Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 152326
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
326 PM PST MON DEC 15 2014

.SYNOPSIS...SOUTHERLY FLOW ALOFT WILL PERSIST THROUGH TUESDAY
NIGHT. WEAK DISTURBANCES EMBEDDED IN THIS FLOW WILL BRING A THREAT
OF PRECIPITATION TO THE AREA AT TIMES. A WEAK LOW PRESSURE SYSTEM
WILL MOVE ACROSS REGION ON WEDNESDAY. UNSETTLED CONDITIONS ARE
ANTICIPATED ON THURSDAY...WITH MORE ACTIVE WEATHER IN STORE FOR
FRIDAY THROUGH THIS WEEKEND.

&&

.SHORT TERM...
A SPLIT FLOW PATTERN WILL CONTINUE THROUGH WED. THE SOUTHERN
BRANCH OF THE JET STREAM WILL BE THE MOST ACTIVE...LEAVING THE
PACIFIC NORTHWEST UNDER THE INFLUENCE OF THE WEAKER NORTHERN
BRANCH. AN UPPER TROF WILL REMAIN OFFSHORE AND THIS WILL MAINTAIN
SOUTHERLY FLOW ALOFT ACROSS THE AREA. EXPECT WEAK DISTURBANCES
EMBEDDED IN THE FLOW ALOFT TO BRING A THREAT OF LIGHT PRECIP TO THE
AREA AT TIMES.

PERSISTENT LOW SURFACE PRESSURE OVER THE OFFSHORE WATERS ALONG
WITH HIGHER PRESSURE EAST OF THE CASCADES WILL KEEP THE LOW LEVEL
FLOW EASTERLY ACROSS THE CWA THROUGH TUE NIGHT. THE DOWNSLOPE FLOW
EAST OF THE CASCADE CREST WILL RESULT IN SOME DRYING AT THE LOW
LEVELS...THUS THE THREAT OF RAIN WILL BE LOW FOR MUCH OF THE PUGET
SOUND REGION NORTHWARD.

EXPECT A WEAK COLD OR OCCLUDED FRONT TO MOVE ACROSS THE REGION ON
WED. THE UPPER TROF OFFSHORE WILL MOVE ONSHORE WED NIGHT AND THEN
INTO EASTERN WA BY THU MORNING. AN UPPER RIDGE WILL FOLLOW IN ITS
WAKE. CONFIDENCE IN THE FORECAST FOR THU WAS NOT HIGH DUE TO
DISAGREEMENT IN THE MODELS. THU COULD BE A DRY DAY PER THE GFS OR
A WET DAY PER THE ECMWF. CHOSE TO GO WITH NEAR CLIMO POPS FOR THU
FOR NOW.

.LONG TERM...
THE MEDIUM RANGE SOLUTIONS WERE IN GENERAL AGREEMENT THAT A WET
AND UNSETTLED PATTERN WILL PREVAIL ACROSS THE PACIFIC NORTHWEST
DURING THIS PERIOD. THE MOUNTAINS MIGHT RECEIVE SIGNIFICANT
SNOWFALL FROM THE SATURDAY SYSTEM.

BEYOND SATURDAY...IT APPEARED THAT A MILD AND QUITE MOIST SYSTEM
WILL IMPACT THE REGION ON SUNDAY. RELATIVELY HIGH SNOW LEVELS
COMBINED WITH STRONG UPSLOPE FLOW WILL RESULT IN THE POTENTIAL FOR
HEAVY RAIN ON THE COAST AND IN THE MOUNTAINS ON SUNDAY. UNLIKE THE
LAST SIGNIFICANT RAIN EVENT...THE FLOW WILL BE WESTERLY AND THIS
WOULD HAVE MORE OF AN IMPACT ON THE CASCADES.

THE MODELS BEGAN TO DIVERGE FOR MON...WITH THE ECMWF KEEPING
STRONG...MOIST AND MILD WESTERLY FLOW OVER THE PACIFIC NORTHWEST.
THIS WOULD MAINTAIN THE THREAT OF SIGNIFICANT RAINFALL FOR THE
CWA. THE GFS...ON THE OTHER HAND...WAS DRIER AND HAD LOWER 500 MB
HEIGHTS. THE FORECAST REFLECTED A COMPROMISE BETWEEN THESE TWO
SOLUTIONS.

