FXUS66 KOTX 042250
AFDOTX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SPOKANE WA
250 PM PST FRI DEC 4 2009
.SYNOPSIS...
LIGHT SNOW SHOWERS...BLUSTERY NORTH WINDS AND COLDER TEMPERATURES
WILL DESCRIBE THE WEATHER ACROSS THE INLAND NORTHWEST THIS WEEKEND...
AS A PAIR OF COLD UPPER LEVEL DISTURBANCES MOVES THROUGH THE
REGION. THEN DRY AND UNSEASONABLY COLD WEATHER WILL PERSIST FOR
MUCH OF NEXT WEEK.
&&
.DISCUSSION...
FOR TONIGHT...A DEEP UPPER LEVEL LOW WILL SINK SOUTH FROM THE
CANADIAN ROCKIES ACROSS WESTERN MONTANA OVERNIGHT. VERY COLD AIR
WILL BE AVAILABLE ALOFT WITH 500MB TEMPS -32C. THIS WILL HELP
DESTABILIZE THE ATMOSPHERE OVER THE REGION...AND AID IN THE
DEVELOPMENT OF LIGHT SNOW SHOWERS OR FLURRIES ESPECIALLY IN THE
UPSLOPE AREAS. STILL CONTENDING WITH A STRATUS DECK ACROSS THE
COLUMBIA BASIN. THE NORTHERN AND WESTERN EDGES OF THE CLOUD DECK
HAVE ERODED...ALTHOUGH WITH INCREASED LOW LEVEL NORTHWEST FLOW THE
CLOUDS WILL PERSIST LONGER FROM THE SPOKANE/COEUR D'ALENE METRO
AREA SOUTH ACROSS THE PALOUSE...SOUTHEAST WASHINGTON AND INTO
SOUTHERN PORTIONS OF THE IDAHO PANHANDLE WITH OCCASIONAL FLURRIES.
MEANWHILE...CLOUD TOPS ARE COOLING OVER BRITISH COLUMBIA AS A
SHORTWAVE ROTATES AROUND THE DEEP LOW. THIS WAVE WILL BE TRACKING
INTO NORTHEAST WASHINGTON OVERNIGHT. THIS WILL INCREASE CLOUDS
AND SNOW SHOWERS ACROSS NORTHEAST WASHINGTON MOUNTAINS...AND THEN
THE PANHANDLE MOUNTAINS. NORTH WINDS ARE SLOW TO DEVELOP IN THE
OKANOGAN VALLEY...AND MODELS SUPPORT LIGHTER WINDS THAN PREVIOUSLY
FORECAST OVERNIGHT. LOW TEMPERATURES SHOULD BE SIMILAR TO LAST
NIGHT IN MOST AREAS.
BY SATURDAY...THE COLD CORE OF THE UPPER LEVEL LOW WILL HAVE
SLIPPED SOUTHEAST ACROSS SOUTHERN IDAHO AND OREGON...WHILE A
SECONDARY SHORTWAVE CARVES A TROUGH OVER WESTERN CANADA. THIS
PATTERN WILL LEAVE A TRANSITORY SHORTWAVE RIDGE ACROSS THE
REGION...WITH PLENTY OF LOW LEVEL MOISTURE TRAPPED BENEATH IT.
SURFACE HIGH PRESSURE BUILDS IN BRITISH COLUMBIA...AND ENHANCES
THE NORTH TO NORTHEAST FLOW INTO THE REGION. THE MAIN THREAT OF
SNOW SHOWERS WILL BE ACROSS SOUTHEAST WASHINGTON AND NORTH IDAHO
AS THE INITIAL LOW DEPARTS...BUT LIGHT SNOW SHOWER ACTIVITY WILL
INCREASE ACROSS THE NORTHERN TIER ZONES AS THE SECOND SHORTWAVE
APPROACHES. /RFOX.
...VERY COLD TEMPERATURES...BREEZY WINDS...AND BELOW ZERO WIND CHILLS
EXPECTED OVER THE WEEKEND INTO NEXT WEEK...
