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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 171556

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

.SYNOPSIS...Today will be a transitional day with cool 
temperatures along the coast and warm/above normal temperatures over 
much of the interior. Strengthening onshore flow tonight will set up 
a typical pattern of morning clouds and near normal temperatures 
that will persist into Saturday. Offshore flow will return Sunday 
bringing warmer conditions into at least the early part of next week.


.SHORT TERM...Today will be a transitional day with increasing, but 
limited onshore flow bringing cooler/normal temperatures to the 
coast and warmer/above normal temperatures still lingering over much 
of the interior. Temperatures over the interior - while above normal 
- will likely run 4 to 6 degrees cooler than yesterday with highs 
reaching into the mid to upper 80s. The warm spot today will be from 
the central Puget Sound Lowlands to the South. Strengthening onshore 
flow tonight will bring a more developed push of marine air into the 
interior overnight into Wednesday morning, helping limit 
temperatures to more typical levels. Highs Wednesday will remain in 
the mid 70s over the interior and upper 60s along the coast.

An upper level low will dig into into Southern British Columbia on 
Friday and brush Washington and Oregon as it ejects eastward 
Saturday. Medium range models continue to keep precipitation 
associated with this system well to to the north over British 
Columbia as the upper level low approaches from the north. The 
primary impact for Washington will most likely be a boost to the 
onshore flow Thursday and a deeper marine layer into Friday. 

.LONG TERM...from previous discussion...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 to moderate westerly flow aloft will continue 
today and tonight. At the surface, light onshore flow today will 
become moderate to strong tonight. The air mass remains dry and 
stable except for low level marine stratus along the coast extending 
inland over the southwest interior and down the 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 southwest 6-11 knots this afternoon. CHB


.MARINE...Onshore flow will increase today and Wednesday with gales 
likely tonight through the Strait of Juan de Fuca. Small craft 
advisory strength winds are also likely over the outer coastal 
waters, Admiralty Inlet and northern inland Waters. A similar 
pattern is likely Wednesday night.

High pressure over the northeast 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. CHB


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.