Area Forecast Discussion Issued by NWS Seattle/Tacoma, WA
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000
FXUS66 KSEW 272211
AFDSEW
Area Forecast Discussion
National Weather Service Seattle WA
311 PM PDT Mon Mar 27 2023
.SYNOPSIS...-- Changed Discussion --
Warmer and drier weather will persist through mid week
as a strong storm system tracks to the south. A trend back towards
cooler and wetter weather will then return the latter half of the
week and through the weekend as an upper level trough brings the
next chance for widespread rain and a cold front ushers in cooler
air.-- End Changed Discussion --
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.SHORT TERM /TONIGHT THROUGH THURSDAY/...-- Changed Discussion --
The large scale pattern
this afternoon features a strong area of closed low pressure
centered near 135W and 45N well offshore the WA/OR Coast.
Downstream from this feature exists a weak ridge of high pressure
over the Intermountain West with another broad closed low near
Hudson Bay. Meanwhile, upstream, a ridge of high pressure extends
from the NE Pacific into the Gulf of Alaska with troughing over
the Bering Sea. Near the surface, low pressure corresponds well to
the strong Pacific low initially mentioned, with a stronger
cyclone located to its south with which it is beginning to merge
with (and will soon significantly intensify). Downstream from
this significant feature exists high pressure over much of the
west, with a few weak boundaries over the
Plains/Midwest/Southeast.
It`s a quiet weather day across W WA today as weak high pressure
influences the low levels and the upper closed low over the
Pacific remains displaced well to the west. Abundant sunshine
along with the gradual development of offshore flow has allowed
temperatures to rebound into the 50s. For the rest of today, mid
and high level cloud cover will increase, with increasing offshore
flow/downslope winds off the Cascades as the pressure gradient
tightens between low pressure offshore and higher pressure inland.
Conditions will become windy late this evening and overnight
tonight in the Cascade gaps and East Puget Sound lowlands (where a
wind advisory will remain in place). Isolated power outages and
downed trees will be possible in these areas. Elsewhere, winds
will become breezier.
The strong upper level low and associated strong surface cyclone
will begin to drift towards the N CA Coast on Tuesday, with
easterly/downslope flow peaking across the area into the morning
hours. High clouds will begin to break up over the area, yielding
a period of sunny skies, mostly from southern Puget Sound
northward. Moisture and cloud cover will advect northward from the
low through the day, resulting in increasing cloud cover and even
some areas of light rain scraping Grays Harbor, Lewis, Thurston
and potentially Pierce Counties. High temperatures generally
around 60 in the lowlands but may top off a few degrees cooler in
the counties just mentioned where rain and more persistent cloud
cover are expected.
The upper level low and its sfc reflection will begin to weaken
and sink further south into Wednesday, with wrap around moisture
still expected to linger across Lewis and Pierce Counties,
particularly the southern Cascades. A ridge of upper level high
pressure will begin to build briefly into the area from north to
south on Wednesday, with enough subsidence from it along with high
pressure at the sfc to yield an overall sunny and warm day, with
highs reaching the low 60s. Cloud cover is likely to start to
increase the second half of the day as a weak trough approaches
from the north.
This weak trough will slide across the area on Thursday, but with
a largely continental trajectory, expect minimal moisture with it.
There still could be sufficient lift with it to help generate a
few showers along the coast and higher terrain. Cooler weather
will follow with it with high temperatures a good 5 degrees cooler
in the mid 50s.
This will then give way to much more active and cooler weather in
the long term period.
Kovacik
.LONG TERM /FRIDAY THROUGH MONDAY/...On Friday, an area of low
pressure will deepen over the Gulf of Alaska, with a shortwave
ejecting out of its southern periphery towards the Pacific
Northwest. This system will carry a surface frontal boundary with
it, bringing the next chance for organized widespread lowland rain
and heavy mountain snow to the area.
Rain looks to arrive around the Coast Friday afternoon and evening
and quickly spread inland through the overnight period into early
Saturday. The system will give way to scattered showers across
the area through the day Saturday and with cooler mid level
temperatures, could support convection. Showery and cooler weather
looks to continue through the rest of the weekend and into early
next week as upper troughing remains in place. High temps for the
lowlands trend back to the upper 40s/near 50 for highs while lows
dip into the low-mid 30s. Snow levels hover around 1000ft so not
expecting a rain/snow mix potential for the lowlands at this time,
but widespread mountain snow showers could be heavy at times.
Kovacik-- End Changed Discussion --
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.AVIATION...-- Changed Discussion --
A broad upper trough continues to shift southward over
the offshore waters with south to southeast flow aloft becoming
easterly on Tuesday. The air mass is dry and generally stable. High
and mid level clouds will increase later tonight across the southern
part of the area. Mainly VFR conditions expected into Tuesday with a
few showers possible KHQM-KOLM southward after 12Z Tuesday. Low
level offshore flow increases tonight and Tuesday with gusty E/NE
surface winds expected...especially near gaps in terrain.
KSEA...VFR conditions continuing with high and mid level clouds
increasing overnight. Surface winds northerly 7 to 12 knots shifting
E/NE overnight with gusts of 20 to 25 knots possible by daybreak
Tuesday. 27-- End Changed Discussion --
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.MARINE...-- Changed Discussion --
A rapidly deepening surface low around 300 nautical
miles west of North Bend Oregon this afternoon will continue to
deepen this evening over the Oregon offshore waters while a surface
ridge strengthens over interior British Columbia. The combination of
these will lead to strengthening low level offshore flow into
Tuesday with small craft advisory winds for the coastal waters and
portions of the inland waters. Winds could approach gale force near
and adjacent to the west entrance to the strait early Tuesday. Winds
will gradually ease Tuesday night into Wednesday as the offshore low
weakens and shifts into the Northern California offshore waters.
Quieter conditions are expected to prevail Wednesday and Thursday
before another frontal system arrives from the Gulf of Alaska on
Friday.
Coastal seas expected to remain below 10 feet for the most part.
However, the strengthening system to our south will generate an
increasing S/SW oriented swell on Tuesday. 27-- End Changed Discussion --
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.HYDROLOGY...-- Changed Discussion --
No river flooding expected the next 7 days.-- End Changed Discussion --
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.SEW WATCHES/WARNINGS/ADVISORIES...
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WA...Wind Advisory from 11 PM this evening to 5 PM PDT Tuesday for
East Puget Sound Lowlands.
PZ...Small Craft Advisory from 8 PM this evening to 8 PM PDT Tuesday
for Central U.S. Waters Strait Of Juan De Fuca-West
Entrance U.S. Waters Strait Of Juan De Fuca.
Small Craft Advisory from 11 PM this evening to 5 AM PDT
Wednesday for Northern Inland Waters Including The San Juan
Islands.
Small Craft Advisory from 8 PM this evening to 2 PM PDT Tuesday
for Coastal Waters From Cape Flattery To James Island Out
10 Nm-Coastal Waters From James Island To Point Grenville
Out 10 Nm-Coastal Waters From Point Grenville To Cape
Shoalwater Out 10 Nm.
Small Craft Advisory until 2 PM PDT Tuesday 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.-- End Changed Discussion --
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