Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 301120
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
320 AM PST FRI JAN 30 2015

.SYNOPSIS...AN UPPER RIDGE WILL PROVIDE DRY AND MILD WEATHER THROUGH
SATURDAY. A COUPLE OF SYSTEMS WILL BRING RAIN BACK TO THE AREA
SUNDAY AND MONDAY.

&&

.SHORT TERM...SATELLITE IMAGERY SHOWS UPPER LEVEL RIDGE OVER THE
AREA WITH SOME HIGH CLOUDS OVER THE NORTHERN PORTION OF THE AREA AND
SOME FOG MOSTLY CONCENTRATED OVER THE SOUTHWEST INTERIOR.
TEMPERATURES WERE IN THE 30S AND LOWER 40S AT 3 AM/11Z.

LITTLE CHANGE IN THE FORECAST THIS MORNING WITH THE UPPER LEVEL
RIDGE REMAINING OVER THE AREA TODAY. LOW LEVEL MOISTURE LAYER IS
SHALLOW WHICH WILL LEAD TO A MORNING BREAKOUT IN THE LOCATIONS WERE
FOG FORMED. FOR THE REMAINDER OF THE AREA SUNNY SKIES. SURFACE
GRADIENTS ARE LIGHT OFFSHORE THIS MORNING AND THIS PATTERN WILL
CONTINUE THIS AFTERNOON. THIS WILL PUSH HIGH TEMPERATURES A FEW
DEGREES ABOVE NORMAL TODAY WITH 50S COMMON.

WEAK SHORTWAVE MOVING OVER THE TOP OF THE RIDGE TO THE NORTH OF THE
AREA TONIGHT WITH THE RIDGE AXIS RETROGRADING BACK TO NEAR 130W.
SURFACE GRADIENTS REMAINING LIGHT OFFSHORE AND WITH THE MOSTLY CLEAR
SKIES TEMPERATURES WILL DROP INTO THE 30S IN MOST LOCATIONS WITH THE
COLDER SPOTS NEAR OR JUST BELOW FREEZING. OFFSHORE GRADIENTS WILL
NOT BE STRONG ENOUGH TO PREVENT PATCHY FOG FROM FORMING AGAIN
OVERNIGHT ESPECIALLY IN THE SOUTHWEST INTERIOR.

ANOTHER SUNNY DAY IN STORE FOR SATURDAY WITH THE UPPER LEVEL RIDGE
AXIS OFFSHORE SATURDAY MORNING APPROACHING THE COAST BY LATE
SATURDAY AFTERNOON. TAIL END OF A WARM FRONT MOVING INTO THE
NORTHERN PORTION OF VANCOUVER ISLAND WHILE THE TRAILING COLD FRONT
STILL WEST OF 130W BY 00Z SUNDAY. SURFACE PRESSURES LOWERING A BIT
OFFSHORE WHICH WILL INCREASE THE LOW LEVEL OFFSHORE FLOW OVER THE
AREA. WITH THE INCREASING OFFSHORE FLOW HAVE ADDED A COUPLE OF
DEGREES TO THE HIGH TEMPERATURES SATURDAY VERSUS FRIDAYS READINGS.
THIS WILL PUSH THE WARMEST LOCATIONS...LIKE THE NORTH COAST...INTO
THE LOWER 60S. FOR THE REMAINDER OF THE AREA 50S WILL BE COMMON.

UPPER LEVEL RIDGE MOVING INLAND SATURDAY NIGHT WITH THE APPROACHING
COLD FRONT INSIDE 130W BY 12Z SUNDAY ON THE GFS AND NAM WHILE THE
ECMWF IS A LITTLE SLOWER WITH THE FRONT STILL OUT AROUND 130W. RAIN
OUT AHEAD OF THE FRONT JUST STARTING TO REACH WESTERN WASHINGTON ON
THE FASTER MODEL SOLUTIONS BY 12Z SO WILL STAY WITH A CHANCE OF RAIN
AFTER MIDNIGHT SATURDAY NIGHT. LOWS WITH THE INCREASING CLOUD COVER
WILL BE WARMER THAN THE PREVIOUS TWO NIGHTS...IN THE MID 30S TO
LOWER 40S.

EVEN WITH THE DIFFERENCES IN THE TIMING ALL OF THE MODELS HAVE RAIN
SPREADING OVER WESTERN WASHINGTON ON SUNDAY. THE FASTER NAM/GFS
SOLUTION HAS THE FRONT AT LEAST INTO THE CASCADES BY 00Z MONDAY
WHILE THE SLOWER ECMWF STILL HAS THE FRONT OFFSHORE AT 00Z MONDAY.
ALL OF THE MODELS DO NOT HAVE A LOT OF QPF WITH THE SYSTEM WITH
RAINFALL AMOUNTS OVER THE LOWLANDS BETWEEN A TENTH AND A QUARTER OF
AN INCH. HIGHS WILL BE COOLER...BACK TO NEAR NORMAL AROUND 50
DEGREES.

