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958
FXUS66 KSEW 202207
AFDSEW

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

.SYNOPSIS...
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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.
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&&

.SHORT TERM /TONIGHT THROUGH THURSDAY/...
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Northerly flow aloft for
much of the PNW as an upper-ridge noses into BC and far western
Alberta. We`ve seen pleasant conditions across much of western WA
due to its influence as dry and seasonal temperatures reign. Much
of the same is in the forecast for this evening but, 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
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&&

.AVIATION...
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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.


LH
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&&

.MARINE...
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High pressure over the NE Pacific will maintain onshore
flow across western WA this week. Very typical to the pattern, the
strongest winds are expected through the Strait of Juan de Fuca
especially during the afternoon and evening hours. - Small Craft
Advisories are in effect. A frontal system will cross the waters on
Tuesday which will boost winds and seas. Combined seas over the
coastal waters are expected to rise to near 15 feet by Wednesday
especially over the northern coastal zones. Another frontal system
may pass through area waters on Friday.
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&&

.HYDROLOGY...The daily hydrology discussion has ended until the
start of the next rainy season; it will only be updated as needed.


&&

.SEW WATCHES/WARNINGS/ADVISORIES...
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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.
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&&

$$