Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

Current Version | Previous Version | Text Only | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

000
FXUS66 KSEW 271007
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
300 AM PDT WED AUG 27 2014

.SYNOPSIS...HIGH PRESSURE ALOFT AND WEAK ONSHORE FLOW WILL RESULT IN
ANOTHER MOSTLY SUNNY WARM DAY FOR WESTERN WASHINGTON TODAY. THE
MARINE LAYER WILL DEEPEN TONIGHT AND A SMALL UPPER TROUGH WILL MOVE
THROUGH AREA THURSDAY FOR CLOUDIER COOLER WEATHER. ANOTHER LARGER
UPPER TROUGH WILL BRING A CHANCE OF SHOWERS FRIDAY THROUGH THIS
WEEKEND...AND THAT PATTERN WILL PROBABLY CONTINUE EARLY NEXT WEEK.

&&

.SHORT TERM...THE UPPER LEVEL RIDGE THAT BROUGHT SUNNY WARM WEATHER
TO WESTERN WASHINGTON ON TUESDAY IS MOVING EAST OF THE REGION EARLY
THIS MORNING...WITH ZONAL FLOW DEVELOPING BEHIND IT. SURFACE FLOW
STARTED TO TURN ONSHORE BEHIND THE RIDGE AS WELL...SO MARINE
MOISTURE IN THE FORM OF STRATUS AND PATCHY FOG HAS BEEN MOVING A
LITTLE FURTHER INLAND FROM THE COAST OVERNIGHT.

ZONAL FLOW ALOFT WILL CONTINUE TODAY...BETWEEN AN UPPER TROUGH
MOVING THROUGH NORTHWEST CANADA AND AN UPPER RIDGE OVER CALIFORNIA.
THE AIR MASS WILL REMAIN FAIRLY WARM WITH 500 MB HEIGHTS AROUND 5820
METERS. ONSHORE FLOW WILL BE WEAK FOR MOST OF THE DAY. MARINE
STRATUS WILL PROBABLY MAKE IT THROUGH THE STRAIT OF JUAN DE FUCA TO
AROUND WHIDBEY ISLAND BY DAYBREAK...AND THROUGH THE CHEHALIS GAP TO
THE SOUTH END OF PUGET SOUND AND HOOD CANAL. ELSEWHERE SKIES SHOULD
REMAIN MOSTLY CLEAR...AND THE STRATUS WILL BURN OFF MID TO LATE
MORNING. HIGHS TODAY SHOULD BE ROUGHLY 5 DEGREES LOWER THAN THE
HIGHS TUESDAY.

A MORE DRAMATIC CHANGE WILL OCCUR TONIGHT AND THURSDAY. ONSHORE FLOW
WILL INCREASE AND THE MARINE LAYER WILL DEEPEN TONIGHT...WITH
STRATUS PUSHING WELL INLAND OVERNIGHT. THEN A SMALL UPPER SHORTWAVE
WILL MOVE THROUGH THE AREA THURSDAY. THERE COULD BE SOME DRIZZLE
ALONG THE COAST IN THE MORNING...AND SOME SHOWERS IN THE NORTH
CASCADES IN THE AFTERNOON. FOR WESTERN WASHINGTON ON THE WHOLE...IT
WILL JUST BE A MOSTLY CLOUDY AND COOLER DAY.

THIS TREND WILL CONTINUE THURSDAY NIGHT AND FRIDAY...WITH THE FLOW
ALOFT BACKING TO SOUTHWESTERLY AS A LARGER UPPER TROUGH MOVES INTO
THE OFFSHORE WATERS. LOW LEVEL FLOW WILL REMAIN ONSHORE...WITH A
FAIRLY DEEP MARINE LAYER OVER WESTERN WASHINGTON. FRIDAY WILL BE
ANOTHER MOSTLY CLOUDY DAY AND SOMEWHAT COOLER YET. THERE COULD BE
SOME LIGHT SHOWERS LATE FRIDAY AND FRIDAY NIGHT ESPECIALLY AT THE
COAST...IN THE MOUNTAINS...AND ACROSS THE NORTH PART OF WESTERN
WASHINGTON.   MCDONNAL

.LONG TERM...THE UPPER TROUGH WILL MOVE INLAND SATURDAY...AND THIS
DAY HAS THE BEST CHANCE FOR WIDESPREAD SHOWERS. THE NORTH COAST...
OLYMPICS...AND NORTH CASCADES IN PARTICULAR SHOULD HAVE
PRECIPITATION. THE LONG WAVE UPPER TROUGH AXIS WILL MOVE WELL EAST
OF THE CASCADES SATURDAY NIGHT...BUT IT LOOKS INCREASINGLY LIKELY
THAT SEVERAL SHORTWAVES WILL AFFECT WESTERN WASHINGTON SUNDAY
THROUGH TUESDAY. THE MODELS DIFFER ON THE TIMING AND STRENGTH OF
THESE FEATURES THOUGH...SO FOR NOW WE CAN REALISTICALLY ONLY
FORECAST A BROAD BRUSH CHANCE OF SHOWERS FOR THAT PERIOD. THE GFS
SHOWS AN ESPECIALLY COOL AND WET SOLUTION FOR TUESDAY...BUT THE
EUROPEAN AND CANADIAN MODELS WAIT TILL WEDNESDAY TO BRING IN A
WEAKER UPPER TROUGH.   MCDONNAL

&&

.AVIATION...WEAK HIGH PRES OVER THE PAC NW WILL MAINTAIN A DRY AND
STABLE AIR MASS ACROSS WESTERN WA TODAY. THE FLOW ALOFT IS WESTERLY.
LOW LEVEL STRATUS CLOUDS COVER THE COAST...STRAIT AND LOWER CHEHALIS
VALLEY THIS MORNING WITH IFR CONDITIONS POSSIBLE. LOW CLOUDS WILL
BURN BACK TO THE COAST THIS AFTERNOON WITH VFR CONDITIONS EXPECTED.
CLOUDS WILL INCREASE TONIGHT AS A COLD FRONT APPROACHES FROM THE
WEST. THIS SYSTEM MAY BRING LIGHT RAIN OR DRIZZLE TO THE REGION THU
MORNING. 33

KSEA...VFR CONDITIONS WILL PREVAIL TODAY. LIGHT WINDS AT THE SURFACE
WILL BECOME NORTHERLY THIS AFTERNOON. 33

&&

.MARINE...HIGH PRES OVER THE NE PAC WILL MAINTAIN ONSHORE FLOW
ACROSS WESTERN WA. HIGHEST WIND AND WAVES WILL BE FOUND THROUGH THE
CENTRAL AND EAST STRAIT OF JUAN DE FUCA. A WEAK COLD FRONT WILL
CROSS WESTERN WA ON THU...FOLLOWED BY ANOTHER WEAK SYSTEM ON FRI.
THE FLOW WILL REMAIN ONSHORE THROUGH THE WEEKEND. 33

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY CENTRAL AND EASTERN STRAIT OF JUAN DE FUCA.

&&

$$

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




USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.