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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 191558

Area Forecast Discussion
National Weather Service Seattle WA
900 AM PDT Tue Mar 19 2019

.SYNOPSIS...A strong upper level ridge and offshore surface flow 
will give two more sunny and warm days. Record highs are likely 
today and Wednesday. Offshore flow will weaken on Thursday resulting 
in lower temperatures. A front will spread light rain northeast 
across the area on Friday. There will be a chance of showers next 
weekend with near normal temperatures.


.SHORT TERM /TODAY THROUGH THURSDAY/...A shallow but very strong 
temperature inversion set up overnight in places exposed to easterly 
downslope flow. KSEA and many other slightly elevated spots in King 
County barely fell below 60 degrees overnight, while valley sites 
were in the 30s and 40s. KSEA will almost certainly set a record for 
highest minimum temperature today--the current record is 50. Will 
have to wait until midnight tonight to be sure.

Today will again be sunny and warm. KSEA was already 63 degrees at 8 
AM, which is the current record high temperature, so new records are 
sure to be set today. Offshore surface flow will keep things toasty 
today, and on Wednesday the thermal trough is right over Puget 
Sound, meaning it will be warm in Seattle again. The forecast high 
for both Tuesday and Wednesday is 75, both records. On Wednesday the 
coast will have a bit of onshore surface flow, and highs there will 
be around ten degrees lower than today.

As offshore flow decreases Wednesday night into Thursday, expect a 
weak southerly surge to develop over the Oregon coast and move north 
into the area. This will give some late night and morning stratus to 
the coast and portions of the interior and will result in lows 
dropping more uniformly into the 40s. Isolated showers or light 
drizzle is possible on portions of the Olympic Peninsula Thursday 
morning. Burke/Albrecht

.LONG TERM /FRIDAY THROUGH MONDAY/...Previous discussion...00Z model 
solutions continue to show the arrival of a negative tilt frontal 
system Friday afternoon that will be followed by a chance of showers 
into Saturday. The ECMWF shows a minimum of shower activity on Sunday
as low level offshore redevelops while the GFS shows some shower 
activity associated with a weak upper low persisting into Sunday. 
A model blend that is weighted toward the ECMWF was used in the 
forecast for now. Another negatively tilted frontal system will 
approach the area from the southwest Monday or Monday night. With
the increase in clouds and moisture along with decreased offshore
flow through the extended forecast period, expect temperatures to
average closer to normal for late March. Albrecht


.AVIATION...Low level offshore flow will persist through today as an 
area of surface high pressure resides east of the Cascades. The air 
mass will be dry and stable with southeasterly flow aloft. VFR 
conditions will continue. Easterly surface winds will increase by 
18z for terminals near the southern portion of the Sound. Winds 
generally easterly today from 5 to 15 knots. Gusts up to 25 knots at 
KSEA and KHQM with offshore flow.

KSEA...Easterly winds 5-15 knots this morning. Increasing offshore 
flow winds this afternoon will lead to gusts up to 25 knots 
possible. VFR conditions continue. JD


.MARINE...Strong upper level ridge will continue over the area with 
persisting offshore flow. Breezy winds will continue near the gaps 
in coastal terrain. Strongest east winds will be at the West 
Entrance of the Strait of Juan de Fuca through today. Increasing 
winds in the Central Strait expected this afternoon continuing into 
early tonight. Light onshore flow expected on Thursday ahead of a 
weakening front spreading into the region on Friday.


.HYDROLOGY...River flooding is unlikely over the next seven days.


PZ...Small Craft Advisory until noon PDT today for Coastal Waters 
     From James Island To Point Grenville 10 To 60 Nm-Coastal 
     Waters From Point Grenville To Cape Shoalwater 10 To 60 Nm-
     Coastal Waters From Point Grenville To Cape Shoalwater Out 
     10 Nm.

     Small Craft Advisory until 8 PM PDT this evening for Central 
     U.S. Waters Strait Of Juan De Fuca-Coastal Waters From Cape 
     Flattery To James Island Out 10 Nm-West Entrance U.S. 
     Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 2 PM PDT this afternoon for Coastal 
     Waters From Cape Flattery To James Island 10 To 60 Nm.