&&

.HYDROLOGY...
RIVER FLOODING IS NOT EXPECTED FOR MUCH OF THE AREA THROUGH DAY 7.
STRONG AND MOIST WESTERLY FLOW WILL BECOME ESTABLISHED OVER THE
PACIFIC NORTHWEST EARLY NEXT WEEK. THIS COMBINED WITH RELATIVELY
HIGH SNOW LEVELS COULD LEAD TO THE POSSIBILITY OF FLOODING ON THE
MORE FLASHY RIVERS SUCH AS THE SKOKOMISH.

&&

.AVIATION...AN UPPER LEVEL RIDGE WELL TO THE EAST OF THE AREA
COMBINED WITH A TROUGH OVER THE OFFSHORE WATERS IS GIVING SOUTHERLY
FLOW ALOFT. AT THE SURFACE...HIGH PRESSURE IS OVER EASTERN
WASHINGTON WITH LOWER PRES OFFSHORE. THE AIR MASS IS STABLE WITH
MAINLY MID AND UPPER LEVEL MOISTURE OVER WESTERN WASHINGTON. THE
LOWER LEVELS ARE MAINLY DRY.

EXPECT CLOUDS TO GRADUALLY THICKEN AND LOWER TONIGHT THROUGH TUESDAY
AS THE OFFSHORE TROUGH MOVES INLAND. BUT DESPITE WIDESPREAD CLOUD
COVER AND SOME OCCASIONAL LIGHT RAIN...EXPECT VFR CONDITIONS TO
PREVAIL WITH MAINLY DRY LOWER LEVELS AS THE LOW LEVEL FLOW IS
OFFSHORE. CEILINGS WILL LOWER FROM THE WEST TONIGHT TO AROUND FLIGHT
LEVEL 040. VISIBILITIES ARE LIKELY TO REMAIN 5SM OR HIGHER THROUGH
TUESDAY EXCEPT ON THE NORTH COAST WHERE PRECIPITATION RATES ARE
HIGHER. ALBRECHT

KSEA...VFR CONDITIONS WILL PREVAIL THROUGH TUESDAY. EXPECT BKN070 TO
LOWER TO OVC040 ABOUT 09Z AND TO PERSIST THROUGH TUESDAY. OCCASIONAL
RAIN AFTER 09Z WILL BE VERY LIGHT. EAST WIND 8-13 KNOTS WILL EASE TO
6-9 KT AFTER 09Z. ALBRECHT

&&

.MARINE...OFFSHORE FLOW TONIGHT WILL GRADUALLY EASE THROUGH TUESDAY
AS A TROUGH OF LOW PRESSURE OFFSHORE MOVES INTO THE WATERS AND
WEAKENS. SMALL CRAFT ADVISORY CONDITIONS WILL CONTINUE OVER THE
COASTAL WATERS AND THROUGH THE STRAIT OF JUAN DE FUCA TONIGHT...THEN
WILL BECOME LIMITED TO THE COASTAL WATERS LATE TONIGHT INTO TUE.

WEAKER PRESSURE GRADIENTS WILL GIVE LIGHT FLOW ON WEDNESDAY.

A COLD FRONT WILL APPROACH THE WATERS LATER THURSDAY AND THURSDAY
NIGHT AND MOVE THROUGH THE REGION ON EARLY FRIDAY. GALE FORCE WINDS
ARE POSSIBLE WITH THIS FRONT...MAINLY OVER THE COASTAL WATERS.

A STRONG WARM FRONT WILL APPROACH THE WATERS FROM THE WEST ON
SATURDAY. THIS FRONT WILL LIKELY GIVE GALES TO THE COASTAL WATERS.
GALES ARE POSSIBLE AHEAD OF THIS WARM FRONT IN THE NORTHERN INLAND
WATERS AND AT THE ENTRANCES TO THE STRAIT OF JUAN DE FUCA. ALBRECHT

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORIES ARE IN EFFECT FOR THE COASTAL WATERS
     AND THE STRAIT OF JUAN DE FUCA.

     SMALL CRAFT ADVISORY FOR ROUGH BAR IN EFFECT TIL 4 AM TUESDAY
     FOR THE GRAYS HARBOR BAR.


&&

$$

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