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Winter Weather Forecast Discussion
 
(Latest Discussion - Issued 0755Z Oct 24, 2014)
 
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Abbreviations and acronyms used in this product
 
Geographic Boundaries -  Map 1: Color  Black/White       Map 2: Color  Black/White


PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
355 AM EDT FRI OCT 24 2014

VALID 12Z FRI OCT 24 2014 - 12Z MON OCT 27 2014


DAYS 1 THROUGH 3...


...PACIFIC NORTHWEST/NORTHERN ROCKIES...

MOISTURE AND LIFT ASSOCIATED WITH A STRONG SHORT WAVE OFF THE CA
THIS MORNING WILL PRODUCE QPF AND SNOWFALL ACROSS THE PACIFIC
NORTHWEST ON DAY 1...AND ACROSS THE PACIFIC NORTHWEST AND NORTHERN
ROCKIES ON DAYS 2 AND 3. FOR THE MOST PART...THE QPF PORTION OF
THE FORECAST WAS BASED ON A BLEND OF THE 00Z GFS/12Z ECMWF AND THE
LATEST WPC QPF. THE THERMAL PORTION OF THE FORECAST WAS BASED ON A
BLEND OF THE 00Z NAM/21Z SREF MEAN AND 12Z ECMWF...AS THE 00Z GFS
SEEMED TOO WARM WITH THE SYSTEM ACROSS THE BOARD.

ON DAY 1...THE BROAD WARM ADVECTION PATTERN AHEAD OF THE SHORT
WAVE OFF THE NORTHERN CA AND PACIFIC NORTHWEST COAST WILL SEND
SNOW LEVELS HIGHER DURING THE PERIOD. SNOW LEVELS RISE FROM 6000
FEET TO ABOVE 8000 FEET ACROSS THE NORTHERN CASCADES...MAINLY
DURING THE SECOND HALF OF THE PERIOD. IN THE WARM AIR ADVECTION
PATTERN...THE BEST LIFT AND MOISTURE OCCURS DURING THE 25/00Z TO
25/12Z TIME FRAME...SO SNOWFALL WAS CAPPED AT 1 TO 3 INCHES ACROSS
THE NORTHERN CASCADES OF WA.

DURING DAY 2...FREEZING LEVELS HOVER NEAR 8000 FEET ACROSS THE
PACIFIC NORTHWEST AND NORTHERN ROCKIES IN THE BROAD WARM AIR
ADVECTION PATTERN. THE BEST MOISTURE AND LIFT CROSSES WA/OR DURING
THE FIRST HALF OF THE PERIOD...AS THE STRONG SHORT WAVE HEADS FOR
THE NORTHERN ROCKIES. THE LOW TO MID LEVEL FLOW SHOULD FOCUS THE
THE MOISTURE ON THE TERRAIN OF WA/OR...AS WELL AS THE BITTERROOTS
OF ID AND THE GRAND TETONS OF WY. THE QPF SUPPORTS A SWATH OF 3 TO
6 INCHES OF SNOWFALL ACROSS THE NORTHERN CASCADES OF WA. THESE
SNOWFALL AMOUNTS ARE SUPPORTED BY ABOUT 25 PERCENT OF THE LATEST
ECMWF ENSEMBLE MEMBERS SHOWING 4+ INCHES OF SNOWFALL. IN
FACT...THERE ARE A FEW MEMBERS OF THE ECMWF ENSEMBLE OUTPUT THAT
SUPPORT 8+ INCHES OF SNOW...BUT THE RELATIVELY HIGH SNOW LEVELS
MAKE THIS A LOW PROBABILITY EVENT. LESSER SNOWFALL AMOUNTS (1 TO 3
INCHES) ARE EXPECTED ACROSS THE HIGHER TERRAIN OF ID AND NORTHWEST
WY.

BY DAY 3...THE SHORT WAVE ELONGATES INTO A LONG WAVE TROUGH THAT
CROSSES THE ROCKIES DURING THE PERIOD. SNOW LEVELS DROP TO BETWEEN
6000 AND 8000 FEET ACROSS THE REGION IN THE BROAD COLD AIR
ADVECTION PATTERN. THE MOISTURE SUPPLY BECOMES THE LIMITING FACTOR
FOR QPF ACROSS THE PACIFIC NORTHWEST AND ROCKIES DURING THIS
TIME...BUT UPSLOPE FLOW SHOULD MAKE THE MOST OF THE AVAILABLE
MOISTURE. THE DROPPING SNOW LEVELS AND QPF SUPPORTS 4 TO 6 INCHES
ACROSS THE NORTHERN CASCADES IN WA...AS WELL AS THE BITTERROOTS OF
ID. THE UPSLOPE FLOW IS MOST PROFOUND ACROSS THE GRAND TETONS OF
NORTHWEST WY...WHERE THE QPF SUPPORTS AN AREA OF 4 TO 8 INCHES OF
SNOWFALL. THERE IS SOME SUPPORT FOR 8+ INCHES OF SNOW ACROSS THE
GRAND TETONS...AS WELL AS THE NORTHERN BITTERROOTS IN ID FROM THE
LATEST ECMWF ENSEMBLE OUTPUT. HOWEVER...LACK OF DEEPER MOISTURE
AND THE SPEED OF MOVEMENT OF THE LONG WAVE TROUGH SHOULD RESULT IN
A LOW PROBABILITY OF 8+ INCHES OF SNOWFALL IN THESE AREAS.

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.


HAYES