Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 292215
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
300 PM PDT TUE JUL 29 2014

.SYNOPSIS...HIGH PRESSURE ALOFT WILL BRING WARM DAYS TO WESTERN
WASHINGTON THROUGH EARLY NEXT WEEK. ONSHORE FLOW WILL ALLOW CLOUDS
ALONG THE COAST TO INTRUDE THROUGH THE STRAIT OF JUAN DE FUCA AND
LOWER CHEHALIS GAP DURING THE LATE NIGHT AND MORNING HOURS.
TEMPERATURES ACROSS GREATER PUGET SOUND WILL REMAIN NEAR TO ABOVE
AVERAGE THROUGH THE WEEKEND.

&&

.SHORT TERM...A STRONG UPPER LEVEL RIDGE OVER THE INTERIOR WRN U.S.
WILL KEEP DRY SW FLOW ALOFT OVER WA THROUGH WEDNESDAY AND THURSDAY.
ONSHORE PRESSURE GRADIENTS WILL BE A LITTLE STRONGER THAN LAST
EVENING SO LOW CLOUDS AND FOG WILL AGAIN DEVELOP IN THE
STRAIT...AROUND THE EAST ENTRANCE...AND THROUGH THE LOWER CHEHALIS
GAP INTO ABOUT SHELTON. BUT MARINE AIR WILL LIKELY NOT FULLY MAKE IT
INTO THE SOUND SO TEMPERATURES WILL START OFF WARM AND SHOULD REACH
THE LOW TO MID 80S BOTH WEDNESDAY AND THURSDAY.

MODELS HAVE LATCHED ON TO A WEAK UPPER LEVEL SHORT WAVE LIFTING NWD
TOWARD THE PAC NOW THURSDAY NIGHT AND SKIRTING E OF THE CASCADES ON
FRIDAY. THIS SYSTEM MAY HELP INDUCE STRONGER LOW LEVEL ONSHORE FLOW
BUT HOW STRONG OF A SWLY PUSH DEVELOPS IS IN QUESTION. TEMPERATURES
SHOULD COME DOWN AT LEAST A COUPLE DEGREES BUT PROBABLY STILL HOVER
NEAR THE LOW 80S...UNLESS THE PUSH IS QUITE STRONG.

ANOTHER EFFECT FROM THE SYSTEM WILL BE TO GENERATE WEAK INSTABILITY
OVER THE MOUNTAINS. THE TRAJECTORY OF THE TROUGH AND ASSOCIATED
VORTICITY MAXIMUM WILL DETERMINE WHETHER THERE WILL BE JUST CUMULUS
OVER THE CASCADES/OLYMPICS OR POSSIBLY SOME ISOLATED THUNDER. MODELS
STARTED PICKING UP ON THIS FEATURE LAST NIGHT AND MOST OF THEM AGREE
THAT IT AFFECTS OUR REGION BUT EACH DIFFERENTLY. THE NAM-12 AND
CANADIAN BOTH INDICATE THE VORT MAX AND ASSOCIATED INSTABILITY LIFT
MAINLY NE THROUGH ERN OREGON AND ERN WA. THIS MIGHT RESULT IN SOME
CU OVER THE CASCADES BUT PROBABLY NOT MUCH THUNDERSTORM ACTIVITY.
THE CANADIAN HAS A TRAILING TROUGH THAT COULD PRODUCE SOME MOUNTAIN
THUNDER BY SATURDAY BUT THAT IS HIGHLY UNCERTAIN. THE ECMWF AND GFS
ARE STRONGER WITH THIS SHORT WAVE WHICH ORIGINATED FROM THE TROPICS.
THESE TWO MODELS HAVE DEEPER MOISTURE AND LIFT OVER THE CASCADES AND
OLYMPICS AS THE TROUGH LIFTS MORE NWD THAN THE OTHER MODELS. SO THE
FORECAST CHALLENGE IS HOW TO WEIGH THE MODELS AND WHETHER OR NOT TO
INTRODUCE THUNDERSTORMS. LOOKING AT THE VAST DIFFERENCES BETWEEN 5
DIFFERENT MODELS MY CONFIDENCE IS QUITE LOW ON WHERE THIS SHORT WAVE
WILL TRACK. DECIDED THE BEST WAY TO HANDLE THIS IS TO INCREASE CLOUD
COVER OVER THE CASCADES/OLYMPICS DURING THE AFTERNOON EVENING HOURS
IMPLYING CUMULUS BUILD UP. WILL WAIT FOR MORE MODEL RUNS BEFORE
INTRODUCING ANY THREAT OF THUNDER.

