Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 170452
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
900 PM PST TUE DEC 16 2014

.SYNOPSIS...A WEAK TROUGH WILL BRING AREAS OF LIGHT RAIN TO WESTERN
WASHINGTON THROUGH WEDNESDAY NIGHT. A STRONGER PACIFIC FRONTAL
SYSTEM WILL MOVE THROUGH THE AREA LATE THURSDAY THROUGH FRIDAY WITH
MORE SIGNIFICANT RAIN. A WET WEATHER PATTERN WILL DEVELOP THIS
WEEKEND...WITH HEAVIER RAIN ESPECIALLY AT THE COAST AND IN THE
MOUNTAINS.

&&

.SHORT TERM...A WEAK SYSTEM IS OVER THE OFFSHORE WATERS THIS
EVENING. THE CORE OF THIS FEATURE IS MOVING INTO SOUTHERN OREGON AND
NORTHERN CALIFORNIA...LEAVING THE MUCH WEAKER PORTION OF IT OFF THE
WASHINGTON COAST. SOME PRECIPITATION IS MOVING INTO THE COASTAL
WATERS AT THIS TIME...BUT WESTERN WASHINGTON IS JUST MOSTLY CLOUDY.
TEMPERATURES AT 8 PM WERE IN THE UPPER 30S TO 40S.

THE SYSTEM OFFSHORE WILL WEAKEN FURTHER AS IT MOVES INLAND
TONIGHT...BRINGING SOME LIGHT RAIN TO MAINLY THE COAST AND OLYMPIC
PENINSULA. AREAS OF LIGHT PRECIPITATION WILL SPREAD TO THE WESTERN
WASHINGTON INTERIOR LOWLANDS AND EVENTUALLY THE CASCADES...AS THE
SYSTEM CONTINUES ITS SLOW EASTWARD PROGRESSION. THEN A WEAK UPPER
TROUGH WILL MOVE THROUGH THE FORECAST AREA WEDNESDAY NIGHT FOR SOME
LIGHT SHOWERS.

TOTAL PRECIPITATION FOR THE 36 HOUR PERIOD FROM TONIGHT THROUGH
WEDNESDAY NIGHT WILL GENERALLY BE 0.10 TO 0.25 INCH. THE SNOW LEVEL
WILL BE AROUND 3000 TO 3500 FT...WITH AROUND 1 TO 3 INCHES OF SNOW
IN THE MOUNTAINS.

A SMALL UPPER RIDGE WILL MOVE QUICKLY ACROSS THE FORECAST AREA ON
THURSDAY. A STRONGER FRONTAL SYSTEM WILL BRING MORE SIGNIFICANT
PRECIPITATION TO THE AREA LATE THURSDAY AND THURSDAY NIGHT...
DECREASING ON FRIDAY. STORM TOTAL PRECIPITATION WITH THAT SYSTEM
WILL BE MOSTLY 0.5 TO 1.5 INCHES IN THE MOUNTAINS AND UP TO 0.75
INCHES IN THE LOWLANDS. THE SNOW LEVEL WILL RISE TO 3500 TO 5000 FT
AS THE HEAVIEST PRECIPITATION OCCURS...SO IT COULD ACTUALLY BRING A
DECENT ROUND OF SNOWFALL TO THE SKI AREAS AND HIGHER PASSES AND
MOUNTAIN ROADS. IN ADDITION IT WILL PRODUCE LOCALLY WINDY CONDITIONS
AT THE COAST AND IN THE NORTH INTERIOR.   MCDONNAL

.LONG TERM...HERE IS THE LONG TERM SECTION FROM THE AFTERNOON
DISCUSSION -- GLOBAL MODELS REMAIN IN GOOD AGREEMENT THAT A MORE
CONSOLIDATED WLY FLOW PATTERN DEVELOPS ACROSS THE PACIFIC OVER THE
WEEKEND. THE STRONG WLY JET WILL DIRECT AN ATMOSPHERIC RIVER OF
MOISTURE INTO WRN WA LATER SATURDAY THROUGH  EARLY NEXT WEEK. THE
INITIAL WARM FRONTAL WAVE WILL BRING STRONG SW FLOW ALOFT AND HEAVY
RAIN TO THE MOUNTAINS SATURDAY AFTERNOON AND NIGHT. MODELS DIFFER ON
HOW LONG THE TRAILING MOISTURE PLUM REMAINS DIRECTED INTO WRN WA.
HOWEVER...MOST MODELS SHOW AT LEAST 24-36 HOURS OF HEAVY MOUNTAIN
RAINFALL BEFORE TAPERING OFF WHICH COULD CAUSE SOME CONCERNS FOR
FLOODING ON PRONE RIVERS. SEE THE HYDRO DISCUSSION FOR FURTHER
DETAILS. RAIN AMOUNTS WILL NOT BE NEARLY HAS HEAVY IN THE LOWLANDS
WHICH SHOULD SHADOW EFFICIENTLY FROM SEATTLE/TACOMA NORTH TO EVERETT
GIVEN THE STRONG W-WSW FLOW. HIGH POPS ARE STILL WARRANTED EVEN WITH
LESS QPF AND IT WILL ALSO BE BREEZY AT TIMES...ESPECIALLY ACROSS THE
N INTERIOR/COAST WHICH IS MORE SUSCEPTIBLE TO STRONG SE SURFACE
GRADIENTS.

