High Wind Watch 4, NWS Seattle, WA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
900 PM PST TUE DEC 1 2015

.SYNOPSIS...AN ACTIVE WEATHER PATTERN WILL AFFECT THE AREA THROUGH
THE WEEKEND. A FRONTAL SYSTEM MOVING INTO WESTERN WASHINGTON THIS
EVENING WILL SHIFT INTO EASTERN WASHINGTON LATER TONIGHT WITH RAIN
CHANGING OVER TO SHOWERS. THE NEXT FRONTAL SYSTEM WILL FOLLOW
WEDNESDAY AFTERNOON AND EVENING WITH A TRAILING COLD FRONT ON
THURSDAY BRINGING LOCALLY WINDY CONDITIONS. A THIRD...POTENTIALLY
WETTER SYSTEM...IS EXPECTED TO AFFECT THE AREA OVER THE WEEKEND.

&&

.SHORT TERM...RADAR AND OBS INDICATE LIGHT TO MODERATE RAIN HAS
SPREAD INLAND THIS EVENING BUT HAS ONLY PICKED UP OVER THE CASCADES
THE PAST COUPLE HOURS. SATELLITE IMAGERY SHOWS BRIGHT CLOUD TOPS
WITH THE FRONTAL BAND WHICH IS HOLDING TOGETHER BETTER THAN THE LAST
FRONT. THE BACK EDGE OF PRECIPITATION IS STILL A FEW HOURS OFF THE
COAST AND PROBABLY WILL NOT SWITCH OVER TO SHOWERS OVER INTERIOR WRN
WA UNTIL AFTER MIDNIGHT. SNOW LEVELS ARE GENERALLY RUNNING ABOVE
5000 FT WEST OF THE CASCADES BUT EASTERLY FLOW IS MAINTAINING SNOW
LEVELS DOWN TO 3000 FEET IN THE PASSES. COLD AIR WILL BE STUBBORN TO
MIX OUT OF THE PASSES IF AT ALL BUT THERE IS A THREAT FOR A FEW
HOURS OF SLEET OR FREEZING RAIN...MAINLY SOUTH OF STEVENS PASS.
MODEL QPF THROUGH EARLY WEDNESDAY MORNING RANGES FROM .40 TO .75
INCHES TRANSLATING TO ABOUT 5-10 INCHES OF SNOW...HIGHEST AT MOUNT
BAKER AND MAYBE PARADISE DEPENDING HOW FAR THE WARMER AIR ALOFT
PENETRATES. SNOW WILL TAPER OFF WEDNESDAY MORNING WITH JUST
SCATTERED SHOWERS EXPECTED MOST THE DAY. HIGHS WILL WARM TO THE LOW
50S.

A WARM FRONT AHEAD OF THE NEXT DEVELOPING LOW PRESSURE SYSTEM WILL
LIFT INTO WRN WA BY EARLY WEDNESDAY EVENING. RAIN AT TIMES WILL
PREVAIL WEDNESDAY NIGHT WITH SOME ADDITIONAL SNOW OR MIXED
PRECIPITATION AFFECTING THE CASCADE PASSES. NEW 00Z MODELS SHOW QPF
AMOUNTS OF 1-2 INCHES WHICH COULD TRANSLATE TO OVER A FOOT OF SNOW
THROUGH THURSDAY. MIXED PRECIPITATION COULD CUT BACK SNOW AMOUNTS IN
SOME AREAS MAINLY SOUTH OF STEVENS BUT CONDITIONS COULD BE QUITE
MESSY. A WINTER STORM WATCH SEEMS PRUDENT FOR THE WEDNESDAY EVENING-
THURSDAY EVENING PERIOD. WILL ISSUE THIS SHORTLY. REGARDING
WIND...MODELS STILL SHOW CONSIDERABLE VARIABILITY IN THE EVOLUTION
OF THE SURFACE LOW PATTERN AND TRACK FOR THURSDAY. GIVEN THE
STRETCHED OUT NATURE OF THE LOW...BREEZY TO LOCALLY WINDY WORDING
SEEMS SUFFICIENT BUT ADVISORY LEVEL WINDS ARE A POSSIBILITY MAINLY
FOR THE COAST AND NORTH INTERIOR LATER THURSDAY. WILL SEE HOW THE
NEXT ROUND OF MODEL RUNS TREND.

MERCER

.LONG TERM...NO UPDATE FROM PREVIOUS DISCUSSION...THE WET AND ACTIVE
PATTERN CONTINUES TO LOOK TO EXTEND THROUGH THE WEEKEND WITH A SET
OF PACIFIC FRONTAL SYSTEMS MOVING ACROSS THE REGION. THE FIRST OF
THESE CONTINUES TO BE FORECAST FOR SATURDAY AND LOOKS TO BE WETTER
THAN EARLIER STORMS THIS WEEK. MEDIUM RANGE MODELS HAVE CONTINUED TO
TREND A BIT WEAKER AND DRIER WITH THIS SYSTEM...BUT ARE STILL
PRODUCING PRECIPITATION ACCUMULATIONS RANGING FROM 4 TO 6 INCHES
OVER THE OLYMPICS AND 3 TO 5 INCHES OVER THE CASCADES. SNOW LEVELS
WILL RISE TO ABOVE 6000 FEET SATURDAY...AND LOWER TO AROUND 5000
FEET SUNDAY. MEDIUM RANGE MODELS HAVE BEEN SHOWING ANOTHER SET OF
POTENTIALLY WET SYSTEMS MOVING INTO THE PACIFIC NORTHWEST EARLY NEXT
WEEK. WHILE CONFIDENCE REMAINS LOW REGARDING THE DETAILS THROUGH THE
EXTENDED...MODELS DO REMAIN CONSISTENT ON THE IDEA OF ACTIVE WEATHER
CONTINUING THROUGH THE EXTENDED PERIOD INTO NEXT WEEK.