SATURDAY NIGHT THROUGH MONDAY NIGHT...MODELS CONTINUE TO INDICATE
VERY COLD TEMPERATURES WILL INVADE THE FORECAST AREA THIS WEEKEND.
BY SUNDAY NIGHT MODELS ARE CONSISTENTLY BRINGING 850MB TEMPERATURES
AS LOW AS -23C ACROSS THE NORTHEAST CORNER AS NORTHERLY FLOW ALOFT
DRAGS AN ARCTIC AIRMASS INTO THE REGION. AN UPPER LOW WILL SLIDE
SOUTH ACROSS WESTERN WASHINGTON/OREGON AND EASTERLY UPSLOPE FLOW
WILL BRING LIGHT SNOW TO THE WESTERN PORTION OF THE FORECAST AREA.
THERE IS NOT A GREAT DEAL OF MOISTURE WITH THIS SYSTEM BUT A TENTH
OF AN INCH OF QPF COULD PILE UP TO A COUPLE OF INCHES OF VERY FLUFFY
SNOW IN THE COLD TEMPERATURES EXPECTED. THE FRESH SNOW COVER ACROSS
THE WESTERN ZONES WILL ACT TO REFLECT SOLAR INSOLATION WITH FURTHER
COOLING AS A RESULT. TEMPERATURES IN THE WESTERN VALLEYS HAVE BEEN
REDUCED A BIT TO ACCOUNT FOR THIS. A NORTH TO NORTHEAST PRESSURE
GRADIENT WILL CHANNEL WINDS DOWN THE NORTH-SOUTH VALLEYS AND THE
PURCELL TRENCH WITH SUSTAINED WINDS OF 15 TO 25 MPH WITH GUSTS OF 30
TO 40 MPH. THE COLD TEMPS COMBINED WITH STRONG WINDS WILL RESULT IN
WIND CHILL TEMPERATURES NEAR ZERO ACROSS MUCH OF THE FORECAST AREA
SATURDAY NIGHT WITH VALUES DIPPING BELOW ZERO SUNDAY NIGHT AND
MONDAY NIGHT. WIND CHILLS WILL BE CLOSE TO WARNING CRITERIA
ACROSS THE NORTHEAST VALLEYS SUNDAY NIGHT. THE CHALLENGE IS
WHETHER THE STRONGER WINDS WILL KEEP TEMPS UP A BIT OR NOT. NO
HIGHLIGHTS ISSUED ATTM BUT MENTIONED IN HWO. WINDS WILL LIGHTEN ON
MONDAY BUT FRIGID TEMPERATURES WILL CONTINUE AT 20 TO 25 DEGREES
BELOW SEASONAL NORMALS. /KELCH
TUESDAY THROUGH FRIDAY...THE SCREAMING MESSAGE IN THIS PERIOD WILL
CONTINUE TO REVOLVE AROUND A COLD BUT RELATIVELY DRY CONTINENTAL
POLAR AIRMASS BROUGHT TO THE INLAND NORTHWEST CARE OF A HIGHLY
AMPLIFIED UPPER LEVEL FLOW PATTERN. THE LONGWAVE CHARTS CONTINUE
TO DEPICT VERY LARGE POSITIVE HEIGHT ANOMALIES CENTERED BETWEEN
140-150W BENEATH THE RIDGE AXIS AND CORRESPONDINGLY LARGE NEGATIVE
HEIGHT ANOMALIES OVER THE WESTERN US. IN BETWEEN THE TWO FEATURES
WILL BE DEEP NORTHERLY FLOW ORIGINATING FROM THE NORTHWEST AND
YUKON TERRITORIES AND DROPPING THROUGH THE INLAND NW. WHILE THERE
IS LITTLE DOUBT THIS WILL DELIVER UNSEASONABLY COLD WEATHER TO THE
REGION... THE QUESTION REMAINS HOW COLD WILL IT GET AND HOW LONG
WILL IT LAST. THE MEDIUM RANGE MODELS REMAIN CONSISTENT ON SHOWING
MONDAY AS THE COLDEST DAY OF THE WEEK...WITH ONLY SUBTLE WARMING
