Forecast Discussion for SEW NWS Office
000
FXUS66 KSEW 291013
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
313 AM PDT Sun Mar 29 2020

.SYNOPSIS...An unsettled pattern will continue for much of the
upcoming week. A weakening front will bring morning rain and
afternoon showers to the area today, before a more organized
system quickly brings the next round of widespread lowland rain
and heavy mountain snow for much of Monday. Shower coverage will
then gradually decrease through the day Tuesday, with the chance
for a few showers each day through the remainder of the week.

&&

.SHORT TERM /TODAY THROUGH TUESDAY/...Looping the early Sunday
morning water vapor imagery depicts broad scale cyclonic flow that
extends roughly from the Aleutian Islands south/southeastward
through the northeastern Pacific and into the western US. Several
distinct shortwave troughs are embedded within this larger scale
flow, one of which is located just offshore the Pacific Northwest
Coast and another, much stronger, shortwave in the Gulf of Alaska.
At the surface, a weakening cold front was noted just offshore
the Pacific Northwest Coast, associated with the aforementioned
upper level shortwave offshore. This is the most significant lower
level feature across the region, although a glance much further
downstream reveals a very strong sfc cyclone and associated parent
upper trough across the Midwest.

For today, the aforementioned weak upper level shortwave trough
will traverse the local area, carrying with it the weakening cold
frontal boundary at the surface. As per current radar imagery
/10z/, widespread rain is located ahead and along the frontal
boundary, and this precipitation will continue to spread east
through the morning hours, leaving a marginally unstable
environment in its wake. This should yield the development of
post-frontal showers this afternoon, with the main mechanisms of
forcing being diurnal heating and somewhat enhanced mid level
lapse rates. Model guidance is suggestive of the development of
a few hundred J/kg of MUCAPE this afternoon, which could
potentially aid in the development of a lightning strike or two.
Given that this is a possibility, have opted to keep slight chance
of thunder in the forecast, though activity looks very isolated
given marginal instability. Also, given the weakening sfc frontal
boundary, will not see a noticeable drop in snow levels today
(snow levels 2000-3000ft) so mountain snow accumulations are not
expected to be impactful.

Things get a bit interesting beginning late this evening and into
the overnight hours. Aforementioned Gulf of Alaska shortwave
trough will have noticeably deepened (and can actually already
currently see tight/strong jet dynamics on its upstream side,
indicative of the potential for the trough to dig) through the day
on Sunday and will begin to approach the local area tonight as it
remains closed. A rather organized sfc frontal boundary will
accompany this system and should begin to see a noticeable
increase in radar returns/precip late this evening from the west.
This system will push rather quickly across the area overnight
tonight into Monday morning. As the front swings through the area,
winds will briefly pick up across much of the area. Given the
progressive nature of the system, have opted to omit any wind
headlines, tho there could be the potential for some borderline
advisory gusts across the usual spots, the Northwest Interior, the
Coast, and Admiralty Inlet Area. The mountains will also pick up
some snow with this round of precip, tho not sure how much the
Passes will be affected-will depend on how quickly snow levels
drop. Snow levels are expected to noticeably drop as the front
plows through Monday morning- likely to between 1500-2500ft.
Fairly widespread post-frontal shower activity is expected then
through much of the day. With mid level temps dropping back down
to below -30C, mid level lapse rates should be steeper and there
could be a better chance for thunderstorm development. Also given
this more convective regime, pockets of heavy snow will be
possible across the mountainous terrain. Given the chance for
decent snow accumulation across the mountains, have issued a
Winter Storm Watch for the mountains tonight through Tuesday
morning. Right now, amounts are borderline for warning criteria,
so will let the next shift monitor additional data today. Rain in
the lowlands will be widespread, especially overnight tonight
through early Monday, with the potential for pockets of heavier
rain within convective showers Monday afternoon/evening, bringing
potential QPF to around a half inch to near an inch.

The potent closed shortwave trough will linger through Tuesday and
keep the chance for nuisance lowland rain showers and mountain
snow showers for much of the day. Overall QPF looks lower than
Monday, so expect most of the impacts to be minimal at this point.

Kovacik


.LONG TERM /WEDNESDAY THROUGH SATURDAY/...The shortwave trough
axis finally pushes through the local area on Wednesday, however,
the broader picture reveals longer wave cyclonic flow still
encompassing the region, and another subtle shortwave is likely to
push through. Showers will be possible again on Wednesday, though
coverage looks to be less than that of the previous few days. We
try to shake this pattern by Thursday, but not quite enough to
prevent some showers from lingering or redeveloping. Friday looks
similar to Thursday. Ensembles appear to struggle with the
placement of broader scale troughing in our vicinity, as well as
the strength of the upstream ridge late this week and into next
weekend. For now will side more with the ECMWF and CMC ensembles,
with argue for placement of the trough slightly further east of
the local area. Will still keep slight chance for showers, but
overall the latter half of the week is far from a washout.

Kovacik

&&

.AVIATION...Expect mostly MVFR ceilings through the period with
local IFR to LIFR possible at times as heavier areas of rain develop
this evening and tonight across the region. The airmass is still
fairly stable with southerly surface winds between 10-15 knots with
locally more gusty conditions developing Sunday. The uptick in winds
is expected to continue into Sunday evening and nighttime hours as
a more robust frontal system moves into the area.

KSEA...High MVFR to low VFR ceilings likely continue through most of
the period with rounds of rain. Periods of IFR to LIFR conditions
possible as rain continues. A stronger frontal system will move into
the area on Sunday evening and bring stronger winds to the terminal.
Winds remain predominantly southerly today continuing into Sunday
from 7-14 knots, gusts increasing more significantly late Sunday to
20 knots. Kristell

&&

.MARINE...Another weak impulse crosses the waters this evening,
bringing another round of small craft advisory winds much of the
coastal and inland waters. A brief lull in winds is expected Sunday
afternoon before a stronger surge of southerly winds moves across
the region later into Sunday night. A gale watch has been issued for
the coastal waters and northern inland waters/east entrance of the
Strait for late Sunday into early Monday due to the strong winds
with this next weather system. Winds subside behind the front, but
seas will remain elevated over the coastal waters through the first
portion of next week. Kristell

&&

.HYDROLOGY...No river flooding is expected over the next 7 days.
An unsettled weather pattern will allow for likely rain chances
through mid week, with estimated QPF amounts around an inch for
the lowlands, and 1-3 inches across the higher terrain. Given
fairly low snow levels, do not expect river flooding impacts
across the mountains. The chance for showers will then linger
through the remainder of the week and into next weekend, with not
much in the way of additional precip accumulation expected.

Kovacik

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Storm Watch from late tonight through Tuesday morning for
     Cascades of Pierce and Lewis Counties-Cascades of Snohomish
     and King Counties-Cascades of Whatcom and Skagit Counties.

     Winter Storm Watch from this evening through late Monday night
     for Olympics.

PZ...Gale Watch from this evening through late tonight 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 7 AM PDT this morning 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 Watch from this evening through Monday morning for East
     Entrance U.S. Waters Strait Of Juan De Fuca-Northern Inland
     Waters Including The San Juan Islands.

     Small Craft Advisory until 3 AM PDT early this morning for East
     Entrance U.S. Waters Strait Of Juan De Fuca-Northern Inland
     Waters Including The San Juan Islands.

     Small Craft Advisory until 2 PM PDT this afternoon for Admiralty
     Inlet-Puget Sound and Hood Canal.

&&

$$

www.weather.gov/seattle

NWS SEW Office Area Forecast Discussion