FXUS66 KOTX 112343 AAA
AFDOTX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SPOKANE WA
343 PM PST FRI DEC 11 2009
.SYNOPSIS...
COOL DRY WEATHER TODAY WILL QUICKLY CHANGE ON SATURDAY TO A COOL
WET PERIOD. THE WILL GRADUALLY BE REPLACED BY A MILDER AND MORE
MOISTURE LADEN AIR MASS. THIS TRANSITION WILL ALLOW A WARM FRONT
TO SURGE TOWARD THE WASHINGTON-OREGON BORDER ON SATURDAY AND
REMAIN OVER THE AREA THROUGH THE WEEKEND. THIS FRONT WILL RESULT
IN MODERATE TO LOCALLY HEAVY SNOW EXTENDING FROM THE BLUE
MOUNTAINS TO THE CENTRAL IDAHO PANHANDLE. A MUCH WETTER FRONT IS
EXPECTED TO CROSS THE AREA ON MONDAY NIGHT INTO WEDNESDAY AND
RESULT IN WIDESPREAD SNOW. TEMPERATURES WILL THEN RISE ABOVE
FREEZING FOR MOST VALLEYS ON WEDNESDAY AND REMAIN THERE THROUGH
THE END OF NEXT WEEK.
&&
.DISCUSSION...
TONIGHT AND SATURDAY...NORTHWEST FLOW WILL CONTINUE TO DOMINATE
THE WEATHER TONIGHT. A SHORT WAVE DISTURBANCE IS MOVING SOUTH
THROUGH BRITISH COLUMBIA AND WILL BECOME A FACTOR IN THE WEATHER
FOR SATURDAY THROUGH THE SHORT RANGE PERIOD. MEANWHILE THE MOIST
WESTERLY JET THAT HAS BEEN OVER NORTHERN CALIFORNIA WILL BEGIN TO
MOVE NORTH AND CARRY ALONG A PAIR OF UPPER LEVEL DISTURBANCES THAT
WILL ALSO AFFECT THE WEATHER SATURDAY THROUGH THE WEEKEND.
FOR TONIGHT...MOISTURE MOVING SOUTH FROM THE NORTHERN COLD FRONT
WILL INCREASE ALONG THE NORTHERN ZONES OVER NIGHT AS THE SHORT
WAVE CONTINUES TO DROP SOUTH. ALSO MOISTURE FROM THE SOUTHERN
STREAM DISTURBANCE WILL BEGIN TO INCREASE THE CLOUDS FROM THE
SOUTH. MOSTLY CLEAR SKIES THIS EVENING WILL GET SQUEEZED FROM BOTH
THE NORTH AND THE SOUTH. THERE WILL BE ENOUGH UP-SLOPING
FLOW TO SUPPORT A FEW LIGHT SNOW FLURRIES ALONG THE CANADIAN
BORDER TOWARDS MORNING AS WELL. TEMPERATURES WILL BE A LITTLE
TRICKY WITH INCREASING CLOUD COVER...BUT AGAIN IT APPEARS AS IF
THE COOLEST OVERNIGHT LOWS WILL BE IN THE BASIN BEFORE THE CLOUDS
INCREASE EARLY SATURDAY MORNING.
FOR SATURDAY...THE FORECAST AREA GETS CAUGHT IN A SQUEEZE BETWEEN
THE NORTHERN COLD FRONT AND THE SOUTHERN WARM FRONT. LOOKING TO
THE NORTH THE MODELS HAVE BEEN VERY CONSISTENT SHOWING THE MID
LEVEL FRONT MOVING TO THE CANADIAN BORDER BETWEEN 18-00Z
SATURDAY. PWAT'S INCREASE TO NEAR 85% OF NORMAL BY SATURDAY
AFTERNOON. AS WE HAVE SEEN OVER THE PAST WEEK OR SO WITH THIS COOL
OF AN AIR MASS IN PLACE IT WILL NOT TAKE MUCH MOISTURE TO SQUEEZE
OUT AN INCH OR TWO OF SNOW ACROSS THE NORTHERN MOUNTAIN ZONES.
