Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 311717
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
915 AM PST SAT JAN 31 2015

.SYNOPSIS...AN UPPER LEVEL RIDGE OVER THE COASTAL WATERS WILL MOVE
ACROSS WESTERN WASHINGTON THIS EVENING THEN SHIFT EAST OF THE
CASCADES LATE TONIGHT. LOW LEVEL TEMPERATURE INVERSIONS WILL SUPPORT
FOG ACROSS THE INTERIOR THROUGH EARLY THIS AFTERNOON. A WEAK FRONTAL
SYSTEM WILL BRING A LITTLE RAIN TO THE AREA ON SUNDAY AND WILL BE
FOLLOWED BY ANOTHER...SLIGHTLY STRONGER SYSTEM...ON MONDAY. SHOWERS
WILL GRADUALLY TAPER OFF MIDWEEK FOLLOWED BY A TRANSITION TO A WET
AND WINDY PATTERN LATE IN THE WEEK THROUGH NEXT WEEKEND.

&&

.SHORT TERM...SATELLITE IMAGERY AND OBSERVATIONS SHOW WIDESPREAD
STRATUS AND FOG ACROSS THE INTERIOR LOWLANDS AND THROUGH THE
CHEHALIS GAP TO AROUND HOQUIAM. THE SAND POINT PROFILER SHOWS A
STRONG AND NEAR-SURFACE BASED INVERSION. PILOT REPORTS FROM EARLIER
THIS MORNING INDICATED THAT TOPS WERE GENERALLY 600-1000 FEET OVER
THE AREA...WHICH AGREES WITH THE LOOK OF THE EARLY MORNING VISIBLE
SATELLITE IMAGES.

THE FORECAST PROBLEM OF THE DAY CONTINUES TO CONCERN HOW LONG THE
FOG AND STRATUS WILL LAST. PRESSURE GRADIENTS ARE OFFSHORE WITH
CROSS CASCADE GRADIENTS NEARLY 3 MB AND THE KUIL-KBLI GRADIENT
NEARLY -2 MB. BUT MUCH OF THE FLOW IS GOING OVER THE TOP OF THE
INVERSION AND HELPING TO MAINTAIN IT. THE LATEST HRRR SHOWS MOST OF
THE FOG AND STRATUS FINALLY BURNING OFF AROUND 22Z...REASONABLE
GIVEN CURRENT CONDITIONS. TEMPERATURES IN THE INTERIOR LOWLANDS FROM
ABOUT EVERETT SOUTHWARD TO THE OREGON BORDER WERE LOWERED ABOUT 3
DEGREES FROM THE PREVIOUS FORECAST -SIMILAR TO FRIDAYS HIGHS- AS A
RESULT...AND A QUICK GRID AND TEXT FORECAST UPDATE WAS DONE FOR THE
FIRST PERIOD OF THE FORECAST.

THE LATEST NAM12 AND GFS FORECASTS SHOW A WEAK INCOMING FRONT NOW
SEEN ALONG 138W MOVING ACROSS WESTERN WASHINGTON LATE SUNDAY
AFTERNOON. THE 00Z ECMWF WAS A FEW HOURS SLOWER IN BRINGING THE
FRONT THROUGH THE AREA. THE PRECIPITATION ASSOCIATED WITH THIS FRONT
APPEARS THAT IT WILL MOVE ONTO THE COAST SHORTLY AFTER 12Z SUNDAY
AND INTO THE INTERIOR BY 15Z. THE FRONT IS COMING IN LATE ENOUGH
THAT SOME PATCHY FOG MAY REFORM IN THE INVERSION SEEN IN THE
INTERIOR AROUND SUNSET...THEN WILL LIFT OR DISSIPATE AS FLOW AND
MID/UPPER LEVEL MOISTURE GRADUALLY INCREASES LATER TONIGHT IN
ADVANCE OF THE INCOMING FRONT. LOWS TONIGHT WILL BE IN THE MID 30S
TO LOWER 40S.

EXPECT RAIN AT TIMES FOR SUNDAY WITH THIS FIRST FRONT...THOUGH THE
RAIN WILL BE QUITE LIGHT WITH THE QPF VALUES LESS THAN A HALF AN
INCH FOR MOST LOCATIONS. HIGHS ON SUNDAY ARE EXPECTED TO BE NEAR
50...AND LOWS SUNDAY NIGHT WITH THE CLOUD COVER AND SOME POST
FRONTAL SHOWERS ONLY IN THE LOWER TO MID 40S.