EXPECTED FOR TUESDAY. AFTER THAT ALL GUIDANCE SHOWS A SLOW
MODERATION AS THE MEAN MID-LEVEL FLOW BACKS FROM DUE NORTH TOWARD
MORE OF AN ONSHORE REGIME. GIVEN THE STRENGTH OF THE OFFSHORE
RIDGE AND THE NOTION THAT IT WILL BE UNDERCUT WELL TO THE SOUTH BY
A MOIST ZONAL JET IT WOULD SUGGEST THAT THE SLOWER THE WARMUP THE
BETTER. SURPRISINGLY THE ECMWF AND GFS HAVE SWAPPED THEIR AIRMASS
MODIFICATIONS SPEEDS WITH THE GFS NOW SHOWING A SLOWER WARMUP AND
THE ECMWF ESSENTIALLY ERODING THE CP AIRMASS BY LATE FRIDAY. IN
FACT BY LATE FRIDAY THE ECMWF WARMS 850 MB TEMPS OVER THE REGION
TO AROUND -3C (FROM A LOW OF -18C ON LATE MONDAY). AT THE SAME
TIME THE GFS IS SHOWING TEMPS HOVERING AROUND -12C WHICH SEEMS A
BIT MORE SENSIBLE AND IS SUPPORTED BY THE CANADIAN MODEL AND THE
GFS ENSEMBLE MEANS. DELAYING THE ONSET OF THE AIRMASS TRANSITION
WILL ALSO DELAY THE ONSET OF MODERATE TO HEAVY PRECIPITATION WHICH
IS USUALLY THE CASE WITH THESE TRANSITIONS. FOR THE MOST PART THE
MODELS HAVE BEEN DELAYING THE ONSET OF THIS PRECIPITATION. AT THIS
POINT WE EXPECT MOST OF NOT ALL PRECIPITATION TO HOLD OFF UNTIL
THE WEEKEND...HOWEVER IF THE ECMWF IS CORRECT IT WOULD ARRIVE
EITHER LATE THURSDAY OR MORE LIKELY ON FRIDAY. FOR NOW WE WILL
HEDGE THE FORECASTS TOWARD CLIMO POPS ACROSS THE SOUTHEASTERN
ZONES...CLOSEST TO THE ZONAL JET POSITION...AND STILL UNDERCUT
VALUES NEAR THE CANADIAN BORDER. EVEN THESE MAY PROVE TOO
HIGH...ESPECIALLY IF THE FORECAST TRENDS TOWARD THE DRIER AND MORE
CONSISTENT CANADIAN MODEL. THE PRECIPITATION DEPICTION OFF THE 12Z
CANADIAN RUN CLOSELY RESEMBLES THE ECMWF FROM YESTERDAY AT THIS
TIME AND THE GFS SEEMS TO BE FOLLOWING SUIT FOR THE LATE THURSDAY
AND FRIDAY PERIODS.
BEFORE THAT TIME THE REGION WILL GENERALLY REMAIN DRY AS IS USUALLY
THE CASE WITH AN IN SITU CP AIRMASS AND PREVAILING N-NW FLOW
ALOFT. BOTH THE GFS AND ECMWF HINT AT A WEAK WAVE MOVING IN FROM
BC ON WEDNESDAY HOWEVER THE PRECIPITABLE WATER VALUES AHEAD OF THE
SYSTEM ARE EXPECTED TO BE NEAR .08 INCHES WHICH IS EXTREMELY DRY.
ALSO...THE NEITHER THE GFS OR ECMWF DEPICT MUCH SATURATION OR
LIFTING THROUGH THE CRITICAL DENDRITIC LAYER. WHILE WE CANNOT
ENTIRELY RULE OUT ANY CHANCE OF PRECIPITATION...AS WAS THE CASE
WITH AN ELEVATED DISTURBANCE THIS MORNING...THE CHANCES OF
MEASURING PRECIPITATION SEEM QUITE SMALL GIVEN THE AFOREMENTIONED
REASONS.
