Quantitative Precipitation Forecast
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FOUS11 KWBC 242026
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
426 PM EDT MON APR 24 2017

VALID 00Z TUE APR 25 2017 - 00Z FRI APR 28 2017


...PACIFIC NORTHWEST/NORTHERN AND CENTRAL ROCKIES...

AN UPPER-LEVEL TROUGH WILL AMPLIFY ACROSS THE WESTERN U.S. THROUGH
TUE AS SHORTWAVE ENERGY MOVES ONSHORE IN THE NORTHWEST THIS
EVENING AND THEN MOVES TO THE FOUR CORNERS REGION BY EARLY TUE
AFTERNOON. THIS SYSTEM IS EXPECTED TO PRODUCE WIDESPREAD
PRECIPITATION ACROSS THE WESTERN U.S., WITH MODERATE TO HEAVY
SNOWS ACROSS PORTIONS OF THE CENTRAL/NORTHERN ROCKIES ON DAY 1
(00Z TUE-00Z WED), WITH A MODERATE TO HIGH RISK OF EXCEEDING 4
INCHES SHOWN ON THE WPC PROBABILITIES, AND A SLIGHT TO MODERATE
RISK OF EXCEEDING 8 INCHES OF SNOW. THE HEAVIEST SNOWS ON DAY 1
ARE EXPECTED TO FALL ACROSS THE BIG HORN MOUNTAINS, WHERE A SLIGHT
TO MODERATE RISK OF EXCEEDING 12 INCHES IS FORECAST.

THE REGION WILL SEE A BRIEF LULL IN PRECIPITATION TUESDAY NIGHT
BEFORE ANOTHER SHORTWAVE AND COLD FRONT ONCE AGAIN SPREAD PRECIP
ACROSS THE WEST ON WED. DAY 2 (00Z WED-00Z THU) WPC PROBABILITIES
SHOW A MODERATE TO HIGH RISK OF 4 INCHES OF SNOW ACROSS PORTIONS
OF THE NORTHERN ROCKIES, WITH A SLIGHT TO MODERATE RISK ACROSS
PORTIONS OF THE SOUTHERN/CENTRAL ROCKIES. PORTIONS OF THE TETONS
AND ABSAROKAS MAY EXCEED 8 INCHES ON DAY 2. BY DAY 3 (00Z THU-00Z
FRI), AS THE NEXT SHORTWAVE MOVES INTO THE ROCKIES AND YET ANOTHER
SHORTWAVE ENTERS THE PACIFIC NORTHWEST, SNOWS ARE ONCE AGAIN
EXPECTED TO INCREASE ACROSS THE CENTRAL AND NORTHERN ROCKIES, WITH
A MODERATE TO HIGH RISK OF EXCEEDING 8 INCHES ACROSS THE TETONS,
AND A MODERATE RISK OF EXCEEDING 4 INCHES ELSEWHERE ACROSS MUCH OF
THE CENTRAL AND NORTHERN ROCKIES. SNOW IS ALSO EXPECTED TO RETURN
TO THE HIGHER ELEVATIONS OF THE CASCADES ON DAYS 2-3, WITH SLIGHT
TO MODERATE RISKS FOR 4 INCHES EACH DAY.

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.

...NORTHERN HIGH PLAINS...

