FXUS66 KPDT 231736 AAB
AFDPDT
AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE PENDLETON OR
936 AM PST WED DEC 23 2009
UPDATED FOR AVIATION DISCUSSION
.UPDATE...HIGH PRESSURE CONTINUES TO BUILD OVER THE REGION WITH FEW
SIGNIFICANT WEATHER CONCERNS TODAY. WHILE MUCH OF THE AREA IS MOSTLY
CLEAR...SOME MID AND HIGH CLOUDS WILL MOVE OVER THE RIDGE LATER THIS
MORNING IN THE NORTHERN PORTION OF THE AREA. AT THE SAME TIME...THE
TAIL END OF A WAVE MOVING INTO CENTRAL MONTANA WILL BRUSH NORTHEAST
OREGON WITH SOME MID LEVEL CLOUDS AND PERHAPS A FEW SNOW FLURRIES IN
THE NORTHERN BLUES AND WALLOWA COUNTY. HAVE MADE SOME SKY COVER
ADJUSTMENTS DOWNWARD THIS MORNING TO MATCH OBSERVED TRENDS.
TEMPERATURES HAVE STARTED OUT COOLER THIS MORNING AND CURRENT
FORECAST HAS A GENERALLY GOOD HANDLE ON HIGH TEMPERATURES TODAY.
HAVE NUDGED TEMPERATURES UP SLIGHTLY IN A FEW PLACES. CURRENT
FORECAST LOOKS GOOD ASIDE FROM ISSUES MENTIONED ABOVE. WILL SEND OUT
A MINOR FORECAST UPDATE SOON.
IN THE LONGER TERM...WITH HIGH PRESSURE OVER THE REGION...LOW MIXING
HEIGHTS AND LIGHT WINDS EXPECTED...CONDITIONS WILL BE RIPE FOR A
PERIOD OF AIR STAGNATION ACROSS MUCH OF THE FORECAST AREA. AFTER
COORDINATION WITH REGIONAL AIR QUALITY AGENCIES...ANTICIPATE ISSUING
AN AIR STAGNATION ADVISORY FOR AREAS BELOW 2000 FEET THURSDAY
MORNING THROUGH SATURDAY MORNING WITH THE AFTERNOON FORECAST
PACKAGE. MAY ULTIMATELY NEED AN ADVISORY INTO EARLY NEXT WEEK BUT
CURRENT THINKING IS TO ISSUE THE ADVISORY THURSDAY MORNING THROUGH
SATURDAY AND SEE HOW THE SITUATION DEVELOPS OVER TIME. PERRY
&&
AVIATION...18Z TAFS...VFR CONDITIONS ARE EXPECTED OVER THE NEXT 24
HOURS AT ALL TAF SITES...EXCEPT FOR KALW WHICH MAY EXPERIENCE
LOWERED VISIBILITIES AND CIGS OVERNIGHT TONIGHT. WINDS WILL BE LESS
THAN 10KTS AND CLOUD CIGS WILL GENERALLY BE 20K FEET OR HIGHER.
THERE MAY BE SOME LOW LEVEL CLOUDS DEVELOP IN THE OVERNIGHT HOURS TO
EARLY MORNING HOURS AT OTHER SITES...BUT LOW CLOUDS ARE NOT EXPECTED
TO INTERFERE WITH FLIGHT CONDITIONS. WEBER
.PREV DISCUSSION... /ISSUED 210 AM PST WED DEC 23 2009/
SHORT TERM...TODAY THROUGH CHRISTMAS DAY. VERY FEW WEATHER CONCERNS
ARE ANTICIPATED IN THE SHORT TERM. INFRARED SHOWS A BAND OF MID
LEVEL CLOUDS OVER NE OREGON ASSOCIATED WITH A SHORTWAVE TROUGH
MOVING SOUTH ALONG THE CANADIAN ROCKIES. ALTHOUGH DOPPLER RADAR IS
NOT PICKING UP ANY RETURNS AND NO PCPN HAS BEEN REPORTED SO FAR IN
THE BLUES OR WALLOWAS...BELIEVE ISOLATED SNOW SHOWERS WITH LITTLE
OR NO ACCUMULATION MAY STILL OCCUR THIS MORNING OVER THE NE MTNS
WHERE A NW FLOW ALOFT PROVIDES WEAK OROGRAPHIC LIFT. OTHERWISE...A
DRY FORECAST IS IN STORE OVER THE FORECAST AREA. AN UPPER LEVEL
RIDGE WILL BUILD OVER THE REGION AND A SURFACE HIGH OVER BC WILL
MOVE SOUTH ACROSS EASTERN WASHINGTON PROVIDING A NE-E SURFACE
GRADIENT. IT WILL BE COLD...VERY STABLE...AND DRY TONIGHT THROUGH
CHRISTMAS DAY. AIR STAGNATION CONCERNS WILL BE ANALYZED BY THE DAY
SHIFT TO DETERMINE IF ADVISORIES ARE NEEDED FOR PORTIONS OF THE
AREA. CURRENT FORECAST HAS PATCHY FREEZING FOG FOR THE BASIN AND
SURROUNDING VALLEYS FOR CHRISTMAS EVE. THIS LOOKS REASONABLE BASED
ON THE STRENGTHENING LOW LEVEL INVERSION AND A FAIRLY MOIST LOW
LEVEL AIR MASS. WILL ALSO ADD PATCHY FREEZING FOG TO CENTRAL OREGON.
