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 241057
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
300 AM PST MON NOV 24 2014

.SYNOPSIS...RAIN WILL SPREAD INTO WESTERN WASHINGTON LATE THIS
MORNING THROUGH THIS EVENING...WITH A STRONG WARM FRONT MOVING
THROUGH TONIGHT. RAINY AND OCCASIONALLY WINDY WEATHER...ALONG WITH
MILD TEMPERATURES...WILL CONTINUE TONIGHT THROUGH WEDNESDAY.
THANKSGIVING SHOULD BE WET TOO...AS THE COLD FRONT FINALLY MOVES
THROUGH. COOLER BUT DRIER WEATHER IS LIKELY THIS WEEKEND.

&&

.SHORT TERM...A DIRTY UPPER RIDGE IS CENTERED OFFSHORE ALONG 130W
EARLY THIS MORNING...WITH WEST-NORTHWEST FLOW ALOFT OVER WESTERN
WASHINGTON. HIGH CLOUDS ARE STARTING TO INCREASE OVER THE FORECAST
AREA AHEAD OF AN APPROACHING WARM FRONT...BUT FOR THE TIME BEING
THERE IS LITTLE TO NO PRECIPITATION. TEMPERATURES FELL INTO THE MID
30S TO MID 40S OVERNIGHT...WITH PATCHY FOG IN THE USUAL SPOTS OF THE
SOUTH PUGET SOUND REGION AND SOUTHWEST INTERIOR.

RAIN WILL SPREAD INTO WESTERN WASHINGTON TODAY AHEAD OF THE STRONG
WARM FRONT THAT IS MAKING ITS WAY OVER THE NORTH SIDE OF THE
OFFSHORE UPPER RIDGE. RAIN WILL START AT THE COAST LATE THIS MORNING
AND SPREAD OVER THE ENTIRE AREA THIS AFTERNOON OR EARLY EVENING. THE
WARM FRONT WILL REACH THE AREA TONIGHT...AND MOIST WARM STRONGLY
BAROCLINIC FLOW WILL CONTINUE THROUGH TUESDAY NIGHT OR EVEN
WEDNESDAY. THE BAND OF MOISTURE SHOULD GRADUALLY LIFT NORTHWARD
TUESDAY NIGHT THROUGH WEDNESDAY NIGHT.

MODELS AGREE WELL ON THE LARGE-SCALE PATTERN...SHOWING LOTS OF
PRECIPITATION LATER TODAY THROUGH TUESDAY NIGHT OR WEDNESDAY WITH
THE SNOW LEVEL MAINLY 6500 TO 7500 FT. THEY DIFFER ON WHERE AND WHEN
THE HEAVIEST PRECIPITATION WILL OCCUR. WE CAN SAY WITH CONFIDENCE
THAT 3 TO 6 INCHES OF PRECIPITATION WILL FALL ALONG THE WEST SLOPES
OF THE CASCADES DURING THE 48 HOUR PERIOD FROM MIDDAY TODAY THROUGH
MIDDAY WEDNESDAY. THE PORTION OF THE CASCADES FROM AROUND STEVENS
PASS SOUTHWARD WILL PROBABLY HAVE THE HEAVIEST PRECIPITATION
INITIALLY...WITH THE FOCUS MOVING NORTHWARD STARTING LATE TUESDAY.
SEE THE HYDROLOGY SECTION BELOW FOR MORE ON THIS.

