Quantitative Precipitation Forecast
Issued by NWS

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FOUS11 KWBC 270726
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
325 AM EDT MON OCT 27 2014

VALID 12Z MON OCT 27 2014 - 12Z THU OCT 30 2014


DAYS 1 THROUGH 3...


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

A MID LEVEL LONG WAVE TROUGH OVER THE ROCKIES EARLY ON DAY 1 WILL
CLOSE OFF OVER THE NORTHERN PLAINS AND UPPER MS VALLEY ON DAY 2. A
STRONG SHORT WAVE NEAR 45.5N 140W AND ITS ASSOCIATED SURFACE LOW
EARLY THIS MORNING WILL WEAKEN AS APPROACHES WA AND SOUTHERN
BRITISH COLUMBIA ON DAY 2. AS THE SHORT WAVE WEAKENS...MID LEVEL
RIDGING BUILDS OVER THE ROCKIES AND GREAT BASIN ON DAY 3. FOR THE
MOST PART...THE QPF PORTION OF THE FORECAST WAS BASED PRIMARILY ON
THE LATEST WPC QPF...WHILE THE THERMAL PORTION OF THE FORECAST WAS
BASED ON A 21Z SREF MEAN/18Z GEFS MEAN/12Z ECMWF BLEND.

AS THE STRONG SHORT WAVE AND ITS ASSOCIATED SURFACE LOW AND
OCCLUDED FRONT APPROACH THE PACIFIC NORTHWEST...THE BULK OF THE
MOISTURE AND LIFT SHOULD COME SHORE ACROSS BRITISH COLUMBIA AND WA
BETWEEN 28/06Z AND 28/12Z. AHEAD OF THE MID AND LOW LEVEL
SYSTEMS...STRONG WARM AIR ADVECTION WILL RESULT IN SNOW LEVELS
RISING FROM NEAR 7000 FEET EARLY ON DAY 1 TO CLOSE TO 10000 FEET
BY THE END OF THE PERIOD. SINCE THE BEST LIFT OCCURS DURING THE
SNOW LEVEL RISE...THE EXPECTED QPF WITH THIS SYSTEM SHOULD PRODUCE
1 TO 3 INCHES OF SNOWFALL OVER THE NORTHERN CASCADES...GENERALLY
ABOVE 9000 FEET.

THE SHORT WAVE WEAKENS AS IT REACHES THE PACIFIC NORTHWEST COAST
DURING DAY 2...AND THE LOW TO MID LEVEL FLOW BECOMES QUASI ZONAL.
THE LOW LEVEL FLOW SHOULD FOCUS THE AVAILABLE MOISTURE (WITH
PRECIPITABLE WATER VALUES NEAR AN INCH OVER WA/OR) ON THE NORTHERN
CASCADES OF WA...AND 1 TO 3 INCHES OF SNOWFALL IS EXPECTED HERE
(AGAIN GENERALLY ABOVE 9000 FEET). MID LEVEL RIDGING BUILDING OVER
THE ROCKIES AND GREAT BASIN SHOULD SHUT OFF THE MOISTURE SUPPLY TO
THE REGION...SO NO SIGNIFICANT SNOWFALL IS EXPECTED ON DAY 3.


...UPPER MS VALLEY...

A MID LEVEL LONG WAVE TROUGH OVER THE ROCKIES EARLY ON DAY 1 WILL
CLOSE OFF OVER THE NORTHERN PLAINS AND UPPER MS VALLEY ON DAY
2...CROSSING THE GREAT LAKES INTO ONTARIO ON DAY 3. THE QPF
PORTION OF THE FORECAST WAS BASED PRIMARILY ON THE LATEST WPC
QPF...WHILE THE THERMAL PORTION OF THE FORECAST WAS BASED ON A 21Z
SREF MEAN/18Z GEFS MEAN/12Z ECMWF BLEND.

AHEAD OF THE CLOSED MID LEVEL SYSTEM ON DAY 2...STRONG WARM AIR
ADVECTION PUMPS MOISTURE ACROSS THE GREAT LAKES AND UPPER MS
VALLEY. AS THE CLOSED LOW ENTERS THE NORTHERN PLAINS...THE
MOISTURE BECOMES WRAPPED NORTH OF THE MID LEVEL SYSTEM...RESULTING
IN THE DEVELOPMENT OF A TROWAL ACROSS SOUTHERN ONTARIO AND
NORTHERN LAKE SUPERIOR. WHILE MUCH OF THE LIFT ASSOCIATED WITH
TROWAL REMAINS NORTH OF THE BORDER...SOME OF THE MEMBERS OF THE
LATEST AVAILABLE MODEL SUITE SUGGEST THAT THE TROWAL LINGERS FOR A
TIME OVER THE ARROWHEAD OF MN.

THE 00Z NAM IS THE MOST BULLISH WITH THE LIFT AND RESULTANT
SNOWFALL...AS IT SHOWS MID LEVEL INSTABILITY ASSOCIATED WITH THE
STRONG ISENTROPIC LIFT REACHING NORTHEAST MN. IN FACT...00Z NAM
MODEL SOUNDINGS OVER THE ARROWHEAD OF MN SHOWED A PERIOD OF LIFT
IN THE DENDRITIC GROWTH ZONE BETWEEN 29/03Z AND 29/06Z...AS THE
COLUMN COOLS AND BECOMES ISOTHERMAL. HOWEVER...IT APPEARS AS
THOUGH THE 00Z NAM IS TOO DEEP WITH THE MID LEVEL SYSTEM...AND
WRAPS THE BEST MID LEVEL MOISTURE/LIFT TOO FAR SOUTHWEST IN
NORTHEAST MN. FOR THIS REASON...THE 00Z NAM SOLUTION WAS NOT
EMPLOYED. THE MODEL CONSENSUS WAS TO BRING JUST ENOUGH MOISTURE
AND LIFT TO NORTHERN LAKE SUPERIOR AND NORTHEAST MN FOR AN INCH OR
SO OF SNOWFALL. TIMING OF THE PRECIPITATION COULD RESULT IN
BOUNDARY LAYER ISSUES...SO ACCUMULATIONS WERE CAPPED IN THIS
REGION.


THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.

HAYES

$$





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