.LONG TERM...SOME DIFFERENCES IN THE MODELS EARLY ON IN THE EXTENDED
WITH THE DIFFERENT TIMING OF THE COLD FRONT SUNDAY INTO SUNDAY
NIGHT. BOTH THE GFS AND THE ECMWF AGREE THAT ANOTHER FAST MOVING
SYSTEM WILL BE ON THE HEELS OF THE COLD FRONT AND MOVE INTO WESTERN
WASHINGTON ON MONDAY KEEPING RAIN IN THE FORECAST. BEYOND MONDAY
FORECAST CONFIDENCE IS LOW WITH THE EXTENDED MODELS SHOWING LITTLE
RUN TO RUN CONSISTENCY. THE ECMWF WHICH ON THE 00Z RUN LAST NIGHT
WAS BRINGING A WARM FRONT INLAND TO THE NORTH OF THE AREA NOW HAS
THE PRECIP AIMED AT OREGON WITH A WEAK TROUGH APPROACHING VANCOUVER
ISLAND AND NORTHWESTERLY FLOW ALOFT OVER WESTERN WASHINGTON ON
TUESDAY. THE GFS ALSO HAS THE PRECIPITATION GOING INTO OREGON WITH A
LITTLE BIT MORE RIDGE OFF THE COAST VERSUS THE ECMWF. THERE IS A
GENERAL AGREEMENT WITH A WEAK RIDGE OVER THE AREA ON WEDNESDAY
FOLLOWED BY A WEAKENING WARM FRONT MOVING UP FROM THE SOUTH ON
THURSDAY. THE GFS ENSEMBLE SOLUTIONS ARE MUCH MORE CONSOLIDATED ON
THE 00Z RUN TONIGHT VERSUS THE PREVIOUS RUNS. EVEN WITH THIS WOULD
LIKE TO SEE A COUPLE MORE RUNS WITH THIS SOLUTION BEFORE GOING WITH
A DRIER FORECAST FOR TUESDAY AND WEDNESDAY. AT THIS POINT WILL STAY
WITH THE CLIMATOLOGY CHANCE POPS. CHANCE POPS CONTINUING ON THURSDAY
WITH THE POSSIBLE WARM FRONT MOVING INTO THE AREA. FELTON

&&

.HYDROLOGY...FLOODING IS NOT EXPECTED FOR THE NEXT 7 DAYS.

&&

.AVIATION...A BROAD SOMEWHAT FLAT UPPER LEVEL RIDGE WILL REMAIN OVER
THE PACNW TODAY AND TONIGHT. A SURFACE HIGH CENTERED OVER E WA WILL
MAINTAIN WEAK LOW LEVEL OFFSHORE FLOW. THE AIR MASS REMAINS DRY AND
STABLE WITH ONLY LOCAL SURFACE MOISTURE IN THE FORM OF FOG AND
STRATUS.

THE RIDGE AND DRY AIR MASS WILL KEEP SKIES MOSTLY CLEAR TODAY. AREAS
OF FOG ARE FORMING OVER THE SW INTERIOR AN S PUGET SOUND WITH PATCHY
FOG ELSEWHERE. THE MORE PERSISTENT FOG/STRATUS OVER THE S PART
SHOULD CLEAR BY MIDDAY WITH PATCHY FOG ELSEWHERE CLEARING A LITTLE
EARLIER.

KSEA...GOOD VFR CONDITIONS ARE STILL EXPECTED TODAY. WEAK MIXING
FROM N WINDS SEEMS TO HAVE HELD THE FOG AT BAY...BUT THE N WIND IS
WEAKENING SO THERE IS STILL A SMALL CHANCE THAT SOME BRIEF FOG WILL
FORM 12Z-16Z. SURFACE WINDS WILL REMAIN N-NE 1-4 KT THIS MORNING AND
3-6 KT THIS AFTERNOON. KAM

&&

.MARINE...A SURFACE RIDGE CENTERED OVER EASTERN WASHINGTON WILL
MAINTAIN WEAK OFFSHORE FLOW THROUGH SATURDAY. PRESSURE GRADIENTS
ACROSS WESTERN WASHINGTON WILL REMAIN WEAK...PRODUCING N TO NE WINDS
ALONG THE COAST AND THROUGH PUGET SOUND...AND MAINLY EASTERLY WINDS
THROUGH THE STRAIT.

MODELS HAVE BEEN CONSISTENT THE PAST FEW RUNS IN BRINGING A WEAK
FRONT ACROSS THE AREA SUNDAY. MODEL SOLUTIONS FOR THE SURFACE WINDS
ARE PRETTY CLOSE...BUT SOME SOLUTIONS KEEP WINDS BELOW SCA LEVELS
WHILE OTHERS SHOW LOW END SCA WINDS.

MODELS ARE STILL HAVING TROUBLE WITH THE NEXT FRONT ON MONDAY. ECMWF
HAS TRENDED WEAKER OVER THE PAST 3 RUNS WHILE THE GFS HAS TRENDED A
LITTLE STRONGER. THE GFS NOW HAS THE POST-FRONTAL W FLOW A LITTLE
STRONGER THAN THE ECMWF...ALTHOUGH BOTH ARE IN THE SCA RANGE. THE
00Z ECMWF SHOWS A SMALL 1009 MB SURFACE LOW FORMING BRIEFLY JUST
BEFORE IT MOVES INLAND ACROSS CAPE FLATTERY. THE 00Z GFS HAS A 1005
MB LOW. KAM

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...NONE.

&&

$$

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