138 FXUS66 KPDT 182248 AFDPDT AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE PENDLETON OR 248 PM PST FRI JAN 18 2013 .SHORT TERM...TONIGHT THROUGH SUNDAY...A DISTURBANCE CONTINUES TO MOVE SOUTHEAST OUT OF BRITISH COLUMBIA AND ALBERTA EAST OF THE STRONG EAST PACIFIC UPPER RIDGE. THIS SYSTEM HAS HAD A MUCH GREATER THAN EXPECTED AFFECT ON THE LOW CLOUDINESS AND FOG UNDER THE INTERIOR NORTHWEST INVERSIONS. AS SUCH THE LOW CLOUDINESS HAS BROKEN UP OVER MOST LOCATIONS IN OREGON, AS WELL AS IN THE WASHINGTON FOOTHILLS AND WASHINGTON CASCADE EAST SLOPES. HAZINESS WHERE IT HAS CLEARED INDICATES THAT THE INVERSIONS REMAIN. THESE INVERSIONS SHOULD STRENGTHEN OVERNIGHT AS THE UPPER RIDGE WILL REBUILDS AND SHIFTS CLOSER TO THE COAST OVERNIGHT INCREASING SUBSIDENCE OVER THE REGION. THUS WILL INDICATE A GRADUAL REFORMATION AND SPREAD OF THE LOW CLOUDINESS AND PATCHY FREEZING FOG TONIGHT. WILL INDICATE FURTHER SPREAD OF THE LOW CLOUDINESS AND PATCHY FOG THROUGH THE REMAINDER OF THIS PERIOD. FOR TONIGHT, SOME AREAS MAY SEE THE DEVELOPMENT OF DENSE FREEZING FOG. WILL NOT GO WITH ANY ADVISORIES AT THIS TIME DUE TO UNCERTAINTY AS TO WHICH LOCATIONS WILL BE AFFECTED. PRESSURE GRADIENTS REMAIN VERY TIGHT ACROSS THE SOUTHERN GRANDE RONDE VALLEY AND WILL THEREFORE EXTEND THE CURRENT WIND ADVISORY FOR ANOTHER 24 HOURS. THE WEAK SYSTEM DID BREAK UP THE LOW CLOUDINESS UNDER THE INVERSIONS, BUT HAS HAD MINIMAL IMPACT ON THE INVERSIONS THEMSELVES. THUS THE AIR STAGNATION ADVISORIES WILL REMAIN IN EFFECT. 90 .LONG TERM...SUNDAY NIGHT THROUGH WEDNESDAY NIGHT...MODELS ARE IN GOOD AGREEMENT IN HAVING THE CURRENT UPPER LEVEL RIDGE CONTINUE OVER THE AREA THROUGH AT LEAST TUESDAY NIGHT. THE RIDGE AXIS WILL BE NEAR THE COAST SUNDAY NIGHT AND WILL MOVE OVER OUR AREA BY TUESDAY AS A FRONTAL SYSTEM APPROACHES THE PACIFIC NORTHWEST. THIS WILL GIVE US A CONTINUATION OF THE CURRENT PATTERN OF LOW CLOUDS AND FOG BELOW 2500 FEET ELEVATION AND MOSTLY CLEAR SKIES ELSEWHERE. TEMPERATURES WILL BE IN A NARROW RANGE FROM 25 TO 32 IN LOWER ELEVATIONS. THE HIGHER ELEVATIONS WILL BE IN THE UPPER 30S AND 40S DURING THE DAY WITH NIGHT TIME LOWS GENERALLY IN THE MID TEENS TO MID 20S AND IN THE SINGLE DIGITS IN THE HIGHER MOUNTAIN VALLEYS. WINDS WILL BE FAIRLY LIGHT. MODELS HAVE SIGNIFICANTLY SLOWED DOWN THE ARRIVAL OF THE FRONT IN THEIR LATEST RUNS AND IT NOW LOOKS LIKE IT WILL ARRIVE AT THE CASCADE CREST WEDNESDAY AFTERNOON AND MOVE ACROSS THE AREA WEDNESDAY NIGHT THOUGH THERE ARE TIMING DIFFERENCES BETWEEN THE MODELS. HAVE A SLIGHT CHANCE OF RAIN AND SNOW IN AND NEAR THE CASCADES TUESDAY NIGHT WITH PATCHY FOG AND FREEZING FOG IN THE LOWER ELEVATIONS. WEDNESDAY HAS A CHANCE OF RAIN WITH SNOW ABOVE 2500 TO 3000 FEET TURNING TO SHOWERS AND ENDING WEDNESDAY NIGHT. AM CONCERNED THAT LINGERING COLD AIR AT THE