.LONG TERM...THE ECMWF/GFS/AND TO SOME EXTENT THE CANADIAN ARE SLOW
TO SHIFT THE SHORT WAVE OUT OF THE AREA. JUST ENOUGH MOISTURE AND
LIFT OVER THE MOUNTAINS COULD PRODUCE SOME LIGHTNING NEAR THE CREST.
HOWEVER...LOOKING AT THE WRFGFS AND EXTENSION MODEL MOST THE QPF
DOES INDEED STAY ALONG THE CREST OR ON THE EAST SIDE THROUGH
MONDAY. WITH MOST THE ACTIVITY LIKELY TO BE JUST OUTSIDE OUR
FORECAST AREA WILL INDICATE INCREASING AFTERNOON/EVENING CLOUDS OVER
THE MOUNTAINS AND LEAVE IT AT THAT FOR NOW.

OTHERWISE...NOT MUCH IMPACT ON THE LOWLANDS. THE ONSHORE FLOW WILL
BRING CLOUDS IN FURTHER OVER THE WEEKEND BUT SHOULD BURN OFF BY
MIDDAY. HIGHS MAY STILL TOP OUT NEAR 80 GIVEN THE 500 MB HEIGHTS
STILL ABOVE 5800M EVEN IF THE WEAK UPPER LOW LIFTS ACROSS.

AFTER THE LOW KICKS OUT THE RIDGE REMAINS FLAT AND THERE IS A CHANCE
OF WLY FLOW ALOFT DEVELOPING. THIS WOULD HELP MODERATE TEMPERATURES
AND BRING MARINE AIR FURTHER INLAND. BUT SOME MODELS MAINTAIN A
STUBBORNLY STRONG RIDGE AND WARMER TEMPERATURES. WILL GO WITH A
COMPROMISE AND KEEP DRY WEATHER AND HIGHS SLIGHTLY ABOVE AVERAGE
WHICH HAS BEEN THE DOMINATE PATTERN. MERCER

&&

.CLIMATE...FROM PREVIOUS DISCUSSION...TODAY MARKS 5 YEARS SINCE THE
ALL TIME RECORD HIGH TEMPERATURE WAS SET AT SEA-TAC WAS SET ALONG
WITH OTHER RECORD HIGHS. SEATTLE(SEA-TAC) 103...NWS SEATTLE
105...CENTRALIA 107...OLYMPIA 104...ELMA 104...EVERETT
100...BELLINGHAM 96...ABERDEEN 95.

IN ADDITION...A TOTAL OF 0.31 INCHES OF RAIN HAS BEEN REPORTED ON
THIS DAY SINCE RECORDS STARTED AT SEATTLE-TACOMA AIRPORT IN 1945
MAKING THIS THE DRIEST DAY OF THE YEAR IN TERMS OF TOTAL RAINFALL
FOR THE DAY AT SEA-TAC.

&&

.AVIATION...AN UPPER LEVEL RIDGE CENTERED OVER THE INTERMOUNTAIN
WEST COMBINED WITH A TROUGH ALONG ABOUT 140W WILL GIVE SOUTHWEST
FLOW ALOFT THROUGH WEDNESDAY NIGHT. AT THE SURFACE...A RIDGE OF HIGH
PRESSURE OVER THE OFFSHORE WATERS COMBINED WITH LOWER PRESSURE EAST
OF THE CASCADES WILL GIVE LOW LEVEL ONSHORE FLOW. THE AIR MASS IS
DRY AND STABLE...EXCEPT FOR A SHALLOW MARINE LAYER ALONG THE COAST.

