FXUS66 KPDT 242011 AAA
AFDPDT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PENDLETON OR
1210 PM PST THU DEC 24 2009
.UPDATE...THE FORECAST AREA REMAINS UNDER A NORTH TO NORTHWEST FLOW
THIS MORNING. HIGH PRESSURE CONTINUES TO BE CENTERED JUST OFF THE
COAST AND ITS INFLUENCE WILL BE BRINGING IN WARMER AIR ALOFT WHILE
TEMPERATURES WILL BE COOLER AT THE SURFACE IN MOST LOCATIONS TODAY.
THIS WILL STRENGTHEN AN INVERSION OVER THE AREA TODAY AND INTO THE
WEEKEND. ALREADY SEEING SIGNS OF THE INVERSION AS LOW STRATUS HAS
DEVELOPED THIS MORNING IN THE YAKIMA AND KITTITAS VALLEYS AND
PORTIONS OF THE COLUMBIA BASIN. DENSE FOG HAS FORMED IN THE LAST
HOUR AROUND WALLA WALLA AND AM NOT SURE WHETHER IT WILL BURN OFF
THIS AFTERNOON. IT IS A PRECURSOR TO MORE WIDESPREAD FOG AND LOW
STRATUS TONIGHT AND ON CHRISTMAS DAY AND SATURDAY. THE INVERSION
WILL LEAD TO POLLUTANTS BEING TRAPPED NEAR THE SURFACE SO THE
CURRENT AIR STAGNATION ADVISORY THROUGH SATURDAY AFTERNOON FOR MUCH
OF THE FORECAST AREA IS WARRANTED AND WILL CONTINUE AS IS. HAVE MADE
SOME CHANGES TO THE WEATHER GRID THIS AFTERNOON FOR FOG AND TO SKY
COVER TO CAPTURE THE LOW STRATUS MORE ACCURATELY. HAVE ALSO ADJUSTED
A FEW TEMPERATURES...GENERALLY DOWN A COUPLE OF DEGREES UNDER THE
STRATUS AND FOG AND UP A COUPLE OF DEGREES IN A FEW PLACES UNDER
SUNNY SKIES THAT WERE STARTING OUT WARMER THAN ANTICIPATED. REST OF
FORECAST LOOKS GOOD. UPDATE OUT SHORTLY. PERRY
&&
.AVIATION...18Z TAFS. SOME CLOUDS ALONG THE EAST SLOPES OF THE
CASCADES TODAY WILL IMPACT YKM DLS WITH CEILINGS 020-040. ALSO SOME
LOW CLOUDS AND FOG IN THE COLUMBIA BASIN IMPACTING PDT ALW. THE
REMAINDER OF THE TAF SITES WILL BE MAINLY CLEAR WITH CIRRUS AT TIMES
TODAY. STRONG LOW LEVEL INVERSIONS WILL LEAD TO INCREASED FOG AND
LOW CLOUD DEVELOPMENT TONIGHT. WILL LIKELY SEE MORE WIDESPREAD IFR
AND LIFR LATE TONIGHT AND FRIDAY MORNING. SURFACE WINDS REMAIN LESS
THAN 10 KT. 94
&&
.PREV DISCUSSION... /ISSUED 230 AM PST THU DEC 24 2009/
SHORT TERM...TODAY THROUGH SATURDAY...AN UPPER RIDGE WILL REMAIN IN
PLACE OVER THE REGION TODAY THROUGH THE CHRISTMAS HOLIDAY WITH
GENERALLY CALM WEATHER CONDITIONS. THE ONLY CONCERNS THROUGH THE
SHORT TERM PERIOD WILL BE AN INCREASE IN COVERAGE OF PATCHY FOG
AND/OR LOW CLOUDS OVER THE COLUMBIA BASIN AND ADJACENT VALLEYS AND
LOW LANDS. THE UPPER RIDGE AXIS IS CURRENTLY OFF THE COAST WITH A
DEEP NORTHERLY FLOW OVER THE FORECAST AREA. THIS IS BRINGING DOWN
