THE FREEZING LINE RETREATS EAST AS THE CIRCULATION MOVES NORTH
ACROSS NORTHERN NT AND THEN THE CANADIAN BORDER BY MORNING. THIS
APPROACH PRODUCES MODERATE TO HIGH PROBABILITIES OF 4 INCHES FOR
NORTHERN VERMONT INTO NORTHEAST NEW YORK...WITH LOW TO MODERATE
PROBABILITIES OF 8 INCHES IN THE SAME AREAS AS WELL.

THE PROBABILITY OF 0.25 INCHES OF ICE IS LESS THAN 10 PERCENT
ACROSS THE CONTIGUOUS US.

PETERSEN

$$




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