Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 222153

Area Forecast Discussion
National Weather Service Seattle WA
253 PM PDT Sun Apr 22 2018

.SYNOPSIS...An offshore upper ridge will build into the Pacific
Northwest on Monday and remain there through Thursday, bringing a
stretch of sunny days to Western Washington. Low level offshore
flow will result in the warmest weather since last September,
with highs in the 70s over much of the area. An upper trough will
bring a return to more seasonal weather Friday and the weekend.


.SHORT TERM...Dry northerly flow aloft has developed over Western
Washington this afternoon ahead of the upper level ridge building
over the offshore waters of the Pacific Northwest. There is still
some fair weather cumulus over the area, but it is a mostly sunny
afternoon with temperatures in the 50s to lower 60s.

The upper ridge axis will move east to the coast tonight, and low
level flow will turn to a weak northerly offshore gradient. Fairly
light winds and clear skies will allow for good radiation
conditions, and overnight lows should be in the mid 30s to lower
40s with patchy morning fog in the usual spots.

The upper ridge will continue building as it shifts inland Monday.
It will continue slowly eastward Tuesday and Wednesday, but
continue to dominate our weather. Offshore flow will strengthen
as a thermally induced surface trough extends northward along the
Pacific Northwest coast. The 500 mb height over Western Washington
will be around 5730 meters Monday through Wednesday; the 850 mb
temperature will rise from around +7C on Monday to around +14C on
Wednesday. As we have been advertising for several days now, this
will result in sunny warm days with highs well into the 70s over
much of the area. McDonnal

.LONG TERM...The models are a little inconsistent -- and hence
there is some uncertainty -- with regard to the transition from
the warm weather to a more typical spring pattern. The models
agree that the upper ridge will shift further east late in the
week, and an offshore closed upper low will begin moving east
toward northern California; that will induce a switch to onshore
flow across Western Washington and bring a chance of showers to
the area in the high-based instability with southerly flow aloft.
The difference in the models is primarily the timing of this
transition, with the GFS showing a switch to onshore flow
beginning Thursday afternoon, and the ECMWF delaying that until
Thursday night. For now we will stick with the current forecast,
which reflects the slower timing of the ECMWF. The models all
agree that the right forecast for Friday through Sunday is onshore
flow and a chance of showers as the low moves inland, followed
most likely by another trough from the west during the weekend.


.AVIATION...An upper ridge will bring dry stable air with just some
patchy late night morning fog or low clouds...otherwise mostly
clear. The flow aloft is northwesterly.

KSEA...Mostly clear with a northerly breeze.


.MARINE...Northerly flow, mainly 5-15kt. Thermally induced low
pressure will set up along the Oregon and Washington coasts and
persist into Wednesday morning. We will probably see small
craft advisory easterly winds at the west entrance to the Strait of
Juan de Fuca by daybreak Tuesday. The thermal low pressure will
move inland later in the week with onshore flow and cooler marine
air pushing back into the area.


.CLIMATE...It is a rarity to get 4 days or more in a row of 70
degree plus weather in Seattle in April. In the 74 years of
records at Sea-Tac it has only happened 4 times (4 days in row 3
times and 5 days once). The last time it happened was the record
crushing 5 day streak from April 17-21, 2016 that included 4 days
in a row in the 80`s. The other 4 day streaks occurred on April
24-27, 1995, April 26-29, 1987 and April 27-30, 1976. Even getting
a streak of 3 days in a row of 70 degree plus weather in April in
Seattle is rare with only 6 streaks of 3 days in a row since
records started in 1945. Felton




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