Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 201000

Area Forecast Discussion
National Weather Service Seattle WA
300 AM PDT Wed Mar 20 2019

.SYNOPSIS...A strong upper level ridge with low level offshore
flow will result in one more sunny and warm day today. Record high
temperatures are likely. Marine air will surge inland tonight into
Thursday resulting in an increase in clouds and moisture and much
cooler high temperatures. Light rain will spread northeast across
the area on Friday. There will be a chance of showers on Saturday
followed by drying to Sunday. Temperatures Friday through the
weekend will be near normal.


.SHORT TERM /TODAY THROUGH FRIDAY/...A strong ridge of high
pressure at the surface and aloft over southeastern British
Columbia is expected to move off to the southeast and slowly
weaken tonight and Thursday. Strong easterly flow continues to
produce dry conditions across the area this morning with a wide
range of temperatures. Places exposed to easterly flow (such as
Seatac Airport) are still in the mid 60s at 2AM! Places that are
sheltered to the wind have radiated out nicely and some colder
locations are in the upper 30s to mid 40s. Temperatures this
afternoon will again exceed record values. The record high at
Seatac Airport today is 69 degrees (we are only 3 degrees shy of
that early this morning) and highs today will likely top out in
the mid and upper 70s. Easterly pressure gradients will be
gradually relaxing this afternoon, so the temptation to match
temperatures that we saw on Tuesday was avoided.

As easterly flow decreases, expect a thermal low to develop on
the north-central Oregon coast and to surge into SW Washington
this afternoon. By late afternoon, expect North Bend OR to Seattle
pressure gradients to go over +4mb, the point at which marine air
surges inland. The NAM seems to do a reasonable job in capturing
the development of a stratus deck at the top of a deepening marine
layer early Thursday morning and in keeping temperatures in the
mid 50s to lower 60s Thursday afternoon. While temperature
forecasts will be on the top end of the guidance envelope for
today, they will be on the lower end of the guidance envelope for
Thursday with the influx of marine air. Despite the marine air, it
appears that it will stay dry on Thursday across the area.

A NW/SE oriented front will lift across the area Friday afternoon
and evening, bringing some light rain to the area. Light NE flow
ahead of the front will allow temperatures on Friday to top out
near the 60 degree mark before precipitation moves in. Albrecht

.LONG TERM /SATURDAY THROUGH TUESDAY/...A weak upper level trough
behind Friday night`s front will give showers to the area on
Saturday. The showers will gradually come to an end Saturday night
as high pressure aloft builds into the area and surface high
pressure builds northward into British Columbia. Sunday now looks
dry with low level NE to E flow increasing ahead of the next
negatively tilted frontal system that will be approaching from the
southwest. Temperatures aloft won`t be as warm as they have been
the past few days, so expect high temperatures on Sunday to stay
in the mid 50s to lower 60s. Rainfall associated with the next
frontal system will move into the area on Monday. Albrecht


.AVIATION...Low level offshore flow will continue today, with VFR
conditions for all terminals. Easterly winds have started to ease
over the last 2 hours, however should pick up again for terminals
along the southern portion of the Sound near 18Z. The air mass
will remain dry and stable, with southerly-southeasterly flow

KSEA...VFR conditions with easterly winds 10-15 knots. Winds
expected to pick up again near 18Z, with gusts up to 25 knots
possible at times. 14


.MARINE...Offshore flow will persist through this evening.
Breezy winds near the gaps of the coastal terrain - strongest
easterly winds will be along the Central Strait of Juan de Fuca
through this morning. Light onshore flow developing on Thursday
ahead of a weakening front spreading into the region on Friday.
Westerly swell 10 to 11 feet will begin to affect the coastal waters
Friday night rising to 12 feet at 16 seconds Saturday.


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


PZ...Small Craft Advisory until noon PDT today for Central U.S.
     Waters Strait Of Juan De Fuca-Coastal Waters From Cape
     Flattery To James Island 10 To 60 Nm-Coastal Waters From
     Cape Flattery To James Island Out 10 Nm-West Entrance U.S.
     Waters Strait Of Juan De Fuca.


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