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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 201059

Area Forecast Discussion
National Weather Service Seattle WA
359 AM PDT Sat Apr 20 2019

.SYNOPSIS...An upper level ridge will build into the area today
and remain through Sunday. The next front will approach the area
on Monday then weaken Monday night and Tuesday as it moves across
the area. Post frontal showers could linger into Wednesday. High
pressure will build back into Western Washington on Thursday.


.SHORT TERM /TODAY THROUGH MONDAY/...Satellite imagery shows
clouds lingering over about the southern 2/3rds of the area early
this morning. Doppler radar has a small area of showers between 
about Olympia and Bremerton associated with a dying convergence 
zone and an upper level trough axis moving through the area. 
Temperatures at 3 am/10z were in the 40s and lower 50s.

As a result of the stubborn convergence zone have made a few 
changes to the forecast this morning. While the zone is in the 
process of weakening early this morning at this point it does not 
look like the zone will totally fizzle out until 12z-15z so have 
added isolated showers to the forecast where the zone is now and 
just to the east of the current location. The persistent 
convergence and showers has added more low level moisture to the 
air mass so have also added more cloud cover to the southern 
portion of the area this morning. Some subsidence with the upper 
level ridge building into the area later today but the subsidence 
is weak and the low level flow is light onshore so have only gone 
partly sunny for the southern portion of the area this afternoon.
To the north where skies have already cleared expect a sunny day.
With the additional cloud cover have taken a couple of degree off
of the high temperatures today from about Seattle southward. This
puts highs today in the mid 50s to lower 60s across the area. 

Upper level ridge remaining over the area tonight into Sunday but
the low level flow remains onshore. With the added low level 
moisture from the last few hours, which will not completely go 
away today with the weak ridging, have increased the morning cloud
cover Easter Sunday morning making for a partly sunny forecast.
Lows tonight mostly in the 40s. With the additional cloud cover on
Sunday have kept highs mostly in the mid 50s to lower 60s. 

Flat upper level ridge remaining over Western Washington Sunday
night. 00z model runs are stronger and a little faster with the
next system with light precipitation just off the North Coast by
12z Monday. Mid and high level clouds out ahead of the front
moving over the area Sunday night. This cloud cover will help keep
lows in the 40s for the most part. 

A little bit of a tug of war going on Monday with the front 
trying to move closer to the area while the upper level ridge 
strengthens a little. The end result is the front will have a hard
time making much eastward progress on Monday but will be close 
enough in the afternoon to have a chance of showers all the way to
the Cascade crest with showers all day along the North Coast. 
Highs on Monday once again in the mid 50s to lower 60s. 

.LONG TERM /TUESDAY THROUGH FRIDAY/...What is left of the front
drags across the area on Tuesday keeping at least a chance of
showers in the forecast. The GFS has some lingering showers into
Wednesday morning so will keep a chance of showers in the forecast
for the first half of Wednesday. Another break in the action, a
little like this weekend, for Thursday and Friday. No big upper
level ridge for a prolonged break in the wet weather but nothing
organized out in the Pacific. Previous runs had the possibility 
of some showers on Friday. Models haven't been showing much run to
run consistency lately so for now will keep the slight chance of 
showers in the forecast for Friday. If the model trends continue 
look for the forecast for the end of the week to dry out. 
Temperatures will be near normal through the period with highs in
the mid 50s to mid 60s and lows mostly in the 40s. Felton 


.AVIATION...Lingering troughing and some convergence has allowed for 
a congregation of showers to continue across the southern portion of 
Puget Sound early this morning. An inherited tempo group has been 
extended to account for these showers from OLM to PWT and over to 
SEA and BFI. Luckily, most ceilings have remained VFR most of the 
night thus far. Do think MVFR is possible going into Saturday 
morning across terminals near the Sound where showers are ongoing 
and also near the coast. KBLI may be the only site that remains 
mostly clear thru most of the day Saturday. Elsewhere, BKN to OVC 
ceilings this morning should scatter out by the afternoon and 
improve/remain VFR. Northerly winds today should begin to shift back 
to southerly by the evening at under 10kts.

KSEA...Showers in the vicinity through the early morning hours. Cigs 
mostly VFR but could drop to MVFR during the morning. Cigs should 
then become more scattered by the afternoon. North wind this morning 
4-8kts should shift to the south by the evening and continue under 



.MARINE...A broad area of high pressure is readily detected offshore 
of the Washington and Oregon coastlines early this Saturday morning. 
Gradients across the outer coastal waters appear to be strong enough 
to continue the small craft advisory through the morning and 
afternoon hours. In addition to this, will be watching the far 
Northern Inland Waters this morning for potential for small craft 
winds. Some of the higher-res guidance suggest a period of time this 
morning where winds reach SCA for these waters. Observations across 
the Strait of Georgia /albeit very sparse/ are beginning to show 
gusts to small craft in recent hours. For now, have capped winds at 
20 kts and will monitor through the morning. Elsewhere, winds should 
be fairly light given light gradients.

By this evening, a westerly push down the Strait of Juan de Fuca is 
expected, with the central and eastern Strait expected to reach 
small craft for wind through Sunday morning. Have gone ahead and 
issued this with the morning package. SCA winds across the coastal 
waters should ease thru the evening. Onshore flow will continue thru 
Sunday with little change in the pattern-winds will be highest 
across the Strait but should fall below SCA by 18z. The next frontal 
system will approach the area on Monday.



.HYDROLOGY...River flooding is not expected across the area for 
the next 7 days.


PZ...Small Craft Advisory from 5 PM this afternoon to 8 AM PDT Sunday 
     for Central U.S. Waters Strait Of Juan De Fuca-East 
     Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 11 PM PDT this evening 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.