290
FXUS66 KSEW 222104
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
204 PM PDT Sun Sep 22 2024

.SYNOPSIS...A flat ridge will give way to a trough over British
Columbia tonight. The system will bring a chance of light rain
showers overnight for the northern half of the coverage area. A
eastward tilted ridge will build back high pressure into the
region Monday into Tuesday, with flow becoming slightly offshore
leading to warmer temperatures via a thermal trough along the
coastline. A couple of frontal systems will move through Wednesday
into Friday, with widespread precipitation expected across the
entire coverage area.

&&

.SHORT TERM /TONIGHT THROUGH WEDNESDAY/...Welcome to Autumn 2024!
A flat ridge over most of the state this morning is moving east
into the Rockies this afternoon. A weak shortwave trough will pass
close by over British Columbia late today into the overnight. It
is a pretty weak one, given it being sandwiched in between two
different ridges, and it not being able to dig southward over
Washington. High cloud coverage has already settled in much of the
region this afternoon with the system approaching. A weak warm
front will also pass to the north with this shortwave. This will
keep precipitation chances primarily in the northern portion of
the coverage area tonight (affecting the North Cascades, lowland
areas north of Everett, Olympics and most of the coastline). Areas
getting precipitation tonight will get scattered showers, with
lowland areas receiving several hundredths of an inch of QPF, and
North Cascades/Neah Bay areas potentially seeing locally heavier
amounts over a quarter of an inch.

A stronger upper level ridge will move into the region Monday into
Tuesday, tilting northeastward as it moves inland over much of
central Canada. At the same time, a thermal trough will build
underneath the ridge along the coastline. This will cause the flow
to turn slightly offshore, resulting in increasing high
temperatures for the first part of the work week. The cloud
coverage holding for part of Monday morning will most likely put a
hinder on daytime temperatures (and will only see highs in the
upper 60s to low/mid 70s once cloud coverage clears). Cloud
coverage will be significantly less Tuesday morning, which will
not put as much of a damper on afternoon temperatures with this
pattern. Highs will top out in the mid/upper 70s to low 80s, with
the warmest temperatures in the South Interior. HeatRisk remains
predominately minor (given the short duration of the warmer
temperatures).

The pattern changes once again going into Wednesday, as the ridge
rotates counter-clockwise giving way to a deepening trough over
the Pacific. This will swing a significant cold front through the
region on Wednesday (dropping high temperatures by around 10
degrees). Southwest flow out ahead of the trough will help tap in
moisture for widespread precipitation during the day. Some of the
models hint heavy precipitation rates in the far north Cascades,
and the Olympics/coastline early Wednesday morning. Any convective
banding/thunder that takes place within the precipitation bands
may be able to produce locally heavy rainfall rates. The
predictability in the amounts of rain remain tough to determine at
this point (the 25th and 75th percentiles in the NBM spread about
0.75-1 inch from the mean and 50th percentiles over the
coastline). Details on possible rainfall will be refined getting
closer to the event.

.LONG TERM /THURSDAY THROUGH SUNDAY/...The ensembles agree
significantly on the cool/wet pattern continuing through the end
of the week. Another shortwave trough will drive another frontal
system through the region Thursday/Friday. Again, convective
banding with thunder/locally heavy rainfall totals will be
possible with this second round, but predictability in amounts
remain uncertain (due to wide spreads in the ensembles in possible
precipitation amounts). In addition, snow levels falling to around
6,000 to 7,000 feet Thursday/Friday may lead to some snow
accumulations at higher mountain elevations in the Cascades. The
wet weather will wrap up Friday, and next weekend looks drier with
a mix of clouds and sun. High temps will stay cool in the low to
mid 60s (significantly cooler in the 40s and 50s at higher
elevations).

HPR

&&

.AVIATION...Westerly winds will transition more northwesterly
tonight into Monday as a weak frontal system moves into British
Columbia and ridging builds offshore. VFR high and mid clouds this
afternoon, with areas of MVFR/IFR along the coast. Cigs will slowly
lower to MVFR initially near the BC border and along the coast,
before falling to MVFR elsewhere. LIFR/IFR cigs are expected tonight
into Monday morning along the coast, with MVFR/IFR for interior
areas. Light rain is also possible into early Monday morning, mainly
along the Olympic Peninsula and north of Snohomish County. Cigs will
slowly improve to VFR BKN Monday afternoon, with lower cigs
continuing along the coast. Areas of lower vsbys are expected
tonight as well along the coast. Lighter winds through tonight will
increase from S/SW on Monday.

KSEA...VFR cigs with high clouds. Mid level clouds will increase
this evening into tonight. Cigs are expected to lower to MVFR
tonight (around 09-10z) and continue through early Monday afternoon.
There is also a 25% of IFR cigs Monday morning, mainly from 13-18z.
Cigs will slowly lift and scatter later Monday afternoon. Light W/SW
winds this afternoon may become variable this evening before S/SW
winds tonight into Monday morning. JD

&&

.MARINE...A weak frontal system will move into British Columbia
tonight into early Monday. High pressure will then build back into
the region Monday and Tuesday as a thermally induced trough expands
northward along the Oregon coast. Weak offshore and northerly flow
expected Monday night through Tuesday. A frontal system will then
move into the Coastal Waters Wednesday, bringing a return of onshore
flow and increased winds, especially along the coast and Strait of
Juan de Fuca. A stronger front looks to move into the area Thursday
afternoon into early Friday, perhaps bringing SCA winds for the
Coastal Waters and localized SCA winds for interior waters.

Seas of 4 to 6 feet will build to 7 feet for the outer Coastal
Waters on Monday. Seas will then continue to build from 7 to 9 feet
Tuesday afternoon through Wednesday. This period will need to be
monitored for near 10 foot seas for the outer Coastal Waters. Seas
build further Thursday afternoon into Friday, with GEFS
probabilistic wave guidance suggesting a 50 to 70 percent chance of
seas greater than 12 feet over the Coastal Waters. JD

&&

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

$$