SURFACE IN THE LOWER ELEVATIONS MAY CAUSE THE RAIN TO FALL AS FREEZING RAIN INITIALLY BUT DO NOT HAVE ENOUGH CONFIDENCE IN THIS HAPPENING TO ADD IT TO THE FORECAST THAT FAR OUT. LATER SHIFTS WILL HAVE TO MONITOR THAT POSSIBILITY. PERRY THURSDAY THROUGH FRIDAY NIGHT...THE ENSEMBLE MEAN FORECASTS OF 500MB HEIGHT CONTOURS FROM THE LATEST 12Z RUNS OF THE GFS AND ECMWF MODELS ARE IN VERY GOOD AGREEMENT SHOWING A MID/UPPER LEVEL LONGWAVE RIDGE OF HIGH PRESSURE ON THURSDAY AND THURSDAY NIGHT FOR DRY CONDITIONS. THE ENSEMBLE MEAN FORECASTS OF BOTH MODELS FOR FRIDAY AND FRIDAY NIGHT ARE IN EXCELLENT AGREEMENT IN THAT BOTH ARE SHOWING THE RIDGE MIGRATING EAST TO THE NORTHERN ROCKIES, WHICH ALLOWS A MID/UPPER LEVEL LONGWAVE TROUGH TO MOVE INTO THE PACIFIC NORTHWEST FOR INCREASING CHANCES OF PRECIPITATION. SNOW LEVELS WILL REMAIN LOW ENOUGH FOR SNOW ON FRIDAY/FRIDAY NIGHT WITH THE BEST CHANCE FOR SNOW IN THE MOUNTAINS. THE NEW 12Z CANADIAN MODEL FORECAST SHOWS THE SAME FEATURES AS THE 12Z GFS AND ECMWF RUNS FOR THURSDAY THROUGH FRIDAY NIGHT AND THUS MY OVERALL CONFIDENCE IN THE FORECAST FOR THIS PERIOD IS ABOVE NORMAL. POLAN && .AVIATION...PREVIOUS DISCUSSION...18Z TAFS...A WEAK DISTURBANCE HAS LOWERED CEILINGS SOMEWHAT AT COLUMBIA BASIN TAF SITES, KYKM AND KDLS THIS MORNING. SHOULD SEE SOME MODEST IMPROVEMENT THIS AFTERNOON AND TONIGHT. KYKM, KALW AND KPDT WILL REMAIN AT VLIFR TO IFR CONDITIONS, KPSC AT IFR AND MVFR LEVELS AND KDLS WILL REMAIN AT MVFR CONDITIONS FOR THE NEXT 24 HOURS. KBDN AND KRDM WILL REMAIN AT VFR LEVELS WITH SOME HIGH CIRRUS THROUGH THIS AFTERNOON THEN CLEAR OUT TONIGHT. THERE IS A CHANCE FOR KPDT TO CLEAR AND RAISE TO VFR LEVELS FOR A FEW HOURS THIS MORNING AND EARLY AFTERNOON AS THE DISTURBANCE CROSSES THE AREA BUT DID NOT INCLUDE IN THE TAFS AS THE CHANCE OF CLEARING WAS TOO LOW FOR MUCH CONFIDENCE THAT IT WILL HAPPEN. WINDS WILL REMAIN LESS THEN 10 MPH THROUGH THE NEXT 24 HOURS. PERRY && .PRELIMINARY POINT TEMPS/POPS... PDT 25 28 26 29 / 0 0 0 0 ALW 28 30 29 31 / 0 0 0 0 PSC 28 32 28 33 / 0 0 0 0 YKM 27 31 27 32 / 0 0 0 0 HRI 28 31 28 32 / 0 0 0 0 ELN 23 29 24 30 / 0 0 0 0 RDM 20 50 22 47 / 0 0 0 0 LGD 20 33 21 34 / 0 0 0 0 GCD 21 37 21 36 / 0 0 0 0 DLS 31 36 31 37 / 0 0 0 0 && .PDT WATCHES/WARNINGS/ADVISORIES... OR...AIR STAGNATION ADVISORY UNTIL 10 AM PST TUESDAY ORZ042-505-507-508. AIR STAGNATION ADVISORY UNTIL 10 AM PST TUESDAY BELOW 4000 FEET FOR THE ORZ509. AIR STAGNATION ADVISORY UNTIL 10 AM PST TUESDAY ORZ041-044. WIND ADVISORY UNTIL 4 PM PST SATURDAY FOR LADD AND PYLES CANYONS IN THE ORZ049. WA...AIR STAGNATION ADVISORY UNTIL 10 AM PST TUESDAY WAZ029. AIR STAGNATION ADVISORY UNTIL 10 AM PST TUESDAY WAZ024-026>028. AIR STAGNATION ADVISORY UNTIL 10 AM PST TUESDAY WAZ520-521. && 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 $$ 90/83/99/83