FXUS66 KPDT 252230
AFDPDT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PENDLETON OR
230 PM PST FRI DEC 25 2009
.SHORT TERM...TONIGHT THROUGH SUNDAY NIGHT...A VERY LARGE MID/UPPER
LEVEL RIDGE OF HIGH PRESSURE WILL PERSIST OVER THE REGION THROUGH
SUNDAY ESPECIALLY OVER THE LOWER COLUMBIA BASIN AND ADJACENT
VALLEYS. SUBSIDENCE ALOFT ASSOCIATED WITH THE RIDGE WILL MAINTAIN A
STRONG SURFACE BASED INVERSION WHICH CONTINUE TO TRAP MOISTURE IN
THE BOUNDARY LAYER GENERALLY BELOW 3500 FEET RESULTING A LOW STRATUS
CLOUD DECK WHICH PRODUCES DENSE FOG WHERE IT MEETS LOCAL TERRAIN. IN
ADDITION...SURFACE HIGH PRESSURE IN THE FORECAST AREA WILL CONTINUE
VERY LIGHT WINDS RESULTING IN LITTLE MOVEMENT OF THE COLD AIR MASS
TRAPPED IN THE COLUMBIA BASIN AND ADJACENT VALLEYS RESULTING IN
STAGNANT AIR CONDITIONS. AS SUCH AN AIR STAGNATION ADVISORY REMAINS
IN EFFECT UNTIL 4 PM PST SATURDAY AFTERNOON.
A CLOSED MID/UPPER LEVEL LOW APPROACHES THE COAST ON SUNDAY
AND THEN MOVES NORTH JUST OFFSHORE SUNDAY NIGHT. THIS WEAK SYSTEM
WILL INTRODUCE A SLIGHT CHANCE OF SNOW ALONG THE IMMEDIATE EAST
SLOPES OF THE CENTRAL AND NORTHERN OREGON CASCADES SUNDAY AND SUNDAY
NIGHT. OTHERWISE...THE FORECAST AREA WILL REMAIN DRY THROUGH SUNDAY
NIGHT. POLAN
.LONG TERM...MONDAY THROUGH WEDNESDAY NIGHT. MODELS HAVE THEIR
DIFFERENCES FOR THIS TIME PERIOD ESPECIALLY ON TIMING. HOWEVER IT
APPEARS THAT A FEW WEAK SYSTEMS WILL MOVE THROUGH THE REGION. THIS
SHOULD BRING SOME LIGHT PRECIPITATION AND MOSTLY CLOUDY SKIES.
COLUMBIA BASIN STRATUS AND FOG WILL LIKELY CONTINUE INTO MONDAY THEN
DIMINISH ON TUESDAY. TEMPERATURES WILL REMAIN A LITTLE BELOW NORMAL
WITH LIGHT WINDS. 94
THURSDAY AND FRIDAY...NORTHWEST FLOW WILL CONTINUE THURSDAY WITH A
FEW UPSLOPE SHOWERS ALONG THE MOUNTAINS OF EASTERN OREGON AND THE
EAST SLOPES OF THE CASCADES. ANOTHER DISTURBANCE WILL MOVE IN LATE
THURSDAY EARLY FRIDAY AND WILL BRING A CHANCE FOR SNOW SHOWERS
CHANGING OVER TO RAIN SHOWERS. CURRENT MODELS INDICATE FAIRLY
STRONG WINDS AT THE 850MB LEVEL AND THAT WAA WITH THE APPROACHING
SYSTEM WILL ALLOW FOR THE COLD AIR TO MIX OUT OF THE LOWER
ELEVATIONS. HOWEVER...MODEL SOUNDINGS INDICATE THAT THERE MAY BE A
SLIGHT CHANCE OF FREEZING RAIN BEFORE THE WARM AIR PUSHES MIXES DOWN
FRIDAY...BUT TOO FAR IN THE EXTENDED TO ADD TO THE TEXT FORECAST.
WEBER
&&
.PREVIOUS AVIATION DISCUSSION...18Z TAFS. LARGE AREA OF STRATUS AND
FOG OVER THE REGION WILL CONTINUE TO PRODUCE MVFR AND IFR CONDITIONS
AT ALL TAFS SITES FOR THE NEXT 24 HOURS. LOCAL LIFR CONDITIONS ALSO
POSSIBLE. SURFACE WINDS 5 KT OR LESS. 94
&&
.PRELIMINARY POINT TEMPS/POPS...
PDT 19 29 17 30 / 0 0 0 0
ALW 20 28 19 30 / 0 0 0 0
PSC 20 32 19 31 / 0 0 0 0
YKM 18 31 16 29 / 0 0 0 0
HRI 19 31 17 30 / 0 0 0 0
ELN 17 29 18 31 / 0 0 0 0
RDM 14 27 20 31 / 0 0 10 10
LGD 15 25 16 29 / 0 0 0 0
GCD 17 36 16 34 / 0 0 10 10
DLS 22 34 21 33 / 0 0 10 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
SATURDAY : GREEN
SUNDAY : GREEN
MONDAY : 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
$$
99/94/94