JOHNSON
LONG TERM...FRIDAY NIGHT THROUGH NEXT WEDNESDAY...A STRONGLY
AMPLIFIED HIGH PRESSURE RIDGE WILL MAINTAIN CONTROL OF THE WEATHER
OVER THE CWA FOR AT LEAST THE BEGINNING OF THE EXTENDED PERIOD. THE
RIDGE AXIS WILL BE LOCATED NORTH-SOUTH ACROSS WEST CENTRAL OREGON
AND WASHINGTON FRIDAY NIGHT AND SATURDAY WITH GOOD AGREEMENT BETWEEN
THE VARIOUS MODELS AT THAT TIME PERIOD. THE RIDGE AXIS WILL
GRADUALLY SHIFT EASTWARD ON SATURDAY ALLOWING CONTINUED MODERATION
OF TEMPERATURES. HOWEVER...THIS MODERATION WILL BE MORE NOTICED IN
THE MOUNTAINS IN FOG/STRATUS FREE AREAS AND WHERE SURFACE BASED
TEMPERATURE INVERSIONS WILL BE THE WEAKEST. BY SUNDAY THE ECMWF AND
GFS MODELS BEGIN TO DIVERGE FROM EACH OTHER SIGNIFICANTLY. THE GFS
WANTS TO KEEP A LARGE CLOSED UPPER HIGH OVER THE PACIFIC NORTHWEST
WHILE THE ECMWF PUSHES THE HIGH FURTHER NORTH INTO CANADA AND
PINCHES IT OFF WITH AN UPPER LOW FORMING JUST OFF THE COAST. THE
MODEL DIFFERENCES BECOME EVEN MORE PRONOUNCED ON MONDAY WITH THESE
TWO MODELS DIVERGING EVEN MORE IN THE PLACEMENT OF UPPER HIGHS AND
LOWS. DUE TO THE UNCERTAINTY FROM SATURDAY ONWARD HAVE DECIDED TO
MAKE VIRTUALLY NO CHANGES TO THE EXTENDED FORECAST. CHANGES ARE
MINOR BEFORE THAT TIME PERIOD AS WELL IN THE MEDIUM RANGE FORECAST
PERIOD THROUGH SUNDAY...WITH BASICALLY A DRY REGIME WITH INCREASING
FOG AND STRATUS IN THE COLUMBIA BASIN AND ADJACENT VALLEYS/LOW
LANDS. 88
&&
.PRELIMINARY POINT TEMPS/POPS...
PDT 35 17 26 18 / 0 0 0 0
ALW 39 20 26 19 / 0 0 0 0
PSC 35 16 27 19 / 0 0 0 0
YKM 33 16 28 16 / 0 0 0 0
HRI 35 16 28 18 / 0 0 0 0
ELN 36 15 28 15 / 0 0 0 0
RDM 37 13 37 14 / 0 0 0 0
LGD 33 8 25 11 / 0 0 0 0
GCD 34 13 31 15 / 0 0 0 0
DLS 40 22 33 22 / 0 0 0 0
&&
.PDT WATCHES/WARNINGS/ADVISORIES...
OR...NONE.
WA...NONE.
&&
THREAT INDEX
TODAY : GREEN
THURSDAY : GREEN
FRIDAY : GREEN
GREEN: NONE OR LIMITED NEED FOR WATCHES, WARNINGS, OR ADVISORIES.
YELLOW: A FEW WATCHES, WARNINGS, OR ADVISORIES POSSIBLE OR IN EFFECT.
RED: NUMEROUS WATCHES, WARNINGS, OR ADVISORIES POSSIBLE OR IN EFFECT.
FOR ADDITIONAL WEATHER INFORMATION, CHECK OUR WEB SITE AT...
WWW.WEATHER.GOV/PENDLETON
$$
83/88/89
