Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 141622
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
922 AM PDT SUN SEP 14 2014

.SYNOPSIS...HIGH PRESSURE ALOFT COMBINED WITH LOW LEVEL OFFSHORE
FLOW WILL GIVE SUNNY DAYS AND CLEAR NIGHTS THROUGH MONDAY. THE DRY
AIR AND OFFSHORE FLOW WILL GIVE UNSEASONABLY WARM DAYTIME HIGH
TEMPERATURES TODAY AND MONDAY. A CHANGE IN THE WEATHER PATTERN IS
EXPECTED STARTING MONDAY NIGHT AS THE HIGH PRESSURE ALOFT SHIFTS
INLAND...AN UPPER LEVEL TROUGH MOVES NORTHWARD ALONG THE COAST...AND
MARINE AIR PUSHES INTO THE INTERIOR FROM THE SOUTHWEST. COOLER
WEATHER WITH A THREAT OF SHOWERS IS FORECAST TO RETURN TO THE AREA
WEDNESDAY THROUGH FRIDAY AS A FEW PACIFIC WEATHER SYSTEMS MOVE
ONSHORE.

&&

.SHORT TERM...THE UPPER RIDGE THAT HAS BEEN SITTING ALONG 135W FOR
THE PAST SEVERAL DAYS IS SHOWING SIGNS OF SLOWLY MOVING EASTWARD
TODAY. IN THE MEAN TIME...COOL SURFACE HIGH PRESSURE EXTENDING FROM
SOUTHEASTERN BRITISH COLUMBIA AND EASTERN WASHINGTON INTO THE GREAT
PLAINS IS GIVING LOW LEVEL OFFSHORE FLOW AND IS HELPING TO MAINTAIN
A THERMAL TROUGH THAT EXTENDS FROM JUST OFF NORTHWESTERN VANCOUVER
ISLAND SOUTHWARD INTO THE NORTHERN CALIFORNIA COASTAL WATERS. THE
PERSISTENT OFFSHORE FLOW AND THE EASTWARD PROGRESSION OF THE
OFFSHORE RIDGE IS ALLOWING THE LOWER LEVELS TO GRADUALLY WARM. 850
MB TEMPERATURES TODAY AROUND 15C WILL RISE TO OVER 20C BY MONDAY
AFTERNOON. WHILE THE SUN IS NOT AS STRONG AS MID SUMMER...THIS WILL
RESULT IN TEMPERATURES WARMING INTO THE 80S TODAY AND POSSIBLY TO
AROUND 90 FROM ABOUT SEATTLE SOUTHWARD IN THE INTERIOR ON MONDAY.

A 571 DAM 500 MB LOW SITTING NEAR 37N 137W IS FORECAST BY SHORT TERM
GUIDANCE TO OPEN UP AND EJECT NWD THROUGH THE OREGON OFFSHORE WATERS
MONDAY AFTERNOON THEN THRU THE WASHINGTON COASTAL WATERS ON TUE. THE
PASSAGE OF THE UPPER LOW WILL DEEPEN THE MARINE LAYER OVER THE
OREGON COASTAL WATERS AND INDUCE A MARINE PUSH MONDAY EVENING. THE
WARMTH IN THE INTERIOR ON MONDAY WILL HELP TO ENHANCE THE ONSHORE
GRADIENTS MONDAY NIGHT. WITH SLY FLOW ALOFT AND SOME INSTABILITY
ALOFT TUE...EXPECT THE STRATUS TO BREAK MIDDAY IN THE INTERIOR...BUT
THE PRESENCE OF THE MARINE AIR WILL HOLD TEMPERATURES IN THE MID 70S
- 10 TO 15 DEGREES COOLER THAN MONDAY. MOISTURE ALOFT MON NIGHT AND
TUE WITH THE UPPER TROUGH WILL BE LIMITED TO ABOVE 10000 FT AND MU
CAPE VALUES ARE MINIMAL OVER THE AREA AS THE TROUGH PASSES...SO
TSTMS WILL CONTINUE TO BE LEFT OUT OF THE FORECAST FOR LATE MON
NIGHT INTO TUE AS THE SHORT WAVE PASSES.

