738
FXUS66 KSEW 020958
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
258 AM PDT Tue Jul 2 2024

.SYNOPSIS...High pressure begins building offshore with northwest
flow aloft over western Washington today. This upper ridge will
strengthen significantly and move east over the region later this
week, ushering in a very hot and dry weather pattern late this
week. This pattern may remain in place through early next week,
maintaining  an extended period of very hot conditions.

&&

.SHORT TERM /TODAY THROUGH THURSDAY/...Upper level high pressure
remains centered well offshore this morning, maintaining northwest
flow aloft across western Washington. Meanwhile, onshore low-level
flow continues to support the expansion of low stratus along the
coast. Some lingering clouds remain over portions of the interior
near Puget Sound after a few showers developed in an area of weak
convergence. With morning clouds in place and the residual onshore
component to the wind continuing today, expect rather seasonable
temperatures that may run a few degrees cooler than yesterday for
most. Not much change to the pattern into Wednesday with temperature
again bumping up a few degrees warmer as the offshore ridge strengthens
and we see less in the way of cloud cover for the morning that day.

A more significant shift in the pattern is in store starting
around the Independence Day holiday for the region as the ridge of
high pressure offshore begins to shift eastward toward the region.
This will bring afternoon temperatures into the upper 70s to mid
80s across much of the interior and signal the start of several
days of well above normal temperatures.

.LONG TERM /FRIDAY THROUGH MONDAY/...Strong agreement in the
ensemble guidance continues to boost the likelihood of very hot
temperatures through the long term period with several days
reaching the Moderate category of HeatRisk. The high amplitude
upper level ridge will likely remain parked over the region into
the weekend, with much of the differences in the ensembles only in
the precise amplitude and/or placement of the ridge axis, but
there`s very strong support for the strong ridge in place in the
vicinity of the region. Saturday and Sunday look to the be hottest
day for most, with better than a 60% percent chance for the
central Puget Sound region eclipsing the 90 degree mark (and
higher from Olympia south along I-5 into Lewis County and eastward
into the Cascade valleys and foothills). Hedged the forecast a
bit closer to the NBM 50th percentile from the deterministic NBM
for the interior for the Friday to Sunday time period, which did
result in a slightly hotter forecast for many locations. Depending
on the overnight low temperatures, it`s even possible for some
locations to have a day or two in the Major level of HeatRisk late
in the weekend if any significant relief from onshore flow is
shut off. It`s notable that there are some fairly large spreads in
the potential outcomes by Sunday and Monday, as much will depend
on the precise placement of the ridge axis and the degree of
offshore flow that develops. Beyond the heat related concerns,
expect that daytime relative humidities will steadily move closer
to critical thresholds, resulting in the potential for increased
fire weather concern (especially in proximity to the Independence
Day holiday). Cullen

&&

.AVIATION...Upper level ridge centered offshore will continue to
build into the region today with northwest flow aloft. The low level
flow remains onshore. MVFR ceilings in stratus remain confined to
coastal areas this morning and this is expected to persist through
the morning. Areas of stratocumulus between 3500 and 6000 feet will
remain across the interior lowlands this morning as the result of
remnant convergence. Ceilings along the coast are expected to
briefly lift to VFR after 20Z while VFR ceilings across interior
areas scatter out this afternoon. Onshore flow will lead to
the return of low MVFR in stratus along the coast and locally inland
tonight and Wednesday A.M.

KSEA...Broken stratocumulus deck between 4000 to 6000 feet
expected to linger through the morning before scattering out this
afternoon and evening. Presently thinking there`s about 50/50 chance
of MVFR ceilings redeveloping again Wednesday A.M. between 12Z and
16Z. Surface winds light S/SW veering once again to W/NW this
afternoon.    27

&&

.MARINE...Summer pattern taking shape this week as broad surface
ridge takes up residence over the offshore waters with lower
pressure across the interior. This will keep onshore flow in place
with westerlies likely reaching small craft advisory levels in the
central/east strait each of the next couple afternoons/evenings.
The strong surface ridge offshore will also lead to some gusty
northwest winds over the outer coastal waters. The fetch
of NW winds will generate some seas of 8 to 10 feet tonight into
early Wednesday. Thermally induced low pressure expanding northward
across the interior Thursday into Saturday will lead to lighter
onshore, or occasionally weak offshore, flow.   27

&&

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

     Small Craft Advisory from 5 PM this afternoon to 3 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 until 5 AM PDT early this morning for
     Admiralty Inlet-Northern Inland Waters Including The San
     Juan Islands.

     Small Craft Advisory until 11 AM PDT Wednesday for Coastal
     Waters From Cape Flattery To James Island 10 To 60 Nm-
     Coastal Waters From James Island To Point Grenville 10 To
     60 Nm-Coastal Waters From Point Grenville To Cape
     Shoalwater 10 To 60 Nm.

&&

$$