Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 231105
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
300 AM PST SUN NOV 23 2014

.SYNOPSIS...A FRONT WILL MOVE THROUGH WESTERN WASHINGTON THIS
MORNING...WITH RAIN TURNING TO SCATTERED SHOWERS THIS AFTERNOON.
SNOW WILL CONTINUE IN THE MOUNTAINS. AFTER A BRIEF LULL TONIGHT AND
MONDAY MORNING...A WARM FRONT WILL RENEW RAINFALL ACROSS THE AREA
MONDAY AFTERNOON...WITH WET WEATHER THROUGH WEDNESDAY. TYPICAL LATE
NOVEMBER WEATHER IS LIKELY THANKSGIVING THROUGH SATURDAY.

&&

.SHORT TERM...STRONG MOIST WESTERLY FLOW ALOFT CONTINUES OVER
WESTERN WASHINGTON EARLY THIS MORNING...JUST AHEAD OF A FLAT UPPER
RIDGE OVER THE OFFSHORE WATERS. A SMALL BUT RATHER VIGOROUS FRONTAL
SYSTEM IN THE WESTERLY FLOW IS MOVING INTO THE REGION...WITH RAIN
FALLING OVER MOST OF THE FORECAST AREA. TEMPERATURES AT 2 AM RANGED
FROM 45 TO 50. THE SNOW LEVEL WAS JUST A BIT OVER 3000 FT.

THE COLD FRONT AND ITS UPPER SHORTWAVE TROUGH WILL MOVE ACROSS
WESTERN WASHINGTON FAIRLY QUICKLY THIS MORNING. IT WILL BRING MORE
RAIN OF COURSE...ALONG WITH BREEZY TO WINDY CONDITIONS. MOIST
WESTERLY FLOW WILL CONTINUE BEHIND IT. STEADY RAIN WILL TURN TO
SCATTERED SHOWERS IN THE AFTERNOON...AND THE WINDS WILL EASE. THERE
WILL PROBABLY BE ANOTHER PUGET SOUND CONVERGENCE ZONE. SHOWERS WILL
BECOME LIGHT AND SPOTTY TONIGHT.

FAIRLY STEADY PRECIPITATION SHOULD CONTINUE ALL DAY IN THE
CASCADES...FIRST WITH THE FRONTAL PRECIPITATION THIS MORNING AND
THEN WITH MOIST WESTERLY FLOW IN ITS WAKE. THE SNOW LEVEL WILL BE
AROUND 3500 FT. SNOQUALMIE PASS -- AT 3000 FT -- MIGHT NOT GET MUCH
MORE SNOW...AS IT WILL PROBABLY BE JUST BELOW THE ELEVATION FOR
SIGNIFICANT SNOW ACCUMULATION. STEVENS PASS HAS A BETTER CHANCE FOR
HEAVY SNOW...SINCE IT IS JUST ABOVE 4000 FT AND THE CONVERGENCE ZONE
WILL PROBABLY ENHANCE PRECIPITATION THERE THIS AFTERNOON AND
EVENING. THE WINTER STORM WARNING FOR THE CASCADES WILL REMAIN IN
EFFECT.

THE UPPER RIDGE WILL BUILD OVER THE AREA TONIGHT...WHILE A WARM
FRONT MOVING OVER THE NORTH SIDE OF THE RIDGE MOVES INTO WESTERN
WASHINGTON. THIS WILL MARK THE BEGINNING OF A PERIOD OF HEAVIER RAIN
AND HIGHER SNOW LEVELS -- IN THE 6000 TO 7000 FT RANGE -- THAT WILL
PROBABLY CONTINUE THROUGH TUESDAY NIGHT. THE NORTH PART OF THE
FORECAST AREA WILL RECEIVE THE MOST PRECIPITATION...AND ESPECIALLY
THE NORTH CASCADES. SOME OF THE MESOSCALE MODEL RUNS SHOW AS MUCH AS
6 INCHES THERE DURING THE 48-HOUR PERIOD ENDING LATE TUESDAY
NIGHT.   MCDONNAL

