Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 302225
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
325 PM PDT TUE JUN 30 2015

.SYNOPSIS...A BROAD UPPER LEVEL RIDGE WILL REMAIN OVER THE PACIFIC
NORTHWEST INTO EARLY NEXT WEEK...ALLOWING VERY WARM AND DRY WEATHER
TO CONTINUE.

&&

.SHORT TERM...STRATUS CLOUDS HAVE RETREATED BACK TO THE IMMEDIATE
COAST THIS AFTERNOON...BUT A ROUGHLY 40-60 MILE WIDE AREA OF STRATUS
EXTENDS OFFSHORE FROM THE COAST. THIS WILL BE A READY SUPPLY OF
MOISTURE TO ADVECT INLAND THIS EVENING. 2 PM PRESSURE GRADIENTS WERE
STILL WEAKLY ONSHORE WITH 1.4 MB HQM-SEA AND 1.5 MB UIL-BLI. ARW AND
NAM MODELS SHOW STRATUS MOVING PARTWAY INLAND UP THE LOWER CHEHALIS
VALLEY...PROBABLY NOT QUITE REACHING SHELTON WEDNESDAY MORNING.

A BROAD UPPER LEVEL RIDGE WILL REMAIN OVER THE WESTERN U.S. THROUGH
THURSDAY NIGHT...CENTERED OVER THE SW U.S.  W FLOW ALOFT WILL
PREVAIL OVER THE PACNW DURING THE PERIOD WITH A COUPLE OF WEAK UPPER
LEVEL SHORTWAVES EMBEDDED IN THE FLOW TRAVERSING THE AREA. AFTER THE
WEAK UPPER LEVEL SHORTWAVE TROUGH OVER SOUTHERN B.C. MOVES E
TONIGHT...A WEAK UPPER LEVEL SHORTWAVE RIDGE WILL CROSS THE AREA
WEDNESDAY. THIS WILL ALLOW THE RIDGE TO REBUILD A BIT WITH 500 MB
HEIGHTS RISING TO AROUND 5880 METERS...ALLOWING WEDNESDAYS MAX TEMPS
TO REBOUND INTO THE UPPER 70S AND 80S.

ANOTHER WEAKER UPPER LEVEL SHORTWAVE TROUGH WILL FOLLOW ON
THURSDAY...CROSSING W WA AROUND 12Z. THIS ONE WILL HAVE EVEN LESS
IMPACT AS MAX TEMPS ARE EXPECTED TO WARM ANOTHER DEGREE OR SO.

A MORE SUBSTANTIAL BUT DRY UPPER LEVEL TROUGH...EXTENDING S FROM AN
UPPER LEVEL LOW OVER NORTHERN B.C...WILL BRUSH W WA FRIDAY THROUGH
EARLY SATURDAY. THE GFS IS THE STRONGEST MODEL COMPARED TO THE ECWMF
AND CANADIAN...AND LOWERS 500 MB HEIGHTS DOWN TO ABOUT 5790 METERS
COMPARED TO 5820 TO 5850 METERS ON THE OTHER TWO MODELS. THIS WOULD
TRANSLATE TO A BIT MORE AIR MASS COOLING AND A NEW BATCH OF STRATUS
REACHING THE COAST. THE MAX TEMP FORECAST IS A COMPROMISE BETWEEN
THE ECMWF AND GFS WITH HIGHS IN THE 80S.

THROUGH THE ENTIRE PERIOD A SURFACE HIGH ABOUT 600 NM OFFSHORE WILL
MAINTAIN WEAK TO MODERATE LOW LEVEL ONSHORE FLOW OVER W WA. THIS
WILL PRODUCE LATE NIGHT AND MORNING STRATUS ALONG THE COAST AND OVER
THE ADJACENT LOWER CHEHALIS VALLEY. WITH ONSHORE PRESSURE GRADIENTS
FROM A W-NW DIRECTION IT WILL BE HARDER FOR STRATUS TO PUSH E OVER
PUGET SOUND. KAM

.LONG TERM...COMPARED TO THE ECMWF AND CANADIAN MODELS...THE GFS IS
SLOWER IN MOVING THE NORTHERN B.C. LOW OFF TO THE EAST ON SUNDAY AND
IN ADDITION BRINGS ANOTHER TROUGH DOWN THE B.C. COAST ON MONDAY.
THIS KEEPS W WA UNDER AN UPPER LEVEL TROUGH...WITH A COOLER AIR MASS
AND LOWER 500 MB HEIGHTS. THE ECMWF AND CANADIAN MODELS ARE FASTER
WITH THE DEPARTING CANADIAN LOW AND ARE ABLE TO REBUILD THE UPPER
LEVEL RIDGE MORE QUICKLY...RESULTING IN WARMER TEMPERATURES OVER W
WA. THE GFS LOOKS A LITTLE TOO STRONG WITH THE TROUGH SO THE
TEMPERATURE FORECAST IS CLOSER TO THE OTHER MODELS AND THE STATUS
QUO FOR LATE THIS WEEK. WEAK ONSHORE FLOW WILL CONTINUE. KAM

