FXUS66 KSEW 241735

Area Forecast Discussion
National Weather Service Seattle WA
935 AM PST Mon Feb 24 2020

.UPDATE...Convergence zone continues to linger across portions of
the central Cascades at this hour. Obs across the area show that
this activity is light, with liquid accumulations generally 0.1"
or less over the past 3 hours across the area. Due to this, the
Winter Storm Warning for the central Cascades will be allowed to
expire at 10am. Light snow showers will likely continue into the
early afternoon hours before the convergence zone dissipates,
yielding light snowfall accumulation and little in the way of
additional impacts.



.AVIATION...Northwesterly flow aloft continues across the region
today with an increasingly stable air mass in place. Mostly VFR
conditions across the region with scattered showers this morning,
though local pockets of MVFR may remain through the morning.
Additionally, remains of a convergence zone over eastern Snohomish
County will result in locally heavier precipitation and reduced
flight conditions into the afternoon. Otherwise, expect a broken to
overcast lower VFR cloud deck to persist through much of the day.

KSEA...Expect predominantly VFR conditions through the period with
broken to overcast high clouds and occasional scattered deck around
3000 ft remaining. Surface winds remain mostly southerly through the
evening at 5 to 10 kt.



.MARINE...Weak high pressure rebuilds across the waters today,
with winds gradually easing through the morning. A weak front
approaches the coastal waters, but will fall apart as it does.
Winds are expected to remain below advisory criteria with this
system, but southerly winds increase a bit. After this, weather
impacts are not expected over the waters through much of the week
with relatively light winds. Seas remain around 15 ft this morning,
but will gradually subside below 10 ft by early Tuesday.
A larger swell arrives around mid-week, but only builds seas to
generally 9 to 11 ft over the coastal waters.



.PREV DISCUSSION... /issued 322 AM PST Mon Feb 24 2020/

SYNOPSIS...Lingering lowland rain showers and Cascade snow will
continue this morning before conditions dry out this afternoon as
high pressure builds in. Another weak front will move across the
area Tuesday into Wednesday. After a quick break Thursday into
Friday, a more organized front may approach the area late Friday,
bringing unsettled weather through much of the weekend.

morning shows the shortwave that was responsible for yesterday`s
weather has since dug into the northern and central Rockies, with
a broad ridge building into the Pacific Northwest in its wake. At
the surface, yesterday`s frontal system has moved well east of the
area, with high pressure from the Pacific building into the local
area in its wake.

Recent radar loops show some lingering shower activity over the
area early this morning, associated with an unstable post-frontal
environment. Mainly seeing some pin-prick showers west of Puget
Sound across the Olympic Peninsula and just offshore, with some
more significant precip seen east of Puget Sound from northern
King County northward, of which includes a Puget Sound
Convergence Zone. Hi-res model guidance has struggled with the
development of this convergence zone, but with recent observations
still showing gale force winds down the Strait of Juan de Fuca,
with weaker southerlies across Puget Sound, it seemed realistic to
think one would develop.

Into and through the morning hours today, will need to carefully
monitor the PSCZ. Given the strong winds down the Strait, would
think the zone would shift south before eventually dissipating.
However, until this happens it will likely cause some additional
impacts across the central Cascades. Since its development,
Stevens Pass has picked up a few more inches of snow, with light
snow also in close proximity to Snoqualmie Pass. With the
convergence zone lingering, have opted to extend the Winter Storm
Warning for the central Cascades through 10am/18z this morning.
The remaining Winter Storm headlines will be allowed to expire at
4am/12z this morning. The convergence zone will also have to
monitored across the lowlands of King and Snohomish Counties.
Snow levels will drop to below 1000 feet through the morning
hours, tho perhaps not quite as low as originally forecast.
Nevertheless, any convective-like areas within the convergence
zone may drive snow levels down even further/cool the atmospheric
column within. This may provide for a brief window of opportunity
for a rain/snow mix, especially across the higher hilltops. At
this time, no impacts are expected. And this is all assuming the
PSCZ can maintain itself into the morning hours.

Going into the afternoon hours today, ridging overhead, providing
for strong height rises, with high pressure at the surface should
provide for rapidly improving conditions. This return to dry,
benign weather will carry into Tuesday, although will likely see
some additional cloud cover move in ahead of the next frontal
system. By Tuesday, the upper level ridge will dampen in response
to a weak shortwave passing through the flow. This shortwave will
be carrying a weak surface front with it, which is expected to
bring a few showers to the area Tuesday afternoon and evening. QPF
with this system is unimpressive and some locations may not even
see much of anything. Showers are likely to linger into Wednesday
morning before dissipating, leaving dry, cloudy conditions in
place for the remainder of the day.


level ridge tries to reestablish itself, which may be enough to
shun most of the precip and deeper layer moisture to the north of
the area. There will still exist the chance that a few showers
could sneak into the far northern sections of the area.

On Friday, upper level troughing looks to become better
established across the Pacific, with an organized frontal system
noted in the low levels. The front`s arrival to the local area may
hold off until Friday evening, but it does look to mark the return
to unsettled weather. BUT, this will not be until we get to enjoy
a relatively mild late February day, with highs on Friday
approaching 60! The frontal system will likely yield periods of
wet weather through Sunday morning, with perhaps then a brief
break in the action until another front moves into the area early
next week.


HYDROLOGY...No river flooding is expected over the next 7 days.
Lingering precip this morning will dissipate through the day, with
snow levels dropping below 1000 feet. A weak front will bring a
few light showers to the area Tuesday into Wednesday, with snow
levels recovering. QPF will be light with this system, with some
locations not guaranteed to see much of anything. After a break
in the weather Thursday into Friday, a more organized system may
approach the area for the weekend. Not seeing a deep moisture feed
with this system, so while precip may be more widespread, precip
totals are not expected to be significant enough to pose a threat
for river flooding. Forecast QPF over the next 7 days per WPC
guidance suggests 1-2 inches across the higher terrain and less
than an inch for the lowlands.



WA...Winter Storm Warning until 10 AM PST this morning for Cascades
     of Snohomish and King Counties.

     Small Craft Advisory until 1 PM PST this afternoon for San Juan

PZ...Small Craft Advisory until 1 AM PST Tuesday 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.

     Small Craft Advisory until 6 PM PST this evening for Grays
     Harbor Bar-West Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 1 PM PST this afternoon for Admiralty
     Inlet-Central U.S. Waters Strait Of Juan De Fuca-East
     Entrance U.S. Waters Strait Of Juan De Fuca-Northern Inland
     Waters Including The San Juan Islands-Puget Sound and Hood




NWS SEW Office Area Forecast Discussion

Discussion Script by Ken True at Saratoga Weather - WFO Script by SE Lincoln Weather