Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 231522
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
822 AM PDT Mon Apr 23 2018

.SYNOPSIS...An upper ridge has built over Western Washington
today; the ridge will shift into Eastern Washington tonight
through Thursday as it continues to build. At the surface, high
pressure over Southeastern British Columbia combined with
developing thermal low pressure along the coast will give
increasing low level offshore flow. The ridge aloft and offshore
flow at lower levels will give highs into the 70s across much of
the area Tuesday through Thursday. An upper level low will
approach from the south late in the week bringing a return to more
seasonal weather for the weekend.

&&

.SHORT TERM...Other than a band of thin cirrus from far
southwestern Washington into the coastal waters, conditions are
clear across Western Washington this morning. It has started off
rather cold this morning with lows in outlying valleys down into
the lower 30s while urban areas fell into the lower 40s. 500 mb
heights will build to around 570 dam this afternoon, and low level
flow is turning easterly with a 1035 mb high siting just southwest
of Banff. The warming aloft and offshore flow combined with bright
late April sunshine will allow temperatures to warm rapidly today
and reach well into the 60s to around 70 for a late afternoon
high. Statistical guidance shows highs a couple of degrees higher
than our current forecast for today, but with the increase in
offshore flow, northerly breezes off the cool waters this
afternoon will likely keep temperatures from reaching their
potential today.

Normally colder protected valleys tonight will see temperatures
fall into the 30s again, but the urban areas and locations exposed
to easterly flow should see lows about 5-6 degrees warmer tonight
than this morning. Then, highs will reach well into the 70s on
Tuesday for the warmest temperatures so far this year.

06Z and 12Z model solutions show temperatures perhaps a degree
cooler on Wednesday as a weak shortwave trough rides by to the
north of the area and temporarily relaxes the offshore flow. The
ridge then amplifies and offshore flow increases on Thursday.
Temperatures on Thursday will likely reach into the upper 70s and
may exceed 80!

Current forecasts look good for now. May make adjustments to the
high temperatures for Wednesday and Thursday if the remainder of
the 12Z guidance looks consistent to what was shown in the 06Z
solutions. Albrecht

.LONG TERM...From the previous long term discussion: Extended
model solutions are once again inconsistent beginning Thursday
night into the weekend. On Thursday low level offshore flow comes
to an end with the transition to onshore flow late in the day.
This will cool off the Coast dramatically with highs in the
interior similar to Wednesday`s readings. Both the ECMWF and GFS
remain at odds with what to do with a shortwave spinning out of
the upper level low over northern California. The GFS has the
shortwave rapidly intensifying Thursday night over the area with
over a half inch of rain from about Seattle south. The ECMWF is
much weaker, slower and further south with this feature keeping it
mostly over Oregon and southwest Washington Friday night into
Saturday. The ECMWF then develops an upper level low to the north
of Washington on Sunday while the GFS has a weak open trough. Both
models transition the low level flow to onshore Thursday night
and Friday cooling high temperatures over the interior back down
into the 60s with highs back to near normal, mid 50s to lower 60s
for the weekend. With the lack consensus precipitation wise in the
models will stay with the broadbrush chance of showers forecast
for Friday through Sunday. Felton

&&

.AVIATION...An upper level ridge is over the region with dry
stable air. At the surface, there is a 1036mb high centered over
southeast British Columbia and thermally induced low pressure
extending north along the Oregon coast.

KSEA...Clear skies and a northerly breeze.

&&

.MARINE...High pressure over British Columbia and Eastern
Washington along with thermally induced low pressure along the
Oregon and Washington coasts will persist until about Thursday
morning. Northerly flow will become more easterly and small craft
advisory strength easterlies are forecast at the West Entrance
of the Strait of Juan de Fuca around Cape Flattery by daybreak
Tuesday. The thermally induced low pressure will move inland
on Thursday with onshore flow developing Thursday night and
Friday.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...None.
&&

$$

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