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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 241021

Area Forecast Discussion
National Weather Service Seattle WA
321 AM PDT Mon Jun 24 2019

.SYNOPSIS...Low level onshore flow will weaken today. An upper
level trough will approach the area Tuesday and be over the 
Pacific Northwest Wednesday through Friday. The trough will move 
east of the area on Saturday.


.SHORT TERM /TODAY THROUGH WEDNESDAY/...Satellite imagery shows 
cloudy skies over most of the area this morning with some clearing
over the extreme southern sections. Weak low level convergence 
over the central Puget Sound with some light showers/drizzle 
between Everett and Sea=Tac airport. Temperatures at 3 am/10z were
in the upper 40s to mid 50s. 

Not much going on weather wise today over Western Washington. Low
level onshore flow weakening with flow aloft turning southwesterly
later in the day. The weaker onshore flow and resulting shallower
marine layer will lead to some sunshine this afternoon over most
of the area. Weak convergence over the central Puget Sound will
come to an end this morning. Have included areas of drizzle for
the morning between Seattle and Everett. With some sunshine this
afternoon highs will be a little warmer than Sunday but still 
below normal, mostly in the 60s. 

Upper level trough digging south offshore tonight into Tuesday
with the trough off the coast by Tuesday afternoon. Low level flow
light overnight into Tuesday morning which will reduce the amount
of low cloud coverage but with the trough approaching mid and high
level moisture increasing. Will word the forecast partly sunny on
Tuesday. The trough will be close enough Tuesday afternoon to 
help destabilize the air mass enough to not be able to rule out 
showers by late afternoon. Lows tonight in the mid 40s to mid 50s.
Highs on Tuesday in the 60s to lower 70s.

Trough turning east Tuesday night and will move over Western
Washington on Wednesday. By Wednesday afternoon convective 
temperatures are a couple degrees below the max temperatures, 
precipitable water values are up around and inch, and lifted 
indexes are near 0. High temperatures will be a little tricky both
Wednesday and Thursday as just a little sun this time of year in 
the afternoon can pop the temperatures by a few degrees. With 
shower activity increasing during the afternoon will keep high 
temperatures only in the lower to mid 60s. Lows Tuesday night in 
the upper 40s to mid 50s. 

.LONG TERM /THURSDAY THROUGH SUNDAY/...Good agreement in the
extended models for Thursday and Friday with both the GFS and the
ECMWF keeping the trough in the area through Friday. Previous 
runs had the trough moving out of the area on Friday. Some 
differences in the models over the weekend with the ECMWF bringing
a weak upper level trough over the area while the GFS has Western
Washington in a lull with light flow aloft over the area. Both 
models have low level onshore flow in the lower levels. Will keep 
showers in the forecast through Friday with a continuation of 
below normal high temperatures. For the weekend morning clouds 
and afternoon sunshine with highs warming back to near normal, 60s
and lower 70s Saturday and a couple of degrees warmer Sunday. 


.AVIATION...Current satellite around 09z shows western Washington 
blanketed in clouds this morning. Most of the sub-VFR ceilings 
reside around the eastern portion of Puget Sound from KPAE south 
thru KSEA and KBFI, while VFR cigs are holding across the remaining 
terminals. Current forecast is for most sites to be MVFR by sunrise, 
with gradual lifting back to VFR in the afternoon. Contrary to the 
last two days, think there is a better chance for some scattering, 
though likely not until around or after 00z Tues.

Last evening, a Puget Sound Convergence Zone developed over Snohomish 
and northern King County. It has since dropped south but is 
weakening. This trend will continue with light rain expected within 
the area of convergence over the next few hours, with drizzle 
possible elsewhere through the morning hours. Although a few light 
showers may develop this afternoon, looks to be a dry day overall. 
Winds will initially be W/SW for most (aside from variable winds 
near the weakening convergence zone thru 12z), but look to shift 
more N this evening with speeds 5-10kts.

KSEA...A weakening PSCZ may bring a light shower and some drizzle to 
the terminal thru about 12z this morning, along with variable winds. 
Thereafter, morning MVFR ceilings will rise back to VFR this 
afternoon and have the chance to scatter around 00z Tues. Conditions 
should be dry overall after the morning hours. Winds prevail S/SW 
thru the afternoon at 5-10kts, shifting more N this evening.



.MARINE...As has been the case over the last several days, onshore 
flow will continue for much of the week across western Washington as 
a broad area of low pressure moves into the region. Gale warning 
from last evening across the central and eastern Strait of Juan de 
Fuca was allowed to expire at 11PM last night. This leaves current 
headlines as of 09z at small craft advisory winds for the central 
and eastern Strait, Admiralty Inlet, and Northern Inland Waters. 
Small craft will drop off at 12z for latter two waters, and at 15z 
for the Strait as gradients noticeably relax.

Pressure gradient will remain weaker going into the afternoon and 
evening hours today compared to the previous several days, however, 
a low-end small craft event appears likely across the central and 
eastern Strait so have issued an advisory from 21z Mon-10z Tues. 

Borderline to low-end small craft advisory winds then appear likely 
each afternoon and evening across across the central and eastern 
Strait. Headlines do not appear to be needed for any of the remaining 
marine zones for much of the week as winds and waves remain 
relatively benign.



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


PZ...Small Craft Advisory until 8 AM PDT this morning for Central 
     U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. 
     Waters Strait Of Juan De Fuca.

     Small Craft Advisory from 2 PM this afternoon to 3 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 5 AM PDT early this morning for 
     Admiralty Inlet-Northern Inland Waters Including The San 
     Juan Islands.