Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 202238

Area Forecast Discussion
National Weather Service Seattle WA
338 PM PDT MON JUN 20 2016

.SYNOPSIS....A weak upper low just offshore will move through the
area tonight for a chance of showers. Weak high pressure will
build over the region Tuesday. Another upper level low will move
through the area Thursday night bringing showers Thursday and
Friday. A weak upper level ridge will build over Western
Washington for warmer and drier weather this weekend into early
next week.


.SHORT TERM...Mid level convection has been present this afternoon
through much of Western Washington, especially over the Cascades
and eastern Puget Sound region. Several mammatus clouds have been
sighted in the Puget Sound area today. However, so far, most
precipitation west of the crest has been in the form of virga,
with no accumulations at the ground. This convection is spurred by
an upper level low spinning just offshore and moisture in the

Expect shower activity to pick up tonight as the upper level low
moves overhead and then inland. I have kept a chance of
thunderstorms in for portions of the Cascades tonight, as the
SREF ensembles continue to indicate. The HRRR and other high
resolution models show a short-lived Puget Sound Convergence zone
setting up over Snohomish and north King Counties in the early
morning hours.

Tuesday will see some drying with zonal flow over the area. Expect
improving conditions through the day for partly sunny skies and
temperatures again near normal under onshore flow conditions.
Wednesday into Wednesday night a weak surface front will stall off
the coast and then move inland bringing a chance of precipitation
mainly to the coast and Olympics.

Nearly all models are in agreement that a deep upper low will move
through Western Washington late Thursday night. Earlier solutions
were showing it moving through a bit earlier but now only the NAM
is showing an earlier arrival. In agreement with discussions from
the national centers, I have gone with the other models` solutions
and disregarded the NAM. I lowered POPs a bit on Thursday morning
to coincide with the later arrival time. Smith

.LONG TERM...Models are indicating residual showers, especially in
the mountains, on Friday from the upper level low moving out of
the area. The GFS, ECMWF, and GEM models are all showing an
improving trend for the weekend and early next week with a ridge
building in and drier and warmer weather in store. Smith


.AVIATION...An upper level low will move inland tonight for
isolated to scattered showers across Western Washington. There is
a slight chance of thunderstorms over the Cascades where
instability is the greatest. MVFR conditions are likely early
Tuesday morning as the marine layer spreads inland. Otherwise,
Tuesday will be drier with weak ridging over the region. 33

KSEA...VFR conditions this evening. Winds will flip to S/SW by
06Z with -SH in the vicinity. MVFR conditions are possible by
09-12Z the the marine layer pushes inland. 33


.MARINE...Low level onshore flow will increase this afternoon and
evening as low pressure shifts into Eastern Washington. Small
Craft Advisory winds are expected over the Outer Coastal Waters,
Central/East Strait and Northern Inland Waters. Winds will ease
late tonight into Tuesday morning. A weak front will pass through
the waters on Wednesday. Onshore flow will prevail through the end
of the week. 33


.SEW Watches/Warnings/Advisories...
PZ...Small Craft Advisory until 5 AM PDT Tuesday for Coastal Waters
     From Cape Flattery To James Island 10 To 60 Nm-Coastal
     Waters From James Island To Point Grenville 10 To 60 Nm-
     Coastal Waters From Point Grenville To Cape Shoalwater 10
     To 60 Nm-Northern Inland Waters Including The San Juan

     Small Craft Advisory until 9 AM PDT Tuesday for Central U.S.
     Waters Strait Of Juan De Fuca-East Entrance U.S. Waters
     Strait Of Juan De Fuca.

     Small Craft Advisory until midnight PDT tonight for Admiralty



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