854
FXUS66 KSEW 241530
AFDSEW
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
830 AM PDT THU JUL 24 2008
SYNOPSIS
A WEAK UPPER LEVEL RIDGE WILL MOVE ACROSS THE AREA TODAY
BRINGING AFTERNOON SUNSHINE AND WARMER TEMPERATURES. ONSHORE
PRESSURE GRADIENTS WILL INCREASE FRIDAY AFTERNOON AS THE UPPER RIDGE
SHIFTS EAST INTO MONTANA. ONSHORE FLOW AND A PASSING UPPER LEVEL
TROUGH WILL GIVE CLOUDS AND COOL CONDITIONS TO THE AREA OVER THE
WEEKEND. THE UPPER LEVEL TROUGH AND ONSHORE FLOW WILL PERSIST INTO
MUCH OF NEXT WEEK GIVING NIGHT AND MORNING MARINE CLOUDS TO THE AREA
AND TEMPERATURES 2 DEGREES OR SO BELOW NORMAL EACH DAY.
SHORT TERM
A WEAK UPPER LEVEL RIDGE IN THE COASTAL WATERS THIS
MORNING WILL MOVE ACROSS WESTERN WASHINGTON TODAY THEN SHIFT EAST
INTO MONTANA BY LATE FRI. SUBSIDENCE AND WARMING ALOFT WITH THE
APPROACH OF THIS UPPER RIDGE HAS SQUASHED THE EXISTING MARINE LAYER
SOMEWHAT AND HAS RESULTED IN THE FORMATION OF AREAS OF LOW STRATUS
AND FOG IN THE INTERIOR LOWLANDS. WITH BASES AROUND 005 AGL AND TOPS
015...EXPECT A RAPID 17Z BREAKOUT FOLLOWED BY CLEAR SKIES THIS
AFTERNOON. TEMPERATURES GOT A COOL START WITH LOWER 40S IN PORTIONS
OF THE SOUTHWEST INTERIOR AND LOWER TO MID 50S ELSEWHERE. BASED ON
THE COOL START...A COOL WEDGE BELOW 3000 FT...AND CURRENT
OBSERVATIONS AT KSEA AND KSMP...EXPECT HIGH TEMPERATURES TODAY TO BE
2-3 DEG BELOW CURRENT FORECAST -- 75 OR 76 AT SEATAC...77 OR 78 AT
OLYMPIA AND UPPER 60S AROUND EVERETT. CURRENT FORECASTS ALSO APPEAR
TOO HIGH AROUND THE WATER IN THE NORTH INTERIOR.
MAINLY CLEAR CONDITIONS WITH THE PASSING UPPER RIDGE AND A RATHER
DRY ATMOSPHERE POINT TO A COOL NIGHT TONIGHT. GFS MOS FROM 06Z
SHOWED ANOTHER COLD NIGHT IN NORMALLY COLDER OUTLYING LOCATIONS LIKE
KOLM...KAWO AND KUIL...AND ALSO SHOWED TEMPERATURES IN THE LOWER TO
MID 50S ELSEWHERE. CURRENT FORECASTS APPEAR A COUPLE DEG TOO WARM.
THE TEMPERATURE FORECAST FOR FRIDAY LOOKS TRICKY. ONSHORE PRESSURE
GRADIENTS WILL BE RAPIDLY DEVELOPING DURING THE AFTERNOON AS THE
UPPER RIDGE SHIFTS EAST AND AN UPPER TOUGH AND ASSOCIATED FRONTAL
SYSTEM NOW NEAR 138W APPROACHES. GENERALLY WE WOULD EXPECT COOLING
ALONG THE COAST AND NEAR THE STRAIT WHILE TEMPERATURES IN THE
CENTRAL PUGET SOUND AND IN THE MOUNTAINS PEAK. THIS IS SHOWN WELL BY
THE LATEST GUIDANCE AND MODEL TEMPERATURE FORECASTS AND APPEARS BEST
DEPICTED BY THE MM5GFS SOLUTION.
WILL MAKE A MORNING UPDATE FOR THE FIRST THREE PERIODS IN ORDER TO
REFINE TEMPERATURES.