TO OUR SOUTH THE MODELS HAVE BEEN BOUNCING AROUND WITH HOW
FAR NORTH TO BRING THE WARM FRONT. WITH THE EXCEPTION OF THE ECMWF
THE 12Z AND NOW THE 18Z MODEL RUNS BRING THE WARM FRONT NORTH TO
THE LOWER BASIN EAST TO ABOUT THE SOUTHERN BORDER OF SHOSHONE
COUNTY BY 18Z AND THEN STALLS IT THROUGH 12Z SUNDAY MORNING. WARM
OVER-RUNNING PRECIPITATION IS EXPECTED TO BEGIN RIGHT AROUND
SUNRISE ACROSS THE BLUES AND CAMAS PRAIRIE AND SLOWLY MOVE NORTH
INTO THE EASTERN BASIN AND MAINLY THE PALOUSE. PRECIPITATION TO
THE NORTH AND TO THE SOUTH KEEPS A DRY SLOT EITHER SIDE OF A LINE
FROM ABOUT WENATCHEE TO COUER D'ALENE. IF EITHER FRONT MOVES 25
MILES MORE/LESS THAN EXPECTED IT CAN AND WILL RESULT IN A MUCH
DIFFERENT FORECAST. THESE ARE FORECASTS THAT ARE VERY EASY TO
BUST. VERTICAL PROFILES WILL KEEP PRECIPITATION AS ALL SNOW. SNOW
RATIOS WERE RIGHT AROUND 17 TO 1 ACROSS THE NORTHERN ZONES AND 15
TO 1 ACROSS THE SOUTHERN ZONES. THIS WILL JUST BE THE BEGINNING OF
THE SNOW AS THIS STORM SYSTEM IS EXPECTED TO LAST FOR 24-36
HOURS...WITH MORE WINTER WEATHER EXPECTED TUESDAY. HIGHLIGHTS HAVE
BEEN HOISTED TO COVER THIS EXTENDED PERIOD OF WINTER WEATHER. /TOBIN
SATURDAY NIGHT THROUGH SUNDAY NIGHT...AN EXTENDED PERIOD OF LIGHT TO
MODERATE SNOW WILL AFFECT THE SOUTHEAST PORTION OF THE FORECAST
AREA OVER THE WEEKEND AS A WARM FRONT STALLS OVER THE SOUTHEAST
ZONES. HEAVY SNOWFALL WILL ACCUMULATE OVER PARTS OF SOUTHEAST
WASHINGTON AND THE SOUTHERN HALF OF THE IDAHO PANHANDLE. GENERALLY
4 TO 6 INCHES WILL ACCUMULATE IN THE VALLEYS WITH 8 TO 14 INCHES
IN THE MOUNTAINS. LIGHTER AMOUNTS WILL AFFECT THE NORTHERN
PANHANDLE. THE WINTER WEATHER WATCH HAS BEEN UPGRADED TO A WARNING
FOR HEAVY SNOW AND THE NORTHERN PANHANDLE HAS BEEN ADDED AS A SNOW
ADVISORY. THERE IS MORE UNCERTAINTY AS TO REACHING CRITERIA FOR
THE COEUR D'ALENE AREA BUT THERE MAY BE LOCAL AMOUNTS TO 4 INCHES
IN THE EASTERN HALF OF THAT ZONE.
MODELS CONTINUE TO DROP AN ARCTIC FRONT SOUTH ACROSS THE NORTHERN
ZONES...WHICH WILL ACT TO KEEP THE SOUTHERN FRONT STALLED ACROSS
THE SOUTHEAST PORTION OF THE FORECAST AREA. THIS WILL LEAVE A DRY
SLOT IN BETWEEN WITH MUCH LOWER SNOWFALL AMOUNTS FOR THE CENTRAL
PORTION OF THE FORECAST AREA...ESPECIALLY THOSE LOCATIONS IN THE
VICINITY OF INTERSTATE 90. MODELS SHOW A DEFINITE DRY SLOT FROM
THE EAST SLOPES EAST TO SPOKANE COUNTY AND TO SOME EXTENT INTO
KOOTENAI COUNTY IDAHO. NAM/GFS/GEM ARE CONSISTENT IN LIMITING QPF
TO ABOUT A TENTH OF AN INCH TO THE SPOKANE AREA THROUGH SUNDAY
NIGHT AND 15 HUNDREDTHS TO 2 TENTHS IN THE COEUR D'ALENE AREA.
