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Winter Weather Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 2004Z Dec 18, 2014)
 
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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
304 PM EST THU DEC 18 2014

VALID 00Z FRI DEC 19 2014 - 00Z MON DEC 22 2014


DAYS 1 THROUGH 3...


...WEST COAST/NORTHERN AND CENTRAL ROCKIES...

A SERIES OF SHORT WAVES IN A VERY FAST MID LEVEL FLOW AFFECT THE
WEST COAST FROM WA INTO NORTHERN CA...AS WELL AS THE NORTHERN AND
CENTRAL ROCKIES...DURING THE NEXT THREE DAYS. THE 00Z ECMWF HAS
CONTINUED THE TREND OF THAT MODEL TO HANDLE THE SHORT WAVE TIMING
BETTER THAN OTHERS...SO THE THERMAL FIELDS ARE BASED ON A MULTI
MODEL BLEND (WHICH INCLUDES THE 00Z ECMWF). THE QPF PORTION OF THE
FORECAST WAS BASED ON THE MOST RECENT WPC QPF.


DAY 1...

TWO SHORT WAVES AFFECT THE WEST COAST AND NORTHERN ROCKIES ON DAY
1. THE NORTHERN SHORT WAVE (WHICH IS THE STRONGER OF THE TWO)
CROSSES THE WA/OR COASTS NEAR 19/12Z...THEN TRACKS INTO THE
NORTHERN ROCKIES TOWARD THE END OF THE PERIOD. THE DEEPEST
MOISTURE AHEAD OF THE SHORT WAVE OCCURS DURING THE FIRST HALF OF
THE PERIOD...AND THE LOW LEVEL FLOW FOCUSES THE MOISTURE ON THE
HIGHER TERRAIN OF THE CASCADES OF WA AND OR...AS WELL AS THE BLUE
MOUNTAINS. SNOW LEVELS HOVER BETWEEN 5000 AND 6000 FEET...WITH THE
HIGHEST VALUES ACROSS THE SOUTHERN OR CASCADES. SNOWFALL AMOUNTS
OF 6 TO 10 INCHES WERE PLACED ALONG THE CASCADES OF NORTHERN WA
AND SOUTHERN OR...WITH 4 TO 8 INCHES EXPECTED OVER THE BLUE
MOUNTAINS.

FURTHER EAST...THE SHORT WAVE IMPACTS THE GRAND TETONS IN WY
DURING THE LAST PORTION OF DAY 1. THE MOISTURE IS NOT AS ROBUST AS
IT WILL BE ACROSS WA/OR...AS SNOW LEVELS REMAINS NEAR 6000 FEET.
THE COMBINATION OF SYNOPTIC SCALE LIFT AND AVAILABLE MOISTURE IN
THE COLUMN SHOULD PRODUCE 3 TO 6 INCHES ACROSS THE GRAND TETONS.

THE SOUTHERN SHORT WAVE CROSSES NORTHERN CA EARLY ON DAY 1...WHICH
WEAKENS AS IT TRACKS ACROSS THE GREAT BASIN. SNOW LEVELS RISE TO
BETWEEN 6000 AND 7000 FEET IN THE DIFFERENTIAL WARM AIR ADVECTION
AHEAD OF THE SHORT WAVE ACROSS NORTHERN CA...RESULTING IN AREAS OF
3 TO 6 INCHES OF SNOWFALL ACROSS THE NORTHERN SIERRA NEVADA RANGE.
FURTHER EAST...MOISTURE BECOMES THE LIMITING FACTOR FOR SNOWFALL
ACROSS NORTHERN NV...BUT ENOUGH MOISTURE REMAINS IN THE COLUMN FOR
A SMALL AREA OF 2 TO INCHES OF SNOWFALL OVER THE INDEPENDENCE
MOUNTAINS.


DAY 2...

