130
FXUS66 KSEW 181003
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
303 AM PDT Thu Jul 18 2024

.SYNOPSIS...High pressure aloft will maintain dry conditions
along with a gradual warming trend across Western Washington into
the coming weekend. An upper level trough moving into British
Columbia early next week is expected to cool temperatures and
increase night and morning cloudiness.

&&

.SHORT TERM /TODAY THROUGH SATURDAY/...An upper level disturbance
that brought a few thunderstorms to the Cascades is now pushing
eastward across northern Idaho this morning. This initiated a
marine push yesterday evening, but onshore gradients have weakened
over the past few hours. The OTH/SEA gradient has weakened
considerably...which may explain why stratus is making slower
progress into Puget Sound than previously expected. Any low
clouds that do get into the interior will burn off fairly quickly.
Coastal areas should also see a little sunshine by afternoon.
High temperatures will range from the 60s coast to the 70s and
lower 80s inland.

Another weak upper trough will swing across the coastal waters on
Friday. This won`t have much affect on the forecast apart from
tempering an inevitable warming trend. Upper ridging gains
amplitude on Saturday with 500 millibar heights rebounding into
the lower to mid 590s dam. With only light onshore flow and 850
millibar temperatures near or above 20C, temperatures at or above
90 F look likely for interior areas from Seattle southward on
Saturday.

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...The ridge weakens somewhat
on Sunday and onshore flow increases, but this should still
equate to a fairly warm day across the interior of Western
Washington. Deterministic models and ensembles still remain firm
with the idea of an upper trough moving into British Columbia
early next week. Given the time of year and depth of the trough,
the main effects for the region will be cooler temperatures and
more widespread night and morning cloudiness. There could be some
local drizzle near the coast or the odd shower in the North
Cascades, but that would be the extent of any precip as things
stand. Nonetheless, this would be our first venture into anything
temperature wise that resembles normal in almost three weeks.  27

&&

.AVIATION...Upper level ridge to the east and an upper level
trough offshore giving Western Washington southwesterly flow
aloft. In the lower levels high pressure centered offshore with
onshore flow.

Stratus covering most of the lowlands west of the Puget Sound at
09z/2 am with MVFR ceilings. Onshore flow weakening this morning.
This has slowed down the eastward expansion of the stratus and
lowered the chances of the stratus getting east of Puget Sound.
Stratus that does form east of the Puget Sound after 13z
dissipating by 16z. Stratus retreating back to the coastline by
19z leaving clear skies for the afternoon except right along the
ocean beaches. Stratus moving inland again tonight but with weaker
onshore flow good chance it will not get east of Puget Sound
again Friday morning.

KSEA...Stratus still well west of the terminal at 09z. With the
weakening onshore flow if the stratus does arrive it will be brief
between 14z-16z. Mostly clear skies later this morning through
tonight. Southerly wind 4 to 8 knots becoming west southwesterly
21z-03z. Felton

&&

.MARINE...High pressure centered over the offshore waters
extending over the remainder of the waters through the first part
of next week with varying degrees of onshore flow. Westerly
pushes through the Strait of Juan de Fuca each night with small
craft advisory winds tonight and again Sunday and Monday evening.
Felton

&&

.CLIMATE...With two weeks left to go in July Seattle-Tacoma
airport has received zero rain this month. Not even a trace. It is
pretty rare for July to not of gotten any rain at all by this
point of the month. It has only happened 4 times in 80 years of
records, this year, 2023, 2017 and 1985. The record dry month of July
for Seattle is a trace in 2021, 2017, 2013, 1960 and 1958. Felton

&&

.FIRE WEATHER...We`ll see a return of warmer and dry conditions
through the end of the week, with afternoon minimum RHs again
dipping closer to critical thresholds by Friday and Saturday in
the Cascades. At this point, there`s not an overwhelming signal
for significantly increased winds or instability, so just
something to monitor for now. Another disturbance may again bring
some renewed threat for thunderstorms by late in the weekend
across some portion of the mountains, but confidence isn`t
particularly high for now. Finally, expect a somewhat cooler and
more humid pattern to emerge into early next week as a broad
trough of low pressure affects the region. Cullen/27

&&

.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.

&&

$$