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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 181641

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

.SYNOPSIS...Strong onshore flow will prevail today for cooler
weather. A low pressure system will approach the area from the
northwest on Thursday. The tail end of this system will slowly
move across the area on Friday before exiting the region late 
Saturday. Temperatures will be near normal through Saturday.


The forecast was updated to delay clearing over the interior.
Thanks to strong onshore flow, temperatures will be cooler (near
normal) across much of the CWA.

From the previous discussion: 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

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...An upper level trough passing to the north will 
result in westerly flow aloft and moderate to strong low level 
onshore flow today. The onshore flow has produced a relatively deep 
marine layer over the lowlands of Western Washington overnight. Low 
MVFR and IFR ceilings this morning will lift into midday, but be 
hard pressed to burn off over the interior until after 21Z. The 
coast will similarly see ceilings lift through the day, but will 
likely not see clearing. Stratus will redevelop over the interior 
again this evening with low MVFR or IFR ceilings again across the 
area Thursday morning.

KSEA...MVFR ceilings will lift through midday, then scattering out 
late in the afternoon after 21Z. Stratus will redevelop tonight with 
low MVFR/IFR ceilings into Thursday morning. Surface winds will 
remain S-SW, 7 to 14 knots. 


.MARINE...Moderate onshore flow today will strengthen this afternoon 
and evening. Small craft criteria westerlies through he central and 
eastern Strait of Juan de Fuca this afternoon will likely reach gale 
levels this evening. 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 central and eastern Strait of Juan 
de Fuca. Strongest winds in the strait will occur during the late 
afternoon and evening hours each day. The flow may turn offshore 
early next week.


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.