AS SHORTWAVE ENERGY CROSSES THE WEST TONIGHT, A SURFACE LOW IS
EXPECTED TO DEEPEN ACROSS THE CENTRAL PLAINS ALONG A SURFACE
FRONTAL BOUNDARY. AN ADDITIONAL ARCTIC FRONTAL BOUNDARY REMAINS
BANKED AGAINST THE EASTERN SLOPES OF THE NORTHERN/CENTRAL ROCKIES,
AND WILL SERVE AS A FOCUS MECHANISM FOR PRECIPITATION AS SYNOPTIC
SCALE LIFT INCREASES OVERNIGHT. ADDITIONALLY, HIGH PRESSURE NOSING
INTO THE NORTHERN PLAINS FROM CANADA WILL SUPPORT INCREASING
LOW-LEVEL EASTERLY UPSLOPE FLOW FROM MONTANA SOUTH ACROSS WY AS
WELL AS WESTERN SD. EXPECT PRECIP TO DEVELOP OVERNIGHT ACROSS THE
BLACK HILLS AND ADJACENT AREAS OF THE NORTHERN HIGH PLAINS. PRECIP
MAY FALL AS RAIN INITIALLY, BUT MODELS ARE IN GENERAL AGREEMENT ON
COOLING THE LOWER LEVELS OF THE ATMOSPHERE LATE TONIGHT AND
CONTINUING INTO TUE. THE HEAVIEST SNOWFALL AMOUNTS ARE EXPECTED
ACROSS THE BLACK HILLS, WHERE TEMPERATURES WILL SUPPORT
ACCUMULATIONS EARLIEST, AND TERRAIN MAY SOMEWHAT ENHANCE RATES
GIVEN LOW-LEVEL UPSLOPE FLOW. WPC DAY 1 PROBABILITIES SHOW A
MODERATE RISK OF 8 INCHES ACROSS THE BLACK HILLS, WITH A SLIGHT
RISK OF 12 INCHES. AT LOWER ELEVATIONS EAST ACROSS SD AND
POTENTIALLY NWRN NE, WPC PROBABILITIES SHOW A SLIGHT RISK OF
EXCEEDING 4 INCHES OF SNOW.

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.

...UPPER MIDWEST/UPPER GREAT LAKES...

A MIXED PRECIPITATION EVENT IS LOOKING INCREASINGLY LIKELY ACROSS
PORTIONS OF THE UPPER MIDWEST AND UPPER GREAT LAKES DURING DAYS
2-3. AS A SLOW-MOVING WAVE OF LOW PRESSURE MOVES NORTHEASTWARD
FROM THE SOUTHERN PLAINS TO THE MIDWEST, ABUNDANT MOISTURE WILL BE
TRANSPORTED NORTH FROM THE GULF OF MEXICO INTO THE REGION, AS
SHOWN BY PWAT ANOMALIES OF +2 TO +2.5 STANDARD DEVIATIONS BY TUE
NIGHT CONTINUING INTO WED/WED NIGHT. AT THE SAME TIME, A STRONG
ARCTIC SURFACE HIGH WILL REMAIN ANCHORED ACROSS SOUTHERN HUDSON
BAY, WITH BRISK NORTHEASTERLY LOW-LEVEL FLOW TRANSPORTING COLD AIR
SOUTH ACROSS THE UPPER MIDWEST AND UPPER GREAT LAKES. THE SHALLOW
NATURE OF THIS COLD AIR WILL MAKE FOR A VARIETY OF P-TYPES,
INCLUDING THE POTENTIAL FOR SNOW, SLEET, AND FREEZING RAIN.
SIGNIFICANT UNCERTAINTY REMAINS WITH RESPECT TO THE EXACT TRACK OF
THE SURFACE LOW AND THE INTENSITY/DEPTH OF THE WARM LAYER ABOVE
THE SURFACE. DATA FROM THE ECMWF ENSEMBLE SHOW AN 850A HPA TEMP
SPREAD OF 4-5 DEG C ACROSS THE REGION LATE WED. AS SUCH,
CONFIDENCE IS LOW WITH RESPECT TO THE EXACT PROPORTION OF
PRECIPITATION THAT WILL FALL IN EACH P-TYPE. FOR THIS FORECAST
PACKAGE, THERMAL PROFILES WERE BASED ON A MULTI-MODEL BLEND
INCLUDING THE 12Z GFS/00Z ECMWF/09Z SREF. WPC PROBABILITIES SHOW A
SLIGHT RISK OF 4 INCHES OF SNOW ON DAYS 2 AND 3 FOR PORTIONS OF
NORTHERN MN. ADDITIONALLY, A SLIGHT TO MODERATE RISK OF 0.25 INCH
OF ICE IS SHOWN ON DAY 2 ACROSS NORTHEASTERN MN. ICE IS POSSIBLE
FARTHER EAST ON DAY 3, BUT CONFIDENCE IS LESS WITH RESPECT TO
AMOUNTS. CURRENT DAY 3 PROBABILITIES SHOW A SLIGHT TO MODERATE
RISK OF 0.10 INCH OF ICE FROM NORTHERN WI INTO THE WESTERN UPPER
PENINSULA OF MICHIGAN.

RYAN

$$




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