Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
000
FXUS66 KSEW 311210
AFDSEW

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

.SYNOPSIS...UPPER RIDGE OVER WESTERN WASHINGTON TODAY. RADIATIONAL
COOLING AND LOW LEVEL TEMPERATURE INVERSIONS WILL SUPPORT AREAS OF
FOG OVER MUCH OF THE INTERIOR LOWLANDS THIS MORNING. A SERIES OF
WEAK SYSTEMS WILL BRING RAIN BACK TO THE AREA SUNDAY AND MONDAY.
SHOWERS WILL GRADUALLY TAPER OFF MIDWEEK FOLLOWED BY A TRANSITION
BACK TO A MORE ACTIVE PATTERN LATE NEXT WEEK.

&&

.SHORT TERM...SATELLITE IMAGERY SHOWS LOW CLOUDS AND FOG OVER MUCH
OF THE I-5 CORRIDOR FROM THE CANADIAN BORDER TO THE WILLAMETTE
VALLEY...ALONG THE NORTH COAST AND THROUGH THE CHEHALIS VALLEY
(PIC0). LOCATIONS WITHOUT THE FOG AND LOW CLOUDS HAVE MOSTLY CLEAR
SKIES. TEMPERATURES WERE MOSTLY IN THE MID 30S TO LOWER 40S AT
3AM/11Z.

FORECAST PROBLEM FOR THE DAY IS HOW LONG THE FOG AND LOW CLOUDS WILL
LAST. SANDPOINT PROFILER SHOWS THE LOW LEVEL INVERSION HAS BEEN
WEAKENING OVERNIGHT. SURFACE GRADIENTS ARE LIGHT OFFSHORE AND WITH
AN OFFSHORE FRONT APPROACHING TODAY THE LOW LEVEL OFFSHORE FLOW WILL
INCREASE. CURRENT KOLM-KBLI AND KUIL-KBLI GRADIENTS ARE -1 AND -2
MB. BY 00Z MONDAY THESE NUMBERS WILL BE CLOSER TO -2 AND -3 MB
(PIC1). MORNING PILOT REPORT FROM SEA-TAC PUTS THE TOPS BETWEEN 600
AND 1000 FEET. WITH THE INCREASING OFFSHORE FLOW...WEAKENING OF THE
INVERSION...AND THE DEPTH OF THE LOW LEVEL MOISTURE WILL STAY WITH
THE FOGGY CONDITIONS THIS MORNING BUT GO FOR SOME SUNSHINE THIS
AFTERNOON ACROSS THE LOWLANDS. HIGH CLOUDS OUT AHEAD OF THE
APPROACHING FRONT WILL REACH THE AREA THIS AFTERNOON SO WILL GO WITH
PARTLY SUNNY WORDING FOR THE AFTERNOON FOR THE MOST PART. HIGHS WILL
BE IN THE UPPER 40S TO MID 50S.

WARM FRONT MOVING BY TO THE NORTH OF THE AREA TONIGHT (PIC2)...CLOSE
ENOUGH TO HAVE A CHANCE OF RAIN OVER AT LEAST THE NORTHERN PORTION
OF THE AREA AFTER MIDNIGHT. MID/HIGH LEVEL CLOUDS WILL NOT BE THICK
ENOUGH EARLY ON TO PREVENT MORE FOG FROM FORMING ESPECIALLY IN THE
SOUTHWEST INTERIOR. LOWS WILL BE MOSTLY IN THE MID 30S TO LOWER 40S.

SOME DIFFERENCES IN THE MODELS REGARDING HOW FAST THE OFFSHORE
FRONTAL SYSTEM WILL MOVE EASTWARD, THE GFS AND NAM ARE THE FASTER
SOLUTIONS (PIC3) WHILE THE ECMWF REMAINS THE SLOWEST OF THE THREE.
CURRENT SATELLITE IMAGERY HAS THE PARENT LOW FOR THE FRONTAL SYSTEM
WAY BACK NEAR 45N/157W (PIC5) WITH THE LOW ACTUALLY MOVING WESTWARD
IN THE LAST FEW HOURS. WITH THE MOVEMENT OF THE PARENT LOW PREFER
THE SLOWER ECMWF SOLUTION. ALL OF THE MODELS BRING RAIN INTO WESTERN
WASHINGTON ON SUNDAY. BOTH THE NAM AND THE GFS MOVE THE COLD FRONT
THROUGH WESTERN WASHINGTON SUNDAY AFTERNOON WHILE THE SLOWER ECMWF
HOLDS OFF UNTIL SUNDAY EVENING. GOING WITH THE SLOWER SOLUTION WILL
HAVE RAIN AT TIMES FORECAST FOR SUNDAY WITH A RAIN LIKELY FORECAST
FOR SUNDAY NIGHT. THIS SYSTEM IS NOT PARTICULARLY WET FOR THIS TIME
OF THE YEAR WITH THE QPF VALUES LESS THAN A HALF AN INCH FOR MOST
LOCATIONS. HIGHS ON SUNDAY NEAR 50 WITH LOWS SUNDAY NIGHT WITH THE
CLOUD COVER ONLY IN THE LOWER TO MID 40S.

ANOTHER SYSTEM RIGHT ON THE HEELS OF THE ONE SUNDAY NIGHT MOVING
INTO WESTERN WASHINGTON ON MONDAY (PIC4) KEEPING RAIN IN THE
FORECAST. THIS SYSTEM IS A LITTLE STRONGER THAN THE ONE FOR SUNDAY
AND WILL PRODUCE A LITTLE MORE PRECIPITATION AS WELL AS BREEZY
CONDITIONS OVER THE NORTHWEST INTERIOR. SNOW LEVELS WILL LOWER DOWN
TO 4000-4500 FEET IN THE MOUNTAINS. HIGHS IN THE LOWLANDS WILL ONCE
AGAIN BE NEAR 50.

.LONG TERM...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 (PIC5). 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 (PIC6). 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 (PIC7)(PIC8). 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...0 AT SEATTLE-TACOMA AIRPORT. FELTON

&&

.AVIATION...AN UPPER RIDGE TODAY WILL GIVE WAY TO A WEAK FRONT
SUNDAY. LIGHT LOW LEVEL OFFSHORE FLOW. FOG IN THE LOWLANDS (PIC0)
OTHERWISE JUST INCREASING CIRRUS TODAY.

KSEA...FOG CONTINUES. AN INCREASE IN OFFSHORE FLOW MAY HELP PUSH IT
OUT OF THE AREA LATER THIS AFTERNOON.

&&

.MARINE...LIGHT FLOW AND AREAS OF FOG. STRONGER OFFSHORE FLOW LATER
THIS AFTERNOON MAY HELP TO PUSH THE FOG AWAY (PIC1). THE FOG WILL BE
DONE SUNDAY WITH STRONGER FLOW AND A FRONT (PIC3)...AND ANOTHER
FRONT WILL REACH THE AREA MONDAY (PIC4).

&&

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

&&

$$

WWW.WEATHER.GOV/SEATTLE

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















USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.