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National Weather Service meteorologists use a wide variety of tools such as observations,
satellite imagery, radar imagery, and computer models to aid in making forecasts. The Enhanced
Area Forecast Discussion is intended to provide additional insight into our traditional Area
Forecast Discussion (AFD) by including embedded links to some of the key tools used in the
forecast process. These links are specific to the forecast issue of the day and are updated as the
forecast discussion is updated.
For comments or feedback please
contact the Seattle WFO
Webmaster
For more information on the terms used in the Enhanced AFD, please see
Frequently Asked Questions (updated 20 May, 2008).
For help decoding the image legends, please see
Legends Help.
Previous Version
ZCZC SEAGPHAFD ALL
TTAA00 KSEA 151810
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
930 AM PST THU JAN 15 2009
.SYNOPSIS...A STRONG UPPER LEVEL RIDGE RIDGE (image) WILL REMAIN OVER
THE PACIFIC NORTHWEST THROUGH THIS WEEKEND (image). A WARM DRY AIR
MASS ALOFT WILL CONTINUE TO GIVE A STRONG INVERSION. AREAS OF LOW
CLOUDS AND FOG WILL PERSIST OVER THE LOWLANDS WHILE CLEAR SKIES AND
MILD TEMPERATURES CAN BE EXPECTED AT THE COAST AND IN THE MOUNTAINS.
OFFSHORE FLOW WILL PERSIST...MAINLY NEAR THE WESTERN STRAIT OF JUAN
DE FUCA AND NEAR THE COAST.
&&
.SHORT TERM...STRATUS CONTINUES TO SLOWLY LOWER...AIR MASS IS MURKY
IN THE LOWLANDS. CLEAR AND GOOD MIXING COAST AND WESTERN STRAIT OF
JUAN DE FUCA...OFFSHORE GRADIENTS HAVE INCREASED...AT 9AM SEA-EAT WAS
-6.2MB AND UIL-BLI -4.7MB...BUT IT WILL STILL BE A BIT OF A STRUGGLE
FOR THE WARM AIR ALOFT AND EASTERLY FLOW TO START MIXING OUT THE
PUGET SOUND BASIN...MAYBE A COUPLE HOURS OF IMPROVEMENT AND ONLY IN
A FEW AREAS...THEN FOG OR STRATUS WILL SET UP QUICKLY AGAIN EVENING.
THE WEST SLOPES OF THE CASCADES CONTINUE TO HAVE A GOOD CHANCE OF
AFTERNOON SUNSHINE. OUR MAX TEMP FCSTS WERE TOO WARM YESTERDAY ALL
ALONG THE WEST SLOPES OF THE CASCADES AND ON THE EAST SIDE...BUT IT
IS SO WARM ALOFT THAT JUST A LITTLE MIXING WILL MAKE A SPECTACULAR
DIFFERENCE. THE FCST FOR THE MOUNTAINS AND COAST IS EASY
ENUF...SUNNY...AND THE INVERSION IS ONLY VERY SHALLOW ON THE COAST
SO WARM TEMPS ABOVE (IT IS 60F ON THE UIL SOUNDING AT ONLY 1200FT)
ARE CERTAIN TO MIX DOWN TO SOME DEGREE IN MANY AREAS OUT THERE. THE
PROFILER HERE SHOWS THE TEMP JUMPING FROM +2C BELOW THE INVERSION AT
300M TO +15C UP ABOVE THE STRATUS IN THE WARM AIR...THAT IS QUITE A
JUMP. WHETHER THE 10-15KT EAST WIND SKIMMING ACROSS THE TOP OF THE
STRATUS CAN ENTRAIN DRY AIR INTO THE LAYER AND THEN ALLOW SOME
MIXING THIS AFTERNOON REMAINS TO BE SEEN. BUT EVEN IF WE DO MIX UP
TO SOME DEGREE THIS AFTERNOON THE STRONG INVERSION IS SURE TO
QUICKLY REESTABLISH RIGHT AWAY EACH EVENING...AND AN INVERSION EVER
GETTING CLOSER TO THE SURFACE WILL TRAP POLLUTANTS EVEN MORE
EFFECTIVELY. OUR TEMP FCSTS ARE A MIX OF STATISTICAL GUIDANCE AND
THE MODELS WITH COOLEST TEMPS IN THE LOWLANDS...AND WARMTH IN THE
MOUNTAINS AND ON AND COAST...AS WELL AS SOME OPTIMISM FOR THE WEST