COLD AIR FROM CANADA. THIS WILL CAUSE ANY FOG THAT FORMS TO BE
FREEZING FOG WHICH WILL LIKELY CAUSE RIME ICE TO FORM ON OBJECTS
AND ROAD SURFACES. THIS MAY CAUSE SOME SLICK CONDITIONS...ESPECIALLY
OVER BRIDGES AND OVERPASSES. IN ADDITION THE UPPER RIDGE WILL NOT
ALLOW MUCH MIXING TO TAKE PLACE...ESPECIALLY IN AREAS OF FOG AND/OR
LOW STRATUS...WHICH WILL CAUSE STAGNANT CONDITIONS AND TRAPPED
POLLUTANTS. AS SUCH AN AIR STAGNATION ADVISORY IS AND WILL REMAIN IN
EFFECT FOR THE NEXT SEVERAL DAYS AT LEAST FOR MOST VALLEYS AND LOW
LANDS OF THE FORECAST AREA. IN FOG FREE AREAS SUCH AS THE MOUNTAINS
TEMPERATURES WILL SLOWLY MODERATE THROUGH SATURDAY AND WARM UP A
DEGREE OR TWO EACH DAY AS THE UPPER RIDGE AXIS SLOWLY SHIFTS
EASTWARD CAUSING LESS OF A NORTHERLY FLOW WITH TIME. OTHERWISE
TEMPERATURES WILL REMAIN STEADY AND COLD THROUGH THE PERIOD. 88
LONG TERM...SATURDAY NIGHT THROUGH THURSDAY...THE RIDGE WILL BEGIN
TO BREAK DOWN ON SATURDAY NIGHT AS A PACIFIC TROUGH TRIES TO
UNDERCUT THE LARGE AREA OF HIGH PRESSURE CENTERED OVER WESTERN
CANADA. THERE ARE DIFFERENCES AMONG THE FORECAST MODELS...BUT THE
GENERAL CONSENSUS IS THAT THE MAIN ENERGY ASSOCIATED WITH THIS
TROUGH WILL DIG TOO FAR SOUTH TO BE ABLE TO BRING SIGNIFICANT
PRECIPITATION TO THE LOCAL AREA...OR EVEN TO BREAK THE SURFACE
INVERSION OVER THE COLUMBIA BASIN. CONSEQUENTLY...ONLY A FEW SNOW
SHOWERS WILL BE POSSIBLE ALONG THE CASCADES AND IN CENTRAL
OREGON...AND FREEZING FOG WILL REMAIN LIKELY ACROSS THE LOWER
ELEVATIONS THROUGH AT LEAST MONDAY MORNING.
THERE IS STILL MUCH UNCERTAINTY IN THE LONG TERM FORECAST. THE ECMWF
AND THE GFS ARE GREATLY OUT OF PHASE TUESDAY THROUGH THURSDAY.
THEREFORE...HAVE CHOSEN TO LEAVE A CHANCE OF SNOW IN THE
CLIMATOLOGICALLY FAVORED MOUNTAINOUS AREAS...WHILE KEEPING THE LOWER
COLUMBIA BASIN MOSTLY DRY. TEMPERATURES WILL REMAIN COOLER THAN
NORMAL. 82
&&
.PRELIMINARY POINT TEMPS/POPS...
PDT 27 17 27 20 / 0 0 0 0
ALW 29 19 26 21 / 0 0 0 0
PSC 28 18 29 20 / 0 0 0 0
YKM 27 15 28 18 / 0 0 0 0
HRI 29 17 29 19 / 0 0 0 0
ELN 29 15 27 18 / 0 0 0 10
RDM 37 13 37 15 / 0 0 0 10
LGD 27 11 31 15 / 0 0 0 10
GCD 32 17 36 17 / 0 0 0 10
DLS 35 20 32 22 / 0 0 0 0
&&
.PDT WATCHES/WARNINGS/ADVISORIES...
OR...AIR STAGNATION ADVISORY UNTIL 4 PM PST SATURDAY FOR ORZ041>044-
049-501-504-505.
WA...AIR STAGNATION ADVISORY UNTIL 4 PM PST SATURDAY FOR WAZ024-
026>029-501-502.
&&
THREAT INDEX
TODAY : GREEN
FRIDAY : GREEN
SATURDAY : 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
$$
83/82/94
