Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 241041
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
330 AM PDT THU JUL 24 2014

.SYNOPSIS...UPPER LEVEL LOW PRESSURE OVER WASHINGTON WILL SHIFT
NORTHEASTWARD INTO SOUTHERN ALBERTA TODAY. SHOWERS THIS MORNING WILL
GRADUALLY TAPER OFF THIS AFTERNOON...EXCEPT WHERE A PUGET SOUND
CONVERGENCE ZONE DEVELOPS. HIGH PRESSURE WILL BUILD OVER THE REGION
ON FRIDAY AND DOMINATE THE PATTERN THROUGH EARLY NEXT WEEK. THIS
WILL MAINTAIN DRY WEATHER AND MUCH WARMER TEMPERATURES.

&&

.SHORT TERM...WATER VAPOR IMAGERY CLEARLY SHOWS THE POSITION OF THE
UPPER LEVEL LOW OVER CENTRAL WA EARLY THIS MORNING. MODELS AGREE
THAT THE MAIN LOW CENTER WILL TRACK NE AND MOVE INTO SRN ALBERTA
THIS AFTERNOON. THERE ARE STILL SEVERAL FEATURES OF INTEREST THAT
WILL AFFECT THE WEATHER TODAY EVEN AS THE LOW SHIFTS AWAY FROM THE
AREA. THE LATEST RADAR LOOP INDICATES SHOWERS ARE MOSTLY
CONCENTRATED OVER THE S COAST...SW INTERIOR...AND THROUGH PUGET
SOUND EASTWARD OVER THE CASCADES. CONVECTION HAS WEAKENED NOTABLY
OVERNIGHT WITH NO LIGHTNING STRIKES SINCE LAST EVENING INDICATING
LOWER TOPPED SHOWERS. THE VORT MAX RESPONSIBLE FOR SOME OF THE
SHOWER ACTIVITY WILL ROTATE E OF THE CASCADES EARLY THIS MORNING SO
PRECIPITATION COVERAGE SHOULD GENERALLY DECREASE. ANOTHER AREA TO
WATCH IS THE BAND OF WRAP AROUND MOISTURE/RAINFALL OVER
B.C./VANCOUVER ISLAND AND JUST BEGINNING TO AFFECT THE EXTREME N
INTERIOR. THE WRFGFS APPEARS TO MATCH UP BEST WITH CURRENT RADAR
TRENDS AND IS WETTER ACROSS THE N INTERIOR THIS MORNING. DECIDED TO
GO WITH A BLEND OF MOS POPS WEIGHTED TOWARD THE WRFGFS
IDEA...CATEGORICAL ACROSS THE N AND LIKELY POPS SOUTH.

SHOWERS WILL GENERALLY BE ON THE DECREASE THIS AFTERNOON BUT MOIST
ONSHORE FLOW WILL MAINTAIN SHOWERS IN THE MOUNTAINS. CHANCE POPS
SHOULD SUFFICE FOR MOST OF THE LOWLANDS WITH LOSS OF DYNAMICS AND
COOL TEMPERATURES LIMITING DIURNAL CONVECTION. HOWEVER...BOTH THE
NAM-12 AND WRFGFS DEVELOP A PUGET SOUND CONVERGENCE ZONE DURING THE
AFTERNOON AS THE FLOW THROUGH THE STRAIT EXTENDS INTO THE N SOUND
AND ENCOUNTERS THE SOUTHERLIES. BOTH THE NAM AND WRFGFS INDICATE THE
CONVERGENCE ZONE WILL GET GOING DURING THE AFTERNOON AND CONTINUE
THROUGH THE EVENING. NOT ALL MODELS SHOW THE CONVERGENCE ZONE
LASTING INTO THE EVENING AND EVEN THE NAM-12 HAS NOT BEEN CONSISTENT
FROM RUN TO RUN. WILL OPT TO LEAVE THE EVENING FORECAST UNCHANGED
AND ALLOW THE NEXT SHIFT TO FINE TUNE THE TIMING AND POPS.

MOIST WESTERLY FLOW CONTINUES INTO FRIDAY. THERE COULD BE A FEW
SHOWERS IN THE MOUNTAINS BUT IT WILL BE DRY FOR MOST THE AREA. MOST
AREAS WILL GET SOME SUNSHINE IN THE AFTERNOON. HIGHS WILL WARM UP
INTO THE LOW TO MID 70S...EXCEPT COOLER ALONG THE COAST/STRAIT.
UPPER HEIGHTS RISE FRIDAY NIGHT AS THE FOUR CORNERS HIGHS NUDGES
NWD. THE RIDGE WILL PROVIDE A STRETCH OF DRY WEATHER AND WARMING
TEMPERATURES. HIGHS ON SATURDAY COULD REACH CLOSE TO 80...ABOUT 15
DEGREES WARMER THAN WEDNESDAY`S CHILLY HIGHS IN THE MID 60S.

