631
FXUS66 KSEW 181655
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
955 AM PDT Sun Aug 18 2024

.SYNOPSIS...A strong upper level low will slowly begin moving
northeast off the coast of Washington today. Showers with a few
thunderstorms will continue early Sunday morning along and east of
the Interstate 5 corridor. More showers and thunderstorms will
develop west of Interstate 5 late Sunday morning, and spread
across portions of the coverage area Sunday afternoon. While a
couple of the storms may be strong, severe weather is not
expected. The remainder of the work week remains unsettled with
more chances for showers and thunderstorms as upper level
troughing continues.

&&

.SHORT TERM /TODAY THROUGH TUESDAY/...No significant changes made
to the forecast this morning. The previous discussion can be found
below along with updated aviation and marine discussions:

Same upper level low from Saturday will slowly begin moving
northeast, from off the coast of Oregon to off the coast of
Washington during the day. A small embedded shortwave trough will
move from Oregon into Washington during the late morning and
afternoon. This will help push a jet streak from Oregon into
Washington during the day. An associated vorticity max will also
aid in the jet streak for another chance of showers and
thunderstorms late Sunday morning and afternoon.

The expected scenario (based on HREF and CAMs) is for an area of
scattered showers and thunderstorms to develop in far Northwest
Oregon/Southwest Washington during the late morning hours as the
vorticity max moves up the coast into a modestly unstable airmass.
The storms will likely increase in coverage with additional
individual cells or multicell clusters. SRH/hodograph profiles
show quick movement again to the north with activity that develops
in the Southwest Interior. Most of the activity will track north
into the Olympics into Canada. Some models show convection east of
I-5, but the confidence is lower. The best chance for convection
in this region is primarily north of I-90 into the North Cascades.

Some of the instability will recover for afternoon convection, as
temperatures rebound into the mid and upper 70s, and dew points
remain in the upper 50s to low 60s. CAPE values increase to
200-300 J/kg for areas east of I-5, and 300-500 J/kg west of I-5.
PWATs still remain relatively high with anomalies between an inch
to an inch and a quarter. DCAPE takes a hit (dropping to 500
J/kg), as well as lapse rates dropping to 6.2 to 6.5 degrees/km.
Taking this all into account, the thunderstorms will generally
produce moderate to heavy rain, with a couple stronger storms
potentially producing small hail and gusty winds. QPF totals will
be highest in the Olympics, with a quarter to a third of an inch
of rain possible. Amounts are light in remaining areas, with
several hundredths to a tenth of an inch of rain possible.
Lightning is not expected to be as widespread as last night, but
there will be the danger of fires being started due to lightning
strikes, and not enough rain to extinguish the fires (see fire
discussion).

Monday through Tuesday: the upper level low will move northeast
into Canada, with troughing still remaining just off the coast.
The majority of Monday will remain dry (some clearing will take
place late Sunday evening/Monday morning, so will have to watch
for areas of fog). The trough however will begin to slide east
over the state on Tuesday. This will return the chance of showers
across the entire coverage area late Monday night into Tuesday.
There is a slight chance of an isolated thunderstorm over the
Olympics, otherwise the thunderstorm threat remains low for
Tuesday. High temperatures will cool down from Monday into
Tuesday, with temperatures ranging from the mid 70s to the low
70s. Lows will remain mild with widespread mid and upper 50s.

.LONG TERM /WEDNESDAY THROUGH SATURDAY/...The likelihood of
troughing remaining over the northwest remains high through next
weekend in the ensembles/deterministic models. Another large-scale
upper level low will track south from Canada into Oregon Wednesday
through Saturday. The models show it attempting to cutoff, but
then absorbing back into the main meridional pattern over Canada.
Wednesday will likely see widespread showers with scattered
thunderstorms across the region, with a break Thursday afternoon
as the flow temporarily becomes neutral behind the low. Another
broad shortwave may bring showers across the region again Friday
into the first part of Sunday. The ensembles early next week show
the potential for a zonal to ridge flow pattern taking place of
the troughing (which may warm up temperatures from the expected
low to mid 70s Wednesday through Saturday).

HPR

&&

.AVIATION...Southerly winds continue this morning with isolated
shower, mainly along the coast. A few pockets of lower ceilings
but should trend toward VFR this afternoon. Another round of
isolated thunderstorms possible this afternoon over the coast and
interior - although far less coverage compared to last night.

KSEA...VFR today. Southerly winds may be breezy at times this
afternoon. There will again be a low potential (10%) for a
thunderstorm to reach the terminal today but confidence is low. 33

&&

.MARINE...Low pressure remains well offshore with southerly winds
developing over the coastal waters. Isolated thunderstorms
possible over the coastal waters and interior. Weak west pushes
through the Strait the next several afternoons but widespread
advisory conditions are not expected. Elsewhere winds become
increasingly southeast through Monday with low pressure remaining
centered over the outer coastal waters. With stronger winds in the
lower atmosphere and some lingering instability near the low
center, could see a few occasional gusts to 25 kt over portions of
the coastal waters on Sunday, but these don`t appear frequent or
widespread enough to warrant an advisory at this time. Seas will
build a bit through the early part of the week in response to the
persistent southerly winds and remain choppy but only building to
around 6 feet. Diurnal west pushes through Strait of Juan de Fuca
also likely through the week each afternoon and evening, possibly
approaching advisory strength around midweek. 33/Cullen

&&

.FIRE WEATHER...An upper level low will continue to remain in the
region Sunday into Monday. Showers continue in Puget Sound and the
Cascades this morning with isolated thunderstorms along the crest.
Lightning has been frequent with these morning storms, and with
the speed of the storms, the rainfall may not be enough to
extinguish any fires that start.

Another disturbance is expected to trigger scattered
thunderstorms this afternoon from the Southwest Interior into the
Olympics, and potentially also including the North Cascades.
Rainfall amounts with afternoon convection will be significantly
lower (best chance of wetting rain in the Olympics), but the
threat of lightning will be significantly lower. Still could see
gusty winds if a storm or two get strong enough.

HPR

&&

.HYDROLOGY...There will be an additional round of showers and
thunderstorms late Sunday morning and afternoon. Activity is
primarily expected west of I-5, but a couple pop up cells are
possible east of I-5, and north of I-90. The highest QPF totals
will be in the Olympics, with a quarter to a third of an inch of
rain from storms moving through. Remaining areas will only see
several hundredths to a tenth of an inch of rain. The threat for
heavy rainfall triggering a debris flow/flash flooding will be low
with the afternoon activity.

The remainder of the week will see several chances of showers and
thunderstorms. Precipitation amounts remain low except for
possible convection Wednesday in Western Skagit/Whatcom Counties.

HPR

&&

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

$$