Quantitative Precipitation Forecast
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QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
336 PM EDT SUN APR 12 2015

VALID 00Z MON APR 13 2015 - 00Z THU APR 16 2015


DAYS 1 THROUGH 3...


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

A STRONG SHORT WAVE APPROACHES THE PACIFIC NORTHWEST LATE ON DAY
1...AS IT PUSHES A SURFACE FRONT ONSHORE. THE SHORT WAVE ELONGATES
INTO A LONG WAVE TROUGH AS IT CROSSES THE NORTHERN ROCKIES DURING
DAY 2...SLOWING ACROSS THIS AREA DURING DAY 3. THE LONG WAVE AIDS
IN SPINNING UP SURFACE LOW PRESSURE OVER EASTERN MT...WHICH TRACKS
INTO WESTERN ND AND SOUTH CENTRAL CANADA DURING DAYS 2 AND 3.
THERE WAS DECENT MODEL AGREEMENT DURING DAY 1. HOWEVER...AFTER
THAT TIME...THE 12Z NAM BECOMES TOO SLOW...AND THE 00Z ECMWF IS
TOO STRONG. IN AN ATTEMPT TO MITIGATE SOME OF THESE
DIFFERENCES...THE THERMAL PORTION OF THE FORECAST WAS BASED ON A
MULTI MODEL BLEND...WEIGHTED MORE HEAVILY TOWARD THE 12Z GFS. THE
QPF PORTION OF THE FORECAST WAS BASED ON THE MOST RECENT WPC QPF.

DAY 1...
MID LEVEL RIDGING BUILDS OVER THE PACIFIC NORTHWEST IN BETWEEN
SHORT WAVES DURING THE FIRST PART OF DAY 1...WHICH SLOWS THE
APPROACH OF PRECIPITATION AHEAD OF THE NEXT SHORT WAVE IN THE
FLOW. WARM AIR ADVECTION IN ADVANCE OF THE SHORT WAVE ALLOWS SNOW
LEVELS TO RISE TO BETWEEN 6000 AND 7000 FEET OVER THE PACIFIC
NORTHWEST. THE BEST LIFT WITH THE SHORT WAVE ARRIVES OVER WA/OR
BETWEEN 14/06Z AND 14/12Z... AND THE LOW LEVEL FLOW FOCUSES THE
MOISTURE ON THE HIGHER TERRAIN. THE HIGHER SNOW LEVELS PLACE 3 TO
6 INCHES OF SNOWFALL ABOVE 6000 FEET ACROSS THE OLYMPIC RANGE AND
THE NORTHERN CASCADES IN WA. THESE AMOUNTS ARE SUPPORTED BY
MEMBERS OF THE LATEST ECMWF ENSEMBLE OUTPUT.

DAY 2...
THE SHORT WAVE ELONGATES INTO A LONG WAVE TROUGH AS IT CROSSES THE
PACIFIC NORTHWEST AND NORTHERN ROCKIES DURING DAY 2. AS MID LEVEL
HEIGHTS FALL WITH THE PASSAGE OF THE LONG WAVE TROUGH...SNOW
LEVELS CRASH...DROPPING AS LOW AS 4500 FEET OVER THE REGION DURING
THE PERIOD.  OVER WESTERN WA/OR...THE LOW LEVEL WEST NORTHWEST
FLOW FOCUSES PACIFIC MOISTURE ON THE HIGHER TERRAIN. INCREASING
MOISTURE AND FALLING SNOW LEVELS SUPPORTS A SWATH OF 4 TO 8 INCH
SNOWS ACROSS THE WA AND OR CASCADES...AS WELL AS THE NORTHERN
OLYMPIC RANGE... ESPECIALLY ABOVE 4500 FEET. THESE AMOUNTS ARE
WELL SUPPORTED BY MEMBERS OF THE LATEST ECMWF ENSEMBLE
OUTPUT...WHICH SHOW LOCAL 8+ INCH AMOUNTS ACROSS THE NORTHERN WA
AND NORTHERN OR CASCADES...AND RESULT IN AT LEAST A SLIGHT CHANCE
OF 8+ INCHES OF SNOWFALL.

FURTHER EAST...THE MOISTURE SOURCE IS NOT AS ROBUST ACROSS THE
NORTHERN ROCKIES DURING THE SECOND HALF OF THE PERIOD...WITH
PRECIPITABLE WATER VALUES GENERALLY NEAR 0.25 INCHES.
HOWEVER...LOW PRESSURE FORMING ON THE PACIFIC FRONT CROSSING THE
ROCKIES INCREASES LOW LEVEL MOISTURE CONVERGENCE...AND UPSLOPE
FLOW INCREASES BEHIND THE FRONT AFTER 14/18Z. THE COMBINATION OF
SYNOPTIC SCALE ASCENT ASSOCIATED WITH THE LONG WAVE TROUGH AND
UPSLOPE FLOW MAKE THE MOST OF THE AVAILABLE MOISTURE TO PRODUCE 4
TO 8 INCHES OF SNOWFALL ACROSS THE NORTHERN SAWTOOTH AND
BITTERROOT RANGES IN ID. THESE AMOUNTS ALSO TRANSLATE INTO A
MODERATE PROBABILITY OF 4+ INCHES OF SNOW. LESSER AMOUNTS
(GENERALLY BETWEEN 3 AND 6 INCHES OF SNOWFALL) ARE EXPECTED OVER
THE BLUE MOUNTAINS OF NORTHEAST OR AND THE ROCKY MOUNTAIN FRONT
RANGE IN WESTERN MT.

DAY 3...
THE MID LEVEL LONG WAVE TROUGH TRUNDLES ACROSS THE NORTHERN AND
CENTRAL ROCKIES ON DAY 3...AS THE SURFACE LOW TRACKS FROM EASTERN
MT INTO SOUTH CENTRAL CANADA. AGAIN...MOISTURE REMAINS THE
LIMITING FACTOR FOR SNOWFALL HERE...AS PRECIPITABLE WATER VALUES
REMAINS CLOSE TO 0.25 INCHES. AS WAS THE CASE ON DAY 2...THE
COMBINATION OF SYNOPTIC SCALE ASCENT AND UPSLOPE FLOW ACROSS THE
NORTHERN AND CENTRAL ROCKIES MAKES UP FOR THE GENERAL LACK OF
MOISTURE FOR WIDESPREAD 2 TO 4 INCH SNOWFALL ACROSS THE HIGHER
TERRAIN OF UT...WY AND THE CO ROCKIES.

THE BEST UPSLOPE IS FOCUSED OVER THE TERRAIN IN WY...WHERE LOCAL 6
TO 10 INCH SNOWFALL AMOUNTS ARE POSSIBLE ACROSS THE GRAND TETONS
AND BIG HORN RANGES. THESE AMOUNTS ALSO REPRESENT AT LEAST A
SLIGHT CHANCE OF 8+ INCHES OF SNOWFALL. THE LATEST ECMWF ENSEMBLE
OUTPUT SUPPORTS 4+ INCH SNOWFALL AMOUNTS ACROSS THESE AREAS (AS
DOES THE 09Z SREF OUTPUT)...BUT INTERESTINGLY NEITHER MODEL SHOWED
INDICATIONS FOR HIGHER AMOUNTS. LOCAL 3 TO 6 INCH AMOUNTS ARE ALSO
EXPECTED ACROSS PORTIONS OF THE CENTRAL WASATCH RANGE IN UT AND
THE ROCKIES IN NORTH CENTRAL CO.

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.

HAYES


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