ANOTHER FRONTAL SYSTEM IS FOLLOWING QUICKLY BEHIND SUNDAYS. THIS
SECOND SYSTEM WILL MOVE ACROSS THE AREA ON MONDAY. THE MONDAY SYSTEM
WILL BE A LITTLE STRONGER THAN THE SUNDAY ONE...BRINGING MORE RAIN
AND BREEZY CONDITIONS ALONG THE COAST AND IN THE NORTH INTERIOR.
HIGHS ON MONDAY WILL AGAIN BE AROUND 50 DEGREES. ALBRECHT

.LONG TERM...FROM THE PREVIOUS LONG TERM SOLUTION...THERE IS MORE
GENERAL AGREEMENT IN THE EXTENDED MODELS THIS MORNING VERSUS
YESTERDAYS 00Z RUN BUT THERE ARE STILL SOME BIG INCONSISTENCIES FOR
THE MIDDLE OF NEXT WEEK. ON TUESDAY THE ECMWF ATTEMPTS TO BUILD A
WEAK RIDGE OVER WESTERN WASHINGTON WHILE THE GFS HAS THE RIDGE
FURTHER TO THE WEST AND HAS A SURFACE LOW DEVELOP OVER VANCOUVER
ISLAND WITH A WEAK FRONTAL BOUNDARY DEVELOPING OVER NORTHWEST
WASHINGTON. ON WEDNESDAY THE ECMWF BRINGS A WARM FRONT UP INTO THE
AREA FROM THE SOUTH WHILE THE GFS DOESN`T EVEN HAVE THIS FEATURE AND
INSTEAD HAS A DRY FORECAST WITH AN UPPER LEVEL RIDGE OFF THE COAST
AND LOW LEVEL OFFSHORE FLOW. THERE IS BETTER AGREEMENT THAT
BEGINNING THURSDAY THE PATTERN WILL TURN WARMER AND WETTER FOR
WESTERN WASHINGTON WITH A PAIR OF SYSTEMS MOVING THROUGH THE AREA
THURSDAY AND FRIDAY. SNOW LEVELS LOOK TO BE ABOVE 5000 FEET
BEGINNING THURSDAY RISING TO AS HIGH AS 7000 FEET ON FRIDAY. WITH
THE DIFFERENCES IN THE MODELS WILL HAVE TO GO WITH CHANCE POPS FOR
TUESDAY AND WEDNESDAY. WILL STAY WITH THE RAIN AT TIMES/ RAIN LIKELY
FORECASTS FOR THURSDAY THROUGH FRIDAY WITH BETTER MODEL CONSENSUS.
FELTON

&&

.HYDROLOGY...FLOODING IS NOT EXPECTED THROUGH THURSDAY.

A SERIES OF SYSTEMS MOVING BY JUST TO THE WEST OF THE AREA COMBINED
WITH A NEARLY STALLED FRONTAL SYSTEM AROUND WESTERN WASHINGTON MAY
PRODUCE LOCALLY HEAVY RAIN AT TIMES STARTING LATER THURSDAY OR
FRIDAY AND CONTINUING THROUGH NEXT WEEKEND. THIS MAY INCREASE THE
THREAT OF FLOODING ON SOME AREA RIVERS STARTING FRIDAY OR NEXT
WEEKEND. SNOW LEVELS WILL RISE TO AROUND 7000 FEET BY FRIDAY
THURSDAY AND REMAIN HIGH THROUGH NEXT WEEKEND. ALBRECHT/FELTON

&&

.CLIMATE...65 YEARS AGO THIS MORNING SEATTLE RECORDED ITS COLDEST
TEMPERATURE ON RECORD...0 AT SEATTLE-TACOMA AIRPORT. FELTON

&&

.AVIATION...AN UPPER RIDGE WILL REMAIN OVER THE REGION. NW FLOW
ALOFT WILL BECOME W TONIGHT. CONTD WEAK LOW LEVEL OFFSHORE FLOW.
MEANWHILE...FAIRLY WIDESPREAD FOG WITH VSBYS AOB 1SM AND CIGS BLO
500 FT WILL SLOWLY LIFT THIS AFTERNOON. LESS FOG IS EXPECTED TONIGHT
DUE TO INCREASING MID AND HIGH CLOUDS IN ADVANCE OF A LOW PRESSURE
SYSTEM OVER THE NERN PACIFIC.

KSEA...LIFR VSBYS AND/OR CIGS ARE EXPECTED TO PERSIST TIL ABOUT
2000 UTC. ANTICIPATE VSBYS TO IMPROVE INTO THE MVFR CATEGORY RANGE
THIS AFTERNOON. LIFR CIGS SHOULD SCATTER OUT ABOUT 0000 UTC
SUNDAY.

&&

.MARINE...THERE WAS FAIRLY WIDESPREAD FOG WITH VISIBILITIES OF 1 NM
OR LESS ACROSS THE INTERIOR WATERS THIS MORNING. EXPECT THE FOG TO
PARTIALLY LIFT THIS AFTERNOON.

MEANWHILE...FALLING SURFACE PRES OFFSHORE WITH HIGHER PRES E OF
THE CASCADES WILL RESULT IN EASTERLY FLOW THROUGH TONIGHT.
EXPECT A WEAK COLD OR OCCLUDED FRONT TO MOVE ACROSS THE AREA ON
SUNDAY...FOLLOWED BY A SLIGHTLY STRONGER FRONTAL SYSTEM ON MON.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY IN EFFECT TIL 6 AM SUNDAY FOR THE
      COASTAL WATERS FROM CAPE FLATTERY TO JAMES ISLAND OUT TO
      60 NM AND THE WEST ENTRANCE TO THE STRAIT OF JUAN DE FUCA.

&&

$$

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