CURRENT FORECASTS ARE IN GOOD SHAPE. NO UPDATES ARE PLANNED. ALBRECHT

.LONG TERM...FROM THE PREVIOUS LONG TERM DISCUSSION...THE EXTENDED
MODELS CONTINUE TO HAVE TROUBLE WITH THE SOLUTION FOR WEDNESDAY
THROUGH FRIDAY. THE 00Z MODEL RUN CONTINUES THE TREND OF LACK OF
CONSISTENCY IN THE MODEL SOLUTIONS FOR THIS TIME FRAME. THE
INDIVIDUAL MODELS ARE ALSO SHOWING A LACK OF RUN TO RUN CONSISTENCY.
IN PREVIOUS RUNS THE ECMWF WAS THE FASTER AND WETTER OF THE MODELS
WITH THE NEXT SHORTWAVE SPINNING OUT OF THE UPPER LEVEL TROUGH
OFFSHORE. ON THE 00Z RUN THE GFS IS NOW FASTER AND WETTER WITH THE
SHORTWAVE WITH ALL OF WESTERN WASHINGTON WET BY 00Z THURSDAY. THE
ECMWF HAS SLOWED DOWN THE FEATURE WITH ONLY THE COASTAL SECTIONS WET
BY 00Z THURSDAY. THERE IS SOME GENERAL AGREEMENT ON THURSDAY WITH
ALL OF THE MODELS INDICATING SOME TROUGHINESS AND SHOWERS. THE GFS
NOW STARTS BUILDING AN UPPER LEVEL RIDGE OVER THE AREA ON FRIDAY
DRYING THINGS OUT QUICKER THAN THE ECMWF. ALL OF THE MODELS SHOW
ANOTHER RIDGE IN PLACE FOR SATURDAY WITH 500 MB HEIGHTS BUILDING
ABOVE 580 DECAMETERS AND THE LOW LEVEL FLOW TRYING TO GO OFFSHORE
AGAIN. CONFIDENCE IN THE EXTENDED FORECAST NOT VERY HIGH AT THIS
POINT. FOR NOW WILL STAY WITH THE BROAD BRUSH CHANCE OF SHOWERS
FORECAST THROUGH THE PERIOD. IF THE 12Z MODEL RUNS CONTINUE THE
TREND LOOK FOR THE FORECAST TO BE WARMER AND DRIER FOR AT LEAST NEXT
SATURDAY. FELTON

&&

.FIRE WEATHER...A THERMAL PRESSURE TROUGH CONTINUES TO RESIDE NEAR
THE COAST THIS MORNING...BUT IT CONTINUES A SLOW DAY-TO-DAY
WEAKENING TREND AND SHOULD DISSIPATE ON MONDAY AFTERNOON. AS A
RESULT...EAST AND NORTHEAST WINDS ARE ALSO ON A DOWNWARD TREND. RED
FLAG CONCERNS THIS MORNING ARE NOW LIMITED TO ONLY THE MOST EAST
WIND-PRONE LOCATIONS SUCH AS ELLIS MTN ON THE NW OLYMPIC
PENINSULA...AS WELL AS LOCATIONS JUST WEST OF THE MAJOR CENTRAL
CASCADE PASSES. EVEN THOSE LOCATIONS SHOULD HAVE THEIR WINDS DROP
BELOW CRITERIA THIS AFTN...AND THEY SHOULD REMAIN THAT WAY TOMORROW.
SO LAST OF THE RED FLAG WARNINGS EXPIRE THIS AFTERNOON.

DESPITE THE DOWNWARD TREND ON EAST WIND...ENOUGH EAST WIND WILL
PERSIST THROUGH MONDAY TO KEEP A DRY AIR MASS IN PLACE WITH A
WARMING TREND. MID-LEVEL HAINES 5 AND 6 CONDITIONS ARE EXPECTED ON
MONDAY AFTERNOON...LEADING TO AN ACTIVE BURNING ENVIRONMENT. THERE
IS A SMALL RISK OF ISOLATED LIGHTNING EARLY TUESDAY MORNING...BUT
THE RISK IS TOO SMALL TO MENTION YET IN THE OFFICIAL FORECAST. AFTER
THAT...DEEPER MARINE AIR WILL SPREAD ONTO THE COAST LATE TUESDAY AND
DEEPEN OVER THE INTERIOR ON WEDNESDAY.     HANER

&&

.AVIATION...THE UPPER RIDGE OFFSHORE WILL MOVE OVER THE AREA
OVERNIGHT. LIGHT AND VARIABLE FLOW ALOFT. CONTD LOW LEVEL OFFSHORE
FLOW. VFR CONDITIONS WILL PERSIST.

KSEA...WINDS WILL REMAIN LIGHT NLY.

&&

.MARINE...
AN ELONGATED AREA OF LOW PRES OFF THE COAST WILL WEAKEN ON MON. THIS
WILL ALLOW HIGHER PRES TO BUILD OVER OREGON MON NIGHT...RESULTING IN
THE DEVELOPMENT OF ONSHORE OR SOUTHERLY FLOW. A 1011 MB HIGH ON THE
WASHINGTON COAST AND LOW PRES E OF THE CASCADES WILL MAINTAIN
ONSHORE FLOW ON TUE.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...RED FLAG WARNING FOR WIND AND LOW HUMIDITY THROUGH 3 PM
      THIS AFTERNOON IN FIRE WEATHER ZONES 650 AND 659.
PZ...NONE.

&&

$$

WWW.WEATHER.GOV/SEATTLE

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











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