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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 180933

Area Forecast Discussion
National Weather Service Seattle WA
233 AM PDT Wed Jul 18 2018

.SYNOPSIS...An upper level trough passing by to the north of the
area, combined with strong low-level onshore flow, will give 
cooler conditions along with night and morning clouds through the
end of the week. The onshore flow will weaken on Saturday 
allowing for an increase in sunshine and warming conditions. 
Offshore flow will likely return on Sunday bringing very warm to 
hot conditions into the area for the first part of next week.


.SHORT TERM...Strong onshore flow overnight is allowing marine air
to flood Western Washington early this morning. Stratus has 
developed eastward along the Strait of Juan de Fuca while stratus 
has pushed rapidly northeast through the Chehalis Gap into the 
interior from about Everett southward. The stratus will continue 
to march northward into the north interior early this morning and 
will cover most areas below about 2000 ft elevation by 7 AM. 
Stratus is expected to burn back to the Pacific coast during the 
early afternoon hours leaving partly sunny conditions in the 
interior. With the marine air and the morning clouds, temperatures
will be much cooler in the interior today with highs topping out 
in the lower to mid 70s most areas. The coastal areas will remain 
in the 60s. 

An upper level low centered near Whitehorse, Yukon early this
morning is forecast by all global models to drive SE along the
southeastern Alaskan and British Columbia coastlines into Southern
British Columbia Friday afternoon. As the upper low approaches the
area, lowering heights and strengthening westerly flow aloft will
result in the marine layer deepening to around 5000 feet Thursday
into Friday. Clouds will return tonight and persist a little 
later into the afternoon on Thursday resulting in a couple degrees
additional cooling on Thursday. Friday will likely be a repeat of
Thursday. Forecast soundings show the depth of the marine layer 
to be insufficient to result in any precipitation through the end 
of the week. Enjoy the cool down and the increase in cloudiness 
because it appears that this cool spell won't last more than a few
days. Albrecht

.LONG TERM...The upper level low over southern British Columbia
late Friday shifts eastward into Alberta on Saturday. As the low 
moves out, developing high pressure over British Columbia will 
result in the development of offshore flow over the area late 
Saturday into Sunday and the development of a low-level thermal 
trough over Western Washington. 

Models are generally consistent through Monday showing
strong warming Sunday with temperatures into the 80s and further
warming Monday with highs approaching or possibly exceeding 90
over portions of the area. The subsidence combined with a strong
July sun and long days will mean that temperatures at night will
struggle to fall below 60 degrees in the urban areas from Everett
to Tacoma Sunday night. After Monday the models are beginning to
diverge a bit with the new 00Z ECMWF shifting the thermal trough
off to the east for some cooling and the GFS keeping conditions 
quite warm over the area through Wednesday. The ECMWF is a new 
solution and isn't consistent with its 12Z run. The forecast will 
show continued heat into Tuesday and Wednesday with not a great 
deal of relief at night. 

No precipitation is expected over the area for the next 8+ days.


.AVIATION...Westerly flow aloft today as an upper level trough moves 
by to our north through northern British Columbia. Moderate to 
strong low level onshore flow continues with a deepening marine 
layer covering the lowlands of Western Washington. Low MVFR and IFR 
ceilings in stratus can expected across the region this morning. The 
stratus will burn off over the interior after 21Z but remain in 
place along the coast and several miles inland over the Chehalis 
Gap. Stratus will push back inland this evening for a return to low 
MVFR or IFR ceilings in stratus across the area Thursday morning.

KSEA...Strong marine push well underway and thermal profile on our 
profiler suggests the marine layer now extends up through at least 
3000 feet. This will take some time to burn off today. Expecting 
MVFR ceilings to stick around through 21Z or 22Z then scattering out 
for the late afternoon into the evening. Stratus returns promptly by 
06Z or so tonight for more low MVFR/IFR. Surface winds S-SW 7 to 14 
knots. 27


.MARINE...Onshore flow will continue across Western Washington
today then increase tonight. Gales are possible through the
central and east Strait of Juan de Fuca this evening and a Gale 
Warning is in effect. Winds will ease late tonight. 

High pressure over the NE Pacific will maintain onshore flow 
through the remainder of the week. Highest wind and waves will be 
over the outer Coastal Waters and through the strait. Small Craft 
Advisory westerlies are likely in the strait during the late 
afternoon and evening hours each day. The flow may turn offshore 
early next week. 33


PZ...Small Craft Advisory until 5 AM PDT Thursday 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.

     Small Craft Advisory until 6 PM PDT this evening for Central 
     U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. 
     Waters Strait Of Juan De Fuca.

     Gale Warning from 6 PM this evening to 5 AM PDT Thursday for 
     Central U.S. Waters Strait Of Juan De Fuca-East Entrance 
     U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from 6 PM this evening to 5 AM PDT Thursday 
     for Admiralty Inlet-Northern Inland Waters Including The 
     San Juan Islands.