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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 202214

Area Forecast Discussion
National Weather Service Seattle WA
314 PM PDT Tue Aug 20 2019

.SYNOPSIS...An upper level ridge over the Pacific Northwest will
slide east through this afternoon as a front approaches the area.
The front will spread rain across Western Washington tonight
through Wednesday. Dry weather by Thursday afternoon with a weak
high pressure over the area. A series of systems will pass
generally to our north Friday into the weekend keeping
temperatures near normal and bringing a chance of showers mainly
to the coast and northern areas.


.SHORT TERM /TONIGHT THROUGH FRIDAY/...Satellite imagery shows
high clouds over Western Washington this afternoon ahead of the
next frontal system. Dry weather through this evening as the upper
level ridge slides east. Temperatures generally in the 70s to low
80s currently with a few mid 80s by this afternoon. 

A fairly potent weather system near 45N/132W will move northeast
and track into the central B.C. coast tonight. The front will
slide onshore a bit quicker than expected with showers chances
along the coast tonight pushing eastward into the Puget Sound area
by daybreak. Models still fluctuate on QPF amounts, but most
generally in the range of a quarter of an inch to possibly a half
inch in spots around the Sound with around an inch along the 
coast. The front will move quickly through Western Washington and
weaken over the Cascades by evening. Mesoscale models highlight
the possibility for the development of a convergence zone over 
Snohomish/King Counties Wednesday night into early Thursday 
morning. Drier weather expected with some sunshine by Thursday 
afternoon as a weak area of high pressure builds over the area. 
Temperatures will be cooler on Wednesday with highs in the 60s 
with temperatures into the mid 70s on Thursday.

The upper level ridge over the area Thursday will become flattened
by Friday with the approach of another upper level trough.
Although the bulk of the energy with this system will generally be
north of the area, there will be a few showers possible across the
northern tier on Friday. Temperatures on Friday will be in the mid
60s to mid 70s across the area. JD

.LONG TERM /SATURDAY THROUGH TUESDAY/...Another weather system
north of the area looks to bring a chance for POPs on Saturday
with the GFS being a bit drier than the ECMWF. Onshore flow
expected through the weekend with temperatures near normal.
Guidance does suggest high pressure aloft building into the area
on Monday. Pattern change is possible heading into the middle of 
next week if the current trends are maintained with higher 
amplitude ridging over the Pacific Northeast and warmer 
temperatures. JD


.AVIATION...An approaching frontal system has brought fairly 
widespread mid and high level clouds this afternoon. Expect 
increasing low level southerly flow most areas ahead of the front 
late  tonight. The front will reach the coast after midnight with 
low clouds and rain spreading inland through the morning. Conditions 
will remain VFR overnight with ceilings lowering to MVFR levels 
along the coast after 06z followed by much of the interior 14-16z. 
IFR conditions are possible Wednesday morning at some locations as 
rain moves onshore and inland. 

KSEA...Mid and high level clouds with VFR conditions through 
tonight. Ceilings will to MVFR levels after daybreak with rain as a 
cold front moves into the central Puget Sound. Southwesterly winds 2 
to 6 kt becoming 8 to 10 kt after 06z. 


.MARINE...Southerly winds over most waters tonight with weak onshore 
flow through the strait. Increasing southerlies ahead of the front 
will result in small craft advisory conditions most waters as the 
front moves into the area tonight and Wednesday morning. Onshore 
flow and westerlies will follow behind the front through the 
remainder of the week with periods of small craft conditions 
possible in the central and eastern strait. A weak front will brush 
the area later Friday and Saturday with minimal affect on the


.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 11 AM PDT Wednesday for Coastal 
     Waters From Cape Flattery To James Island 10 To 60 Nm-
     Coastal Waters From Cape Flattery To James Island Out 10 Nm-
     Coastal Waters From James Island To Point Grenville 10 To 
     60 Nm-Coastal Waters From James Island To Point Grenville 
     Out 10 Nm-Coastal Waters From Point Grenville To Cape 
     Shoalwater 10 To 60 Nm-Coastal Waters From Point Grenville 
     To Cape Shoalwater Out 10 Nm.

     Small Craft Advisory from 11 PM this evening to 5 PM PDT 
     Wednesday for Admiralty Inlet-Northern Inland Waters 
     Including The San Juan Islands.

     Small Craft Advisory from 6 PM this evening to 4 AM PDT 
     Wednesday for Central U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from midnight tonight to 10 AM PDT 
     Wednesday for Puget Sound and Hood Canal.