THE GFS/ECMWF BOTH SHOW A LULL OR BREAK IN PRECIPITATION AROUND
MONDAY OR MONDAY NIGHT. ANOTHER SURFACE LOW FEATURE TRACKS INTO WRN
WA TUESDAY WITH MORE RAIN. IF THERE IS A BREAK AS SHOWN BY CURRENT
MODELS...THEN ADDITIONAL RAINFALL WILL PROBABLY NOT BE
HYDROLOGICALLY SIGNIFICANT. IF THE LOW IS FASTER WITH LITTLE OR NO
BREAK...THEN IT WILL ADD TO FLOODING CONCERNS.

RAIN TRANSITIONS TO SHOWERS SOMETIME MID WEEK. THE 00Z RUNS OF THE
GFS/ECMWF SHOWED A COLDER NLY FLOW PATTERN DEVELOPING. 12/18Z RUNS
ARE COOL AND DRY BY WEDNESDAY BUT NOT NEARLY AS COLD. IT WILL ALL
DEPEND ON THE DEVELOPMENT OF A HIGH OVER KONA LOW. RIDGING OUTSIDE
OF 140 W WOULD DRIVE COLDER NLY FLOW OVER THE PAC NW. INSIDE 140 W
WOULD STILL LIKELY BE DRY BUT NOT AS COLD. TEMPERATURES ARE TRENDED
DOWNWARD SLIGHTLY WITH HIGHS IN THE LOWER 40S BY WEDNESDAY AND LOWS
IN THE 30S. MERCER

&&

.HYDROLOGY...RIVER FLOODING IS NOT EXPECTED THROUGH SATURDAY. MOIST
WESTERLY FLOW WILL DEVELOP SATURDAY NIGHT AND CONTINUE THROUGH
AROUND MONDAY...WITH HEAVY MOUNTAIN RAIN POSSIBLE. MODELS SHOW 3-6+
INCHES OF RAIN IN 36 HOURS FROM SATURDAY NIGHT THROUGH EARLY
MONDAY... WHICH COULD BE ENOUGH TO CAUSE FLOODING ON SOME FLOOD
PRONE RIVERS.   MCDONNAL

&&

.AVIATION...AN UPPER LEVEL TROUGH NEAR THE COAST WILL GIVE LIGHT
SOUTHWEST FLOW ALOFT OVER WESTERN WASHINGTON TONIGHT AND WEDNESDAY.
LOW LEVEL OFFSHORE FLOW WILL GRADUALLY EASE AS A SURFACE TROUGH
MOVES ONSHORE AND SURFACE RIDGING EAST OF THE CASCADES WEAKENS. THE
AIR MASS WILL REMAIN MOIST AND STABLE WITH AREAS OF VERY LIGHT
PRECIPITATION MOVING THROUGH AT TIMES.

BRIEF CLEARING AND INCREASING LOW LEVEL MOISTURE WILL LEAD TO SOME
AREAS OF STRATUS AND/OR FOG OVER THE EXTREME SOUTH SOUND AND
SOUTHWEST INTERIOR OVERNIGHT WHERE SURFACE VSBYS COULD DROP BELOW
1SM AT TIMES THROUGH 15Z. FROM THE CENTRAL PUGET SOUND
NORTHWARD...MAINLY VFR CEILINGS ARE EXPECTED OVERNIGHT BUT WITH
PLENTIFUL HIGH AND MID LEVEL MOISTURE. LATEST MODELS BRING SOME
LIGHT PRECIPITATION INLAND WITH THE SURFACE TROUGH WEDNESDAY
AFTERNOON AND THERE COULD BE SOME HIGHER END MVFR CEILINGS
ASSOCIATED WITH IT.

KSEA...VFR CEILINGS BELOW 050 A TIMES OVERNIGHT INTO WEDNESDAY WITH
SOME LIGHT PRECIP POSSIBLE TOWARD DAYBREAK. EAST-SOUTHEAST WIND 5 TO
10 KNOTS.

&&

.MARINE...OFFSHORE GRADIENTS CONTINUE TO RELAX WITH A WEAK SURFACE
TROUGH APPROACHING THE COASTAL WATERS. SMALL CRAFT ADVISORIES WILL
CONTINUE FOR THE OUTER COASTAL WATERS WHERE BORDERLINE ADVISORY
WINDS AND SEAS NEAR 10 FEET REMAIN POSSIBLE. FOR THE REMAINDER OF
THE WATERS...A PERIOD OF QUIET WEATHER IS FORECAST FOR WEDNESDAY
WITH A WEAK BROAD TROUGH OVER THE REGION. A BETTER ORGANIZED AND
STRONGER FRONT WILL APPROACH THE WATERS LATE THURSDAY. A GALE WATCH
FOR THE COASTAL WATERS MIDDAY THURSDAY THROUGH THURSDAY NIGHT
REMAINS IN EFFECT FOR THIS. A STRONGER FRONT ON SATURDAY COULD
PRODUCE GALES OVER THE NORTHERN INLAND WATERS...ENTRANCES TO THE
STRAIT OF JUAN DE FUCA...AND COASTAL WATERS. LONG PERIOD SWELL IN
EXCESS OF 20 FEET COULD REACH THE COAST ON FRIDAY. 27

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY OUTER COASTAL WATERS.
     SMALL CRAFT ADVISORY FOR ROUGH GRAYS HARBOR BAR ON WEDNESDAY.

&&

$$

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





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