FXUS66 KPDT 221748 AAB
AFDPDT

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE PENDLETON OR
947 AM PST TUE DEC 22 2009

UPDATED FOR AVIATION DISCUSSION

.UPDATE...UPPER LOW MOVING SOUTH ACROSS OREGON WILL MOVE WELL SOUTH 
OF THE AREA THIS EVENING. SOME MOISTURE ASSOCIATED WITH THIS SYSTEM 
WILL BRING A FEW SNOW SHOWERS MAINLY TO CENTRAL OREGON THIS 
AFTERNOON. CURRENT FORECAST IS ON TRACK AND WILL ISSUE A ZFP UPDATE 
TO FRESHEN THE WORDING AND TO ADJUST HIGH TEMPERATURES FOR TODAY 
DOWN SLIGHTLY. 78

.&&

AVIATION...18Z TAFS...VFR CONDITIONS ARE EXPECTED TO PREVAIL OVER 
THE NEXT 24 HOURS. KRDM MAY SEE ANOTHER BRIEF SNOW SHOWER OR TWO 
THIS MORNING...LOWERING CIGS AND VIS...BUT SHOULD REBOUND QUICKLY. 
WINDS WILL BE LIGHT LESS THAN 15KTS AT ALL TAF SITES. CIGS WILL BE 
MAINLY FEW TO SCATTERED MID LEVEL CLOUDS...WITH PERHAPS AN EARLY 
MORNING LOW LEVEL STRATUS DECK. WEBER

&&

.PREV DISCUSSION... /ISSUED 230 AM PST TUE DEC 22 2009/ 

SHORT TERM...TODAY THROUGH WEDNESDAY NIGHT.  A CLOSED UPPER LOW IS 
CENTERED OVER THE FORECAST AREA.  THE EASTERN SIDE OF THE FORECAST 
AREA IS UNDER A SOUTHERLY FLOW ALOFT WHILE THE WESTERN SIDE ALONG 
THE CASCADE RANGE IN UNDER A NORTHERLY FLOW ALOFT.  AT THIS 
TIME...DOPPLER RADAR SHOWS A BAND OF PRECIPITATION FROM THE BLUE 
MTNS EASTWARD.  SNOW ACCUMULATIONS THIS MORNING IN THE WALLOWA 
VALLEY COULD REACH AROUND 2 INCHES...JUST SHY OF ADVISORY CRITERIA. 
PRECIPITATION WILL TAPER OFF THIS MORNING IN OUR EASTERN MOUNTAINS.  
HOWEVER...THERE APPEARS TO BE A WAVE ON THE WEST SIDE OF THE LOW 
THAT COULD BRING SCATTERED SNOW SHOWERS OVER CENTRAL OREGON THIS 
AFTERNOON.  WILL INCREASE POPS INTO THE CHANCE CATEGORY FOR CENTRAL 
OREGON.  ACCUMULATIONS SHOULD BE LIGHT BUT WOULD NOT BE SURPRISED IF 
LOCAL ACCUMULATIONS AROUND AN INCH ARE REPORTED.  ELSEWHERE...SKIES 
WILL BE PARTY CLOUDY WITH A FEW FLURRIES POSSIBLE.

SKIES WILL BE CLEARING TONIGHT AS THE SYSTEM MOVES SOUTH AND OUT OF 
THE REGION WITH COLD OVERNIGHT LOW TEMPERATURES--TEENS TO LOWER 20S 
WITH A FEW AREAS IN THE SINGLE DIGITS IN THE MOUNTAINS.  AS AN UPPER 
RIDGE OFFSHORE SHIFTS EASTWARD...EASTERN WA/OR WILL BE ON THE EAST 
SIDE OF THE RIDGE AND UNDER A NORTHERLY FLOW ALOFT ON WEDNESDAY.  
COLD DRY AIR WILL SPREAD ACROSS THE REGION BY WEDNESDAY NIGHT.  THIS 
WILL INHIBIT FOG DEVELOPMENT...YET MODELS ARE HINTING AT SOME LOW 
LEVEL MOISTURE IN CENTRAL OREGON LATE WEDNESDAY NIGHT.  PATCHY 
FREEZING FOG OR STRATUS IS POSSIBLE IN THE LOWER DESCHUTES PLATEAU 
AS A N-NE GRADIENT PROVIDES SOME WEAK LOW LEVEL UPSLOPE.  JOHNSON

LONG TERM...THURSDAY THROUGH NEXT TUESDAY...HIGH PRESSURE ALOFT 
WILL GRADUALLY BUILD INTO THE REGION ON THURSDAY FROM THE WEST. A 
NORTHERLY FLOW ALOFT AHEAD OF THE RIDGE WILL KEEP TEMPERATURES ON 
THE COOL SIDE...WHICH WILL PERSIST THROUGH AT LEAST CHRISTMAS DAY. 
BY SATURDAY THE RIDGE AXIS WILL SHIFT EASTWARD AND INLAND INTO THE 
PACIFIC NORTHWEST. THIS WILL ALLOW SOME MODERATION IN TEMPERATURES 
BUT ONLY A LITTLE. A WEAK WEATHER DISTURBANCE WILL TRY TO MOVE INTO 
AND THROUGH THE UPPER RIDGE BUT WILL WEAKEN SIGNIFICANTLY AS IT 
ENCOUNTERS IT. IN ADDITION IT WILL LIKELY NOT BE STRONG ENOUGH TO 
MIX OUT ANY AREAS OF FOG COMPLETELY IN THE LOWER COLUMBIA BASIN AND 
ADJACENT LOW LANDS. THE MAIN EFFECT WILL BE JUST SOME INCREASING MID 
AND HIGH CLOUDS WITH POSSIBLE LIGHT RAIN OR SNOW SHOWERS NEAR THE 
CREST AND UPPER EAST SLOPES OF THE CASCADES AND OTHER MOUNTAINS 
ACROSS EASTERN OREGON MONDAY AND TUESDAY. THE UPPER RIDGE WILL TRY 
TO REESTABLISH ITSELF AGAIN BY THE MIDDLE OF NEXT WEEK...BUT AT THIS 
TIME THERE IS UNCERTAINTY OF HOW STRONG IT WILL BECOME AND IF IT 
WILL BE ENOUGH TO END ANY RAIN OR SNOW SHOWERS ACROSS THE FORECAST 
AREA. 88

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  37  23  35  20 /  10  10  10   0 
ALW  39  25  35  22 /  10  10  10   0 
PSC  42  23  34  20 /  10  10   0   0 
YKM  36  16  33  17 /   0   0   0   0 
HRI  41  23  35  20 /  10  10   0   0 
ELN  36  18  31  18 /   0   0   0   0 
RDM  37  14  37  13 /  30  20   0   0 
LGD  35  20  33  18 /  20  10  10   0 
GCD  33  18  33  19 /  20  20  10   0 
DLS  41  27  39  24 /  20  10   0   0 

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...NONE.
WA...NONE.
&&

THREAT INDEX
TODAY : GREEN
WEDNESDAY : GREEN
THURSDAY : 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

$$

78/89/89