155
FXUS66 KSEW 172158
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
258 PM PDT Wed Jul 17 2024

.SYNOPSIS...A trough of low pressure in eastern Washington will
trigger isolated to scattered thunderstorms across the Cascades
this afternoon, with a lesser chance in the Olympics of storms.
The fire danger continues in these areas through tonight due to
the threat of lightning and gusty winds from these storms.
Remaining areas will see partly sunny skies through the afternoon.
High pressure will build back in through the end of the week, with
Saturday being the warmest day with south Puget Sound areas
reaching 90 degrees. Onshore flow will strengthen early next week,
cooling temperatures down.

&&

.SHORT TERM /TONIGHT THROUGH SATURDAY/...Upper analysis shows
southwesterly flow aloft between a ridge over Montana, and a
trough to the west over the Pacific Ocean. There is a cutoff upper
level low feature over western Washington this afternoon which has
been dipping in some vorticity advection into the region (hence
the cloud cover over much of the state this morning and
afternoon). Instability remains in place over much of the region
this afternoon (particularly in the Cascades and Olympics), thanks
to a thermal trough still position on the west side of the state.
CAPE values still remain sufficient for convection going to the
afternoon (especially as daytime heating takes out some of the
capping - temperatures already in the 80s this afternoon).

Radar has been most active this morning/afternoon south of Lewis
County, where and area of scattered thunderstorms continue to move
in a northerly direction up the Cascade Mountains. These
thunderstorms are expected to continue their journey north through
the remainder of the Cascades this afternoon and evening.
Precipitation amounts are rather lackluster with this activity,
with only a few hundredths of QPF expected in total. Lightning and
gusty winds are expected to be the primary concerns with the
storms. This will keep critical fire danger in the Cascades and
Olympics through tonight (see fire discussion for more details).

The low perturbation feature will move out of Washington by
Thursday, with flow aloft remaining southwesterly. Cloud coverage
should burn off quickly Thursday morning with light southwest
winds, and highs in the low to mid 80s along the I-5 corridor
(60-70 along the coast). Patchy fog is possible in a couple low
lying areas during the morning hours, but will burn off quickly.
Friday is very similar to Thursday. There is a weak upper level
trough that will move through B.C. and northern Washington, but
as of now will pass through dry. Winds will be lighter and more
northerly on Friday.

Saturday is expected to the warmest day in the forecast, with a
ridge over the western states building up well into Canada.
Southerly flow aloft will increase which will bring warmer air
into the region. Highs as of right now are most likely to top out
in the upper 80s to 90 degrees (hottest temperatures in south
Puget Sound). NBM gives a 25 percent chance of temperatures being
4 to 5 degrees warmer (low to mid 90s). Heat risk at this time is
minor to moderate (with the moderates being in urban areas). Lows
Saturday morning and Sunday morning may only drop to 60 in urban
areas.

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...Pattern remains largely
dry in the extended forecast. A small shortwave trough tracking up
the west coast of Washington Sunday may trigger a few showers, but
confidence is too low at this time to include in the forecast.
With an upper level low tracking towards the region first part of
next week, the return of some onshore flow will cool high temperatures
down into the upper 70s to low 80s.

HPR

&&

.AVIATION...Flow aloft largely out of the south turning southwest
into Thursday. The airmass is unstable as showers and a 20-30%
chance for thunderstorms are in the forecast for this
afternoon/evening. At the terminals, mostly VFR across the area
aside from KHQM where MVFR stratus is present. Marine stratus will
march inland overnight into Thursday with MVFR to IFR conditions
expected. Can`t rule out isolated pockets of LIFR as well. Any
lingering stratus should burn off by 18-21z Thursday with a return
of widespread VFR.


KSEA...VFR this afternoon as a mid-level cloud deck tracks
northward. Showers and thunderstorms over the Cascades this
afternoon-evening could be in the approach path for planes arriving
from the east. Going forward, marine stratus will return overnight
(08-10z Thursday) and remain before burning off by 18-19z. South-
southwest winds throughout the TAF period between 5-10 kt.

McMillian

&&

.MARINE...Onshore flow increasing this evening as surface high
pressure builds over area waters. High pressure centered over the
offshore waters Thursday through the weekend with varying degrees of
onshore flow. A SCA is posted for the central and eastern Strait of
Juan de Fuca lasting into early Thursday morning. An additional push
is in the cards for Thursday evening and again over the weekend
with the potential for more headlines.

Seas 3 to 5 ft increasing to 6 ft over the weekend.

McMillian

&&

.FIRE WEATHER...A few clusters of thunderstorms near the Cascades
have developed this afternoon. Expect this to continue with the
threat area lifting north through the evening hours. The risk is
slightly lower for the Olympics, but will maintain the collection of
red flag warnings unchanged at this point in time. 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.

Cullen

&&

.CLIMATE...In 80 years of records at Seattle-Tacoma airport this
was the warmest first half of July. The average temperature of
72.0 degrees was fractionally warmer than the previous warmest in
2015. The average high temperature of 85.3 degrees was two degrees
warmer than the previous mark also set in 2015. The average low
temperature of 58.7 degrees was 4th warmest ( 60.6 2015, 58.9
2019 and 58.8 2007 ). Felton

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Red Flag Warning until 11 PM PDT this evening for East Portion
     of the Olympic Mountains-West Portion of the Olympic
     Mountains-West Slopes of the Central Cascades Generally
     above 1500 Feet-West Slopes of the North Cascades Generally
     above 1500 Feet.

PZ...Small Craft Advisory until 5 AM PDT Thursday for Central U.S.
     Waters Strait Of Juan De Fuca-East Entrance U.S. Waters
     Strait Of Juan De Fuca.

&&

$$