.LONG TERM...LONG RANGE MODELS ARE FAIRLY CONSISTENT IN
STRENGTHENING A LARGE HIGH PRESSURE RIDGE OVER THE INTERIOR WRN U.S.
SUNDAY THROUGH THE MIDDLE OF NEXT WEEK. IT APPEARS THE RIDGE AXIS
WILL EXTEND THROUGH THE NRN ROCKIES WHICH SHOULD RESULT IN SW FLOW
ALOFT OVER WA. THIS UPPER FLOW PATTERN FAVORS LIGHT W-NW SURFACE
GRADIENTS WHICH MAY BRING SOME CLOUDS TO THE COAST BUT PROBABLY NOT
MAKING MUCH PROGRESS INLAND. GFS MOS GIVES LOW TO MID 80S SUNDAY
THROUGH THE MIDDLE OF NEXT WEEK. THE ECMWF IS STRONGER AND FURTHER
WEST WITH THE RIDGE WHICH COULD RESULT IN TEMPERATURES APPROACHING
90 DEGREES. EITHER WAY IT WILL BE A BIG CHANGE IN THE PATTERN WITH
DRY WEATHER AND ABOVE AVERAGE TEMPERATURES. MERCER

&&

.AVIATION...THE UPPER LEVEL LOW OVER THE WASHINGTON COAST WILL
CONTINUE TO LIFT NORTH AND EAST INTO EASTERN BRITISH COLUMBIA THIS
MORNING. LIGHT FLOW ALOFT WILL BECOME NORTHWEST AND STRENGTHEN AS
THE UPPER LEVEL LOW SHIFTS TO THE NORTHEAST. LOW LEVEL ONSHORE FLOW
WILL STRENGTHEN THIS AFTERNOON AND EVENING.

THE AIR MASS OVER WESTERN WASHINGTON WILL REMAIN UNSTABLE AND MOIST
THIS MORNING WITH SHOWERS PERSISTING AS THE UPPER LEVEL LOW MOVES
INLAND. VFR CONDITIONS WILL GENERALLY LOWER EARLY THIS MORNING TO
MVFR LEVELS WITH SOME IMPROVEMENT DURING THE AFTERNOON
HOURS. THE EXCEPTION WILL BE IN AREAS WITH LINGERING
SHOWERS...ESPECIALLY OVER THE NORTH AS WRAP AROUND MOISTURE AND
UPPER LEVEL DYNAMICS CONTINUE TO WORK THEIR WAY AROUND THE UPPER
LEVEL LOW AND INTO THE AREA. IN ADDITION...MESOSCALE MODELS SHOWING
THE DEVELOPMENT OF A PSCZ OVER THE CENTRAL SOUND LATE IN THE
AFTERNOON AND EVENING.

KSEA...VFR CONDITIONS WILL LOWER TO MVFR LEVELS OVER THE NEXT FEW
HOURS. EXPECT SHOWERS TO LINGER THROUGH THE MORNING WITH SOME
IMPROVEMENT IN THE AFTERNOON. MESOSCALE MODELS SHOWING PSCZ FORMING
OVER THE CENTRAL SOUND DURING THE AFTERNOON AND EVENING HOURS WITH
SOME CHANCE THAT IT WILL AFFECT THE METRO TERMINALS. SOUTH WINDS 8
TO 10 KT WILL BECOME SOUTHWEST BY MIDDAY.

&&

.MARINE...THE SURFACE LOW MOVING ONSHORE THIS MORNING WILL WEAKEN
AND CONTINUE TO MOVE INLAND. MODERATE NORTHWESTERLY WINDS OVER THE
OUTER COASTAL WATERS WILL EASE THIS AFTERNOON...WHILE THE ONSHORE
FLOW THROUGH THE STRAIT WILL STRENGTHEN THIS AFTERNOON PRODUCING
15-25 KT WESTERLIES THROUGH THE CENTRAL AND EAST STRAIT THROUGH
TONIGHT. SIMILARLY SMALL CRAFT ADVISORY SOUTHWESTERLIES WILL AFFECT
THE NORTHERN INLAND WATERS THIS EVENING BEFORE EASING AFTER
MIDNIGHT.

A MORE TYPICAL ONSHORE FLOW PATTERN WILL PERSIST THROUGH THE WEEKEND
WITH SMALL CRAFT ADVISORY WESTERLIES IN THE STRAIT AT
TIMES...ESPECIALLY DURING THE AFTERNOON AND EVENING HOURS.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY OUTER COASTAL WATERS...CENTRAL AND EAST
     STRAIT...NORTHERN INLAND WATERS.

&&

$$

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










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