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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 162224

Area Forecast Discussion
National Weather Service Seattle WA
323 PM PDT Mon Jul 16 2018

.SYNOPSIS...Dry and warm conditions will linger over much of Western 
Washington into Tuesday. A weak upper level trough will brush the 
area Tuesday afternoon helping boost low level onshore flow setting 
up a return to normal high temperatures for Wednesday. A typical 
onshore flow pattern will follow through the weekend with night and 
morning clouds and normal high temperatures. 


.SHORT TERM...Another very warm day for much of the interior 
today with very limited onshore flow bringing cooling only to the 
immediate coast and far western Strait of Juan de Fuca. Temperatures 
through out the remainder of the interior have risen well above 
normal again today reaching into the upper 80s and low 90s, 
especially for locations away from the water. Expect onshore flow to 
remain limited again tonight, setting up Tuesday to be a transition 
day with normal temperatures along the coast and strait, and 
slightly cooler - but still above normal temperatures for much of 
the interior. A dry, weak upper level trough will brush the area 
Tuesday afternoon boosting onshore flow Tuesday night into Wednesday 
morning. The more mature onshore flow, coupled with the zonal flow 
aloft will help a shallow marine layer penetrate further into the 
interior early Wednesday morning resulting in morning low clouds 
west and south of Puget Sound and a return to near normal high 
temperatures. Highs over the interior will remain in the mid to 
upper 70, and the 60s along the coast and strait.  

.LONG TERM...Medium range models continue to agree on a fairly 
typical zonal/onshore flow pattern persisting through Thursday. This 
will mean night and morning clouds for at least parts of the 
interior and near normal temperatures. Models continue to advertise 
another upper level trough sliding down the British Columbia coast 
late Thursday into Friday morning before it shifts inland Friday 
afternoon and evening. Models continue to show it shifting inland 
while still over British Columbia which will keep Western Washington 
dry. It's possible that there may be enough energy aloft to result 
in a chance of showers - mainly over the north Cascades - but for 
now the forecast remains dry. The most likely impact will be some 
increased mid and high clouds as the upper level trough moves past 
the area to the north and an enhancement to the low level onshore 
flow Friday night. The bottom line is that the typical pattern of 
night and morning clouds with normal temperatures will keep through 
Friday and possibly Saturday. Upper level heights look to build 
Sunday coupled with a weaker onshore flow will result in a bit of 
warming especially by Sunday.  


.AVIATION...Weak westerly flow aloft will continue tonight, and 
become a little stronger Tuesday. Low level flow is weakly onshore 
and will increase a bit tonight. VFR clear conditions across the 
area this evening. Marine stratus will return to the coast later 
tonight, and also reach down the strait and into the southwest 
interior. Ceilings will be IFR around 1k under the stratus. VFR 
clear conditions will continue over the rest of the interior. The 
stratus should evaporate back to the coast by noon or so. CHB

KSEA...VFR clear. Northerly winds 8-10 kt. CHB


.MARINE...Flow has turned onshore this afternoon, but winds are 
forecast to be less than 20 kt over all waters tonight. Onshore flow 
will increase Tuesday, with gales possible in the strait Tuesday and 
Wednesday nights. Small craft advisory strength northwest winds will 
be possible on the coast at times. Moderate onshore flow will 
continue the rest of the week, with at least small craft advisory 
strength westerlies in the strait each afternoon and evening. CHB


PZ...Gale Watch from Tuesday afternoon through late Tuesday night for 
     Central U.S. Waters Strait Of Juan De Fuca-East Entrance 
     U.S. Waters Strait Of Juan De Fuca.