238 
FXUS66 KOTX 212256
AFDOTX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SPOKANE WA
256 PM PST SAT NOV 21 2009

.SYNOPSIS...
A STRONGER AND COLDER STORM SYSTEM WILL BRING THE POTENTIAL FOR
MODERATE TO HEAVY SNOWFALL OVER MUCH OF THE REGION TONIGHT AND
SUNDAY. A SERIES OF STORM SYSTEMS WILL CONTINUE TO PASS THROUGH
THE INLAND NORTHWEST EVERY 24 TO 36 HOURS THROUGH AT LEAST THE
MIDDLE OF NEXT WEEK.


&&

.DISCUSSION...

...LIGHT TO MODERATE SNOWFALL EXPECTED TONIGHT THROUGH SUNDAY 
ACROSS THE INLAND NW...

TONIGHT THROUGH SUNDAY...AFTERNOON WATER VAPOR SHOWING A STRONG
PACIFIC STORM CHARGING TOWARD THE WASHINGTON COAST WITH A CLASSIC
BAROCLINIC LEAF SIGNATURE INDICATIVE OF THE ORGANIZED DEEPENING
SURFACE CYCLONE AND ASSOCIATED CONVEYOR BELTS. MODELS HAVE COME INTO
BETTER AGREEMENT REGARDING THE INCOMING STORM SYSTEM WHICH NOW
TRACK THE SURFACE LOW OVER THE KITTITAS VALLEY...ALONG THE I-90
CORRIDOR...BEFORE TRACKING TO THE NORTHEAST ACROSS THE NORTHERN
IDAHO PANHANDLE INTO SOUTHEASTERN BRITISH COLUMBIA. THE LOW WILL
BOTTOM OUT THIS EVENING WHILE CROSSING ONTO THE WASHINGTON COAST
JUST BELOW 990MB...THEN GRADUALLY FILL AND WEAKEN OVERNIGHT INTO
TOMORROW. THE HEAVIEST PRECIPITATION OVERNIGHT WILL FOCUS ALONG
THE CASCADES...WATERVILLE PLATEAU...OKANOGAN REGION...AS THE LOW
NEARS THE COAST TONIGHT AND INCREASING EASTERLY FLOW ENHANCES LIFT
UNDER A BLANKET OF DEEP ISENTROPIC ASCENT. AS THE LOW JUMPS THE
CASCADES AFT 12Z SUNDAY...THE PRECIPITATION SHIELD WILL BEGIN TO
SHIFT TOWARD FAR EASTERN WASHINGTON AND IDAHO. RADAR IMAGERY WILL 
LIKELY ILLUSTRATE A WELL DEFINED DEFORMATION BAND ALONG THE
NORTHERN PERIPHERY OF THE LOW WHICH IS WHERE THE 6 HOUR LIQUID
AMTS WILL NEAR THREE-TENTHS OF AN INCH THEN AS THE LOW EJECTS TO
NORTHEAST MIDDAY TOMORROW...CONTINUED STRONG UPSLOPE INTO THE ID
PANHANDLE WILL RESULT IN THE SECOND CHANCE FOR MODERATE
PRECIPITATION RATES...MAINLY FOCUSING INTO THE PANHANDLE
MTNS...IDAHO PALOUSE...AND COEUR D ALENE AREA. SNOWFALL TOTALS
WILL GENERALLY RUN 2-5 INCHES IN THE VALLEYS WITH 6-10" IN THE
MOUNTAINS. WITH THE HIGHEST TOTALS ACROSS THE CENTRAL ID PANHANDLE
AS THE SECOND WAVE OF PRECIPITATION FORMS IN THE WAKE OF THE LOW.
THIS IS WHERE WINTER WEATHER WARNINGS WERE POSTED AND VALLEYS CAN
EXPECT 3-6" WITH 8-10" IN THE MOUNTAINS.

CURRENT OBSERVATIONS FROM THE EAST SLOPES...OKANOGAN HIGHLANDS...AND 
WENATCHEE AREA SUGGEST TEMPERATURES HAVE WARMED INTO THE MID TO
UPPER 40'S BELOW 2000' WITH DEWPOINTS IN THE UPPER 20'S TO LOWER
30'S. CALCULATED WETBULB COOLING SUGGEST TEMPERATURES WILL REMAIN
IN THE MID TO UPPER 30'S WITH THE ONSET OF PRECIPITATION SO ALL
WINTER HIGHLIGHTS IN THE AFOREMENTIONED ZONES WERE FOCUSED MAINLY
ABOVE 2000'.

