FXUS66 KSEW 031008
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
308 AM PDT Tue Aug 3 2021

.SYNOPSIS...A shortwave trough will bring the potential for a few
lowland showers and mountain thunderstorms today as is crosses the
CWA during peak heating. A ridge will build northward from the
desert SW in its wake for Wednesday, likely advecting in more 
smoke aloft from California. More substantial troughing from the
Gulf of Alaska and BC Coast will approach on Thursday, eventually
helping to swing a frontal boundary through the area on Friday and
providing the first shot at wetting rainfall in quite some time.

&&

.SHORT TERM /TODAY THROUGH THURSDAY/...Water vapor imagery early
this Tuesday morning reveals areas of troughing aloft over the NE
Pacific: An area of closed low pressure can be seen over the Gulf
of Alaska, with a weak shortwave trough noted well to the south
just east of 130W off the Coastal Waters of WA/OR, with another
area of closed low pressure noted well offshore the Coast of
California. Downstream from these Pacific troughs lies a ridge of
high pressure over the desert SW and thru the Intermountain West,
with troughing then dominant east of the Rockies over much of the
eastern CONUS. In the low levels, onshore flow continues over the
PNW as high pressure continues offshore and weak troughing
continues east of the Cascades. High pressure is noted over 
portions of the Rockies and through much of the middle of the 
country, with a stationary boundary located along the Gulf Coast 
states.

Conditions are fairly tranquil over the W WA as of 3AM, with just
a few pinprick showers noted in the Cascades and down towards
Lewis Co. Through the morning hours, the aforementioned shortwave
trough offshore the WA/OR Coast will approach the Coast, with the
flow aloft continuing from the SW. Model guidance continues to
suggest the advection of smoke aloft from the fires in CA. 
Although skies are forecast to be clear, smoke will continue to
produce hazy sky conditions. Marine stratus will also develop 
along the Coast and Lower Chehalis Valley, eventually eroding 
into the afternoon.

The fly in the ointment today will be the shortwave and its
potential for the production of some showers and possibly mountain
thunderstorms. Most model guidance is rather lackluster in terms
of POP generation today, however, a few are hinting at the
potential. Provided decent vorticity associated with the
shortwave, combined with its optimal placement over the CWA during
peak heating, think lift supplied by it combined with some upslope
flow component over the terrain will be enough to spark some
showers this afternoon and evening amidst sufficient instability.
Right now the best chance for showers appears to be confined to 
the Central & North Cascades, however, the potential for 
development all the way west into the San Juans cannot be ruled 
out. Have opted to include this line of thinking in the forecast 
update this morning, with also the addition of thunder potential 
for the Cascades, which matches well with current SPC outlook. All
this being said, can't even completely rule out a few showers 
around Puget Sound, but confidence was too low to include mention 
of precip currently. Main threats with mountain thunderstorms will
be lightning and erratic winds, which combined with dry fuels 
will raise fire weather concerns for potential new starts. 
Activity should move east of the Cascades overnight. 

By Wednesday morning, the shortwave will have pushed east of the
Cascade crest, with slight mid level height rises expected over W
WA as the desert SW ridge builds northward in the shortwave's 
wake. Have opted to add mention of haze again for Wednesday as mid
and upper level flow remains out of the S/SW as the trough 
offshore California moves closer inland. Wednesday will be dry.

For Thursday, smoke aloft should move east as the more substantial
troughing that was over the Gulf of Alaska begins to move
southward. This will begin to push a sfc frontal system towards
the local area. Most of Thursday currently looks dry and right now
do not even see much of an indication of substantial lower level
moistening per latest NAM and GFS Bufr soundings. Have therefore
cut back on POPs near the Coast for Thursday until the evening
hours. This will then carry us into the long term period.

Kovacik

.LONG TERM /FRIDAY THROUGH MONDAY/...The beginning of the long
term forecast period on Friday will feature the approach of an
area of closed low pressure from near Haida Gwaii. This low
continues to look strong enough to swing an organized frontal
boundary across W WA late Thursday night through much of Friday.
This is still likely to bring a wetting rain to most, if not all,
areas on Friday. Current QPF totals from this system look to range
from around 0.10-0.25 inches over the lowlands, with higher
amounts in the mountains. Showers look likely to linger overnight
Friday into Saturday, particularly for areas east of the Sound. 

Thereafter, through the remainder of the weekend and into early
next week, ensemble guidance is in pretty good agreement that the
trough will slowly moves east of the area, riding along the
US/Canada border, with a fairly high amplitude ridge building over
the Pacific. Enough residual troughing may influence the area
enough to keep for a chance of showers across the northern half of
the CWA into Monday (essentially the San Juans and Skagit and
Whatcom counties). High temperatures will remain in the 70s over
the lowlands (60s Coast).

Kovacik

&&

.AVIATION...South to southwest flow over western WA today with 
low level onshore flow near the surface. Low level stratus clouds 
will mainly affect the coast this morning with possible IFR to 
MVFR cigs, otherwise VFR conditions expected. There's a slight 
chance of high-based showers and thunderstorms near the Cascades 
this afternoon and early evening. 33

KSEA...VFR today with north wind around 5-12kt. 33

&&

.MARINE...Varying degrees of onshore flow through the period with
strongest wind/waves through the Strait of Juan de Fuca. A gale 
Warning is in effect for the Strait of Juan de Fuca tonight. A 
frontal system will affect the area Thursday and Friday with 
increasing southerly winds over the interior waters. 33

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 5 AM PDT early this morning for 
     Central U.S. Waters Strait Of Juan De Fuca-East Entrance 
     U.S. Waters Strait Of Juan De Fuca.

     Gale Warning from 5 PM this afternoon to 5 AM PDT Wednesday for 
     Central U.S. Waters Strait Of Juan De Fuca-East Entrance 
     U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from 5 PM this afternoon to 5 AM PDT 
     Wednesday for Northern Inland Waters Including The San Juan 
     Islands.

&&

$$

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