&&

.CLIMATE...WITH ONLY ONE DAY LEFT IN THE MONTH SEATTLE WILL END UP
WITH THE WARMEST JUNE ON RECORD. THROUGH THE 29TH THE AVERAGE HIGH
FOR THE MONTH IS 78.6 DEGREES. THE OLD RECORD FOR THE MONTH WAS 75.8
DEGREES IN 1992. WITH THE WARM LOW TEMPERATURES THE LAST FEW DAYS
SEATTLE IS NOW ON PACE TO BREAK THE RECORD FOR THE WARMEST AVERAGE
LOW TEMPERATURE FOR THE MONTH OF JUNE. THROUGH THE 29TH THE AVERAGE
LOW FOR THE MONTH HAS BEEN 56.3 DEGREES. THE RECORD WAS 55.7 DEGREES
SET IN 2013. THE AVERAGE TEMPERATURE FOR THE MONTH THROUGH THE 29TH
IS 67.5 DEGREES. THE OLD RECORD WAS 64.9 DEGREES SET IN 1992.

TO PUT THIS RECORD WARM JUNE IN PERSPECTIVE...THE CURRENT AVERAGE
MAX TEMPERATURE OF 78.6 DEGREES WOULD BE THE 12TH WARMEST JULY ON
RECORD AND THE 9TH WARMEST AUGUST ON RECORD. RECORDS STARTED AT SEA-
TAC IN 1945.

IN ADDITION TO THE WARM WEATHER IT HAS ALSO BEEN A VERY DRY JUNE
WITH ONLY 0.23 INCHES OF RAIN AT SEA-TAC. THIS WILL BE THE 4TH
DRIEST JUNE ON RECORD. THE MAY 1ST THROUGH JUNE 30TH RAINFALL TOTAL
IS 0.81 INCHES. THIS WILL BE THE DRIEST ON RECORD. THE OLD RECORD
WAS 1.26 INCHES IN 1992.

NUMEROUS OTHER RECORDS WILL BE BROKEN ACROSS THE AREA THIS MONTH.
THE RECORDS WILL BE SUMMARIZED TOMORROW IN A PUBLIC INFORMATION
STATEMENT. FELTON

$$

.AVIATION...A WEAK UPPER TROUGH MOVING THROUGH SOUTHERN BRITISH
COLUMBIA WILL BRUSH WASHINGTON THIS EVENING. HIGH PRESSURE ALOFT
WILL STRENGTHEN LATER TONIGHT AND WEDNESDAY. LIGHT WESTERLY FLOW
ALOFT. LOW LEVEL ONSHORE FLOW WILL INCREASE LATE TODAY AND CONTINUE
TONIGHT. THE AIR MASS IS MAINLY DRY EXCEPT FOR MOISTURE IN THE
SHALLOW SURFACE-BASED MARINE LAYER. IFR STRATUS TONIGHT WILL LIKELY
STAY CONFINED TO THE COAST AND STRAIT OF JUAN DE FUCA AFFECTING
MAINLY THE KHQM TERMINAL. PATCHY LOW CLOUDS COULD ALSO FORM IN PLACE
ACROSS THE INTERIOR BUT IS UNLIKELY AT THE TERMINALS.

KSEA...STRATUS ALONG THE COAST MAY PARTIALLY INTRUDE INTO THE WRN
PORTION OF THE SW INTERIOR BUT IS NOT EXPECTED TO REACH THE
TERMINAL. NW WIND 5-8 KT THROUGH 04Z...THEN N-NE TONIGHT. DTM

&&

.MARINE...ONSHORE FLOW WILL INCREASE THIS EVENING AS A SYSTEM
BRUSHES THE PACIFIC NORTHWEST. GALE FORCE WESTERLIES ARE
POSSIBLE TONIGHT IN THE CENTRAL STRAIT OF JUAN DE FUCA AND EAST
ENTRANCE STRAIT. IN ADDITION SMALL CRAFT ADVISORY WINDS ARE
LIKELY OVER THE OUTER COASTAL WATERS AND IN ADMIRALTY INLET TONIGHT.

VARIABLE DEGREES OF ONSHORE FLOW ARE LIKELY THE NEXT FEW DAYS OVER
THE COASTAL WATERS AND IN THE STRAIT. PERIODS OF SMALL CRAFT
STRENGTH WINDS ARE POSSIBLE...ESPECIALLY DURING THE EVENINGS IN THE
CENTRAL/E STRAIT OF JUAN DE FUCA. WINDS WILL BE LIGHTER AND MORE NLY
OVER THE INLAND WATERS THROUGH THE WEEK. DTM

&&

.SEW WATCHES/WARNINGS/ADVISORIES...

WA...NONE.
PZ...GALE WARNING CENTRAL AND EASTERN STRAIT OF JUAN DE FUCA.
     SMALL CRAFT ADVISORY ADMIRALTY INLET AND THE OUTER COASTAL WATERS.

&&

$$

YOU CAN SEE AN ILLUSTRATED VERSION OF THIS FORECAST DISCUSSION AT
WWW.WEATHER.GOV/SEATTLE/GAFD/LATEST_WEBAFD.HTML


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