[MCD] SWOMCD

[MCD at goshenarc.org]

ACUS11 KWNS 040516
SWOMCD
SPC MCD 040516 
WAZ000-041115-

MESOSCALE DISCUSSION 2497
NWS STORM PREDICTION CENTER NORMAN OK
1116 PM CST FRI DEC 03 2004

AREAS AFFECTED...NRN WA

CONCERNING...WINTER MIXED PRECIPITATION 

VALID 040516Z - 041115Z

HVY SNOW WITH 1-1.5 INCH PER HOUR RATES WILL DEVELOP FROM NORTH TO
SOUTH ACROSS THE NRN WA CASCADES FROM 07-12Z. DURATION OF HVY SNOW
SHOULD BE FROM 3-4 HOURS AS FORCING ONLY GRADUALLY SHIFTS SWD.
FARTHER EAST...MDT SNOW WITH HOURLY RATES AROUND 1/2 INCH WILL BE
POSSIBLE ACROSS THOSE AREAS OF NERN WA ABOVE 2000 FT BEGINNING
BETWEEN 09-12Z. LIGHT FREEZING RAIN WILL ALSO BE POSSIBLE IN THE
COLUMBIA BASIN WHERE SFC SUB-FREEZING TEMPERATURES WILL EXIST BELOW
ELEVATED WARM LAYER.

RECENT WV IMAGERY SHOWS A COMPACT UPPER TROUGH CENTERED NEAR 50 N
142 W MOVING SEWD AROUND 40 KTS. STRENGTHENING UPPER LEVEL
DIVERGENCE NOTED BY COOLING CLOUD TOPS ON IR IMAGERY AHEAD OF
ASSOCIATED TROUGH SHOULD BEGIN TO APPROACH THE COAST OF WA BETWEEN
06-08Z. AS THIS MOVES INLAND...INCREASING LOW-MID LEVEL SWLY FLOW
AHEAD OF AN ASSOCIATED COLD FRONT WILL SUPPORT MOISTENING LOW LEVELS
AND STRONG OROGRAPHIC ASCENT INTO THE NRN CASCADES MTNS STARTING
AFTER 08Z. DIGGING NATURE OF THE UPPER TROUGH SUGGESTS FRONTOGENETIC
FORCING WILL LIKELY ALSO CONTRIBUTE TO STRONG VERTICAL MOTION IN THE
MID LEVELS FAVORING A DEEP LAYER OF VERTICAL MOTION WHICH WILL BE
SUPPORTIVE OF HVY SNOWFALL RATES THROUGH 12Z. DESPITE INCREASING
WARM ADVECTION...SNOW LEVELS SHOULD REMAIN AROUND 4000 FT.

INCREASING ASCENT FURTHER EAST WILL AID IN LOW-MID LEVEL SATURATION
AND DEVELOPMENT OF LIGHT-MDT PRECIPITATION BETWEEN 09-12Z DOWNWIND
OF THE CASCADES ACROSS NERN WA. ELEVATED LOW LEVEL WARM/MOIST
ADVECTION ZONE WILL CONTINUE TO DEVELOP INTO THE REGION AHEAD OF
SEWD MOVING COLD FRONT. ATOP SHALLOW SUB-FREEZING SFC LAYER...THIS
WILL SUPPORT A PTYPE OF FREEZING RAIN OVER THE COLUMBIA RIVER BASIN
REGION. HOWEVER THE STRENGTH OF THE WLY WINDS IN THIS 850-700 LAYER
AND LEE SIDE SHADOWING WILL ALLOW FOR ONLY GRADUAL
SATURATION...LIMITING HOURLY AMOUNTS. IN THE MTNS/FOOTHILLS OF NERN
WA...WHERE DEEPER COLD AIR LAYER WILL EXIST SNOW WILL BE THE
PREDOMINANT PTYPE. LESS LEE SIDE SHADOW EFFECTS AND INCREASING
UPSLOPE FLOW WILL SUPPORT MDT SNOW DEVELOPMENT BY 12Z.

..CROSBIE.. 12/04/2004

...PLEASE SEE WWW.SPC.NOAA.GOV FOR GRAPHIC PRODUCT...

...NOTICE...
SPC WINTER WEATHER MESOSCALE DISCUSSIONS WILL BE ISSUED ON A 
SCHEDULED BASIS...FOUR TIMES A DAY...BETWEEN 05-06 UTC...
11-12 UTC...17-18 UTC...AND 23-00 UTC AS SIGNIFICANT SHORT
RANGE WINTER WEATHER EVENTS UNFOLD.  THIS SCHEDULE WILL BE
USED FROM DECEMBER 1 THROUGH FEBRUARY 28 TO ALLOW PARTNERS
AND USERS TO BETTER ANTICIPATE THE TIMING OF SPC WINTER WEATHER
MESOSCALE DISCUSSIONS.

ATTN...WFO...OTX...PDT...SEW...

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