THIS COULD ARGUE FOR A SNOW ADVISORY FOR IDZ02 BUT NOT FOR
SPOKANE. THE 12Z RUN OF THE ECMWF IS NOW THE OUTLIER BRINGING THE
SWATH OF HEAVIER QPF NORTH INTO THE CD'A AREA. FOR NOW WILL LEAVE
IDZ02 OUT OF THE ADVISORY AREA AND WILL LET THE NEXT SHIFT RE-
EVALUATE WITH THE NEXT MODEL RUN.
NORTHERN IDAHO PANHANDLE SNOW AMOUNTS PROVE TO BE A BIT TRICKIER.
WHILE THERE IS PERSISTENT WESTERLY UPSLOPE FLOW ALONG WITH
ISENTROPIC UPGLIDE THROUGH 12Z SUNDAY...FAVORABLE CONDITIONS FOR
MODERATE SNOWFALL...THE LOWER PWATS ARGUE FOR LIGHTER AMOUNTS.
ALTHOUGH THE FETCH APPEARS TO ACQUIRE A BIT OF MARINE INFLUENCE ON
SATURDAY THAT IS LOST BY SATURDAY NIGHT. WE COULD EASILY REACH
ADVISORY CRITERIA DURING THE OVERNIGHT HOURS WITH LIGHTER
ACCUMULATIONS FOR THE SECOND HALF OF THE EVENT.
SNOWFALL GRAPHIC CAN BE SEEN IN THE WEATHER STORY ON OUR WEBPAGE.
THE NORTHERN ZONES WILL GET A SECOND SHOT OF LIGHT SNOW SUNDAY NIGHT
AS AN UPPER LOW SLIDES SOUTHEAST ACROSS BRITISH COLUMBIA AND SENDS A
RIBBON OF SHORTWAVE ENERGY ACROSS THE NORTHERN TIER. ADDITIONAL
ACCUMULATIONS WILL BE LIGHT AS PWATS DROP TO ABOUT 25 PERCENT OF
NORMAL. BY SUNDAY AFTERNOON THE STALLED FRONT STARTS TO MOVE SOUTH
OF THE REGION AND SNOW WILL TAPER OFF ACROSS THE SOUTHEAST ZONES.
TEMPERATURES WILL CONTINUE BELOW SEASONAL NORMALS. /KELCH
MONDAY THROUGH FRIDAY...SIGNIFICANT WINTRY WEATHER IS EXPECTED FOR
THE EARLY TO MIDDLE PART OF NEXT WEEK. THERE IS GENERAL AGREEMENT
IN THE MODELS OF A PROLONGED OVERRUNNING EVENT BEGINNING AS EARLY
AS MONDAY NIGHT AND CONTINUING INTO THE MIDDLE OF THE WEEK.
ON MONDAY ARCTIC AIR WILL BE MOVING ACROSS NORTHERN WA/IDAHO
KEEPING HIGH TEMPS IN THE TEENS TO LOWER 20S WHILE OVER SE
WASHINGTON TEMPS WILL BE A LITTLE WARMER IN THE UPPER 20S TO LOWER
30S. THEN A SERIES OF WARM FRONTS WILL GRADUALLY ERODE THE COLD
AIR. PRECIPITATION WILL BE LIKELY ACROSS THE ENTIRE AREA INCLUDING
THE COLUMBIA BASIN AS ISENTROPIC LIFT WITH EACH WARM FRONT
COUNTERACTS THE TYPICAL RAIN SHADOW EAST OF THE CASCADE CREST.
