Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 220352
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
852 PM PDT Thu Mar 21 2024

.SYNOPSIS...Upper level low well offshore will slowly move east
reaching Western Washington Saturday. Bands of showers rotating
around the low will weaken as they reach the area until the low
gets closer Friday night into Saturday. Low moving east Sunday
with short lived upper level ridge over the area Sunday night. A
series of weather systems will move across Western Washington
Monday through Thursday.

&&

.SHORT TERM /TONIGHT THROUGH SUNDAY/...Current temperatures in the
upper 40s to lower 50s across the area as of 9 PM. Surface
analysis reveals an occluded surface low moving closer towards
130W this evening with a warm front drawing northward across the
offshore waters. Light showers and sprinkles are working their way
northward across western Washington as this front lifts north.
Made a few minor adjustments to temperatures, PoPs, and weather
grids to match latest trends with this light precipitation. Given
near southerly 850 mb flow, moisture continues to be limited
across the Cascades this evening and only expected nuisance
showers and sprinkles for the region with greatest likelihood
along the coast and southern Olympics.

Upper level low will slowly drift east tonight into Friday with
the low off the Northern Oregon coast by late Friday afternoon.
As the low moves east the flow aloft over Western Washington will
become more southerly. Band over Southwest Washington this
afternoon will lift north northeast this evening keeping showers
along the coast and a chance of showers in the forecast for most
of the interior into early Friday morning. As the flow aloft
becomes more southerly this will keep the next band rotating out
of the low from getting very far to the east creating a brief
break in the shower activity over the southeast portion of the
area Friday morning. As the low continues to move east shower
chances will increase Friday afternoon. Nothing really organized
spinning out of the low at this time but the air mass will be
slightly unstable keeping the showers along the coast with a
chance of showers inland scenario intact. Lows tonight in the 40s.
Highs Friday in the mid 50s to lower 60s.

Vort max spinning out of the low moving over Western Washington
Friday night. The highest pops in the short term forecast will be
during this time along with the highest QPF values. A quarter to
a half inch of rain forecast for most locations Friday night. Lows
in the 40s.

Upper level low weakening and moving over Western Washington
later Saturday. With the low overhead showers will remain in the
forecast. Snow levels, which will be above pass levels tonight
through Friday night, dropping to 3500 to 4000 feet but
precipitation amounts are light leading to only 2 to 4 inches of
new snow over Stevens and White pass. Highs with the low overhead
a little cooler, in the upper 40s to mid 50s.

Low sagging south and dissipating by Sunday morning. Upper level
ridge building offshore with the ridge extending into British
Columbia. Ridge not strong enough to prevent a few lingering
showers especially Saturday night into Sunday morning. Lows
Saturday night in the 40s. Highs Sunday in the mid 50s.

.LONG TERM /MONDAY THROUGH THURSDAY/...Brief upper level ridge
moving through Western Washington Sunday night for a short dry
spell. Models and ensembles in good agreement with an organized
front moving into Western Washington Monday followed by another
system Tuesday night/Wednesday morning. Both the GFS and ECMWF
have a deep surface low associated with this feature but there is
little agreement on the position of the low. The GFS has the low
near the coast Wednesday while the ECMWF has the low some 500
miles back to the northwest. Both solutions will not create very
much wind across the area but the GFS solution is close enough to
keep an eye on the situation.

Surface low filling Wednesday night into Thursday with the
possibility of another front arriving later Thursday keeping the
wet late March pattern going for the end of next week.

Davis/Felton

&&

.AVIATION...Southwesterly flow aloft shifting more southerly by
Friday morning with a broad upper level trough offshore and weak
ridging inland. Mainly VFR conditions in place over much of W WA
this evening although spots along the coast and locations in and
near showers are dipping down into MVFR to IFR. Latest guidance for
terminals more prone to lower cigs shows conditions continuing to
deteriorate overnight and Friday morning, remaining in solidly in
MVFR to IFR categories. Otherwise, there is some suggestion that
cigs may improve somewhat for locations that have dipped down due to
showers more into VFR conditions. The return of showers to the area
by Friday afternoon will likely drag cigs down once more, although
this lowering may be limited to either low-end VFR or high-end MVFR.

KSEA...MVFR conditions in place now...and with the prospect of
another round of showers within the next hour or so /03-05Z/, these
conditions may remain in place or, at the very least, any
improvement may be stunted. As indicated above, models do suggest at
least some improvement into VFR overnight and into Friday morning.
The increase of rain showers Friday afternoon may result in some
lowering, but should still remain VFR. Generally southeasterly winds
at 5-10 kts tonight into Friday morning, becoming more northerly by
Friday afternoon.

18/Mazurkiewicz

&&

.MARINE...A weak frontal system offshore will remain in place with
little to no significant impacts for the coastal waters. An
additional disturbance will move through the area waters this
weekend, but again, conditions will remain under any headline
thresholds. The marine pattern will generally be quiet into the
beginning of next week, possibly turning active by midweek.

Combined seas 3 to 6 feet will continue through much of the
weekend and into the beginning of next week, before possibly
rising midweek.

18/Mazurkiewicz

&&

.HYDROLOGY...No river flooding expected in the next 7 days.

&&

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

$$


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