Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 272228 CCA
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
330 PM PDT WED APR 27 2016

.SYNOPSIS...INSTABILITY SHOWERS OVER THE MOUNTAINS WILL END THIS
EVENING. HIGH PRESSURE WILL BUILD OVER THE REGION THURSDAY WITH
MOSTLY SUNNY AND SLIGHTLY WARMER WEATHER. AN UPPER TROUGH WILL
GENERATE SHOWERS ACROSS WESTERN WASHINGTON LATER THURSDAY NIGHT AND
FRIDAY. A STRONG UPPER RIDGE WILL BRING DRY WEATHER AND A WARMING
TREND SATURDAY THROUGH MONDAY. ONSHORE FLOW MAY RESUME TUESDAY WITH
A CHANCE OF SHOWERS AS A FRONTAL SYSTEM APPROACHES WESTERN
WASHINGTON.

&&

.SHORT TERM...MOSTLY CLOUDY SKIES CONTINUE ACROSS WRN WA LATE THIS
AFTERNOON UNDER A WEAKLY UNSTABLE AND MOIST AIR MASS. RADAR SHOWS
SOME ISOLATED SHOWERS ALONG THE HOOD CANAL/E SIDE OF THE OLYMIPICS
AND ALSO OVER THE CASCADES. ONLY A FEW SPOTS GOT MEASURABLE
RAIN...MAINLY OVER THE MOUNTAINS OR FAR SOUTH INTERIOR. CLOUDS WILL
DECREASE WITH LOSS OF DIURNAL INSTABILITY LATER THIS EVENING.

A BRIEF SHORT WAVE RIDGE TRAVERSES THE REGION THURSDAY. ONSHORE FLOW
WILL MAINTAIN SOME CLOUDS ACROSS THE AREA BUT IT SHOULD REMAIN DRY
WITH SLIGHTLY WARMER HIGH TEMPERATURES.

MODELS ARE STILL IN AGREEMENT THAT THE NEXT PROGRESSIVE TROUGH
APPROACHES THE PAC NW THURSDAY NIGHT. THE GFS/ECMWF AND MESO MODELS
ALL SHOW A PRE-FRONTAL CONVERGENCE ZONE DEVELOPING AFTER MIDNIGHT
BETWEEN SOUTH SKAGIT AND NORTH KING COUNTIES. THE FRONT REACHES THE
COAST FRIDAY MORNING WHICH MODELS SHOW TO BE NEARLY VERTICALLY
ALIGNED WITH THE 500 MB TROUGH AXIS. THE SYSTEM PUSHES EAST OF THE
CASCADES IN THE AFTERNOON. MOS GUIDANCE GENERALLY POINTS TO LIKELY
POPS ACROSS THE AREA AND THERE IS GOOD RUN TO RUN MODEL CONSISTENCY.
POPS WERE BOOSTED TO LIKELY CATEGORY ACROSS THE ENTIRE AREA WITH
MOST PLACES LIKELY TO MEASURE. THE COAST...MOUNTAINS...AND
CONVERGENCE ZONE CORRIDOR MAY SEE UP TO A QUARTER INCH WITH LIGHTER
AMOUNTS OVER THE NORTHERN LOWLANDS. HIGHS WILL BE SLIGHTLY BELOW
AVERAGE IN THE 50S.

ENERGY ON THE BACK SIDE OF THE TROUGH SLOWS THE SYSTEM FRIDAY NIGHT
BUT INSTABILITY WILL STILL DECREASE WITH LOSS OF DAYTIME HEATING.
KEPT CHANCE POPS FOR THE EVENING THEN CONFINE LOW CHANCE POPS TO
JUST THE MOUNTAINS LATE FRIDAY NIGHT. HIGH PRESSURE BUILDING
OFFSHORE WILL START TO MIGRATE EASTWARD TOWARD THE AREA BRINGING AN
END TO SHOWERS BY SATURDAY MORNING.

A STRONG RIDGE WILL BUILD NE INTO B.C. AND OVER THE PAC NW SATURDAY.
500 MB HEIGHTS WILL WILL REACH NEAR 5750M BY AFTERNOON AND LOW LEVEL
NORTHERLY FLOW. AREA NEAR THE WATER WILL BE HELD BACK FROM REACHING
POTENTIAL HIGHS GIVEN THE AFTERNOON NORTH BREEZE. BUT WILL BE WELL
ABOVE AVERAGE IN THE 60S...POSSIBLY EVEN NEAR 70 WELL INLAND NEAR
THE WATER.

.LONG TERM...MODELS SHOW THE UPPER RIDGE NARROWING AND SHIFTING NE
OVER B.C. AND THE PAC NW SUNDAY. THE RIDGE WILL BROADEN BUT BEGIN TO
SHIFT EWD ON MONDAY AS A LOW OVER THE GULF OF AK STARTS TO DIG SWD.
THE FLOW WILL BE OFFSHORE AND DRIER AIR AND SUNNY SKIES. HIGHS
SHOULD EASILY REACH THE 70S WITH THE WARMEST TEMPERATURES AROUND
PUGET SOUND ON MONDAY WITH LOW 80S POSSIBLE.

MOST MODELS INDICATE THE NEXT APPROACHING TROUGH WILL INDUCE A FULL
OR PARTIAL MARINE PUSH MONDAY NIGHT OR TUESDAY. HIGHS SHOULD DROP
OFF SIGNIFICANTLY BUT THE TIMING AND STRENGTH OF THE TROUGH IS
HIGHLY UNCERTAIN. THERE IS A LOT OF UNCERTAINTY GOING INTO MID WEEK
BUT THE TREND APPEARS TO BE MOVING TOWARD A WEST COAST LONG WAVE
TROUGH. THE FORECAST TRENDS COOLER WITH A CHANCE OF SHOWERS TUESDAY
AND WEDNESDAY BUT STILL ABOVE AVERAGE. MERCER

&&

.AVIATION...CLOUDS OVER WRN WA LEFTOVER FROM A WEAK FRONT THAT MOVED
ASHORE EARLIER TODAY WILL THIN AND BREAK UP INTO THE EVENING
HOURS...SCATTERING OUT BY LATE EVENING. PATCHES OF LOW CLOUDS OR FOG
COULD SET UP IN THE MOST FOG PRONE SPOTS LATE TONIGHT...OLM AWO FOR
EXAMPLE.

KSEA...THE CLOUDS WILL SLOWLY THIN AND BREAK UP THIS EVENING.
PRES GRADIENTS ARE LIGHT.

&&

.MARINE...ONSHORE FLOW WILL CONTINUE WITH HIGH PRESSURE OVER
THE COASTAL WATERS AND LOWER PRES INLAND. ANOTHER WEAK FRONT WILL
BREAK UP OVER WRN WA ON FRIDAY.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORIES ARE IN EFFECT FOR THE CENTRAL/EASTERN
 STRAIT OF JUAN DE FUCA AND THE ADMIRALTY INLET TONIGHT.


&&

$$

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