Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 241603

Area Forecast Discussion
National Weather Service Seattle WA
903 AM PDT Sat Aug 24 2019

.UPDATE...A weak shortwave trough is progressing rather quickly
across the state of Washington this morning and is readily
apparent in water vapor imagery. This shortwave is embedded
within a larger scale cyclonic flow regime that essentially
extends from Alaska into the Northern Rockies, with best/strongest
shortwave energy across BC. Associated with some of this stronger
energy in BC is a weak sfc reflection in the Haida Gwaii vicinity
that has carried a trailing cold front southward towards the
coastal waters of the Pacific Northwest. Some shower activity is
seen along the Washington coast with this weakening frontal
boundary. These showers will continue to push inland over the next
few hours this morning before gradually diminishing with eastward
progress. Therefore, best chance of rain into much of the
afternoon with remain west of Puget Sound, with mostly cloudy
skies expected across the area with some sun breaks.

Behind the weak front later this afternoon into the evening will
be a decent onshore push of westerly winds. Some hi-res guidance
is showing a weak convergence zone developing in the northern
Sound vicinity as westerly winds down the Strait of Juan de Fuca
push into the north Sound and converge with SW low level flow in
the central/south Sound. Therefore have added a slight chance
showers in this timeframe across this area and into portions of
King and Snohomish Counties. Any QPF resulting from shower
activity will be light, assuming this zone even develops.

Little change in the pattern overnight into Sunday morning, though
heights will begin to quickly rise late in the day as a ridge
across the Pacific into California and the desert SW begins to
build north.

Aside from an adjustment to the POPs/WX forecast to account for
potential convergence zone later today, the rest of the forecast
is on track and was left as is. Previous discussion included below
with an aviation and marine discussion update.



.PREV DISCUSSION... /issued 311 AM PDT Sat Aug 24 2019/

.SYNOPSIS...A weak upper level trough will move by to the north
today and tonight. An upper level ridge will begin building
offshore on Sunday. The ridge will continue to build and shift
over Western Washington Tuesday and Wednesday with the low level
flow going light offshore. The ridge will weaken Thursday and
Friday with low level onshore flow returning to the area.


.SHORT TERM /TODAY THROUGH MONDAY/...Satellite imagery shows
cloudy skies over Western Washington at 3 am/10z. Most of the
cloud cover is of the mid and high variety. Temperatures were in
the 50s to lower 60s.

Lots of cloud cover today into tonight as a weak trough moves by
to the north. Could be enough lift with the trough to produce a
few showers from about a Bellingham to Astoria line westward today
and mainly over the Cascades this evening. Any precipitation that
does develop will be light. With the cloud cover high temperatures
will continue to be a little below normal, in the mid 60s to mid
70s. Lows tonight mostly in the 50s.

Increasing low level onshore flow later tonight into Sunday with
the trough east of the area. Low clouds developing along the coast
spreading inland Sunday morning. Marine layer not deep enough to
get the stratus all the way to the Cascade crest but deep enough
to keep skies mostly cloudy through the morning hours. Upper level
ridge beginning to build offshore with 500 mb heights rising over
Western Washington but too late to warm high temperatures Sunday
with readings once again in the mid 60s to mid 70s.

Upper level ridge continuing to build offshore Sunday night into
Monday with the surface gradients going northwesterly. This will
limit the cloud cover in the morning on Monday. Monday morning
the coolest morning of the month with lows in the mid 40s to mid
50s. Highs on Monday warming a little, into the 70s.

.LONG TERM /TUESDAY THROUGH FRIDAY/...Upper level ridge moving
over Western Washington Tuesday and Wednesday with 500 mb heights
in the lower to mid 580 dms. Low level flow turning offshore but
00z runs are weaker with this development versus previous runs.
Models still have 850 mb temperatures warming into the plus 16 to
plus 20C range but with the weaker offshore flow scenario will
have the warmest locations near 90 with the remainder of the area
in the mid 70s to upper 80s. The weaker offshore flow scenario
leads to a quicker return to low level onshore flow on Thursday
as the upper level ridge begins to weaken. Upper level ridge
continues to weaken on Friday with low level onshore flow. The
models have a weak surface low over the Oregon and Washington
coastal waters limiting the strength of the onshore flow. For now
will go with a 2 day weak push scenario with around 5 degrees of
cooling both Thursday and Friday. Felton


.AVIATION...Westerly flow aloft will become more northwesterly today
as an upper level trough departs the area. Onshore flow at the
surface. Generally VFR cigs at this hour with MVFR along the coast
and across a swath of the Puget Sound near KSEA and KBFI. Expect VFR
cigs by late morning across the area with mid to high level clouds
through the afternoon. Winds generally southerly becoming more
northerly for some by later this afternoon. Winds 5-10 knots.

KSEA...MVFR cigs have developed with MVFR possibly through the next
few hours. Expect improvement to VFR by late morning/midday. Mid to
high clouds this afternoon. Southwesterly winds becoming more
northerly into this evening. Winds 5-10 knots. JD


.MARINE...Onshore flow continues through today. A weak front will
move across the area today with another westerly push down the
Strait this evening into tonight. Gradients look stronger with this
push but still under gale criteria so have Small Craft Advisory for
the central and eastern Strait of Juan de Fuca through tonight.
Onshore flow through Monday with offshore flow Tuesday and through
much of next week. JD


.HYDROLOGY...The daily hydrology discussion has ended until the
start of the next rainy season; it will only be updated as needed
until then.


PZ...Small Craft Advisory from 4 PM this afternoon to 5 AM PDT Sunday
     for Central U.S. Waters Strait Of Juan De Fuca-East
     Entrance U.S. Waters Strait Of Juan De Fuca.


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