Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 241033
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
330 AM PDT WED AUG 24 2016

.SYNOPSIS...
A strong upper level ridge of high pressure over the north central
Pacific will continue to steadily build east...resulting in
continued warming across Western Washington through Friday.
Temperatures will reach well above normal along with increased fire
danger in the lowered humidities and increased winds. This will come
to a sharp end on Saturday as an upper level low pressure diving
south through BC into Washington will bring in sharply cooler
temperatures and a few light showers.

&&

.SHORT TERM...Not much in the way of stratus out there this early
morning for much of Western Washington...and temps cooling
nicely...albeit short lived as high temperatures today are expected
to be approximately 5 degrees warmer...on average...over the area
today.

That said...fair to say that the strong upper level ridge continues
to hold court over the Pacific NW and will continue its fiery grip
into the second half of the week...as temperatures will continue to
warm Thursday and Friday. Looking at current forecast highs...with
interior lowland temperatures once again in the upper 80s and
southwestern interior temps in the lower 90s...looks borderline for
any heat advisories for those days but will send out a Special
Weather Statement as a precursor should the day shift opt to go with
headlines and to heighten public awareness. Given that the previous
heat event occurred so recently...would assume that public awareness
is likely already halfway there though. The ridge is not the only
factor into the upcoming heat...as models continue to show offshore
flow both days in addition to the potential development of a thermal
pressure trough at the surface.

This gives rise to another combination of concern...with the hot
weather resulting in lower relative humidities coupled with offshore
flow and the potential for a thermal pressure trough...fire weather
concerns also rise to the forefront. While NE winds look
borderline...the low RH values coupled with widespread mid-level
Haines values of 6 certainly display that this is a period of
concern...thus the inherited Fire Weather Watch for Thursday and
Friday looks good and will remain in place.

Thus...inherited forecast in generally good shape...only a few minor
tweaks here and there for morning forecast package.  SMR

.LONG TERM...Saturday will be different...very different. Much like
the end of the last run of very warm temperatures...this streak too
will come to a crashing end as high temps on Saturday look to be
approximately 15 degrees cooler than highs on Friday. Imagine that
is how one spells relief. This comes from yet another familiar
pairing...an incoming upper level low and a strong onshore marine
push starting as early as Friday evening. Speaking of the
aforementioned upper low...models finally starting to show a unified
solution with the system dipping down into Western WA Saturday
night...bringing with it the chance for precipitation...mainly over
the north Cascades but spreading a bit more during the day Sunday.
This will make for a pattern change...as current progs suggest a
generally trough-y pattern over the Pacific NW for the remainder of
the forecast cycle with periodic systems and chances for showers
into the second half of next week.  SMR

&&

.AVIATION...A strong upper level ridge will remain over the NE
Pacific today. The flow aloft is northerly. The air mass over
Western Washington is dry and stable. Patchy low clouds will
mainly affect the coast this morning. Otherwise VFR conditions are
expected.

KSEA...North wind rising to 10-15 kt this afternoon. VFR
conditions. 33

&&

.MARINE...Northerly flow will prevail over the waters through
Thursday as a thermal trough forms along the coast. Onshore flow
will develop on Friday as low pressure shifts inland. Onshore flow
will continue through the weekend. 33

&&

.SEW Watches/Warnings/Advisories...
WA...Fire Weather Watch from Thursday morning through Friday evening
     for Black Hills and Southwest Interior Lowlands-Central and
     South Puget Sound Lowlands-North Coastal Lowlands-Southeast
     Puget Sound Lowlands Generally Below 1500 Feet.

PZ...None.
&&

$$

www.weather.gov/seattle

You can see an illustrated version of this discussion at
www.weather.gov/seattle/gafd/latest_webafd.html



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