THE BIG STORY WILL BE THE CLOUDS AND COOL TEMPERATURES FOR THE
WEEKEND AS MARINE MOISTURE SURGES EAST AND DEEPENS TO OVER 5000 FEET
AS AN UPPER TROUGH AND OLD FRONTAL SYSTEM MOVES ACROSS THE AREA. AT
THIS POINT IT APPEARS THAT SAT AND PERHAPS SUN COULD BE AT LEAST AS
COOL AS WHAT WE SAW MON AND TUE...AND SOME AREAS OF MORNING DRIZZLE
CANT BE RULED OUT. WILL ADDRESS THIS IN THE AFTERNOON PACKAGE.
ALBRECHT
LONG TERM...PREVIOUS LONG TERM DISCUSSION FOLLOWS
WEAK
TROUGHINESS HANGING OVER THE AREA ON SUNDAY WITH LOW LEVEL ONSHORE
FLOW FOR ANOTHER CLOUDY AND COOL DAY. MODELS SQUEEZING OUT A LITTLE
BIT OF PRECIPITATION OVER THE AREA WITH THE WEAK TROUGH BUT 500 MB
HEIGHTS AREA STILL ABOVE 570 DM AND THE ONSHORE GRADIENTS ARE NOT
VERY STRONG. WILL LEAN TOWARD CLIMO AND KEEP THE PRECIP OUT OF THE
FORECAST FOR NOW. EXTENDED MODELS IN GOOD AGREEMENT FOR THE FIRST
PART OF NEXT WEEK WITH WESTERLY FLOW ALOFT OVER THE AREA AND LOW
LEVEL ONSHORE FLOW. THIS LEADS TO A MORNING CLOUDS AFTERNOON
SUNSHINE FORECAST WITH TEMPERATURES JUST A TOUCH BELOW NORMAL FOR
THE END OF JULY. FELTON
AVIATION
UPPER RIDGE OVER WESTERN WASHINGTON TODAY MOVING OVER
EASTERN WA TONIGHT. AREAS OF LOW LEVEL MOISTURE WESTERN WASHINGTON
MARINE AREAS SHOULD EVAPORATE BEFORE NOON. AIR MASS STABLE.
AREA OF LOW STRATUS OVER PUGET SOUND AND ADJACENT LAND IS STILL
EXPANDING SOUTHWARD BUT ALSO ERODING FROM THE NORTH. LIKELY IT WILL
MOSTLY BE GONE BY 17Z OR SO. AN UNRELATED CLOUD DECK AROUND 4K FT IN
THE FAR N SHOULD ONLY AFFECT KBLI TODAY. OTHERWISE SKIES BASICALLY
CLEAR THIS AFTERNOON THROUGH TONIGHT.
KSEA...EXPECTING CLEAR SKIES BY 17Z AND SHOULD REMAIN THAT WAY
TONIGHT. WIND NORTHERLY 7-11 KT. CHB
MARINE
LOW LEVEL ONSHORE GRADIENTS DOWN THE STRAIT ACTUALLY
OFFSHORE THIS MORNING. THEY WILL PROBABLY TURN AROUND TO ONSHORE
THIS AFTERNOON OR EVENING BUT LOOK TO BE A BIT WEAKER TONIGHT THAN
LAST NIGHT. SO W WINDS SHOULD REMAIN BELOW SMALL CRAFT ADVISORY
STRENGTH THIS EVENING IN THE STRAIT.
GRADIENTS ARE STRONGER FRIDAY AFTERNOON AND EVENING AND HAVE
INDICATED SMALL CRAFT ADVISORY STRENGTH THEN.
A WEAKENING FRONT APPROACHING THE COAST FRIDAY NIGHT MAY BE STRONG
ENOUGH TO TURN WINDS SOUTHERLY ON THE COAST AND PERHAPS INLAND AS
WELL. FORECASTS REFLECT THIS TREND. MODELS ARE FAIRLY CONSISTENT
WITH THIS FEATURE AND HAVE ALSO BEEN CONSISTENT IN BRINGING A LITTLE
RAIN TO THE COAST AND OLYMPICS. CHB
SEW WATCHES/WARNINGS/ADVISORIES
WA...NONE.
PZ...NONE.
WEATHER.GOV/SEATTLE
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