Quantitative Precipitation Forecast
Issued by NWS

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FOUS11 KWBC 232115
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
514 PM EDT THU APR 23 2015

VALID 00Z FRI APR 24 2015 - 00Z MON APR 27 2015

DAYS 1 AND 2...

...NEW ENGLAND...

A CLOSED MID LEVEL LOW CURRENTLY OVER THE NORTHEAST IS EXPECTED TO
SLOWLY DRIFT EAST THROUGH THE DAY ON FRIDAY AND SATURDAY HOLDING
ANOMALOUSLY COLD AIR IN PLACE OVER THE REGION. SNOW LEVELS ARE
EXPECTED TO RANGE BETWEEN 1500 AND 2500 FT AGL...PERHAPS DROPPING
TO NEAR 1000 FT AGL IN PLACES ACROSS NRN MAINE IN THE EARLY
MORNING HOURS BUT A RAIN/SNOW MIX IS EXPECTED AT TIMES ELSEWHERE.
BEYOND MOUNTAIN TOPS...THE PROBABILITY OF RECEIVING SIGNIFICANT
SNOW OR ICE IS LESS THAN 10 PERCENT.


DAYS 1 TO 3...

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

A CLOSED MID-UPPER LEVEL LOW CURRENTLY WEST OF VANCOUVER WILL
SLOWLY DRIFT TOWARD THE PACIFIC NORTHWEST THROUGH FRIDAY AT WHICH
POINT A STRONGER JET CORE...130+ KTS...WILL PASS WEST OF THE SLOW
MOVING CLOSED LOW FRIDAY NIGHT. THIS JET ENERGY WILL ALLOW UPPER
TROUGH AMPLIFICATION ACROSS THE WRN CONUS WITH ANOTHER CLOSED LOW
LIKELY SETTING UP JUST SOUTH OF THE FOUR CORNERS REGION BY SUNDAY.
THE UPPER LOW IS THEN FORECAST TO PUSH EASTWARD ACROSS NEW MEXICO
WITH THE SURFACE COLD FRONT REACHING THE SRN HIGH PLAINS BY THE
END OF THE DAY 3 PERIOD...00Z/27. QPF WAS BASED ON WPC QPF FOR
DAYS 2 AND 3...WHILE USING A BLEND OF THE SIMILAR 12Z NAM...12Z
GFS..00Z ECMWF FOR DAY 1. WHILE MODELS SHOWED SIMILAR
EVOLUTION...THE 12Z NAM/GFS WERE BLENDED FOR SNOW LEVELS AND
P-TYPE GIVEN THEIR SYNOPTIC PREFERENCE VIA THE PMDHMD. SNOWFALL
WILL BE STRONGLY ELEVATION DEPENDENT AND MAY EVEN VARY WITH TIME
OF DAY GIVEN THE HIGHER SUN ANGLE OF LATE APRIL.

ACROSS THE WASHINGTON CASCADES AND OLYMPICS...SNOW LEVELS WILL
LOWER BELOW 3000 FT AT TIMES TONIGHT AND ON FRIDAY AS THE UPPER
LOW DRAWS NEARER. SNOWFALL IS EXPECTED TO BE SHOWERY IN NATURE
WITH THE GREATEST SNOWFALL ACCUMULATIONS LIKELY OCCURRING ACROSS
THE NRN CASCADES DUE TO A COMBINATION OF STRONGER FORCING AHEAD OF
THE UPPER LOW AND BETTER UPPER JET SUPPORT EARLY ON WITH DIFFLUENT
FLOW THROUGH FRIDAY EVENING.

AS THE AFOREMENTIONED UPPER LEVEL JET CORE ADVANCES TOWARD THE
CNTRL WEST COAST FOR SAT MORNING...LEFT EXIT REGION LIFT AND AN
ANOMALOUS PRECIPITABLE WATER FETCH...+2 TO +3 STANDARDIZED
ANOMALIES...WILL REACH THE CALIFORNIA COAST WITH WLY FLOW TAKING
AIM INTO THE SIERRA NEVADA RANGE FOR A 12 TO 18 HOUR TIME FRAME
WHICH WILL BRING LOCALLY HEAVY SNOW TO HIGHER ELEVATIONS WITH SLOW
LEVELS NEAR 7000-8000 FT.

BEHIND THE COLD FRONT...SNOW LEVELS WILL DROP ACROSS
IDAHO/MONTANA/WYOMING WITH STEADY SWLY FLOW IN PLACE AHEAD OF THE
DEEPENING WESTERN TROUGH. THIS WILL FAVOR THE SALMON RIVER AND
SAWTOOTH MOUNTAINS INTO THE TETONS AND ABSAROKA RANGES WITH
MODERATE PROBABILITIES OF 4 INCHES.

BEYOND SATURDAY NIGHT...COLD AIR WILL CONTINUE TO FILTER INTO THE
WESTERN STATES WITH A DIURNAL SWING IN SNOW LEVELS ACROSS
CALIFORNIA AND NEVADA...BUT THE FOCUS WILL BE AHEAD OF THE 700-500
MB LOW WHICH SHOULD IMPACT NWRN WYOMING INTO THE COLORADO ROCKIES
WITH THE GREATEST SNOWFALL. SNOW LEVELS ACROSS COLORADO SHOULD BE
ABOVE 8000 FT IN GENERAL...BUT STRONGLY DIFFLUENT FLOW WILL
ACCOMPANY THE TRACK OF THE UPPER LOW ALLOWING FOR THE GREATEST
PROBABILITIES OF A FOOT INTO THE SAN JUANS AND WIND RIVER RANGES.
WHILE THERE IS SOME QUESTION ON THE SPEED OF HEIGHT FALLS MOVING
INTO THE CNTRL PLAINS...IT APPEARS THE NAM/GFS TRACK WILL ALLOW
SUFFICIENTLY COLD AIR FOR ACCUMULATING SNOW INTO THE LARAMIE RANGE
OF WYOMING BY 00Z/27.

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.


OTTO

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