Quantitative Precipitation Forecast
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FOUS11 KWBC 230730
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
229 AM EST TUE DEC 23 2014

VALID 12Z TUE DEC 23 2014 - 12Z FRI DEC 26 2014

WEST COAST/INTERMOUNTAIN WEST/ROCKIES...

A STRONG AND FAST MOVING SHORTWAVE TROUGH WILL REACH THE PACIFIC
NORTHWEST BY EARLY WEDNESDAY BEFORE PROGRESSING THROUGH THE
INTERMOUNTAIN WEST BY THURSDAY AND FRIDAY.  WHILE MODEST SPREAD
CONTINUES TO BE SEEN WITHIN THE GUIDANCE...TRENDS TOWARD GREATER
AMPLIFICATION SEEM THE FAVOR THE SLOWER HALF OF THE GUIDANCE ONCE
THE TROUGH MOVES INLAND THAT WOULD INCLUDE USING LIQUID EQUIVALENT
AMOUNTS AND THERMAL PROFILES LARGELY DERIVED FROM THE THE 12Z
ECMWF AND 12Z ECMWF ENSEMBLE MEAN...AS WELL AS PORTIONS OF THE 00Z
NAM...WHILE THE 00Z GFS APPEARS TO BE TOO FAST.  THIS APPROACH
RESULTS IN MODERATE AND HIGH PROBABILITIES OF 12 INCHES OF SNOW
FOR THE NORTHERN CASCADES OF WASHINGTON FOR DAY 1.  BY DAYS 2 AND
3 (WED AND THU)...MODERATE PROBABILITIES OF 8 INCHES OF SNOW
EXTEND ACROSS THE HIGHER PEAKS OF THE INTERMOUNTAIN WEST TOWARD
THE CENTRAL ROCKIES...INCLUDING PORTIONS OF OREGON
CASCADES/SIERRAS/TETONS/WASATCH...AS WELL AS PORTIONS OF THE
COLORADO ROCKIES...WHILE LOW PROBABILITIES BEGIN TO EXTEND INTO
THE HIGH PLAINS OF WYOMING/NEBRASKA/
SOUTH DAKOTA...AS THE SURFACE LOW REORGANIZES IN THE LEE OF THE
COLORADO ROCKIES.

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

UPPER MIDWEST...

MODELS HAVE TRENDED A LITTLE FASTER AND EAST WITH THE LOW TRACKING
OUT OF IOWA AND ACROSS EASTERN WISCONSIN...WHILE THE WARMER
BOUNDARY LAYER TEMPERATURES OF THE ECMWF HAVE BEEN VERIFYING
BETTER THAN THE COLDER NAM/GFS/SREF MEAN.  THUS...SNOW AMOUNTS
HAVE BEEN REDUCED SLIGHTLY FROM THE PREVIOUS FORECAST...BUT
OTHERWISE SHOW A SIMILAR CONFIGURATION.  THE RESULT IS FOR A
NARROW AXIS OF LOW PROBABILITIES OF 4 INCHES FROM JUST SOUTH OF
MINNEAPOLIS NORTHEASTWARD TO THE UPPER PENINSULA OF
MICHIGAN...WHERE A NARROW SWATH OF MODERATE PROBABILITIES FOR 4
INCHES EXIST.

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

WESTERN OH VALLEY INTO MICHIGAN...

A NARROW BUT POTENTIALLY IMPACTFUL SNOW EVENT MAY DEVELOP ACROSS
PORTIONS OF ILLINOIS INTO NORTHERN MICHIGAN WEDNESDAY INTO
WEDNESDAY NIGHT...THE DETAILS OF WHICH INCLUDING LOCATION AND
AMOUNTS ARE SOMEWHAT UNCLEAR AT THIS TIME.  USED LIQUID EQUIVALENT
AMOUNTS AND THERMAL PROFILES WEIGHTED 2/3RDS TOWARD THE 12Z ECMWF
AND 1/3 TOWARD THE 12Z ECMWF ENSEMBLE MEAN TO ACCOUNT FOR THE SOME
OF THE FASTER LOW TRACKS.  THE 00Z NAM AND 00Z GFS ARE CONSIDERED
TOO SLOW AND FAST RESPECTIVELY...AND THUS WERE NOT USED...AND HAVE
ALSO SHOWN A COLD BIAS NEAR THE RAIN/SNOW LINE OVER THE PLAINS.
THAT SAID...THE ECMWF SHOWS THE POTENTIAL FOR A NARROW AXIS OF
MODERATE TO PERHAPS EVEN HEAVY SNOW (PERHAPS BRIEF) FROM CENTRAL
ILLINOIS TO NEAR CHICAGO...WITH A SECONDARY AXIS OVER NORTHERN
MICHIGAN.  THE POTENTIAL IS MOSTLY A RESULT OF HIGH RATES PERHAPS
CONTRIBUTED FROM WEAK CONVECTION THAT MAY OVERWHELM THE BOUNDARY
LAYER TEMPERATURES THAT ARE GENERALLY FORECAST TO REMAIN FREEZING.
 IF THESE RATES DEVELOP FOR A SUFFICIENTLY LONG
PERIOD...ACCUMULATING SNOWS ARE LIKELY.  FOLLOWING A CONFERENCE
CALL WITH SEVERAL WFO`S...SUFFICIENT CONSENSUS FOR THESE CONCERNS
DEVELOPED TO SUPPORT A NARROW AXIS OF MODERATE PROBABILITIES FOR 4
INCHES ACROSS CHICAGO...WITH A SURROUNDING AREA OF SLIGHT
PROBABILITIES.  THE DETERMINISTIC AMOUNTS ALSO REFLECTED THESE
PROBABILITIES WITH A NARROW AXIS OF 1 TO 2 INCHES EXTENDING
SOUTHWARD FROM CHICAGO.

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

JAMES

$$




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