THE OLYMPICS SHOULD GET LESS PRECIPITATION IN THIS PATTERN --
SOMETHING LIKE 2 TO 3.5 INCHES. AMOUNTS OF 1 TO 2 INCHES SHOULD BE
COMMON ACROSS THE WESTERN WASHINGTON LOWLANDS. IT WILL BE BREEZY TO
LOCALLY WINDY AT TIMES...MAINLY AT THE COAST AND IN THE NORTHWEST
INTERIOR. HIGHS ON TUESDAY AND WEDNESDAY WILL BE IN THE MID TO UPPER
50S.   MCDONNAL

.LONG TERM...THE TRAILING COLD FRONT SHOULD MOVE THROUGH WESTERN
WASHINGTON THANKSGIVING AND THURSDAY NIGHT...WITH PRECIPITATION
TAPERING OFF IN ITS WAKE ON FRIDAY. PRECIPITATION WITH THAT FEATURE
WILL BE FAIRLY DECENT TOO. IT LOOKS INCREASINGLY LIKELY THAT AN
UPPER TROUGH WILL SWING INTO THE AREA FROM THE NORTHWEST FRIDAY
NIGHT OR SATURDAY...FOLLOWED BY DRIER COLDER NORTHERLY FLOW ALOFT
AND LOW LEVEL NORTHEAST FLOW COURTESY OF A SURFACE HIGH IN SOUTHERN
BRITISH COLUMBIA.   MCDONNAL

&&

.HYDROLOGY...AS MENTIONED ABOVE 3 TO 6 INCHES OF PRECIPITATION IS
LIKELY ALONG THE WEST SLOPES OF THE CASCADES DURING THE 48 HOURS
FROM MIDDAY TODAY THROUGH MIDDAY WEDNESDAY. THE SNOW LEVEL WILL RISE
TO AROUND 6500 FT TONIGHT AS THE STRONG WARM FRONT MOVES THROUGH...
AND REMAIN IN THE 6500 TO 7500 FT RANGE THROUGH WEDNESDAY NIGHT.

THIS PATTERN OF STRONG MOIST WESTERLY FLOW JUST DOWNSTREAM OF A FLAT
UPPER RIDGE IS ONE OF THE CLASSIC FLOOD-PRODUCING PATTERNS FOR
WESTERN WASHINGTON...ESPECIALLY FOR THE RIVERS FLOWING OFF THE WEST
SLOPES OF THE CASCADES. THIS EPISODE LOOKS SIMILAR TO -- BUT MUCH
LESS SEVERE THAN -- THE PATTERN THAT PRODUCED THE FLOODS OF LATE
NOVEMBER 1990 AND LATE NOVEMBER 1995.

THE PRECIPITATION AMOUNTS EXPECTED ARE HEAVY BUT NOT EXTREME...WHICH
MEANS THAT THE FLOOD THREAT WILL PROBABLY BE LIMITED TO THE MOST
FLOOD-PRONE RIVERS THAT FLOW OFF THE CASCADES. THAT GENERALLY
INCLUDES THE COWLITZ...PUYALLUP NEAR ORTING...SNOQUALMIE...TOLT...
SKYKOMISH...SNOHOMISH...STILLAGUAMISH...AND NOOKSACK RIVERS. THE
RIVERS OF THE OLYMPICS ARE NOT LIKELY TO FLOOD IN THIS SCENARIO...
BUT IT DOES NOT SEEM SMART TO RULE OUT THE POSSIBILITY ON THE
UNIQUELY FLOOD-PRONE SKOKOMISH RIVER.

IF FLOODING OCCURS...IT WOULD PROBABLY BE MINOR. HOWEVER...THE
FLOODING COULD BECOME WORSE IF THE FOCUS OF PRECIPITATION REMAINS
FIXED OVER A PARTICULAR PART OF THE CASCADES FOR MOST OF THE EVENT.
THERE IS TOO MUCH INCONSISTENCY IN THE MODEL SOLUTIONS TO KNOW THOSE
DETAILS AT THIS TIME.

