National Weather Service

Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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411
FXUS66 KSEW 210341
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
841 PM PDT Mon May 20 2024

.SYNOPSIS...Upper-level ridging has produced warmer and drier
conditions today. But, troughing will arrive on Tuesday and
Wednesday with a return of cooler and wetter weather. A brief dry
break is possible on Thursday, before cool and unsettled
conditions return Friday and may persist into the start of the
weekend.

.SHORT TERM /TONIGHT THROUGH THURSDAY/...-- Changed Discussion --Forecast remains on track
this evening with dry conditions but increasing mid clouds ahead
of the next weather system. See the remainder of the previous
short term discussion below.

Clouds will increase ahead of the next disturbance bringing widespread
rain and cooler temperatures into Tuesday. As mentioned, conditions
are expected to deteriorate as early as the wee hours of the morning
on Tuesday for the coast before spreading inland. By the morning rush
hour, an upper-trough is expected to bring widespread rain across
much of western Washington. This disturbance appears to be rather
impressive for mid-late May. QPF forecasts indicate a half inch
or more for much of the lowlands and 1 to 2 inches for both the
Cascades and Olympics (including the coast) through Tuesday night.
High temps are forecasted to be upwards of 10-12 F below average
with values in the mid to upper 50s. Lingering showers with 4,000
ft snow levels are in the forecast for Wednesday as guidance hints
at the upper-low centered directly over south- central WA and
north- central OR. We`ll warm up slightly but still running cooler
than normal with highs in the upper 50s to lower 60s. Overnight
lows are to fall into the 40s for both nights.

Thursday will be a bit of a transition day with transient ridging
sliding overhead. We`ll see a brief return of drier conditions
along with near average temperatures. However, it won`t be long
lasting as unsettled conditions return soon after.

.LONG TERM /FRIDAY THROUGH MONDAY/...The next upper-low looks to
arrive on Friday with another round of precipitation of cooler
conditions. Chance PoPs and below normal temperatures continue
into the weekend as the region remains under the influence of
troughing aloft. Ensemble guidance hinting at a possible shift in
the pattern early next week with upper ridging rebounding over
the Intermountain West and upper troughing positioned over the
Gulf of Alaska and NE Pacific.

McMillian-- End Changed Discussion --

.AVIATION...North-northwesterly flow aloft as an upper level trough
exits the region and a weak positively tilted ridge builds in its
wake. VFR conditions across the region with broken cloud cover along
the coast and over the Cascades. Expect VFR conditions to continue
through the evening. A frontal system will approach from the
northwest, bringing with it rain showers and MVFR ceilings area-
wide. Widespread rain will approach the coast early this evening,
spreading inland through the overnight hours. Some local IFR
conditions may be possible, in particular along the coast.

Onshore/westerly flow will continue through this evening, becoming
more south/southwesterly tonight as the system approaches. Winds
will pick up tomorrow, with gusty winds possible, in particular
through the interior terminals.

KSEA...VFR conditions this afternoon with scattered cloud cover. VFR
conditions continue through early this evening with increasing
clouds. MVFR conditions and rain showers beginning around 12Z
Tuesday and persisting through the day tomorrow. Westerly winds
becoming southerly this evening around 4 to 8 kt, increasing 10 to
15 kt tomorrow, with gusts to 25 kt possible as the system passes
through the region.

.AVIATION...-- Changed Discussion --
North-northwesterly flow aloft as a weak positively
tilted ridge will be pushed out by an incoming shortwave trough. VFR
conditions across the region with increasing clouds from the
northwest ahead of the next frontal system. A few radar echos are
already visible near Vancouver Island. Expect VFR conditions to
continue through the evening. A frontal system will approach from
the northwest, bringing with it rain showers and MVFR ceilings area-
wide. Widespread rain will approach the coast early tonight,
spreading inland through the overnight hours. Some local IFR
conditions may be possible, particularly along the coast. Some
visibility restrictions will be possible at times in heavier showers.

LH-- End Changed Discussion --

.MARINE...-- Changed Discussion --High pressure over the NE Pacific will maintain onshore
flow across western WA this week. Very typical to the pattern, the
strongest winds will continue through the Strait of Juan de Fuca
this evening; the Small Craft Advisories will continue through early
tomorrow morning. A frontal system will cross the waters on Tuesday
which will boost winds and seas, likely warranting additional
headlines. Combined seas over the coastal waters are expected to
rise to near 15 feet by Wednesday especially over the northern
coastal zones. Waves will be steep with short period waves
dominating. Another frontal system may pass through area waters on
Friday.

KC/LH-- End Changed Discussion --

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 5 AM PDT Tuesday for Central U.S.
Waters Strait Of Juan De Fuca-East Entrance U.S. Waters
Strait Of Juan De Fuca.