000
FXUS66 KSEW 031650
AFDSEW

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

.UPDATE...Mainly sunny across most of the area this morning with
low clouds along the immediate coast and into the north Puget
Sound. Some clouds at the mid-levels across the Cascades this
morning with mid level instability, even a weak short located over
portions of southern King County in the Cascades. A shortwave
located offshore will move towards W WA today. Instability will
increase over the Cascades and Olympics into the afternoon, and at
the mid levels across the interior. The combination of increasing
instability and the timing of the shortwave may promote the
development of convection, specifically over the Cascades with
upslope flow. There is uncertainty in the short range guidance,
limiting any reflectivity, however, HREF highlights a decent
chance of MUCAPE/DBZ probability along the Cascade crest,
primarily in the North Cascades this evening. The lowlands, mainly
east of Puget Sound will need to be focused on as well with the
shortwave into this evening.

&&

.SYNOPSIS...A shortwave trough will bring the potential for a few
lowland showers and mountain thunderstorms today as it 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 Friday for potential wetting
rain.

&&

.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...Mostly clear but hazy skies throughout Western
Washington this morning. There are some low level marine stratus
hugging the coast this morning but this is expected to mostly
scatter out and lift through this morning. A shortwave will be
moving through the area this afternoon, and will bring some
increased cloudiness to the area. While there is a chance of some
thunderstorms as this wave moves through, they should be mostly
confined to the mountainous regions and away from most of the
terminals. General low level flow will remain onshore throughout
today, with direction depending on terminal location. Looking
ahead to tomorrow, marine stratus is expected to make a further
push inland resulting in morning cloudiness.

KSEA...VFR conditions with hazy skies will continue through most
of today. After 2100 UTC today there will likely be some increased
cloudiness due to a shortwave moving through. Showers from this
wave will likely remain outside of the terminal area. While
surface winds are expected to remain mostly northerly, there could
be some variability as the aforementioned shortwave moves through
this afternoon.

Butwin

&&

.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/Butwin

&&

.FIRE WEATHER...An upper ridge is located across the interior
Pacific NW today. A system will move across the area later today
into tonight. Increasing instability today for an unstable
environment with Mid-Haines values of 6 for much of W WA. There is
a slight chance for t-storms along the Cascades this afternoon
into this evening for LALs of 2. Some uncertainty in the forecast
for the extent of thunderstorms, which will need to be monitored,
with the dry fuels and mid-haines of 6 for the area.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...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.

&&

$$

www.weather.gov/seattle

NWS SEW Office Area Forecast Discussion