WE HAVE DECIDED TO UPDATE THE HYDROLOGIC OUTLOOK FOR NOW...RATHER
THAN GOING TO A FLOOD WATCH. THE DAY SHIFT SHOULD BE IN A BETTER
POSITION TO PIN DOWN A FLOOD WATCH LATER TODAY...AFTER THE 12Z SUITE
OF MODEL QPF IS IN AND THE NORTHWEST RIVER FORECAST CENTER HAS RUN
THE HYDROLOGIC MODEL.   MCDONNAL

&&

.AVIATION...MODERATE W FLOW ALOFT. AT THE SURFACE...A WARM FRONT
FROM THE NW COAST OF CALIFORNIA INTO THE PACIFIC WILL LIFT INTO THE
AREA LATE THIS AFTERNOON THEN BECOME STATIONARY OVER CENTRAL
PORTIONS OF WA TONIGHT. THE LOW LEVEL AIR MASS IS STABLE AND WILL
SEE INCREASING MOISTURE THIS AFTERNOON THROUGH TONIGHT.

CONDITIONS ARE MAINLY VFR FROM ABOUT KSEA NORTHWARD EARLY THIS
MORNING. SOUTH OF KSEA IN THE INTERIOR...AN AREA OF STRATUS WITH
BASES AROUND FL 010 AND VIS 1-3 SM BR HAS DEVELOPED. THE LOW CLOUDS
AND MIST WILL MIX OUT 16Z-18Z AS LOW LEVEL FLOW INCREASES IN
RESPONSE TO THE APPROACHING WARM FRONT. MID AND HIGH CLOUDS WILL
LOWER TO AROUND FL030 MOST AREAS THIS AFTERNOON THEN TO 010-015 LATE
THIS AFTERNOON AND THIS EVENING AS PRECIPITATION INCREASES FROM THE
SW. ALBRECHT

KSEA...STRATUS AND VISIBILITY RESTRICTIONS THIS MORNING ARE EXPECTED
TO REMAIN S OF THE TERMINAL. EXPECT SCT-BKN HIGH CLOUDS TO GRADUALLY
LOWER TO AROUND 030 AT 21Z THIS AFTERNOON AND ABOUT 015 AROUND 00Z
LATE THIS AFTERNOON. SE WIND 5-8 KT IS EXPECTED TO INCREASE TO
S15G25 KT 21Z THIS AFTN. WIND AT FL 050 WILL INCREASE TO SW 35 KT
THIS AFTERNOON AND W 45 KT LATE THIS EVENING. ALBRECHT

&&

.MARINE...SE TO S WIND ARE EXPECTED TO INCREASE LATER THIS MORNING
AS A WARM FRONT THAT SITS FROM THE NW CALIFORNIA COAST TO AROUND 46N
140W LIFTS NWD TOWARD THE WATERS. THE WARM FRONT WILL REACH THE
CENTRAL PORTIONS OF THE WATERS THIS EVENING THEN WILL BECOME
STATIONARY TONIGHT INTO MIDDAY TUE. THE WARM FRONT IS THEN EXPECTED
TO LIFT NORTHWARD INTO SOUTHERN BRITISH COLUMBIA TUE AFTERNOON AND
EVENING. STRONG S-SW WIND 20 TO 30 KT CAN BE EXPECTED TO THE S OF
THE WARM FRONT AS IT LIFTS NWD.

THE FRONT TO THE NORTH OF THE AREA IS EXPECTED TO DROP BACK
SOUTHWARD THROUGH THE WATERS ON FRI AS A COLD HIGH PRES SYSTEM MOVES
INTO THE INTERIOR OF BRITISH COLUMBIA. BY LATER FRI INTO THE WEEKEND
EXPECT STRONG NELY FLOW TO DEVELOP OVER THE WATERS WITH FRASER
OUTFLOW OVER THE NORTHERN INLAND WATERS.

HAZARDOUS SWELL WILL CONTINUE ALONG THE COAST THROUGH THIS MORNING
AND WILL SLOWLY SUBSIDE THIS AFTERNOON AND TONIGHT. ALBRECHT

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY ALL WATERS EXCEPT THE CENTRAL STRAIT.
     SMALL CRAFT ADVISORY FOR ROUGH GRAYS HARBOR BAR CONDITIONS.

&&

$$

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.