240
FXUS66 KSEW 140350
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
850 PM PDT Thu Jun 13 2024

.UPDATE...
Evening forecast on track with increasing southwesterly flow aloft
ahead of approaching upper level trough. Evening 00Z sounding from
KUIL shows moisture remaining AOB 700 mb this evening and still
mostly very dry in the lowest levels. Expecting temps to cool well
into the 40s again in many areas for another comfortably cool
mid-June night.

&&

.SYNOPSIS...Ridging will slide east with troughing developing
offshore into Friday. A weak front will move through on Friday.
Troughing overhead over the weekend will bring unsettled weather,
including potential thunderstorms. Troughing will then slide east
into the middle of next week.


&&

.SHORT TERM /TONIGHT THROUGH MONDAY/...Dry weather this afternoon
with high clouds overhead as southwesterly flow increases aloft.
Ridging over the area will continue to slide east tonight and
Friday with troughing developing offshore. Clouds will increase
later tonight into Friday morning, with a weakening front bringing
light rain on Friday. Temperatures will be a few degrees cooler
on Friday with highs in the upper 50s to mid 60s.

An upper low will slide over the Pacific Northwest on Saturday,
with reinforcing troughing on Sunday. This will lead to continued
unsettled weather and cooler temperatures over the weekend. Given
the colder air aloft, precipitation on Saturday and Sunday will
become more convective in nature. Instability will increase
Saturday afternoon, with forecast sfc CAPE currently ranging 200
to 400 J/kg on NBM, sufficient for isolated thunderstorms.
Probabilities range 20 to 35% across Western Washington for
t-storms Saturday, with highest probabilities across central
Puget Sound, due to likely convergence activity during the
afternoon and evening hours. The main threats for any storms that
do form will be lightning, heavy rainfall, gusty winds, and small
hail. Coverage in showers dissipates a bit on Sunday, with the
highest likelihood for thunderstorms mainly south of I-90 and over
the Olympics (probabilities less than 20%). Otherwise, locally
breezy onshore winds over southern Puget Sound expected on
Saturday. Temperatures over the weekend will be cooler in the mid
50s to low 60s.

.LONG TERM /TUESDAY THROUGH FRIDAY/...Troughing will continue to
deepen over the Pacific Northwest on Monday, with ensembles
generally consistent on the location of troughing being interior
Pacific Northwest. Continued shower potential will exist Monday
and Tuesday as the trough moves slowly eastward, with the highest
chance of precipitation over the Cascades. Uncertainty increases
in ensembles Wednesday and Thursday, with ECMWF showing a bit more
of a ridging influence, and continued weak troughing shown on GFS
ensembles. Given this pattern, have continued with NBM guidance
suggesting perhaps drier conditions at times, with precipitation
potential mostly confined to the Cascades. Otherwise, a slow
warming trend is likely Monday through Thursday, although the
degree of warming continues to be uncertain given the fluctuation
in guidance during this period. JD


&&

.AVIATION...An approaching upper level trough of low pressure
will bring increasing southwesterly flow aloft over Western
Washington overnight. While low level onshore flow will also
strengthen, VFR conditions are expected to prevail with the
moisture remaining at higher levels. A weak front will approach
the area Friday, with some MVFR cigs along the coast but elsewhere
it should remain primarily VFR with mid to high level cloudiness.
Shower activity will increase after 18Z Fri.


KSEA...VFR conditions. S/SW winds AOB 10kts will persist. Increasing
mid to high level clouds tonight ahead of an incoming upper level
low pressure system. Increasing shower activity in the 18Z Fri - 00Z
Sat timeframe but largely remaining in VFR conditions.

Johnson

&&

.MARINE...A weak ridge of surface high pressure over the NE Pacific
will gradually weaken through late Friday as a weak frontal boundary
pushes into the area. Gale conditions in a strong onshore in the
overnight hours in the central/eastern strait will be isolated as
winds diminish early Friday morning. Aside from a chance of
thunderstorms Saturday, marine conditions will be rather quiet for
the next few days. Weak surface ridging rebuilds on Sunday ahead of
another trough that is expected to pass well south of area waters.
Heading into early next week, surface ridging offshore and lower
pressure inland is expected to produce varying degrees of onshore
flow.

27/Johnson


&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Gale Warning until 1 AM PDT Friday for Central U.S. Waters
     Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of
     Juan De Fuca.

     Small Craft Advisory until 1 AM PDT Friday for Admiralty Inlet.

&&

$$