Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 240337
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
837 PM PDT MON MAY 23 2016

.SYNOPSIS...Weak high pressure aloft will be over the area Tuesday
and Wednesday for mostly dry weather. An upper low will bring
showers Thursday into the weekend.

&&

.SHORT TERM...There are a few showers on radar this evening,
mainly over the south Sound but there are also areas of clearing,
especially toward the coast. The current mostly cloudy forecast
might be a bit pessimistic, at least for this evening, but low
level onshore flow will probably allow clouds to fill back in
again by Tuesday morning. The HRRR and NAM12 indicate shower
activity will basicallybe gone by midnight with dry conditions
after that.

Weak high pressure aloft will move over the area Tuesday for
mainly dry weather. Afternoon heating might cause a few showers to
bubble up over the Cascades but overall it looks like a dry day
with morning clouds burning off for at least a partly sunny
afternoon.

Wednesday will be about the same as Tuesday with some subtle
changes. The high pressure aloft will weaken slightly as an upper
trough over northern British Columbia approaches from the
northwest. This in turn will cause an increase to the low level
onshore flow for Western Washington and result in more clouds and
less, if any, afternoon sunshine.

The upper low over northern British Columbia will move to
southern British Columbia Thursday. This will bring a chance of
showers to the area again...mainly in the afternoon as the low
nears. Schneider

.LONG TERM...Previous discussion...Model consistency and
continuity is poor for the upcoming Memorial Day weekend.
Ensembles show quite a bit of spread in 850 mb temperature and 500
mb height across the region through the period with some solutions
showing the upper low that drops into the region Thursday night or
Friday persisting through the weekend, and other solutions showing
the low moving off to the east with maybe a ridge over the area by
Memorial Day. The deterministic GFS and ECMWF solutions have both
flipped from the cold solution of last evening to the warmer end
of the ensemble envelope, with the ECMWF flipping less. The
deterministic Canadian GEM shows the first upper low late in the
week shifting east and rapidly being replaced with another rather
deep cut off low on Memorial Day. Given the large ensemble spread
and the presence of a long term trough over the area, a rather
persistent mostly cloudy with a chance of showers forecast is
provided with high temperatures near or a bit below normal.
Albrecht

&&

.AVIATION...A long wave trough remains over the West while an upper
level low spins over MT. The low level air mass remains moist...but
with clouds continuing to scatter this early evening...not much risk
for any showers or sprinkles. Clouds expected to return tonight
though...with patchy MVFR cigs overnight and into Tuesday morning.
Clouds should start to break up once more Tuesday afternoon with the
only real risk for precip being up in the Cascades. 33/SMR

KSEA...VFR conditions through this evening with patchy low clouds
developing overnight. Light winds out of the north then flipping
back to S/SW by 12Z tonight. 33/SMR

&&

.MARINE...Onshore flow will increase tonight as a weak surface low
moves inland. Obs not really supporting inherited Small Craft
Advisory...so may consider taking it down...just not sure if it will
be in time for evening update or if it will have to wait til morning
forecast package. High pressure over the NE Pacific will maintain
onshore flow across western WA through the end of the week. Small
Craft Advisory west winds are likely through the Strait of Juan de
Fuca each day, mainly during the late afternoon and evening hours.
33/SMR

&&

.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.

&&

$$

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You can see an illustrated version of this discussion at
www.weather.gov/seattle/gafd/latest_webafd.html


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