Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 311041 CCA
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
330 AM PDT FRI JUL 31 2015

.SYNOPSIS...A STRONG UPPER LEVEL RIDGE WILL BE OVER THE PACIFIC
NORTHWEST THROUGH SATURDAY. THE RIDGE WILL WEAKEN AND SHIFT A LITTLE
EAST SUNDAY INTO MONDAY. AN UPPER LEVEL TROUGH WILL MOVE OVER THE
AREA BY THE MIDDLE OF NEXT WEEK. TEMPERATURES WILL MODERATE TO NEAR
NORMAL WITH A CHANCE OF A FEW SHOWERS WEDNESDAY THROUGH THE END OF
NEXT WEEK.

&&

.SHORT TERM...A STRONG RIDGE OF HIGH PRESSURE WITH 500 MB HEIGHTS
NEAR 5900M WILL CONTINUE OVER THE AREA TODAY. THERE IS VERY LITTLE
CHANGE IN THE OVERALL AIR MASS AND THE LOW LEVEL FLOW REMAINS
OFFSHORE...EXCEPT NEAR COAST. THE ONLY CHANGE IS THE LOCATION OF THE
THERMALLY INDUCED TROUGH JUST W OF PUGET SOUND EXTENDING OVER THE
OLYMPICS. NOT SURPRISINGLY...MOS HAS TRENDED WARMER FOR TODAY WITH
HIGH TEMPERATURES ACROSS INTERIOR WRN WA SIMILAR TO YESTERDAY.
EXPECT WIDESPREAD MID TO UPPER 90S FROM SEATTLE SOUTHWARD ALONG THE
I-5 CORRIDOR. ONSHORE FLOW ALONG THE COAST WILL RESULT IN HIGHS ONLY
REACHING THE UPPER 70S. OVER THE N INTERIOR...HIGHS WILL BE IN THE
70S ALONG THE WATER AND 80S JUST INLAND.

THE RIDGE FLATTENS SLIGHTLY ON SATURDAY BUT 500MB HEIGHTS ONLY COME
DOWN TO ABOUT 5860M OVER WRN WA. THERMALLY INDUCED LOW PRESSURE WILL
SHIFT A BIT FURTHER INLAND OVER THE INTERIOR WATERS. ONSHORE
GRADIENTS WILL BE A BIT STRONGER ON THE COAST WITH SOME WLY FLOW
DEVELOPING IN THE STRAIT DURING THE EVENING. HOWEVER...THE MAIN
MARINE PUSH FOR THE INTERIOR WILL LIKELY HOLD OFF UNTIL SUNDAY
EVENING.

LATEST MODELS STILL SHOW UPPER HEIGHTS DROPPING ON SUNDAY BUT MODELS
ARE IN PRETTY GOOD AGREEMENT THAT THE THERMAL HEAT LOW WILL PERSIST
OVER PUGET SOUND. THE NAM-12 INDICATES THE TROUGH AXIS WILL SHIFT
FROM THE FAR S SOUND OVER CENTRAL PUGET SOUND. ALTHOUGH HEIGHTS ARE
LOWER...THE LOW LEVEL AIR MASS WILL PROBABLY ONLY MODERATE SLIGHTLY
SINCE THE DAY AHEAD OF A MARINE PUSH IS USUALLY QUITE WARM. WENT OUT
ON A LIMB A BIT WITH THE FORECAST HIGHS NEAR 90 FOR THE SW INTERIOR
AND THE SEATTLE METRO AREA S AND E. SEA-TAC COULD REACH 90 AND THAT
WOULD AGAIN TIE THE RECORD FOR A 5 DAY STRETCH OF 90S IF IT WERE TO
OCCUR. BECAUSE THE MARINE PUSH WILL PROBABLY HOLD OFF UNTIL SUNDAY
EVENING...THE HEAT ADVISORY WILL BE EXTENDED THROUGH SUNDAY EVENING
FOR THE SEATTLE METRO AREA SWD ALONG I-5 THROUGH LEWIS COUNTY. EVEN
IF HIGHS FALL SHORT OF 90...UPPER 80S AFTER SEVERAL HOT DAYS AND
MILD NIGHTS WILL STILL WARRANT AN ADVISORY.

IF THE 00Z MODELS ARE RIGHT...A MARINE PUSH WILL DEVELOP SUNDAY
EVENING. GALES ARE POSSIBLE IN THE STRAIT AND MARINE AIR WILL
INFILTRATE THE INTERIOR LOWLANDS. THIS WILL START A TREND OF
MODERATING TEMPERATURES.