.LONG TERM...CONFIDENCE IN THE LONG TERM FORECAST FOR WEDNESDAY
THROUGH SATURDAY IS NOT VERY HIGH. THE BROAD UPPER RIDGE WILL
PROBABLY MOVE EAST OF THE AREA ON WEDNESDAY...LEAVING WESTERN
WASHINGTON NEAR THE CONFLUENCE OF A MOIST SOUTHWEST FETCH AND A
COOLER DRIER NORTHERLY FLOW OVER WESTERN CANADA. EARLIER MODEL RUNS
GENERALLY FAVORED THE IDEA THAT THE DRIER NORTHERLY FLOW WOULD
PREVAIL THANKSGIVING DAY AND FRIDAY. MORE RECENT RUNS...ESPECIALLY
OF THE GFS...SHOW THE WETTER PATTERN DEVELOPING. WE INCREASED POPS
INTO THE LIKELY TO CATEGORICAL RANGE FOR WEDNESDAY...BUT WE HAVE
DECIDED TO STAND PATE ON THE FORECAST FOR THANKSGIVING DAY THROUGH
SATURDAY.   MCDONNAL

&&

.HYDROLOGY...MODELS SHOW AROUND 3 TO 6 INCHES OF PRECIPITATION OVER
THE NORTH CASCADES DURING THE 48 HOUR PERIOD ENDING LATE TUESDAY
NIGHT. THE SNOW LEVEL SHOULD BE 6000 TO 7000 FT. ACCORDING TO THE
LATEST RIVER FORECASTS FROM NORTHWEST RIVER FORECAST CENTER...THIS
COULD DRIVE THE NOOKSACK RIVER TO WITHIN ABOUT A FOOT OF FLOOD
STAGE. IN ADDITION SOME MODELS -- ESPECIALLY THE NAM -- SPREAD THE
PRECIPITATION FURTHER SOUTH IN THE CASCADES...WITH 3 TO 5 INCHES IN
THE BASINS AS FAR SOUTH AS THE SNOHOMISH. THAT SCENARIO WOULD DRIVE
DECENT RISES ON THE SNOQUALMIE...TOLT...SKYKOMISH...SNOHOMISH...AND
STILLAGUAMISH RIVERS.

AT THIS POINT I WOULD SAY THERE IS A SLIGHT THREAT OF MINOR FLOODING
ON AT LEAST THE NOOKSACK AND POSSIBLY THE OTHER RIVERS MENTIONED
ABOVE. WE NEED TO SEE WHAT THE NEXT RUN OF THE RIVER MODELS WILL
SHOW LATER TODAY.   MCDONNAL

&&

.AVIATION...A COLD FRONT OVER THE OFFSHORE WATERS EARLY THIS MORNING
WILL MOVE ONTO THE COAST EARLY THIS MORNING THEN SHIFT WELL EAST OF
THE AREA MIDDAY. WESTERLY FLOW ALOFT WILL BECOME NORTHWESTERLY THIS
AFTERNOON. THE AIR MASS IS MOIST AND UNSTABLE AND WILL GRADUALLY DRY
BEHIND THE FRONT LATER THIS AFTERNOON AND THIS EVENING.

RADAR AND SATELLITE DATA SHOW A RATHER VIGOROUS FRONTAL SYSTEM OVER
THE OFFSHORE WATERS EARLY THIS MORNING. THE FRONT IS MOVING RAPIDLY
EASTWARD AND WILL MOVE ONTO THE NORTH WASHINGTON COAST BY 15Z AND
THROUGH THE INTERIOR OF WESTERN WASHINGTON BY 18Z. LIGHTNING
DETECTION SYSTEM DATA SHOWS SEVERAL STRIKES EXTENDING SSW FROM THE
CENTRAL VANCOUVER ISLAND COAST INTO THE OFFSHORE WATERS. THESE TSTMS
ARE EXPECTED TO REMAIN OVER THE COASTAL WATERS EARLY THIS MORNING
BUT MAY IMPACT THE IMMEDIATE COASTLINE AS THE FRONT MOVES INLAND.

FOLLOWING THE FRONT...EXPECT SCATTERED SHOWERS WITH CEILINGS AND
PRECIPITATION BECOMING INCREASINGLY TIED TO THE TERRAIN AND A PUGET
SOUND CONVERGENCE ZONE. SHOWER ACTIVITY WILL TAPER OFF LATER THIS
AFTERNOON AND THIS EVENING AS THE AIR MASS STABILIZES.

