786
FXUS66 KSEW 161633
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
933 AM PDT Fri Aug 16 2024

.UPDATE...A rather tranquil morning with widespread clouds across
Puget Sound with some clearing along the coast. Clouds will clear
into this afternoon for Puget Sound for additional sunshine.
Temperatures will generally be in the 60s along the coast and 70s
for the interior. Active weather tomorrow, more info found in the
discussion below. No major updates this morning.

&&

.SYNOPSIS...A complex pattern remains in place with a broad trough
across the Pacific Northwest today, while several disturbances are
lined up offshore over the northeastern Pacific. An upper low will
deepen just offshore Saturday, with an increased threat of
thunderstorms and widespread rain over the weekend. Unsettled
conditions then continue across Western Washington through the
first half of next week as cooler conditions and shower chances
prevail in the onshore flow.

&&

.SHORT TERM /TODAY THROUGH SUNDAY/...Not much activity across the
local area this morning weather-wise with some stratus forming and
lowering across portions of the region. Meanwhile, a line of
showers across northeastern parts of the state have built a little
westward and a few showers in the North Cascades will be possible
through daybreak. Otherwise, modest onshore flow will lead to a
gradual improvement in the cloud cover through the day for most
and high temperatures again reaching the 70s. The focus for any
showers or isolated thunderstorms will once again be the North
Cascades this afternoon and evening.

A much more active pattern then takes shape for the weekend as a
deepening upper low slides southward near or just offshore of the
coastline. This will bring an increased threat for thunderstorms
across much of the region by mid-afternoon Saturday with the
threat continuing overnight.

Ensemble guidance continues to suggest probabilities in the
ballpark of 20-30% across a large portion of the area including
the Cascades and the Puget Sound region. Rather steep lapse rates
and the favorable placement of a branch of the jet will enhance
the potential, and some elevated instability may lead to a more
widespread outbreak of lightning. While confidence isn`t high,
there does remain some potential of a few storms reaching severe
thresholds, particularly wind gusts. Heavy rainfall will also be a
primary hazard with these storms and some hail cannot be totally
ruled out either.

These storms will have abundant moisture to work with with
guidance suggesting precipitable water values around 1 inch and
the good news is there should be decent steering to support storm
motion in the range of 15-20 kt, which should limit very heavy
rain for long durations of time in any particular spot. One
exception may be if training storms develop off of the terrain in
the south/southeast flow aloft - this could be particularly
concerning in the context of recent burn scars. Additional details
about fire weather and hydrology related hazards can be found in
the sections below. With moving into the time horizon of several
of the higher resolution convection allowing models, expect to be
able to continue to refine the details around the convective
threat over the next day or so.

While some of the guidance has suggested the bulk of the action
may push through in a more organized feature late in the day
Saturday and into the overnight hours, expect that a few isolated
storms may linger into Sunday. Another round of activity in the
North Cascades will return on Sunday.               Cullen

.LONG TERM /MONDAY THROUGH THURSDAY/...Looking ahead to next week,
ensemble guidance continues to suggest a broad trough across the
western U.S. through much of the week. This will maintain general
onshore flow and support some shower chances through the week.
Expect this to be much less impactful than the conditions over the
weekend, but there may be a day or two that again features thunderstorm
chances mostly confined to the North Cascades. High temperatures
generally remain in the 70s next week with a diurnally driven
pattern of morning clouds and increasing afternoon sunshine
pending the arrival of any disturbances. Cullen

&&

.AVIATION...An upper level low will deepen off the coast Friday into
Saturday, which will increase southerly flow (aided by a jet max on
the lee/east side of the low). With no flow in between lows
currently (one sits in eastern Washington this morning), some cloud
coverage will remain this morning. This is expected to remain
VFR/MVFR for most areas. A couple pockets of IFR/LIFR clouds are
possible in the Cascades, Kitsap Peninsula, and between Forks and
Aberdeen. Fog is not expected (but may be patchy in any areas that
get clear overnight). All cloud coverage will lift and scatter by 19-
20z , with a scattered to broken deck remaining from Kitsap
Peninsula over to Neah Bay (MVFR potentially still at Neah Bay).
Winds will be light out of the southwest today at 4 to 8 kt
(becoming light and northerly in Puget Sound late this afternoon).

Convection for today is expected to remain in the North Cascades.
The threat for thunderstorms increases Saturday night for the entire
coverage area. A few strong storms will be capable of producing
frequent lightning, strong winds with wind shear near the ground,
small hail, and heavy downpours.


KSEA...CIGs currently VFR. Confidence is 30% in the potential in
seeing MVFR CIGs between 15 and 19Z this morning (most likely will
see a scattered to potentially broken 1,500 to 2,000 ft layer during
this time). Improvement to VFR will take place in the afternoon with
mostly clear skies. Light southwest winds 4 to 8 kt will turn
north/northeasterly at 4 to 6 kt after 01Z Saturday.

HPR/McMillian

&&

.MARINE...Setup remains onshore with a low in eastern Washington
today, and a deepening low dropping down from B.C. Canada throughout
the day into Saturday. The next round of pushes through the Strait
of Juan de Fuca through the weekend are expected to be much weak
(not requiring any advisories). Coastal waters will also see light
northerly winds through Saturday. As the low slows offshore early
next week, the southerly flow aloft will make it down to the seas,
flipping winds to the south Sunday through next week over the
coastal waters. Seas will remain at 3 to 5 feet Thursday through
next week.

Additionally, low clouds are possible up and around Neah Bay this
morning that may lower visibilities to any mariners out on the
waters. This is expected to increase for Saturday along and off the
coastal waters as the low deepens. Thunderstorms will be possible
Saturday evening with this system, especially over inner waterways.
A few strong storms will be capable of producing gusty and erratic
winds, small hail, heavy downpours, and frequent lightning.

HPR/McMillian

&&

FIRE WEATHER...The low pressure system moving over the area this
weekend could bring widespread thunderstorms. However, given the
relatively high fuel moisture and widespread rain expected with
the thunderstorms, it is still expected that fire starts due to
lightning will be somewhat limited. But even if there is plenty of
lightning and multiple fire starts, the conditions will curtail
fire growth. With continued cool, cloudy, and high humidity
conditions well into next week, holdover fires may only smolder
for quite sometime until we get our next hot, dry, or windy
conditions. JBB

&&

HYDROLOGY...Impulses coming out of the low pressure trough aloft
will bring very slight thunderstorm chances to the northern Cascades
near the crest this afternoon and evening. There is a very low
probability of flash flooding for that area if a slow moving
thunderstorms sets up over the area Friday. The bigger threat comes
over the weekend.

The low pressure system that moves over the area Saturday into
Sunday will bring plenty of moisture as stated above. Rainfall will
be widespread, but total QPF has 90% of 0 to 0.25in., ranging to a
10% chance of 1in. to 3in.. It will be fairly unstable with
thunderstorm chances having increased up to 75%. If a thunderstorm
or multiple thunderstorms pass over an area, especially a burn scar,
rainfall rates and/or amounts could be enough to trigger flash
flooding and debris flows. JBB

&&

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

$$