267
FXUS66 KSEW 150941
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
241 AM PDT Sat Jun 15 2024

.SYNOPSIS...A low pressure system will track inland today,
ushering in widespread shower and thunderstorm activity that will
continue through the evening along a convergence zone boundary.
Troughing will maintain unsettled conditions through the remainder
of the weekend and into next week as cooler temperatures and
periods of showers persist across western Washington.

* THUNDERSTORMS: Thunderstorm activity will spread across western
  Washington today, with the highest chance (30% to 40%) east and
  south of the Puget Sound this afternoon. Storms will be capable
  of producing small hail, gusty outflow winds to 30 mph, and
  locally heavy rainfall alongside lightning.

* FLASH FLOODING: Localized heavy rainfall is favored this
  afternoon and evening along a convergence zone east of the Puget
  Sound, with the potential for flash flooding and debris flows
  over the Bolt Creek burn scar along US-2 in the Cascade
  Mountains.

&&

.SHORT TERM /TODAY THROUGH MONDAY/...A broad upper level low will
continue to slowly move inland throughout the weekend, maintaining
unsettled and cooler conditions through the short-term. Moisture
will continue to stream inland this morning with onshore flow,
allowing showers to continue as another front approaches the
region. Instability will slowly increase as the sun comes up, with
high resolution forecast model soundings showing steep lapse
rates and as much as 500 to 600 J/kg of CAPE in the early to mid
afternoon over the Puget Sound region. Thunderstorms will be
possible across all of western Washington into the afternoon as
precipitation continues, with storms capable of producing small
hail and graupel, locally heavy downpours, and gusty outflow winds
up to 30 mph alongside lightning. While most precipitation will
shift eastward by the evening, a convergence zone is favored to
set up east of the Puget Sound and persist through early Sunday
morning. Forecast models highlight the potential for locally heavy
rain with the formation of the convergence zone especially over
the Cascades. The Bolt Creek burn scar along US-2 remains
susceptible to debris flows, so a Burn Scar Flash Flood Watch
remains in effect through the evening. The afternoon instability
will also be enhanced by cold temperatures aloft, which will allow
snow levels to lower to near 4000-4500 ft. The wet bulb effect may
even allow snow levels to drop as low as 3000 ft in heavier
showers that develop over the mountains, but confidence is low.
The Cascade mountain passes will likely not see any snow
accumulation, but the highest peaks will see light snowfall
amounts through early Sunday. Cloud cover will stick around for
most of the day today, limiting high temperatures across the
lowlands to the upper 50s and low 60s.

Wet and buoyant conditions will continue on Sunday as a cold
front drops southward across the region, spreading another round
of showers and embedded thunderstorm activity southward throughout
the day. Instablilty will decrease on Sunday as the trough axis
shifts further southeastward, with ensembles showing CAPE values
peaking at 200 to 250 J/kg south of the Puget Sound. Temperatures
Sunday will warm a couple degrees from today, peaking around the
60 degree mark for most of the lowlands.

Upper level troughing will finally exit the region on Monday with
moist northwest flow filling in behind. A shortwave disturbance
passing over the Pacific Northwest will stir up continued light
shower activity over higher terrain as conditions start to dry out
across the lowlands. Temperatures will warm into the mid 60s with
snow levels rising above 6000 ft, diminishing any additional
snowfall over the mountains.

.LONG TERM /TUESDAY THROUGH FRIDAY/...Ensembles highlight a
warming trend through the rest of the week, with temperatures
returning to typical mid June conditions near 70 degrees by
Tuesday and peaking well above normal near 80 degrees by
Thursday. Forecast models have also come into slightly better
agreement over weak flow throughout the week, with any chance of
light shower activity favored over the Cascades each day.

Lindeman

&&

.AVIATION...Southwest flow aloft early this morning becoming more
westerly by this afternoon as an upper level low begins to enter W
WA. As the upper low moves south and east, could see NE winds aloft
Sunday morning. Generally southwesterly winds at the surface will
become zonal with speeds increasing to 10 to 15 kts in the afternoon
with occasional gusts up to 25 kt possible.

VFR conditions in place over the majority of the area this early
morning, although some isolated spots of MVFR have materialized over
the San Juans, near HQM and near PAE. Cigs will continue to
deteriorate throughout the morning hours with most terminals getting
into MVFR by mid-morning...between 14-16Z. Atmosphere starts to
destabilize by late morning, with highest instability stretching
from HQM eastward through the south Sound, then along the I-5
corridor through SEA and possibly as far north as PAE. Will likely
use this area to limit any potential TS mention in 12Z TAFs and
eliminate any TS after 03Z. It should also be worth noting that
confidence will likely not exceed PROB30 for most terminals, however
could see to nudge SEA and BFI up to TEMPOs for the early afternoon
period. One plus to the instability will be lifting cigs, returning
VFR conditions to the area between 18-20Z. Precip activity expected
to start winding down after 03Z tonight.

KSEA...VFR conditions in place for much of the morning with any
precip well of to the west at the time of this writing. Terminal
should squeak through the AM mostly dry, although some showers may
pass to the immediate south...so VCSH continues to seem appropriate.
As discussed above, should see instability peak starting around 19Z
and thus will likely nudge up PROB30 from inherited TAF to TEMPO.
Most activity should shut down by 03Z or slightly after. Winds
remain generally southerly ranging 10-15 kts, however may nudge
upward to 15-17 kts this afternoon with gusts up to 25 possible.
Speeds should ease after 03Z down to approx 8-12 kts.

18

&&

.MARINE...Broad surface troughing will develop across the
waters through midday today before ridging gradually rebuilds over
the coastal waters this evening into Sunday as lower pressure
remains over the interior. This will increase onshore flow with
gusty winds exceeding 20-25 kt this afternoon and evening,
especially in the central and eastern Strait...where a Small Craft
Advisory will go into effect at 2 PM PDT this afternoon through 8 PM
PDT tonight.

By early/mid next week, broad surface ridging will develop over the
coastal/offshore waters, with lower pressure east of the Cascades.
This will maintain onshore flow with westerlies reaching small craft
advisory strength in the central/east strait daily in the late
afternoon and evening. Otherwise, fairly quiet conditions are
expected across area waters.

18

&&

.HYDROLOGY...Widespread showers and thunderstorms will continue to
spread across the region today, with convergence zone showers
favored to set up this afternoon and evening east of the Puget
Sound and continue through early Sunday morning. Locally heavy
rainfall is possible with these showers, and forecast models
continue to show the potential for 1 to 2 inches of rainfall over
the Cascades east of the Sound. The heaviest rainfall is expected
to occur between noon and 9 PM before rainfall rates weaken.
While this storm system is not particularly moist, the convective
nature of these showers could produce rainfall rates that meet or
exceed flash flood trigger thresholds. A Debris Flow Flash Flood
Watch remains in effect through the evening for the Bolt Creek
burn scar, where excessive rainfall may cause debris flows that
could impact US-2.

&&

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

$$