000
FXUS66 KSEW 282037
AFDSEW
Area Forecast Discussion
National Weather Service Seattle WA
137 PM PDT Thu Sep 28 2023
.SYNOPSIS...An unsettled Thursday continues with showers, and a
couple isolated thunderstorms continuing to move through the
region. Heavy rain is the most likely hazard with the strongest
storms, but lightning and graupel cannot be ruled out. The upper
and surface disturbance move out of the region Friday, with high
pressure building in over the weekend. Skies will clear on
Saturday and Sunday, but temperatures will remain cool with highs
in the 50s and 60s, and lows dropping to the 40s, with a couple
30s in the mountains. The next chance of rain returns early next
week with the next disturbance.
&&
.SHORT TERM /TONIGHT THROUGH SUNDAY/...The pattern remains a broad
upper level trough centered over the northwestern CONUS, with a
500 mb shortwave trough swinging through WA from Canada today.
Strong positive vorticity advection with the shortwave, along with
CAPE values in the 200-300 J/kg range, have aided in the
development of showers and a couple thunderstorms across Puget
Sound late this morning and this afternoon. Heavy rain is expected
with the strongest storms/showers, with QPF amounts this afternoon
through tomorrow around a quarter of an inch to a half an inch.
Lightning and graupel remain possibilities with the strongest
storms that are able to elevate (so far have just seen the
lightning). For all remaining areas not affected by convection,
off and on showers remain possible through the remainder of today
into Friday morning. Higher elevations may see some snow with the
precipitation activity this afternoon, mainly in the north
Cascades and around Rainier.
For Friday into Sunday, the upper level trough slides eastward,
replaced by an upper level ridge, and surface high pressure.
Heights will build in the region, and skies will clear from Friday
afternoon into Sunday. Any overnight and morning clouds that do
develop should clear by late morning. Winds behind the trough will
become northwesterly, but generally remain light at 5 to 10 mph.
Temperatures during this period remain steady, with highs in the
50s and 60s expected region-wide. Lows will drop into the 40s,
with a couple 30s possible in higher elevations.
The next disturbance arrives Sunday night into early next week,
bringing the next chance of precipitation into the region. See the
long term section for more details.
HPR
.LONG TERM /MONDAY THROUGH THURSDAY/...The pattern next week
remains somewhat uncertain with disagreements in the
deterministic, and probabilistic models. Most models are
pinpointing an upper level disturbance for Monday into Tuesday,
with precipitation chances. The ridge is expected to broaden over
the Pacific Monday with a trough swinging through Monday into
Tuesday. With some uncertainties again with timing of the trough
pattern (in terms of persistence into the week), the confidence
for rain is highest Monday into Tuesday, and much lower Wednesday
into Thursday. Amounts appear to be highest in the Cascades with a
half an inch of QPF possible Monday into Tuesday. Remaining areas
appear to receive around a tenth of an inch of QPF. As confidence
increases regarding the timing of the disturbance, will include
updates regarding the details. Again, temperatures remain steady
with highs in the 50s and 60s, and lows in the 40s and 50s.
HPR
&&
.AVIATION...Westerly flow aloft with upper level troughing. Surface
flow across Western WA is west to southwesterly around 5 kt.
Scattered showers are present across the region. An area of
convergence and limited but sufficient instability has maintained a
line of more intense showers and some occasional lightning strikes
in a line north of Bremerton to central Snohomish county. Showers
and, possibly, an isolated thunderstorm will continue until the
evening. Winds will shift to northerly tonight but speeds will
decrease tonight. With lighter winds, clearing skies overnight, and
sufficient moisture from the recent rains, fog and low stratus is
going to be of concern from around 12-18Z for the Puget Sound area
terminals. VFR conditions are expected tomorrow with northerly winds
5-10 kt, potentially gusty at times.
KSEA...Scattered showers just north of the terminal will hang around
for the next hour. Ceilings are hovering around 2500-3500 ft, but
will begin to rise to VFR this evening. Southwesterly winds around
10 kt will switch to northerly around 01Z and begin to decrease
overnight. Low stratus and fog look to develop tomorrow morning from
around 12-18Z. IFR conditions are likely (30%), with periods of LIFR
possible (20%). Ceilings lift to VFR after 18Z with northerly winds
6-10 kt.
LH
&&
.MARINE...Light onshore flow is prevailing over the coastal waters.
Winds have begun to come up in the Strait of Juan de Fuca. A push of
westerlies are expected in the Central and Eastern Strait this
afternoon and evening, prompting a Small Craft Advisory through 8 PM
tonight. A weak low pressure system will swing south into the Oregon
Coast tonight into tomorrow morning. Northwesterly winds look to
pick up down the Strait of Georgia Friday evening, which may lead to
occasional gusts over 20 kt north of the San Juan Islands.
Following, high pressure will build back into the coastal waters
into the weekend. The associated increase in northerly flow over the
outer coastal waters may reach Small Craft level speeds on Saturday.
Winds look to remain west to northwesterly 5-15 kt through the
beginning of next week.
Seas from 7 to 9 ft slowly easing to 4 to 6 ft beginning Friday.
Seas may temporarily rise to 8 to 10 ft in the outer waters
Saturday. Long period swell from the northwest will elevate seas
again to 8 to 10 ft on Tuesday.
LH
&&
.HYDROLOGY...No river flooding is expected over the next 7 days.
&&
.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 8 PM PDT this evening for Central
U.S. Waters Strait Of Juan De Fuca.
Small Craft Advisory until 8 PM PDT this evening for East
Entrance U.S. Waters Strait Of Juan De Fuca.
&&
$$
NWS SEW Office Area Forecast Discussion
Discussion Script by Ken True at Saratoga Weather - WFO Script by SE Lincoln Weather
