Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 241047
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
345 AM PDT THU APR 24 2014

.SYNOPSIS...A COLD FRONT WILL MOVE THROUGH THE AREA EARLY THIS
MORNING. AN UPPER LEVEL TROUGH OFF THE COAST WILL KEEP SHOWERS IN
THE FORECAST LATER TODAY AND TONIGHT. THE TROUGH WILL MOVE INLAND
LATE FRIDAY INTO FRIDAY NIGHT. ANOTHER FRONTAL SYSTEM WILL BRING
ANOTHER ROUND OF MORE ORGANIZED RAINFALL TO THE AREA LATER SATURDAY
INTO SUNDAY MORNING.

&&

.SHORT TERM...SATELLITE IMAGERY SHOWS WARM FRONT EAST OF THE AREA
WITH THE TRAILING COLD FRONT MOVING INTO THE WESTERN WASHINGTON
INTERIOR AT 10Z. TEMPERATURES WITH THE RAIN AND CLOUD COVER ARE IN A
NARROW RANGE...MID 40S TO LOWER 50S AT 3 AM.

COLD FRONT WILL MOVE INTO THE CASCADES EARLY THIS MORNING. PLENTY OF
POST FRONTAL SHOWER ACTIVITY EVIDENT ON THE SATELLITE IMAGERY. AS
THE FRONT MOVES INTO THE CASCADES THE KOLM-KBLI GRADIENT WILL
INCREASE RAPIDLY. WITH A LACK OF WESTERLIES THROUGH THE STRAIT THIS
WILL RESULT IN BREEZY...15 TO 25 MPH...WINDS IN THE INTERIOR TODAY.
SHOWER ACTIVITY DECREASING SOMEWHAT THIS AFTERNOON. GFS MOS HAS 40
PERCENT CHANCE OF THUNDERSTORMS FOR SEATTLE. BUFR SOUNDING INDICATE
LIFTED INDEXES ABOVE ZERO...AROUND PLUS 2...THIS AFTERNOON WITH VERY
LITTLE CAPE OF SHEAR. THE CONVECTIVE TEMPERATURES ARE IN THE LOWER
50S SO A RUMBLE OF THUNDER CAN NOT BE RULED OUT BUT FOR NOW THE
CHANCES TO NOT LOOK HIGH ENOUGH TO MENTION IN THE FORECAST. AIR MASS
OVER THE AREA REMAINING COOL WITH 850 MB TEMPERATURES NEAR 0C
KEEPING DAYTIME HIGHS BELOW NORMAL TODAY...IN THE LOWER TO MID 50S.

NOT MUCH CHANGE IN THE PATTERN OVERNIGHT WITH THE UPPER LEVEL
TROUGH MOVING CLOSER TO THE COAST AND SOUTHWESTERLY FLOW ALOFT
CONTINUING. AIR MASS REMAINING SLIGHTLY UNSTABLE TO KEEP A CHANCE OF
SHOWERS IN THE FORECAST. WITH THE POSSIBILITY OF SOME BREAKS IN THE
CLOUD COVER TONIGHT AND THE COOL AIR MASS REMAINING OVERHEAD LOW
TEMPERATURES IN THE COLDER LOCATIONS WILL GET DOWN INTO THE MID 30S.
FOR THE REMAINDER OF THE AREA UPPER 30S AND LOWER 40S WILL BE
COMMON.

UPPER LEVEL TROUGH MOVING INLAND OVER WESTERN WASHINGTON FRIDAY. JET
WELL TO THE SOUTH OF WESTERN WASHINGTON SO IT WILL TAKE SOME DAY
TIME HEATING TO INCREASE THE SHOWER ACTIVITY ACROSS THE AREA.
TEMPERATURES ALOFT STILL COOL WITH THE 850 MB TEMPERATURES STILL
AROUND 0C MAKING FOR ANOTHER DAY OF BELOW NORMAL HIGH TEMPERATURES
...LOWER TO MID 50S.

TROUGH AXIS MOVING EAST OF THE AREA BY LATE FRIDAY NIGHT WITH THE
SHOWER ACTIVITY DRYING UP OVER THE AREA OVERNIGHT. JET WELL SOUTH OF
THE AREA WITH THE TROUGH BECOMING SLIGHTLY NEGATIVELY TILTED. THIS
WILL SLOW THE EASTWARD MOVEMENT OF THE TROUGH DOWN SO HAVE LEFT SOME
LOW CHANCE POPS IN THE FORECAST FOR THE POSSIBILITY OF THE TROUGH
AXIS HANGING UP OVER THE AREA FRIDAY NIGHT. ANOTHER COOL NIGHT IN
STORE...MID 30S TO LOWER 40S...WITH NO MODERATION OF THE
TEMPERATURES ALOFT.