TEMPERATURES DURING THE PERIOD WILL REMAIN WELL BELOW NORMAL FOR
THIS TIME OF YEAR WITH HIGHS BEGINNING THE WEEK IN THE TEENS AND
ENDING IN THE LOWER TO MIDDLE 20S. LOW TEMPS WILL BE MUCH TRICKIER
AND WILL DEPEND ON THE AMOUNT OF CLOUD COVER...WIND...AND POSSIBLE
SNOW COVER. WITH CLEAR SKIES...LIGHT WINDS...AND FRESH SNOW ON THE
GROUND...TEMPS COULD EASILY SURGE WELL BELOW ZERO...HOWEVER THERE
IS NO GUARANTEE ALL THESE ELEMENTS WILL COME TOGETHER AT ANY ONE
TIME. CHANCES ARE GOOD THAT THE WINDS WILL BECOME LIGHT IN THIS
PERIOD HOWEVER THE CLOUD AND SNOW COVER FORECASTS HAVE MUCH LESS
CERTAINTY. OUR GRIDDED TEMPS REPRESENT A SMOOTHED DOWN APPROACH TO
THE FORECAST...BUT IT WOULD NOT BE SURPRISING TO SEE A HUGE RANGE
IN LOW TEMP VALUES ON ANY GIVEN NIGHT IN THIS PERIOD...ESPECIALLY
ON TUESDAY AND WEDNESDAY NIGHTS. FX
&&
.AVIATION...
STRATUS IS ERODING FROM THE NORTH AND WEST. CIGS FROM KEAT-KMWH-KGEG-
KCOE HAVE LIFTED TO MVFR WHILE VSBYS HAVE REBOUNDED TO VFR. COULD
SEE SOME FLURRIES THROUGH THE DAY WITH THE INCREASED INSTABILITY.
ACROSS THE LOWER COLUMBIA BASIN...PALOUSE...SOUTHEAST WASHINGTON
INTO THE IDAHO PANHANDLE...STRATUS AND FOG WILL LINGER WITH
LIGHTER WINDS AND VEERING UPSLOPE FLOW. EXPECT IFR CONDITIONS AT
KPUW-KLWS THROUGH 00Z...AND GRADUAL IMPROVEMENT OVERNIGHT. THINK
THIS PUSH OF DRY COLD AIR WILL NOT BE STRONG ENOUGH TO ERODE THE
ENTIRE STRATUS DECK...AND IT WILL BE SHUNTED TO THE SOUTH
OVERNIGHT INTO SATURDAY MORNING. WHICH MEANS AS WINDS...ANTICIPATE
MORE SLOSHING AROUND OF THE STRATUS...PROBABLY INTO THE CASCADE
VALLEYS BY SATURDAY WITH INCREASED EASTERLY FLOW. /RFOX.
&&
.PRELIMINARY POINT TEMPS/POPS...
SPOKANE 18 29 14 18 4 11 / 10 20 10 10 0 0
COEUR D'ALENE 20 31 13 17 2 10 / 10 20 10 10 0 0
PULLMAN 20 28 15 20 6 14 / 20 20 10 0 0 0
LEWISTON 22 33 18 26 11 19 / 20 20 10 0 0 0
COLVILLE 23 33 16 21 4 13 / 10 20 40 10 0 0
SANDPOINT 21 30 10 13 -1 6 / 20 20 20 10 0 0
KELLOGG 20 27 9 14 -1 7 / 50 30 10 10 10 0
MOSES LAKE 20 32 15 24 5 16 / 0 10 20 10 0 0
WENATCHEE 23 34 16 23 7 16 / 0 10 60 10 10 0
OMAK 20 33 15 23 5 17 / 0 20 70 0 0 0
&&
.OTX WATCHES/WARNINGS/ADVISORIES...
ID...NONE.
WA...NONE.
&&
$$
