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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 161554

Area Forecast Discussion
National Weather Service Seattle WA
855 AM PDT Mon Jul 16 2018

.SYNOPSIS...High pressure over Western Washington will help 
above normal temperatures linger over the interior into Tuesday. A 
weak upper level trough will brush the area Tuesday afternoon 
helping boost low level onshore flow restoring much normal high 
temperatures for Wednesday. A typical onshore flow pattern will 
persist into the weekend with night and morning clouds and normal 
high temperatures. 


.SHORT TERM...The transition from offshore flow overnight has
helped marine stratus work onto the Central Washington Coast, Grays 
Harbor and the West Entrance of the Chehalis Gap this morning. 
Despite mild overnight temperatures, conditions are actually running 
anywhere from 1 to 5 degrees cooler across the area this morning. 
Expect another warm day today over the interior, but given the 
cooling trend this morning, expect high temperatures to be just a 
bit cooler than yesterday with temperatures reaching into the upper 
80s and low 90s. The coast will see much cooler temperatures today 
with highs reaching into the mid 70s. Onshore flow will continue to 
develop overnight tonight with stratus working into the strait and 
Chehalis Gap overnight. The very limited onshore flow will help cool 
the coast Tuesday. For the interior, it will really be a transition 
day, with cooler - but still above normal temperatures. A weak upper 
level trough will brush the area Tuesday afternoon as it lifts into 
British Columbia. This system will remain dry, bringing a few high 
clouds. More importantly it will boost the developing 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 into the south and central Puget Sound Wednesday morning. 
This will likely equate to some morning low clouds west and south of 
Puget Sound and a return to near normal high temperatures across the 
region. Highs over the interior will remain in the 70, and the 60s 
along the coast and strait.  

.LONG TERM...from previous discussion...Extended models in good 
agreement on Thursday with zonal flow aloft and low level onshore 
flow for another day of high temperatures near normal, 70s for the 
interior and 60s on the coast. Another upper level trough moving 
into the area on Friday which will keep the low level onshore flow 
pattern going into Friday night. The ECMWF is a little deeper with 
the trough versus the GFS resulting in a deeper marine layer. Have 
used a model blend keeping high temperatures near normal for Friday. 
Trough shifting east over the weekend with the low level onshore 
pattern weakening adding a few degrees to the highs for the weekend 
with the warmer locations getting into the lower 80s. Felton


.AVIATION...Weak westerly flow aloft will continue today and 
tonight. Low level flow is weakly offshore and will turn moderately 
onshore tonight. VFR clear conditions across the area today except 
for stratus on the coast and partway into the southwest interior. 
This stratus should evaporate by noon or so. Onshore flow will 
increase this evening for some IFR in stratus near the coast, down 
the strait, and over the southwest interior Tuesday morning, but 
skies should remain VFR clear over the rest of interior of Western 
Washington. CHB

KSEA...VFR clear. Surface winds N-NE 5 to 10 knots becoming NW in 
the afternoon hours. CHB


.MARINE...Weak offshore flow will turn onshore this afternoon. Winds 
area forecast to be less than 20 kt over all waters, except the 
offshore coastal waters this morning. Onshore flow will increase 
Tuesday, with gales possible in the strait Tuesday and Wednesday 
nights. 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


.CLIMATE...Today is the day of the somewhat forgotten first 100 
degree day in Seattle. Since 1945 the official Seattle weather 
records have been recorded at Sea-Tac airport. Before the airport 
the official Seattle weather site was the Federal Building 
downtown. On this day in 1941 the high temperature at the Federal 
Building was 100 degrees. The first triple digit reading in 
Seattle recorded weather history. Since the records have been 
taken at Sea-Tac there have been two 100 degree plus days, July 
20th, 1994 100 degrees and 103 degrees on July 29th, 2009. Felton 


PZ...Small Craft Advisory until noon PDT today 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.