&&

.AVIATION...A FRONTAL SYSTEM OVER THE COASTAL WATERS THIS EVENING IS
BRINGING LIGHT PRECIPITATION TO THE AREA. THE FRONT HAS LITTLE IN
THE WAY OF SURFACE SIGNATURE TO IT...SO PRESSURE GRADIENTS WILL
REMAIN OFFSHORE THROUGH WEDNESDAY. WITH THE LOW LEVEL OFFSHORE FLOW
AHEAD OF AND BEHIND THE FRONT...EXPECT MAINLY VFR CONDITIONS TO
DOMINATE ACROSS THE AREA THROUGH WED NIGHT. CIGS MAY LOWER TO BKN030
FOR A FEW HOURS BEHIND THE FRONT LATE TONIGHT...THEN RISE TO BKN050
17Z WED THROUGH LATE WED NIGHT.

FLOW ALOFT WILL BE SWLY AND THE AIR MASS WILL BE STABLE. ALBRECHT

KSEA...CIGS BKN-OVC050 WILL LOWER TO BKN030-035 WITH AND FOR A FEW
HOURS BEHIND FROPA ABOUT 10Z TONIGHT. EXPECT -RA TO END BY 12Z THEN
RESUME AROUND 04Z THU. SE WINDS 10-12 KT TONIGHT AND EARLY WED WILL
TURN MORE ELY 8-12 KT MIDDAY WED THROUGH WED NIGHT.
ALBRECHT

&&

.MARINE...RADAR AND SATELLITE SHOW A FRONT OVER THE OUTER COASTAL
WATERS AT 830 PM MOVING GRADUALLY EASTWARD. WHILE WINDS APPEAR TO BE
2-4 KT SHORT OF VERIFYING THE GALES ALONG THE COAST...AT THE EAST
ENTRANCE TO THE STRAIT...AND IN THE NORTHERN INLAND WATERS...WILL
ALLOW THE GALE TO CONTINUE UNTIL THE FRONT MOVES BY. FOLLOWING THE
FRONT THERE WILL BE A LULL IN THE ACTION ON WED BEHIND THIS FIRST
FRONT AND A SECOND ONE THAT WILL IMPACT THE AREA WED NIGHT INTO THU.

CONCERNING THE SECOND FRONT...WILL LIKELY NEED TO REISSUE THE GALE
WARNING WITH THE EARLY MORNING PACKAGE AS GALES WILL DEVELOP OVER
THE COASTAL WATERS ABOUT MIDNIGHT WED NIGHT AND SHORTLY THEREAFTER
AT THE EAST ENTRANCE TO THE STRAIT AND IN THE NORTHERN INLAND
WATERS. PER SHORT TERM MODELS...THIS SECOND FRONT APPEARS THAT IT
WILL HAVE A SLIGHTLY STRONGER WIND FIELD THAN THE FRONT NOW MOVING
ACROSS THE REGION.

THE FORECAST FOR THU IS STILL IN QUESTION. THE 00Z GFS20 SOLUTION
ONCE AGAIN SHOWS A 990 MB LOW MOVING NE ONTO THE WASHINGTON COAST
NEAR THE GRAYS HARBOR BAR BEFORE MOVING NE ACROSS THE OLYMPICS AND
INTO BRITISH COLUMBIA. THIS LOW APPEARS THAT IT WILL BE COMPLEX WITH
ANOTHER CENTER MOVING ONTO CENTRAL VANCOUVER ISLAND AHEAD OF IT. IF
THE GFS20 WERE TO VERIFY...GALES MAY BE REQUIRED FOR MUCH OF THE
INLAND WATERS THU OR THU EVENING. CONFIDENCE ON THE EVOLUTION OF
THURSDAYS SYSTEM IS LOW WITH MODELS HAVING A DIFFICULT TIME
CONVERGING ON A SOLUTION.

ANOTHER STRONG FRONT WILL MOVE THROUGH THE WATERS FRI NIGHT INTO SAT
NIGHT OR EARLY SUN. ONCE AGAIN...THE COASTAL WATERS AND THE EAST
ENTRANCE TO THE STRAIT WOULD SEE THE HIGHEST CHANCE OF GETTING GALE
FORCE WINDS WITH THIS FRONT. ALBRECHT

&&

.HYDROLOGY...NO UPDATE FROM PREVIOUS DISCUSSION...RIVER FLOODING IS
NOT EXPECTED THROUGH FRI. THE THREAT OF FLOODING IS POSSIBLE OVER
THE WEEKEND WITH A WET PACIFIC SYSTEM THAT IS EXPECTED TO MOVE
ONSHORE SATURDAY.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...

WA...WINTER WEATHER ADVISORY IN EFFECT FROM 4 PM THIS AFTERNOON TIL
     4 AM WEDNESDAY FOR THE CASCADES.
     WINTER STORM WATCH FOR THE CASCADES FROM 9 PM WEDNESDAY EVENING
     THROUGH MIDNIGHT THURSDAY NIGHT.

&&

WWW.WEATHER.GOV/SEATTLE

AN ILLUSTRATED VERSION OF THIS FORECAST DISCUSSION CAN BE SEEN AT
WWW.WEATHER.GOV/SEATTLE/GAFD/LATEST_WEBAFD.HTML