THE NEXT STRONG SHORT WAVE IN THE NORTHERN STREAM AFFECTS THE
PACIFIC NORTHWEST AND NORTHERN ROCKIES LATE ON DAY 2. AHEAD OF THE
SHORT WAVE... DEEP PACIFIC MOISTURE IS USHERED ONSHORE BY A LOW
LEVEL 50 KNOT FLOW. THE LOW LEVEL FLOW FOCUSES THE MOISTURE ON THE
CASCADES ON WA AND OR...WITH THE GREATEST LIFT/MOISTURE EXPECTED
OVER THE WA CASCADES. WARM AIR ADVECTION SHOULD PUSH SNOW LEVELS
UP TO BETWEEN 6000 AND 8000 FEET ACROSS THE CASCADES...WITH THE
HIGHEST VALUES ACROSS THE CASCADES IN SOUTHERN OR.

GIVEN THAT THE BEST COMBINATION OF MOISTURE/LIFT AFFECTS THE
NORTHERN CASCADES OF WA...A SWATH OF 6 TO 12 INCHES OF SNOWFALL IS
EXPECTED. THESE AMOUNTS ARE WELL SUPPORTED BY THE ECMWF ENSEMBLE
OUTPUT...WHICH HAS SEVERAL MEMBERS SHOWING 8+ INCHES OF SNOWFALL
HERE. FURTHER SOUTH...DUE MAINLY TO THE HIGHER SNOW LEVELS...AN
AXIS OF 3 TO 6 INCHES OF SNOWFALL WAS PLACED ACROSS THE CASCADES
OF SOUTHERN OR.


DAY 3...

THE STRONG SHORT WAVE ACROSS WA INTO NORTHERN ID AND MT DURING DAY
3. THE BEST MOISTURE SLUG AFFECTS THE CASCADES IN WA EARLY. SNOW
LEVELS REMAINS IN THE 6000 TO 7000 FOOT RANGE...AND THE
COMBINATION OF LIFT AND MOISTURE SUPPORTS AN ADDITIONAL 6 TO 12
INCHES OF SNOWFALL IN THIS LOCATION. THE MOISTURE SURGE APPEARS TO
SURVIVE THE TRIP ACROSS THE WA CASCADES...IMPACTING THE BLUE
MOUNTAINS IN OR...AS WELL AS THE BITTERROOTS OF ID. THE STRONG LOW
LEVEL FLOW SUPPORTS UPSLOPE ACROSS THESE RANGES...WHERE THE SNOW
LEVELS RISE IN THE WARM AIR ADVECTION TO BETWEEN 8000 AND 9000
FEET. THERE IS MORE THAN SUFFICIENT MOISTURE AND SYNOPTIC SCALE
ASCENT WITH THE SHORT WAVE TO PRODUCE A LARGE AREA OF 8 TO 14
INCHES OF SNOW ACROSS THESE RANGES...WITH THE HIGHEST AMOUNTS OF
THE BITTERROOTS. THESE AMOUNTS ARE SUPPORTS BY SEVERAL ECMWF
ENSEMBLE MEMBERS SHOWING 12+ INCHES OF SNOWFALL ACROSS BOTH THE
NORTHERN CASCADES AND BITTERROOTS.

FURTHER EAST...THE DEEPER MOISTURE SOURCE BECOMES LIMITED...BUT
UPSLOPE AND STRONG SYNOPTIC LIFT WITH THE SHORT WAVE SHOULD MAKE
UP FOR THE RELATIVE LACK OF MOISTURE. THE BEST UPSLOPE OCCURS OVER
THE GRAND TETONS...WHERE THE QPF SUPPORTS A LARGE AREA OF 6 TO 12
INCHES OF SNOWFALL. THE MOISTURE IS EVEN SCANTIER ACROSS THE GRAND
TETONS...BUT THE BEST LIFT WITH THE SHORT WAVE WILL SQUEEZE OUT
THE MOISTURE IN THE COLUMN TO PRODUCE 3 TO 6 INCHES OF SNOWFALL.


HAYES