.LONG TERM...THE RIDGE WILL WEAKEN AND SHIFT EWD OVER THE ROCKIES BY
MONDAY AS AN UPPER TROUGH DROPS SE TOWARD THE PAC NW. LOWER HEIGHTS
AND ONSHORE FLOW MEAN HIGHS WILL STRUGGLE TO REACH THE LOW 80S AROUND
PUGET SOUND...WITH 70S ACROSS THE N INTERIOR AND STRAIT. HIGHS AT
THE COAST MAY STRUGGLE TO REACH 70.

MODELS TAKE THE UPPER LOW SWD OFF THE PAC NW AND SRN B.C. COASTS ON
TUESDAY. ONSHORE FLOW WILL BE STRONGER AND HIGH TEMPERATURES WILL
FINALLY FALL TO THE 70S OVER ALL THE LOWLANDS...NEAR OR EVEN
SLIGHTLY BELOW NORMAL. THE LOW COULD MOVE CLOSE ENOUGH TO THE COAST
TO BRING A FEW SHOWER TO THE COAST AND MOUNTAINS BY WEDNESDAY. BY
THURSDAY AND FRIDAY THERE MAY BE A BETTER CHANCE OF SHOWERS IF THE
LOW MOVES CLOSER TO THE AREA AS SOME MODELS INDICATE. HIGHS WILL BE
NEAR OR EVEN BELOW NORMAL TOWARD THE END OF NEXT WEEK. MERCER

&&

.AVIATION...AN UPPER RIDGE WILL CONTINUE OVER THE REGION. WESTERLY
FLOW ALOFT AND LOW LEVEL NW FLOW. PATCHY LOW CLOUDS AND FOG ARE
POSSIBLE ALONG THE COAST EARLY THIS MORNING.

KSEA...VFR. CONTD NLY WINDS.

&&

.MARINE...PERSISTENT HIGH PRES OFFSHORE WITH LOWER PRES E OF THE
CASCADES WILL KEEP THE FLOW NORTHWEST TODAY. SMALL CRAFT ADVISORY
WINDS ARE EXPECTED IN THE CENTRAL AND EASTERN STRAIT THIS EVENING.
EXPECT THE FLOW TO BECOME MORE WESTERLY AND STRENGTHEN THIS WEEKEND
LEADING TO THE POSSIBILITY OF GALE FORCE WINDS OVER THE CENTRAL AND
EASTERN STRAIT OF JUAN DE FUCA.

&&

.FIRE WEATHER...NO UPDATE FROM PREVIOUS DISCUSSION...A CRITICAL
PERIOD FOR FIRE WEATHER IS UNDERWAY... GIVEN HOW EXTREMELY DRY THE
AIR MASS IS OVER WESTERN WASHINGTON. MANY LOCATIONS WILL EXPERIENCE
RELATIVE HUMIDITY DOWN IN THE 20S THIS AFTERNOON...EVEN LOCATIONS
THAT NORMALLY STAY MORE HUMID SUCH AS NEAR THE STRAIT OF JUAN DE
FUCA. WHEN YOU GET ABOVE A LOW-LEVEL SUBSIDENCE INVERSION NEAR 2000
FEET MSL...THE AIR MASS IS ALSO MORE UNSTABLE. WITH THAT ADDITIONAL
AGGRAVATING FACTOR...WILL KEEP THE RED FLAG WARNING GOING FOR
ELEVATIONS ABOVE 2000 FEET. LOOKS LIKE LITTLE CHANGE IN THE
CHARACTER OF THE MID-LEVEL AIR MASS UNTIL SATURDAY EVENING...SO HAVE
EXTENDED THE RED FLAG WARNING UNTIL 10 PM SATURDAY. THIS IS THE KIND
OF WEATHER SETUP THAT HELPS SMALL FIRES GROW TO BECOME LARGE FIRES.
HANER

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...HEAT ADVISORY REMAINS IN EFFECT TIL 8 PM SUNDAY FOR PARTS OF
     THE PUGET SOUND REGION AND SOUTHWEST INTERIOR.

     RED FLAG WARNING IN EFFECT TIL 10 PM SATURDAY NIGHT FOR THE
     CASCADES AND OLYMPICS ABOVE 2000 FEET.

PZ...NONE.


&&

$$

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