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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 201557

Area Forecast Discussion
National Weather Service Seattle WA
857 AM PDT Tue Aug 20 2019

.SYNOPSIS...High pressure aloft will shift east of the region
later today as a front approaches the area. The front will spread
rain across Western Washington on Wednesday. Weak high pressure
will build back into the area on Thursday. 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 /TODAY THROUGH THURSDAY/...Mostly sunny skies this
morning with generally light surface flow. Dry weather through
today as an upper level ridge slides east of the area this 
afternoon. High temperatures will be in the 70s to low 80s. High 
clouds will increase through the afternoon ahead of the next 
fairly potent frontal system near 45N/135W. The surface low will 
shift into central B.C. coast tonight as the front holds together 
to bring a soaking rain to coastal areas by later tonight.

The rain will spread into the Puget Sound area by mid morning on
Wednesday. Models have fluctuated a bit on QPF amounts but seem to
be settling around an inch for the coast with possibly a quarter
of an inch to a half an inch or so in spots for the Puget Sound 
area. The front will quickly slide through the area Wednesday and
be east of the Cascades by the evening. Mesoscale models do
suggest convergence zone formation Wednesday night over Snohomish
County and drifting south into King County before dissipating by
daybreak on Thursday. Cloudy skies with cooler temperatures on
Wednesday with highs in the 60s. 

A few lingering showers possible Thursday morning, especially in
the Cascades, otherwise dry weather expected with weak high 
pressure aloft. Some sun during the hours of Thursday afternoon
with highs in the mid 60s to mid 70s across the area. JD

.LONG TERM /FRIDAY THROUGH MONDAY/...Upper ridging gets flattened
fairly quickly by later Friday as the first of series of weather
systems move into British Columbia. For the most part, the weekend
looks dry with exception of a few showers possible near the coast
and across the north interior. Persistent onshore flow will keep 
temperatures near normal. Both the GFS and Euro are hinting at the
return of high pressure aloft by Monday. If the models maintain 
their current trends, we could be in for a bit of a pattern change
to close out August with higher amplitude ridging and much warmer
temperatures. 27 


.AVIATION...An approaching frontal system will bring increasing mid 
and high level clouds and strengthening southwest flow aloft. The 
front is expected to reach the coast this evening will reach the 
coast after midnight with low clouds and rain spreading inland 
through the morning. Conditions will remain VFR across the area 
through much of the Avignon with ceilings lowering to MVFR levels 
along the coast after 06z and then through the interior 12-15z. IFR 
conditions are possible Wednesday morning at some locations as rain 
moves onshore and inland. 

KSEA...Increasing mid and high level clouds today. VFR conditions 
will persist through this evening with ceilings lowering around 
daybreak to MVFR levels and as rain moves through the metro area 
through the morning. Light northerly winds will become southerly 6 
to 8 kt around 18z. 


.MARINE...Weak onshore flow today ahead of a fairly vigorous 
frontal system approaching the coastal waters. 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 later Wednesday and Wednesday night. 

Onshore flow will linger continue through the end 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 winds.


.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 from 5 PM this afternoon to 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 11 PM this evening to 11 AM PDT 
     Wednesday for West Entrance U.S. Waters Strait Of Juan De