131
FXUS66 KSEW 132052
AFDSEW

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

.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 SUNDAY/...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 /MONDAY THROUGH THURSDAY/...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...Southwest flow aloft will increase over Western
Washington ahead of an approaching upper trough. Low level onshore
flow will increase through this evening. VFR prevails through this
evening under increasing high level moisture. Ceilings will lower to
MVFR along the coast early Friday morning with shower activity
developing ahead of a weak frontal boundary. Shower activity will
spread into interior areas Friday afternoon, but ceilings over
interior terminals are expected to remain mainly VFR.

KSEA...VFR. Increasing mid to high level cloud cover through the
afternoon and evening hours ahead of an incoming system. Surface
winds light W/NW backing to S/SW near after 03Z this evening.

27

&&

.MARINE...Weak surface ridging over the coastal and inland
waters will weaken tonight as a broad trough and associated weak
frontal boundary moves into the coastal waters on Friday. A sharp
increase in onshore flow this evening is expected to produce low end
westerly gales for a few hours in the central/east strait. Broad
troughing will remain over the waters on Saturday for relatively
benign conditions apart from the possibility of an isolated
thunderstorm. 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

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Gale Warning from 5 PM this afternoon to 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 from 5 PM this afternoon to 1 AM PDT Friday
     for Admiralty Inlet.

&&

$$