SLOPES OF THE CASCADES WITH THIS OFFSHORE FLOW AND BETTER CHANCE OF
MIXING THERE. THIS IS THE MAX TEMP FCST FOR TODAY (image). 19
.LONG TERM...MAY HAVE TO PUSH BACK THE TREND TO MORE CLIMO LOOKING
FCSTS WITH THE CHANCE OF SHOWERS...THE LATEST RUNS OF THE GFS DO
UNDERCUT THE RIDGE SOMEWHAT BUT A SYSTEM GOES INTO CALIF BY MIDWEEK
WHILE A WEAKER CUT OFF UPPER HIGH REMAINS CENTERED NEAR VANCOUVER
ISLAND...SO THE RIDGE MAY TAKE A COUPLE OF EXTRA DAYS TO BREAK DOWN
NEXT WEEK...SLOWER THAN THE CURRENT FCST. HERE IS THE 12Z GFS VALID
MIDDAY WED (image) NOTE THAT THE 1000-500MB RH LOOKS DRY...BUT LOW
CLOUDS AND FOG IN THE LOWER LEVELS MAY VERY WELL STILL BE IN PLACE
OVER WESTERN WA. 19
&&
.AVIATION...A STRONG SUBSIDENCE INVERSION WILL REMAIN INTACT OVER
WESTERN WASHINGTON FOR THE NEXT 24 HOURS. THE INVERSION APPEARS TO
HAVE LOWERED SOME IN THE PAST 24 HOURS...TODAY'S 12Z KUIL SOUNDING
(image) SHOWED THE WARM NOSE ABOVE THE INVERSION NEAR 900
MB...COMPARED TO YESTERDAY'S 850 MB (image). THIS HAS CAUSED STRATUS
AND CEILINGS BENEATH THE INVERSION TO LOWER A BIT. PRETTY STEADY
CEILING AROUND 600 FT MSL OVER THE CENTRAL AND SOUTH SOUND THIS
MORNING. KSEA-KEAT EASTERLY GRADIENTS HAVE STRENGTHENED FROM
NON-EXISTENT YESTERDAY MORNING INTO THE MODERATE CATEGORY THIS
MORNING. AM COUNTING ON THIS ERODING THE ERN EDGE OF THE STRATUS
ALONG THE FOOTHILLS WITH SOME TEMPORARY SCATTERING GETTING AS FAR
WEST AS I-5 FROM TCM-PAE-BLI LATE THIS AFTERNOON. STRATUS NEAR KOLM
AND ALONG THE NORTH AND EAST SIDE OF THE OLYMPICS SHOULD REMAIN
FIRMLY IN PLACE TODAY. (image)
KSEA...STRATUS DECK HAS JUST ABOUT LOWERED ONTO THE AIRFIELD AS OF
17Z. STRATUS MAY LIFT A FEW HUNDRED FEET DURING THE COURSE OF THE
DAY...BUT CIGS SHOULD REMAIN BELOW 010 THROUGH AT LEAST 22Z.
INCREASING DRY EASTERLY FLOW FROM THE CASCADES INTO THE FOOTHILLS
MAY PUSH SOME SCATTERING OF THE STRATUS AS FAR WEST AS KSEA SOMETIME
AFTER 22Z. BELIEVE ENOUGH SCATTERING WILL TAKE PLACE TO SET THE
STAGE FOR SURFACE-BASED FOG FORMING OVER THE AIRFIELD THIS EVENING
AFTER ABOUT 04Z. HANER
&&
.MARINE...STRENGTHENING HIGH PRES EAST OF THE CASCADES (image) WILL
CONTINUE THE MODERATE OFFSHORE FLOW OVER WESTERN WASHINGTON.
KUIL-KBLI GRADIENT OF -4.7 AT 16Z WILL CONTINUE TO SUPPORT EASTERLY
GALES AT THE WEST ENTRANCE TODAY. LOOKING FOR PEAK WIND SPEEDS
DURING THE DIURNALLY FAVORED PERIOD AROUND 11 AM OR NOON. BASED ON
OBS FROM YESTERDAY AND FAIRLY PERSISTENT GRADIENTS OVER WRN
WA...OPTED TO GO WITH SMALL CRAFT ADVISORIES FOR THE COAST AND
CENTRAL STRAIT AS WELL. WITH LITTLE CHANGE IN THE PATTERN FOR THE
NEXT FEW DAYS...GALE WARNING AND SCA MAY NEED TO BE EXTENDED OR
RE-ISSUED LATER TODAY. HANER
&&
.SEW WATCHES/WARNINGS/ADVISORIES...
WA...AIR STAGNATION ADVISORY FOR THE LOWLANDS OF WESTERN WASHINGTON
EXCEPT FOR THE COAST AND WESTERN STRAIT.
FLOOD WARNING REMAINS IN EFFECT FOR THE CEDAR RIVER AT RENTON
IN KING COUNTY.
FLOOD ADVISORY REMAINS IN EFFECT FOR THE GREEN RIVER IN KING
COUNTY.
PZ...GALE WARNING WEST ENTRANCE TO THE STRAIT OF JUAN DE FUCA.
SMALL CRAFT ADVISORY FOR THE COAST AND CENTRAL STRAIT.
$$
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