112
FXUS66 KSEW 120347
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
847 PM PDT Sun Aug 11 2024

.SYNOPSIS...Upper level troughing over the region will promote
onshore flow and cooler temperatures the next several days.
Isolated thunderstorms will be possible across the North Cascades
this evening and again Monday. More widespread chances of
precipitation may return to western Washington by next weekend.

&&

.SHORT TERM /TONIGHT THROUGH WEDNESDAY/...Isolated thunderstorms
continue in the North Cascades in far eastern portions of Whatcom
and Skagit counties this evening. This remains the main activity
for the evening with stratus again returning overnight. Forecast
remains on track and the previous short and long term sections
follow.

Upper level troughing will persist over the area through the
first half of the week, generally promoting cooler and cloudier
conditions across western Washington. Expect additional rounds of
marine stratus to push inland during the overnight and early
morning hours the next few days. Afternoon high temperatures will
generally top out in the 60s and 70s. Some isolated thunderstorm
chances will still be possible for portions of the North Cascades
on Monday afternoon and evening, with probabilistic guidance
indicating a 10-15% chance of thunderstorm development across the
far northern areas near the Cascade crest and border. Chances
across the Cascades really wane heading into Tuesday as the threat
shifts further off to the east.

.LONG TERM /THURSDAY THROUGH SUNDAY/...Upper level troughing
will remain the dominant influence across the Pacific Northwest
through the long term period. Another low dropping down into the
region from Canada late in the week could bring more widespread
shower chances back to western Washington by the weekend, however
will need to see how guidance continues to trend with this over
the coming days. High temperatures on Thursday look to trend
closer to normal for right now (in the mid to upper 70s), with
temperatures dipping to below normal (upper 60s to low 70s) again
Friday into the weekend.

&&

.AVIATION...West to southwest flow aloft with upper level troughing
over western Washington. Surface winds winds will remain west-
southwesterly 5 to 10 kt throughout the TAF period.

Marine stratus has burned off for some interior locations especially
along the I-5 corridor from BLI to OLM. Other interior locations,
such as the Olympic Peninsula lowlands and the interior lowlands of
Whatcom county have not seen much in the way of clearing. As a
result, while VFR conditions are currently widespread locations in
the San Juans, the Olympic Peninsula /save for CLM/ and along the
coast are reporting MVFR conditions. Another inland push of stratus
is expected to move in overnight with timings for both MVFR/IFR
conditions and the gradual break-up being similar to those
experienced today...making for an early to mid afternoon return to
VFR conditions for most of the CWA.

KSEA...VFR conditions currently in place but will erode overnight,
reaching MVFR at or around 10Z early Monday morning. Timing for
recovering back to VFR conditions to be similar to today...with cigs
lifting starting around 18Z...but the potential for lingering
MVFR/IFR conditions could last until 22Z. Winds remain southwesterly
5 to 10 kt.

18

&&

.MARINE...Broad high pressure over the East Pacific with low
pressure inland will maintain northwesterly onshore flow over the
area waters. Diurnally driven pushes down the Strait of Juan de Fuca
will occur each evening. Currently winds look to remain below any
thresholds through the upcoming week.

Seas 6 to 8 ft at 8 seconds will ease tonight, which will allow the
Small Craft Advisory to expire later this afternoon. Seas subside
Monday back to 3 to 5 ft and remain like so through the week.

LH

&&

.FIRE WEATHER...Elevated fire weather conditions will persist
through this evening across portions of the North Cascades as
another shortwave trough brings the potential for thunderstorm
development. Coverage across the western slopes looks to be less
than that of the previous days, however any thunderstorms that do
develop may produce frequent lightning, brief heavy downpours,
hail, and gusty and erratic winds. Isolated thunderstorms will be
possible across the North Cascades on Monday afternoon and evening
again, however probabilities continue to wane as the greatest
threat moves off to the east.

Broader troughing setting up over the region this week will promote
increased onshore flow and good relative humidities across the region.
Apart from the isolated thunderstorm threat in the far northern Cascades
on Monday, this overall trend will help to alleviate fire weather conditions
through at least midweek. 14

&&

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

$$