WINDS WILL BE BREEZY THROUGH THE PERIOD WITH THE HIGHEST GUSTS 
EXPECTED SUNDAY AFTERNOON UNDER THE COLD ADVECTION REGIME. MODELS
CONTINUE TO HINT 850MB WINDS OF 35-50KTS DOWN THE EAST SLOPES AND
THIS WILL NEED TO BE MONITORED CLOSELY. THESE BREEZY WINDS WILL
ALSO CAUSE AREAS OF BLOWING SNOW AT TIMES BUT WITH THE SNOW BEING
OF A WETTER NATURE...EXPECT THIS WILL BE LOCALIZED. /SB

SUNDAY NIGHT THROUGH TUESDAY NIGHT...MODEL GUIDANCE IS IN GOOD 
AGREEMENT THAT THE PERIOD WILL BEGIN ON A QUIETING NOTE AS
SUNDAYS UPPER LEVEL TROF SHIFTS INTO CENTRAL MONTANA. THE TROF
WILL BE SLOWLY REPLACED BY A WELL-DEFINED UPPER LEVEL RIDGE
RESULTING IN RAPID STABILIZATION OVER THE ENTIRE REGION. THUS THE
POST-FRONTAL SHOWERS OVER THE IDAHO PANHANDLE DUE TO MOIST UPSLOPE
SW FLOW WILL DECREASE RAPIDLY. MODEL CROSS SECTIONS SHOW THE
POTENTIAL INSTABILITY DECREASING FROM AROUND 550 MBS AT 00Z TO
LESS THAN 700 MBS BY 12Z. MEANWHILE RELATIVE HUMIDITY VALUES
DECREASE RAPIDLY ABOVE 700 MBS AS WELL. THIS SUGGESTS THAT MOST OF
THE POST-FRONTAL SHOWER ACTIVITY OVER THE CENTRAL PANHANDLE WILL
DECREASE AFTER 06Z WITH ONLY A FEW PERSISTENT SHOWERS .

GENERALLY DRY WEATHER WILL THEN PERSIST THROUGH MOST OF MONDAY UNTIL 
ANOTHER FRONT PUSHES INTO WESTERN WASHINGTON LATE IN THE DAY.
MOISTURE ASSOCIATED WITH THIS DISTURBANCE WAS CURRENTLY LOCATED
JUST WEST OF 150W AND WAS ADEQUATELY HANDLED BY BOTH THE GFS AND
ECMWF AS OF 22Z. BOTH MODEL SOLUTIONS BRING THE PRE-FRONTAL UPPER
LEVEL RIDGE INTO NORTH IDAHO BY 00Z TUE WITH A SWIFT MOVING
SHORTWAVE TROF PASSING THROUGH THE SOUTHERN THIRD OF BC OVERNIGHT.
THIS SYSTEM SHOULD BE MUCH LESS DYNAMIC AND IMPRESSIVE THAN THE
ONE EXPECTED TONIGHT AND SUNDAY. IN FACT...THE MODELS HAVE BEEN
TRENDING TOWARD KEEPING THE PRECIPITATION FAIRLY LIGHT WITH MOST
OF IT HOLDING OVER THE NORTHERN THIRD OF THE CWA. GIVEN THE TRACK
OF THE UPPER LEVEL TROF...THE PRIMARY FLOW IN THE 850-700 MB LAYER
WILL REMAIN OUT OF THE WEST SO WITHOUT THE BENEFIT OF A STRONG
WARM FRONT OR ISENTROPIC ASCENT WE WOULD ALSO EXPECT TO SEE A FAIR
AMOUNT OF DOWNSLOPE FLOW OVER THE CASCADES. WHILE BOTH THE GFS AND
ECMWF DO HINT AT A WEAK WARM AIR ADVECTION PATTERN IN THE 850-700
MB LAYER ITS NOT NECESSARILY ENOUGH TO OVERCOME THE DOWNSLOPE
IMPACTS OF THE CASCADES. WHILE THIS SYSTEM WONT HAVE THE BENEFIT
OF STRONG DYNAMICAL FORCING IT WILL FEATURE A HIGH AMOUNT OF
MOISTURE...AS EVIDENCED BY PRECIPITABLE WATER VALUES AROUND
150-180 PERCENT OF NORMAL. THERE WILL ALSO BE PERSISTENT WEAK
ISENTROPIC ASCENT. WHATS INTERESTING IN THIS SCENARIO IS THE FRONT
LOSES MUCH OF ITS EASTWARD MOMENTUM AS YET ANOTHER SYSTEM PUSHES
INTO THE COAST BY LATE TUESDAY. THIS NEXT SYSTEM AMPLIFIES YET
ANOTHER SHORTWAVE RIDGE OVER THE AREA...WHICH ALLOWS THE MONDAY
FRONT TO SIT AND GRADUALLY WEAKEN OVER THE REGION AND SHIFT SLOWLY
SOUTH THROUGH TUESDAY . GIVEN THIS SCENARIO WE WOULD EXPECT TO SEE
A LIGHT AND RATHER PROLONGED STRATIFORM PRECIPITATION EVENT MUCH
OF WHICH WILL FALL AS SNOW. GIVEN THE WEAK FORCING WE PREFER THE
DRIER GFS AND ECMWF SOLUTIONS OVER THE RATHER WET NAM. AT THIS
POINT IT APPEARS WINTER HIGHLIGHTS WILL NOT BE NECESSARY. MOST OF
THE PRECIPITATION IS EXPECTED TO FALL NORTH OF I90. FX