SOME INCONSISTENCIES IN THE TIMING OF THE NEXT ORGANIZED FRONT TO
REACH THE AREA. THE GFS AND THE ECMWF ARE THE FASTER
SOLUTIONS...WITH RAIN FROM ABOUT PUGET SOUND WESTWARD SATURDAY
AFTERNOON. THE NAM IS SLOWER KEEPING THE PRECIPITATION OFFSHORE
UNTIL SATURDAY NIGHT. FLOW ALOFT ACROSS THE EASTERN PACIFIC IS ZONAL
ON SATURDAY WHICH FAVORS THE FASTER SOLUTIONS WHICH ARE CURRENTLY IN
THE FORECAST.

.LONG TERM...EXTENDED MODELS IN GOOD AGREEMENT EARLY ON WITH THE
FRONT MOVING THROUGH THE AREA SATURDAY NIGHT WITH WEAK TROUGHINESS
OVER THE AREA ON SUNDAY KEEPING SHOWERS IN THE FORECAST. MODELS
SOLUTIONS START TO DIVERGE ON MONDAY WITH THE GFS BUILDING A WEAK
RIDGE OVER THE AREA WITH A DRYING TREND. THE ECMWF ALSO SHOWS SOME
RIDGE BUT BRINGS A WARM FRONT INTO THE AREA LATER IN THE DAY INTO
MONDAY NIGHT. THE CANADIAN MODEL ALSO HAS A WARM FRONT MOVING INTO
THE AREA. WILL KEEP THE CHANCE OF RAIN FORECAST FOR MONDAY AND
MONDAY NIGHT. EXTENDED MODELS ARE IN GOOD AGREEMENT ON TUESDAY WITH
AN UPPER LEVEL RIDGE BUILDING OVER THE AREA AND LOW LEVEL OFFSHORE
FLOW DEVELOPING. THIS WILL RESULT IN HIGHS ON TUESDAY IN THE 60S
ACROSS THE ENTIRE AREA. THE MODELS DISAGREE ON WEDNESDAY WITH THE
ECMWF AND CANADIAN CONTINUING TO BUILD THE RIDGE OVER WESTERN
WASHINGTON WITH LOW LEVEL OFFSHORE FLOW WHILE THE GFS SHIFTS THE
RIDGE TO THE EAST WITH THE LOW LEVEL FLOW TURNING ONSHORE DURING THE
DAY. WITH THE LACK OF CONSISTENCY HAVE STAYED WITH THE MORE
CONSERVATIVE FORECAST OF ANOTHER DAY WITH HIGHS IN THE 60S. IF THE
ECMWF AND CANADIAN ARE RIGHT THERE IS THE POTENTIAL FOR LOWER TO MID
70S ON WEDNESDAY. FELTON

&&

.HYDROLOGY...RIVER FLOODING REMAINS UNLIKELY FOR THE NEXT 7 DAYS.

THE EXCEPTION REMAINS THE NORTH FORK OF THE STILLAGUAMISH RIVER EAST
OF THE SR 530 LANDSLIDE NEAR OSO WHERE A FLOOD WARNING REMAINS IN
EFFECT UNTIL FURTHER NOTICE. FLOODING CONTINUES FOR THIS RIVER AS A
RESULT OF WATER POOLED BEHIND THE DEBRIS DAM THAT PARTIALLY BLOCKS
THE RIVER.

AT 315 AM...THE NORTH FORK OF THE STILLAGUAMISH NEAR ARLINGTON WAS
AT 4.6 FT...OR 2820 CFS. THE NORTH FORK OF THE STILLAGUAMISH RIVER
NEAR OSO WAS AT 216.4 FT AT 1800 CFS. THE POOL EAST OF THE SLIDE WAS
282.0 FT.