INITIALLY THE ENTIRE COLUMN WILL BE BELOW FREEZING ACROSS ALL OF
THE AREA BRINGING SNOW MON NGT INTO TUESDAY. THEN A MELTING LAYER
BECOMES ESTABLISHED WED/THU AS 850MB TEMPS RISE ABOVE FREEZING
FROM SOUTH TO NORTH. MEANWHILE WITH SFC PRESSURE GRADIENTS
SOUTHEASTERLY AND NOT VERY STRONG...LOW LEVEL MIXING POTENTIAL
WILL NOT BE GREAT ALLOWING VALLEY TEMPS TO TAKE A LONGER PERIOD TO
WARM ABOVE FREEZING. THE GREATEST SLEET AND FREEZING RAIN THREAT
WILL BE IN THE VALLEYS ALONG THE EAST SLOPES OF THE CASCADES WHERE
COLD AIR TYPICALLY TAKES AWHILE TO SCOUR OUT AS WELL AS NEAR THE
CANADIAN BORDER. THE LEAST THREAT WILL BE OVER THE PALOUSE AND
LEWISTON AREA WHERE THE SOUTHEAST GRADIENT TYPICALLY PROMOTED
BETTER MIXING POTENTIAL. MODEL SOUNDINGS IN THE EXTENDED IN THIS
PATTERN TYPICALLY SCOUR OUT COLD AIR TOO QUICKLY SO PATTERN
RECOGNITION WAS A BIG FACTOR IN THIS FORECAST. JW
&&
.AVIATION...
A COMPLICATED WEATHER PATTERN WILL SET UP OVER THE PACIFIC NORTHWEST
THE NEXT 36-48 HOURS. THE DRY NORTHWEST FLOW WILL RESULT IN MAINLY
VFR CONDITIONS AND LIGHT WINDS FOR ALL TAF SITES THROUGH 12Z. A MOIST
WEATHER SYSTEM...CURRENTLY OFF THE NORTHERN CALIFORNIA COAST WILL
MOVE NORTH INTO SOUTHEAST WASHINGTON OVERNIGHT. THIS WILL RESULT IN
INCREASING CLOUDS TO KLWS AND KPUW AFTER 09Z SAT WITH CIGS MAINLY
ABOVR BKN050. EXPECT VSBY/CIGS FOR KPUW AND KLWS TO DROP TO MVFR
CONDITIONS AFT 14Z WITH -SN BEGINNING AND INCREASING AFT 18Z.
FURTHER NORTH AN ARCTIC FRONT WILL BRUSH THE NORTHERN MOUNTAINS AND
BRING A CHANCE OF SNOW FLURRIES WITH POSSIBLE MOUNTAIN OBSCURATIONS.
OTHERWISE THE REMAINING TAF SITES WILL REMAIN VFR. /TOBIN
&&
.PRELIMINARY POINT TEMPS/POPS...
SPOKANE 12 23 18 26 16 21 / 0 20 60 60 50 30
COEUR D'ALENE 14 24 20 28 18 22 / 0 30 70 70 60 30
PULLMAN 16 28 24 29 23 26 / 0 80 90 90 70 70
LEWISTON 17 32 29 34 28 32 / 10 80 80 80 60 70
COLVILLE 12 24 17 27 15 20 / 10 70 60 60 50 20
SANDPOINT 13 23 16 22 16 20 / 10 80 80 80 70 30
KELLOGG 13 25 21 26 19 23 / 0 60 90 90 80 60
MOSES LAKE 7 25 16 30 17 25 / 0 20 20 10 20 20
WENATCHEE 9 23 18 30 18 25 / 0 10 10 10 20 20
OMAK 7 24 15 28 9 15 / 0 20 20 30 30 20
&&
.OTX WATCHES/WARNINGS/ADVISORIES...
ID...WINTER WEATHER ADVISORY FROM 10 AM SATURDAY TO 10 PM PST SUNDAY
FOR NORTHERN PANHANDLE.
WINTER STORM WARNING FROM 10 AM SATURDAY TO 10 PM PST SUNDAY FOR
CENTRAL PANHANDLE MOUNTAINS...IDAHO PALOUSE.
WINTER STORM WARNING FROM 10 AM SATURDAY TO 10 PM PST SUNDAY FOR
LEWIS AND SOUTHERN NEZ PERCE COUNTIES...LEWISTON AREA.
WA...WINTER STORM WARNING FROM 10 AM SATURDAY TO 10 PM PST SUNDAY FOR
WASHINGTON PALOUSE.
WINTER STORM WARNING FROM 10 AM SATURDAY TO 10 PM PST SUNDAY FOR
LOWER GARFIELD AND ASOTIN COUNTIES.
WINTER STORM WARNING FROM 10 AM SATURDAY TO 10 PM PST SUNDAY FOR
NORTHEAST BLUE MOUNTAINS.
&&
$$