THE MARINE LAYER WILL ONTO THE WASHINGTON COASTAL AREAS THIS EVENING
THEN EAST THROUGH THE STRAIT OF JUAN DE FUCA AND IN THROUGH THE
CHEHALIS GAP OVERNIGHT. THE ONSHORE FLOW WILL BE STRONGER THAN LAST
EVENING AND WILL LIKELY RESULT IN STRATUS AND FOG PUSHING INLAND
FARTHER THAN THIS MORNING. THE TERMINAL MOST LIKELY TO BE IMPACTED
WILL BE KPAE...BUT KOLM MAY ALSO GET SOME FOG AND STRATUS WED
MORNING. ANY LOCATION GETTING STRATUS AND FOG WILL LIKELY SEE IFR TO
LIFR CONDITIONS FOR A FEW HOURS WED MORNING WHILE OTHER AREAS WILL
REMAIN CLEAR.  ALBRECHT

KSEA...EXPECT CLEAR SKIES THROUGH TONIGHT. AT THIS TIME IT DOES NOT
APPEAR THAT FOG OR STRATUS WILL REACH THE TERMINAL WEDNESDAY
MORNING...BUT IT WILL LIKELY GET TO WITHIN 10 NM OF THE TERMINAL
OVER THE PUGET SOUND DURING THE EARLY MORNING HOURS. WILL INCLUDE A
FEW002 CLOUD LAYER IN THE MORNING PERIOD TO ACCOUNT FOR THIS. NW
WIND 6 TO 9 KT WILL VEER TO NE 5-7 KT AFTER 04Z THIS EVENING THEN
BECOME VARIABLE AFTER 08Z. ALBRECHT

&&

.MARINE...A RIDGE OF HIGH PRESSURE OVER THE COASTAL WATERS COMBINED
WITH LOWER PRESSURE EAST OF THE CASCADES WILL GIVE VARYING DEGREES
OF ONSHORE FLOW THROUGH SATURDAY. THE ONSHORE FLOW IS EXPECTED TO BE
STRONGER LATE THIS AFTERNOON AND TONIGHT THAN IT HAS BEEN OVER THE
PAST FEW DAYS. SO...A GALE WARNING HAS BEEN ISSUED FOR THE CENTRAL
AND EAST ENTRANCE TO THE STRAIT OF JUAN DE FUCA FROM 5 PM THIS
AFTERNOON UNTIL 5 AM WEDNESDAY MORNING. SMALL CRAFT ADVISORIES HAVE
BEEN ISSUED FOR AREAS ADJACENT TO THE GALE WARNING AREA...THE WEST
ENTRANCE TO THE STRAIT...THE NORTHERN INLAND WATERS...AND ADMIRALTY
INLET. NORTHWESTERLY FLOW OVER THE COASTAL WATERS WILL GIVE SMALL
CRAFT ADVISORY WINDS THERE THIS AFTERNOON AND EVENING.

THE PATTERN IS EXPECTED TO CHANGE LITTLE WED THROUGH SAT. SLIGHTLY
WEAKER ONSHORE PRESSURE GRADIENTS WILL STILL GIVE SMALL CRAFT
ADVISORY CONDITIONS TO THE COASTAL WATERS AND THE CENTRAL AND
EASTERN STRAIT OF JUAN DE FUCA DURING THE LATE AFTERNOON AND EVENING
HOURS EACH DAY. ALBRECHT

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...GALE WARNING CENTRAL STRAIT OF JUAN DE FUCA AND EAST
      ENTRANCE TO THE STRAIT.
     SMALL CRAFT ADVISORY COASTAL WATERS...WEST ENTRANCE STRAIT
      OF JUAN DE FUCA...ADMIRALTY INLET...AND NORTHERN INLAND
      WATERS.

&&

$$

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