Quantitative Precipitation Forecast
Issued by NWS

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FOUS11 KWBC 182017
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
416 PM EDT TUE APR 18 2017

VALID 00Z WED APR 19 2017 - 00Z SAT APR 22 2017


...WESTERN U.S....

RELATIVELY LIGHT SNOWS ARE POSSIBLE ON DAY 1 (00Z WED-00Z THU)
ACROSS PORTIONS OF THE NORTHERN ROCKIES AND THE WASATCH AS A
RELATIVELY WEAK SHORTWAVE PASSES OVERHEAD. WPC PROBABILITIES
INDICATE A SLIGHT TO MODERATE RISK OF EXCEEDING 4 INCHES OF
SNOWFALL ON DAY 1 ACROSS THOSE AREAS.

ON DAY 2 (00Z THU-00Z FRI) A MORE SUBSTANTIAL UPPER-LEVEL TROUGH
WILL BEGIN TO MOVE INTO THE PACIFIC NORTHWEST, WHICH IS THEN
FORECAST TO AMPLIFY AS IT CROSSES THE GREAT BASIN/INTERMOUNTAIN
WEST AND THE ROCKIES LATE ON DAY 2 AND INTO DAY 3 (00Z FRI-00Z
SAT). SNOW LEVELS ACROSS THE WEST ON DAY 2, BEFORE THE TROUGH
BEGINS TO AMPLIFY SIGNIFICANTLY, WILL BE RELATIVELY HIGH. THIS
WILL KEEP SNOWS CONFINED TO THE HIGHER ELEVATIONS OF THE CASCADES,
SIERRAS, AND THE SISKIYOUS ON DAY 2, WHERE A SLIGHT TO MODERATE
RISK OF EXCEEDING 4 INCHES ARE SHOWN FOR THESE AREAS IN THE WPC
PROBABILITIES.

BY DAY 3 THE TROUGH WILL BE IN THE PROCESS OF A SUBSTANTIAL
AMPLIFICATION ACROSS THE INTERIOR WEST, WITH INCREASING MODEL
CONSENSUS ON AN UPPER LOW FORECAST TO CLOSE OFF ACROSS THE CENTRAL
ROCKIES FRI MORNING. LOW-LEVEL EASTERLY FLOW WILL INCREASE ACROSS
THE NORTHERN/CENTRAL HIGH PLAINS BY FRIDAY AS HIGH PRESSURE NOSES
IN FROM THE NORTH AND A NEW SURFACE LOW DEVELOPS ACROSS THE
SOUTHERN PLAINS AHEAD OF THE AMPLIFYING SHORTWAVE. THE LOW-LEVEL
UPSLOPE FLOW ALONG WITH THE AMPLIFYING SHORTWAVE/UPPER LOW
OVERHEAD WILL SUPPORT MODERATE TO EVEN HEAVY SNOWS FOR SOME AREAS
OF THE CENTRAL/NORTHERN ROCKIES ON DAY 3, WITH THE HEAVIEST SNOWS
LIKELY FOR THE BIG HORN MOUNTAINS. DAY 3 WPC PROBABILITIES SHOW A
SLIGHT TO MODERATE RISK OF EXCEEDING 4 INCHES FOR THESE AREAS,
WITH A SLIGHT RISK FOR EXCEEDING 12 INCHES ACROSS THE BIG HORN
MOUNTAINS.

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.

...UPPER GREAT LAKES...

THE SHORTWAVE CROSSING THE ROCKIES TUE NIGHT/WED IS FORECAST TO
EMERGE INTO THE NORTHERN PLAINS WED AFTERNOON AS IT BEGINS TO
AMPLIFY, WITH A SURFACE LOW FORECAST TO DEVELOP ACROSS THE CENTRAL
PLAINS LATE WED. AS THE SURFACE LOW MOVES NORTHEASTWARD ACROSS THE
MIDWEST AND INTO THE UPPER GREAT LAKES WED NIGHT/THU MORNING, AN
AREA OF FRONTOGENETICALLY FORCED PRECIPITATION IS EXPECTED TO
DEVELOP NORTH OF THE LOW. COLDER AIR WRAPPING AROUND THE NORTHWEST
SIDE OF THE SYSTEM ALONG WITH SOME DEGREE OF DYNAMIC COOLING
WITHIN THE ZONE OF STRONGEST VERTICAL MOTION WILL SUPPORT RAIN
MIXING WITH OR CHANGING TO SLEET OR SNOW, WITH GENERALLY LIGHT
ACCUMULATIONS POSSIBLE ON DAY 2 FROM EXTREME NORTHEASTERN MN TO
WESTERN PORTIONS OF THE MICHIGAN UPPER PENINSULA. SOME LAKE EFFECT
ENHANCEMENT IS POSSIBLE, PARTICULARLY ACROSS PORTIONS OF THE UPPER
PENINSULA, WHERE LOCALIZED ACCUMULATIONS EXCEEDING 4 INCHES ARE
POSSIBLE. IN GENERAL, WPC PROBABILITIES SHOW A SLIGHT RISK OF
EXCEEDING 4 INCHES CENTERED ACROSS THE MICHIGAN U.P.

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.

...NORTHERN MAINE...

AS THE LOW PRESSURE SYSTEM ACROSS THE GREAT LAKES ON DAY 2 MOVES
EAST INTO SOUTHERN QUEBEC ON DAY 3, PRECIPITATION FORCED BY WARM
ADVECTION ALONG WITH RAPIDLY FALLING HEIGHTS AND FAVORABLE UPPER
DYNAMICS IS EXPECTED TO DEVELOP ACROSS MUCH OF THE NORTHEASTERN
U.S. THERMAL PROFILES APPEAR MARGINALLY SUPPORTIVE OF SNOW ACROSS
NORTHERN MAINE, WITH LIGHT ACCUMULATIONS EXPECTED. MODELS SHOW
SOME SPREAD WITH RESPECT TO THERMAL PROFILES, WITH THE NAM AND
ECMWF COLDER ACROSS INTERIOR MAINE ON DAY 3 RELATIVE TO THE GFS.
GIVEN THE DEGREE OF SPREAD, WITH SOME POTENTIAL FOR A SLIGHTLY
COLDER AIR MASS IN PLACE, WPC PROBABILITIES SHOW A SLIGHT RISK OF
EXCEEDING 4 INCHES ACROSS NORTHERN MAINE ON DAY 3.

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.

RYAN

$$





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