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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 170918

Area Forecast Discussion
National Weather Service Seattle WA
218 AM PDT Tue Jul 17 2018

.SYNOPSIS...Onshore flow will gradually increase through tonight as
an upper level ridge shifts east of the area. The increase in 
onshore flow will give good cooling to the coastal areas today and
slight cooling to the interior. A more complete inland push of 
marine air will bring much cooler conditions inland on Wednesday.
An upper trough will swing across the area Thursday into Friday or
Saturday bringing near normal temperatures with late night and 
morning clouds. Offshore flow may produce much warmer conditions 
Sunday into Monday of next week.


.SHORT TERM...The upper level ridge and low level thermal trough
that produced dry and very warm to hot conditions across the area
over the past couple of days is moving east of the Cascades early
this morning. Moderate inflow through the gaps is allowing a
tongue of fog and low stratus to creep eastward through the 
Strait of Juan de Fuca, and stratus and fog has moved onto the 
coast and through the Chehalis Gap to near Shelton. The onshore 
flow does not appear strong enough to allow the clouds and fog to 
push much farther eastward into the interior this morning, but the
onshore pressure gradients should produce good cooling along the 
coast and in the north interior near the water and modest cooling
across the rest of the interior lowlands. The warm spot today 
will be in the central Puget Sound Lowlands from about Everett to 
Bremerton and Tacoma where mid to upper 80s can be expected this 

Stronger onshore flow is expected tonight into Wednesday. As a
result, clouds will spread further inland late tonight and 
Wednesday morning and temperatures will top out only in the mid 
70s in the interior. This will be a refreshing change after the 
recent spell of very warm weather! 

An upper level low over SE Alaska early Thursday will slide ESE
into Southern British Columbia on Friday. As this upper low
approaches from the NW, expect stronger onshore flow on Thursday
with morning clouds and highs again topping out in the mid 70s or
so. Albrecht 

.LONG TERM...Medium range models are showing rather good
consistency and continuity today. A moderate upper level trough
will swing by just to the north of the forecast area Friday into
Saturday giving near normal temperatures and typical marine
cloudiness during the late night and morning hours. Highs mainly
in the 70s will rise to around 80 in the interior from Seattle
southward on Saturday as the upper trough moves off to the east
and low level onshore flow decreases.

Both the GFS and the ECMWF show low level pressure rises over
central and southern portions of BC late Saturday behind the upper
trough. These pressure rises will produce offshore flow Sunday
into Monday over Western Washington with adiabatic compression 
warming the lower levels. By Monday and Tuesday of next week, 
models appear consistent in developing a thermal trough over 
Western Washington. Model blend temperatures show mid 80s on 
Sunday warming to near 90 for highs early next week. Albrecht 


.AVIATION...Light southwest flow aloft today as an upper ridge axis 
shifts east of the region. The air mass remains dry and stable 
except for some low level marine moisture along the coast extending 
locally inland through the Chehalis Gap and Strait of Juan de Fuca.
The stratus will burn back to the coast by midday, but increasing 
low level onshore flow this afternoon and evening will bring it 
further inland tonight and Wednesday morning. IFR ceilings in 
stratus will be mainly restricted to areas near the coast this 
morning with VFR for the remainder of the region. Stratus will surge 
inland tonight with IFR ceilings developing around the Puget Sound 
terminals 09Z-12Z Wednesday morning.

KSEA...VFR today. IFR ceilings in stratus expected to return by 12Z 
Wednesday morning. Surface winds light southerly this morning 
becoming SW 6 to 11 knots this afternoon.   27


.MARINE...Onshore flow will increase today and Wednesday with 
gales possible through the Strait of Juan de Fuca. Small Craft 
Advisory winds are also likely over the outer Coastal Waters, 
Admiralty Inlet and Northern Inland Waters. High pressure over the
NE Pacific will maintain moderate onshore flow across the region 
through the rest of this week. Highest wind and waves will be 
through the strait, mainly during the late afternoon and evening 
hours each day. 33


PZ...Small Craft Advisory from 6 PM this evening to 11 AM PDT 
     Wednesday 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.

     Gale Warning from 6 PM this evening to 5 AM PDT Wednesday 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 
     Wednesday for Admiralty Inlet-Northern Inland Waters 
     Including The San Juan Islands.