Heavy Rain in Washington, July 24, 2024

Area Forecast Discussion
National Weather Service Seattle WA
335 AM PDT Wed Jul 24 2024

.SYNOPSIS...Upper level trough over Western Washington through
Thursday. Weak system moving through today will be dry with
increasing onshore flow behind the system tonight. The trough will
move off to the northeast Thursday night. A weaker trough will
take its place over the weekend into the first part of next week.

&&

.SHORT TERM /TODAY THROUGH FRIDAY/...Satellite imagery shows
upper level trough over Western Washington with the trough axis
offshore this morning. Mostly clear skies over the area with
stratus just off the coast at 3 am/10z. Temperatures were in the
mid 50s to lower 60s.

Weak shortwave spinning around the trough moving through later
today. This feature is dry with just some middle level clouds
moving through the area. Increasing onshore flow this afternoon
will keep temperatures from warming too much even with the late
July sunshine. Highs in the 70s for the interior and mid and upper
60s for the coast.

Low level onshore flow tonight with a convergence zone setting up
over Snohomish county. Could see light showers/drizzle under the
zone overnight. Also a slight chance of showers over the North
Cascades as what is left of the shortwave exits the area. Under
mostly cloudy skies lows will be in the 50s.

Onshore flow weakening Thursday with the low level flow turning
northerly in the afternoon. Upper level trough axis moving through
combined with the onshore flow keeping skies mostly cloudy in the
morning. Skies clearing Thursday afternoon with the weakening
onshore flow and trough axis over the Cascades. Cloud cover in the
morning plus a little cooler temperatures aloft with the trough
axis overhead will keep highs in the mid 60s to mid 70s.

Upper level trough moving off to the northeast Thursday night.
Weak low level onshore flow developing overnight. The onshore flow
will not be strong enough to drive the marine layer very far
inland leaving skies mostly clear overnight. Lows in the upper 40s
to mid 50s.

Friday a transition day with the upper level trough well to the
northeast and a weaker upper level trough beginning to develop
late in the day. Once again the low level onshore flow will not be
strong enough to push the marine layer inland. Highs near normal,
70s and lower 80s inland and mid 60s to lower 70s coast.

.LONG TERM /SATURDAY THROUGH TUESDAY/...Extended models in good
agreement with a weak upper level trough over the area for the
weekend and low level onshore flow. This will give the area a
couple days of normal late July weather. Onshore flow could be
strong enough to get the marine layer inland Sunday morning with
skies clearing in the afternoon. Highs in the 70s with lows in the
lower to mid 50s.

Model consistency falls apart Monday and Tuesday. The operational
ECMWF has an upper level trough moving down the British Columbia
coast Monday with an upper level low closing up off the coast
Monday night and Tuesday. The operational GFS has a shortwave
moving through Monday with a flat upper level ridge building
Tuesday. While the ECMWF ensembles are not as strong with the low
there is still a very wet signal for Monday afternoon through
Tuesday with over half of the ensemble solutions indicating
measurable rain. The GFS ensembles have a much weaker signal with
less than 20 percent of the ensemble solutions indicating
measurable precipitation. Fun fact, there has not been measurable
rain in Seattle between July 27th and August 1st in the last 15
years. In addition using the combined record between the Federal
Building in downtown Seattle and Seattle-Tacoma airport there has
been measurable rain on July 30th only 9 times since 1894. Given
the climatology will go more with the GFS solution and only have
slight chance pops for Monday and Tuesday. Highs remaining in the
70s inland and 60s along the coast. Felton

&&

.AVIATION...A low tracking across southern British Columbia is
expected to bring west/southwest flow aloft as it comes ashore.
Satellite imagery is showing some remnants of a dissipating frontal
system off the Washington coast. Areas of marine stratus is visible
off the coast, and will continue to advance to as far as the
Southwest Interior/Olympics early this morning. Widespread MVFR CIGs
are expected with this marine layer with pockets of IFR, lifting to
VFR by late morning. Remaining areas east of the Olympics will see
mostly clear skies to start, with increased VFR cloud coverage by
Wednesday afternoon as the weak front boundary passes through.
Surface winds light and variable this morning, becoming westerly by
early afternoon, and northerly by early evening 4 to 8 kt. Gusty
winds to 20 kt are possible for areas along the Central/East Strait
of Juan de Fuca and Admiralty Inlet (marine push this afternoon).

KSEA...Clear VFR skies to start, with the weak frontal boundary
adding a scattered VFR deck this afternoon and evening (from the
northwest). Variable winds early morning will become
southwest/westerly by 17Z at 5 kt. Expect further shifts to the
northwest at 00-03Z 4 to 8 kt, shifting to the northeast/east late
evening.

HPR

&&

.MARINE...A deep (but weakening) low system will track inland over
southern British Columbia today. A dying frontal boundary will move
across all waterways today with this low. High pressure is also
building behind the low, well off in the Pacific. All in all,
various degrees of onshore flow are expected (the strongest of which
is expected Wednesday afternoon and evening).

Confidence is increasing in the chance of gale gusts in the
Central/East Strait of Juan de Fuca. Expectation from HREF guidance
is for west winds to reach 15 to 25 kt (with periodic gusts to 35
kt). The gale watch was upgraded to a gale warning for the East
Strait of Juan de Fuca, and expanded to include the Central Strait
of Juan de Fuca. Admiralty Inlet was also included with this pattern
with a small craft advisory. Gusty winds may start as soon as 21Z
Wednesday and last through 12Z Thursday. Widespread gusty winds are
not expected in the Northern Inland/Puget Sound Waters (max gusts 10
to 15 kt).

Additional pushes of onshore winds will continue through the
weekend, but are expected to remain below small craft advisory
criteria at this time.

Coastal waters: winds will become northerly behind the low. Winds
next few days remain light with peak gusts 10 to 15 kt (highest
winds in the outer coastal waters). By Friday night/Saturday, winds
may reach 20 kt (with gusts to 25 kt) in the outer coastal waters.
The seas will range from 4 to 6 feet through the rest of the week.

HPR

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Gale Warning from 2 PM this afternoon to 5 AM PDT Thursday for
     Central U.S. Waters Strait Of Juan De Fuca-East Entrance
     U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from 2 PM this afternoon to 5 AM PDT
     Thursday for Admiralty Inlet.

&&