WITH THE STEADY RAIN COMING TO AN END OVER THE SLIDE AREA JUST SOME
SLIGHT RISES ARE EXPECTED THIS MORNING WITH THE RIVER REMAINING
STEADY THIS AFTERNOON AND EVENING. THE LATEST HYDROGRAPH SHOWS THE
STILLAGUAMISH NEAR ARLINGTON CRESTING BELOW 5 FEET. FELTON

&&

.AVALANCHE...PREVIOUS DISCUSSION...FURTHER HEAVY SNOW AND THEN
POTENTIAL RAIN SHOULD BE SEEN IN THE MT RAINIER TO MT HOOD AREA THIS
MORNING. THIS IS VERY LIKELY TO CAUSE WET LOOSE...STORM SLAB AND
WIND SLAB AVALANCHE CONDITIONS IN THIS AREA. AN AVALANCHE WARNING
CONTINUES FOR THE MT RAINIER TO MT HOOD AREA THIS MORNING. BACK
COUNTRY TRAVEL IN AVALANCHE TERRAIN IS THIS AREA IS NOT RECOMMENDED
THIS MORNING.

SIGNIFICANT SNOW AND AVALANCHE CONDITIONS ARE ALSO EXPECTED IN OTHER
AREAS BUT THAT ARE SLIGHTLY LESS THAN WARNING CRITERIA. AVALANCHES
ARE ALSO LIKELY IN THESE OTHER AREAS AS WELL.

PLEASE VISIT NWAC.US FOR DETAILS. GKF

&&

.AVIATION...FLOW ALOFT IS SOUTHWESTERLY WITH A MOIST AND STABLE AIR
MASS BECOMING SOMEWHAT UNSTABLE LATER THIS MORNING. A WARM FRONT
LIFTING NORTHWARD THROUGH PUGET SOUND AT 09Z WILL BE FOLLOWED BY A
COLD FRONT AROUND 15Z. WIDESPREAD RAIN WITH MVFR CEILINGS WILL TURN
TO SHOWERS BEHIND THE COLD FRONT WITH A GRADUAL IMPROVEMENT IN
CEILINGS TOWARD AFTERNOON. SURFACE WINDS WILL BECOME GUSTY FROM THE
SOUTH AND SOUTHWEST THIS MORNING FOLLOWING THE PASSAGE OF THE COLD
FRONT.

KSEA...MVFR CEILINGS 2-3K FT AT TIMES IN RAIN THIS MORNING. RAIN
TURNING TO SHOWERS AFTER 15Z WITH CEILINGS MOSTLY VFR THIS
AFTERNOON...EXCEPT NEAR SHOWERS. SOUTHEAST WIND 7 TO 12
KNOTS...BECOMING S-SW 15 TO 25 KNOTS AFTER 15Z AND REMAINING GUSTY
THROUGH THIS AFTERNOON.

&&

.MARINE...A WARM FRONT IS LIFTING NORTHWARD THROUGH THE INLAND
WATERS AT 09Z WITH A COLD FRONT TO FOLLOW LATER THIS MORNING. GALE
WARNINGS REMAIN IN EFFECT FOR THE COASTAL WATERS TODAY...WITH
SMALL CRAFT ADVISORIES ALL OTHER WATERS. STRONG PRESSURE RISES WILL
FOLLOW A SURFACE LOW AS IT LIFTS INTO VANCOUVER ISLAND THIS MORNING.
THIS WILL LEAD TO SOME GUSTY SOUTH OR SOUTHWEST WINDS OVER THE
INLAND WATERS MUCH OF TODAY. WINDS COULD BRIEFLY APPROACH GALE FORCE
IN PARTS OF PUGET SOUND LATER THIS MORNING.

AFTER A LULL IN THE ACTION FRIDAY...ANOTHER FRONT WILL SPREAD SMALL
CRAFT ADVISORY STRENGTH WINDS OVER THE AREA SATURDAY NIGHT. 27

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...FLOOD WARNING REMAINS IN EFFECT FOR THE NORTH FORK OF THE
     STILLAGUAMISH RIVER EAST OF THE SR 530 LANDSLIDE.

PZ...GALE WARNING COAST.
     SMALL CRAFT ADVISORY ALL OTHER WATERS.
     SMALL CRAFT ADVISORY FOR ROUGH GRAYS HARBOR BAR.

&&

$$

WWW.WEATHER.GOV/SEATTLE

FOR AN ILLUSTRATED VERSION OF THE FORECAST DISCUSSION PLEASE SEE
WWW.WEATHER.GOV/SEATTLE/GAFD/LATEST_WEBAFD.HTML   (ALL LOWERCASE)









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