FXUS66 KSEW 270433
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
833 PM PST Fri Feb 26 2021

.UPDATE...The forecast is overall in good shape this evening. The
synoptic pattern is characterized by broad cyclonic flow that
encompasses the western 2/3rds of the CONUS, within which a vort
lobe was noted to extend from the PNW northward into BC. At the
surface, low pressure resides east of the Cascades and into the
Rockies, with high pressure offshore over the N Pacific, nosing
into W WA.

This evening, local radar imagery continues to pick up a few areas
of lowland rain and mountain snow, most noticeably so over the
Eastern Strait of Juan de Fuca southeastward through Snohomish
County and into the Central Cascades (including Stevens Pass) and
across the Southern Cascades. The area of precipitation that
extends from the Strait and over into the Central Cascades has
taken on some convective type features over the last hour. This
has been confirmed via social media with some reports of small
hail and isolated lightning strikes. Per SPC mesoanalysis, this
area falls in line with lingering divergence aloft in the left
exit region of a departing jet streak, some lingering PVA near the
vort lobe, and along a gradient of stronger mid level lapse
rates as temperatures around H5 remain around -30C. These
conditions are supportive of isolated convection and these threats
(isolated lightning and small hail) will continue for the next
hour or so. Thereafter, through the remainder of the evening and towards
midnight, the most favorable dynamics will move east of the local
area, and the convective threats, along with most of the
precipitation in general will subside. Can expect conditions to
gradually dry out overnight, with a dry first half of Saturday
expected. A weak sfc frontal system will graze the area during the
latter half of the day, with some light showers affecting the
Coast and moving inland Saturday evening.

The Winter Storm Warning for the Central and Southern Cascades
has been extended until 1AM Saturday based on recent radar trends
and reports. Travel is likely to become difficult across the
Passes for the next several hours. The Winter Weather Advisory
for the North Cascades was valid through 6PM this evening and,
again given precipitation trends, was allowed to expire. Previous
discussion is available for reference below, with an updated
marine and aviation section.

Kovacik

&&

.PREV DISCUSSION... /issued 236 PM PST Fri Feb 26 2021/

.SYNOPSIS...A cool showery air mass will remain over Western
Washington through this evening. Onshore flow will ease
overnight. A few bands of heavy snow showers will fall this
evening in the Cascades before tapering off overnight. Saturday
will see a break in the weather. A weak warm front will brush the
area Saturday night and Sunday and a cold front will move through
Western Washington Monday. Additional weak weather systems will
keep a chance of rain in the forecast next week.

&&

.SHORT TERM /TONIGHT THROUGH MONDAY/...There is more more impulse
to go through the area this evening in the northwest flow aloft. A
few more bands of heavy snow showers will develop and move off
into the Cascades before the snow ends overnight. A few hills in
the low elevations might see some wet snowflakes this evening. Conditions
should be dry by daybreak Saturday. The high surf continues for
the ocean beaches tonight. Strong high pressure offshore will
shift toward the area, but we wont get enough protection to keep a
weak warm front and then a cold front out of the area. There is
some indication that the cold front and upper trough will split,
so the system on Monday will probably turn out to be on the weaker
side. 19

.LONG TERM /TUESDAY THROUGH FRIDAY/...Some periods of drippy
early spring weather are likely, especially Tuesday and Wednesday,
but there is still a hint of the next upper trough digging enough
to throw up some ridging for a day or two. If the Euro has the
right idea, perhaps Thursday and Friday could turn out to be nice
days. The Canadian model also shows the upper level winds backing
to southerly and heights rising a bit around Thursday. The GFS
doesn`t have much of a ridge and a front moves ashore around
Thursday night. Naturally the forecast is a simple blend of all
the guidance and isn`t much different from climo. 19

&&

.AVIATION...Northwest flow aloft continues tonight with a moist
and somewhat unstable air mass. Showers and possible thunderstorms
may affect Island, Snohomish and King counties due to a
convergence zone. Shower activity will gradually diminish across
the lowlands after 09z as an upper level disturbance exits the
region. Gusty winds will continue near the coast and Strait of
Juan de Fuca this evening before easing overnight. As low level
winds ease overnight tonight, some pockets of MVFR ceilings could
redevelop, especially around the central and southern portions of
Puget Sound.

KSEA...Showers in the vicinity through 09z tonight with variable
wind and possible MVFR cigs. Drier weather Saturday morning
although may still see lingering low clouds and MVFR conditions.
VFR by the afternoon with S wind to 10 kt.

&&

.MARINE...Strong onshore flow will continue across area waters
tonight. Gale warnings remain in effect for the coastal waters
and the Strait of Juan de Fuca. Elsewhere, small craft advisories
remain in effect. Winds are expected to ease across all area
waters late tonight and into Saturday as weak high pressure builds
over the coastal waters. A weakening front may brush the area on
Sunday, potentially bringing another round of small craft winds to
area waters. Coastal swells of 20 to 24 feet will gradually
subside late tonight and over the weekend. 27

&&

.HYDROLOGY...River flooding is not expected over the next 7 days.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Storm Warning until 10 PM PST this evening for Cascades
of Pierce and Lewis Counties-Cascades of Snohomish and King
Counties.

High Surf Advisory until 10 PM PST this evening for Central
Coast-North Coast.

PZ...Small Craft Advisory until 10 PM PST Saturday for Grays Harbor
Bar.

Small Craft Advisory from 4 AM Saturday to 3 AM PST Sunday for
Coastal Waters From Cape Flattery To James Island 10 To 60
Nm-Coastal Waters From Cape Flattery To James Island Out 10
Nm-Coastal Waters From James Island To Point Grenville 10
To 60 Nm-Coastal Waters From James Island To Point
Grenville Out 10 Nm-Coastal Waters From Point Grenville To
Cape Shoalwater 10 To 60 Nm-Coastal Waters From Point
Grenville To Cape Shoalwater Out 10 Nm.

Gale Warning until 4 AM PST Saturday 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-Coastal Waters From James Island
To Point Grenville 10 To 60 Nm-Coastal Waters From James
Island To Point Grenville Out 10 Nm-Coastal Waters From
Point Grenville To Cape Shoalwater 10 To 60 Nm-Coastal
Waters From Point Grenville To Cape Shoalwater Out 10 Nm-
East Entrance U.S. Waters Strait Of Juan De Fuca.

Small Craft Advisory until 7 AM PST Saturday for Northern Inland
Waters Including The San Juan Islands.

Small Craft Advisory until 4 PM PST Saturday for West Entrance
U.S. Waters Strait Of Juan De Fuca.

Small Craft Advisory until 4 AM PST Saturday for Admiralty Inlet-
Puget Sound and Hood Canal.

&&

$$

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