WEDNESDAY TO SATURDAY...THE MEDIUM-RANGE STARTS WITH A RIDGE OVER
THE WEST...A WARM FRONT NR THE BC BORDER AND A COLD FRONT NR 130W. 
THIS LEAVES PASSING MID/HIGHER CLOUDS AND A CHANCE OF RAIN AND SNOW
ACROSS THE CASCADES AND WA/BC BORDER MTNS INTO WED NT...WITH SOME
EARLY FOG/STRATUS THREAT. THEREAFTER MODELS DIVERGE OVER COLD FRONT
PASSAGE. THE FCST LEANS TO THE GFS...WITH BETTER CONSISTENCY AND 
GEM/DGEX AGREEMENT. THE ECMWF TRENDED TO GFS. ALL THIS TRANSLATES
TO THE COLD FRONT PUSHING TO THE CASCADES THU AM AND EAST THU NT
INTO FRI AM...INCREASING PRECIP CHANCES. A THREAT FOR OROGRAPHIC
RAIN/SNOW SHOWERS CONTINUES FRIDAY BEFORE THE NEXT SYSTEM APPROACHES
LATER SATURDAY. SLIGHTLY ABOVE NORMAL TEMPERATURES WEDNESDAY TREND
CLOSER TO NORMAL BY SATURDAY. /JCOTE


&&

.AVIATION...
A WEAK RIDGE ALOFT WILL PROMOTE VFR CONDITIONS AT ALL TAF SITES
THROUGH 03Z TONIGHT...WITH LINGERING SNOW SHOWERS LOCALLY
OBSCURING MOUNTAINS IN THE IDAHO PANHANDLE AND CASCADE CREST. 
THE NEXT PACIFIC STORM SYSTEM WILL SPREAD IFR CEILINGS TO THE
WESTERN REACHES OF THE REGION AFTER 04Z...SPREADING INTO THE
EASTERN TERMINALS AFT 10Z. THE INCOMING STORM SYSTEM WILL 
ALSO TIGHTEN PRESSURE GRADIENTS LEADING TO INCREASING WINDS 
FROM KPUW-KGEG WITH GUSTS OF 25-35KTS.




&&

.PRELIMINARY POINT TEMPS/POPS...
SPOKANE        31  37  25  38  30  40 /  80  90  10  20  40  20 
COEUR D'ALENE  30  38  27  39  30  40 /  50  90  20  20  50  20 
PULLMAN        31  37  27  40  29  44 /  70  90  10  10  30  20 
LEWISTON       35  42  31  47  32  49 /  40  90  20  10  20  20 
COLVILLE       32  40  27  39  29  40 /  80 100  10  50  60  10 
SANDPOINT      30  37  27  34  27  36 /  30  90  20  50  60  40 
KELLOGG        29  33  28  33  29  37 /  30 100  70  20  50  30 
MOSES LAKE     31  44  24  43  26  45 / 100  50   0  30  30   0 
WENATCHEE      32  43  29  43  32  45 / 100  50   0  30  40   0 
OMAK           31  43  22  41  29  45 / 100  80   0  70  80   0 

&&

.OTX WATCHES/WARNINGS/ADVISORIES...
ID...WINTER WEATHER ADVISORY FROM 4 AM TO 10 PM PST SUNDAY FOR 
     NORTHERN PANHANDLE.

     WINTER STORM WARNING FROM 4 AM SUNDAY TO 1 AM PST MONDAY FOR 
     CENTRAL PANHANDLE MOUNTAINS.

     WINTER STORM WARNING FROM 4 AM TO 10 PM PST SUNDAY FOR COEUR 
     D'ALENE AREA...IDAHO PALOUSE.

WA...WINTER WEATHER ADVISORY FROM 10 PM THIS EVENING TO NOON PST 
     SUNDAY FOR OKANOGAN HIGHLANDS.

     WINTER WEATHER ADVISORY FROM 4 AM TO 10 PM PST SUNDAY FOR 
     NORTHEAST BLUE MOUNTAINS.

     WINTER WEATHER ADVISORY FROM 4 AM TO 4 PM PST SUNDAY FOR 
     NORTHEAST MOUNTAINS...SPOKANE AREA...WASHINGTON PALOUSE.

     WINTER WEATHER ADVISORY FROM 10 PM THIS EVENING TO NOON PST 
     SUNDAY FOR UPPER COLUMBIA BASIN.

     WINTER WEATHER ADVISORY FROM 10 PM THIS EVENING TO 10 AM PST 
     SUNDAY FOR OKANOGAN VALLEY...WATERVILLE PLATEAU...WENATCHEE 
     AREA.

     WINTER WEATHER ADVISORY FROM 10 PM THIS EVENING TO 10 AM PST 
     SUNDAY FOR EAST SLOPES NORTHERN CASCADES.

&&

$$

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