EXPECT CIGS GENERALLY BKN-OVC020-025 AHEAD OF THE FRONT THIS MORNING
AND WITH POST FRONTAL SHOWERS AND A PUGET SOUND CONVERGENCE ZONE
FROM LATE THIS MORNING THROUGH THIS EVENING. OUTSIDE OF SCATTERED
SHOWER ACTIVITY LATER THIS MORNING AND THIS AFTERNOON...EXPECT VFR
CONDITIONS WITH CLEARING AT TIMES. VIS WILL MAINLY BE 5SM OR
MORE...BUT WILL OCCASIONALLY FALL TO 2-4 SM IN HEAVIER PRECIPITATION
WITH THE FRONT AND IN HEAVIER POST FRONTAL SHOWERS. ALBRECHT

KSEA...EXPECT CIGS 020-025 WITH INCREASING -RA EARLY THIS MORNING.
WIND WILL INCREASE TO S 14-18KT WITH G28 KT AS THE FRONT APPROACHES
THE TERMINAL. FROPA CAN BE EXPECTED BETWEEN 16Z AND 17Z AND WILL BE
FOLLOWED BY SW WIND 10-12 KT WITH A FEW GUSTS TO 20 KT THROUGH EARLY
AFTERNOON. WINDS WILL GRADUALLY DIMINISH LATER THIS AFTERNOON. SOME
CLEARING AT TIMES IS LIKELY BEHIND THE FRONT AFTER 17Z. WSW WIND 40
KT AT 5000 FT WILL TURN TO W 45K BEHIND THE FRONT MIDDAY THEN WILL
DIMINISH LATE THIS AFTERNOON AND EVENING. ALBRECHT

&&

.MARINE...WINDS ARE INCREASING AHEAD OF A FRONT OVER THE OFFSHORE
WATERS THAT IS APPROACHING THE COASTAL WATERS EARLY THIS MORNING.
ISOLATED TSTMS ARE BEING OBSERVED WITH THE FRONT OFFSHORE THAT
EXTENDS SOUTHWESTWARD FROM THE CENTRAL COAST OF VANCOUVER ISLAND.

SMALL CRAFT ADVISORY CONDITIONS ARE DEVELOPING OVER ALL OF THE
WATERS THIS MORNING EXCEPT THE CENTRAL STRAIT. THE TREND OF THE MOST
RECENT HRRR MODEL SOLUTIONS IS TO DEVELOP GALES OVER THE COASTAL
WATERS AND INTO THE WEST ENTRANCE OF THE STRAIT AS THE FRONT MOVES
THROUGH THIS MORNING...SO A GALE WARNING WAS ISSUED FOR THOSE
WATERS. A GALE WARNING REMAINS IN EFFECT FOR THE CENTRAL AND EASTERN
STRAIT OF JUAN DE FUCA FOR A SURGE OF STRONG POST FRONTAL ONSHORE
FLOW MID MORNING THROUGH EARLY THIS EVENING.

WINDS WILL RAPIDLY RELAX THIS EVENING...AND WEAK HIGH PRES WILL GIVE
LIGHT WINDS TO THE WATERS BY EARLY MON MORNING.

FLOW WILL SHIFT TO SELY AND INCREASE SOMEWHAT AS A WARM FRONT
APPROACHES THE WATERS FROM THE SW ON MON. THEN THE WARM FRONT IS
EXPECTED TO PULL UP STATIONARY SOMEWHERE FROM BETWEEN CENTRAL
VANCOUVER ISLAND AND THE WASHINGTON WATERS. EXPECT RATHER STRONG SW
FLOW ON THE ORDER OF 20 TO 30 KT TO THE SOUTH OF WHERE THIS FRONT
STALLS MON NIGHT THROUGH WED. ALBRECHT

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...WINTER STORM WARNING CASCADES ABOVE 3000 FT.
PZ...GALE WARNING COASTAL WATERS AND ENTIRE STRAIT OF JUAN DE FUCA.
     SMALL CRAFT ADVISORY REMAINING WATERS.
     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





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