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fxus66 ksew 291626 

Area forecast discussion
National Weather Service Seattle Washington
926 am PDT sun Mar 29 2020 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.


Update...the line of more widespread rain associated with the
front continues to push into and east of the Cascades. Meanwhile,
widespread shower activity continues across much of western
Washington in the unstable, Post-frontal air mass. Given some
cloud breaks already this morning and fairly steep low-level lapse
rates, have added a slight chance mention of lightning to the
forecast this afternoon for most of the region. While it is a
somewhat low probability, there is enough support for a stronger
shower to produce a strike or two. Otherwise, forecast remains on
track and the previous short term discussion provides details
concerning the next significant weather system tonight through
Monday. Cullen


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 quantitative precipitation forecast 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 quantitative precipitation forecast looks lower than
Monday, so expect most of the impacts to be minimal at this point.


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 European model (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.



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. With clearing noted on
satellite across the region this morning, the air mass is
becoming somewhat unstable, so could see a thunderstorm or two
this afternoon for areas across the southern interior. Southerly
surface winds between 10-15 knots with locally more gusty
conditions developing throughout the day. The uptick in winds is
expected to continue into through the evening overnight hours as a
more robust frontal system moves into the area.

Ksea...MVFR to low VFR ceilings likely to continue through most
of the period with rounds of rain. Periods of IFR to LIFR
conditions will be possible in any heavier bands that move in the
vicinity of the terminal. A stronger frontal system will move
into the area this evening, bringing stronger winds to the
terminal. Winds remain predominantly southerly today, at 7-14
knots this morning, increasing to 20-25 knots by this evening.


Marine...a brief lull in winds is expected this afternoon before
a stronger, more organized frontal system moves through the region
tonight and into Monday. With the air mass becoming somewhat
unstable, could see a thunderstorm or two over area waters at
times this afternoon ahead of the next system. The incoming system
will yield a stronger surge of southerly winds across area waters
tonight. Thus, a gale watch remains in effect for the coastal
waters and northern inland waters/East Entrance of the Strait for
late this evening into early Monday. Winds will then subside
behind the front, but seas will remain elevated over the coastal
waters through the first portion of next week. Kristell/Borth

&& river flooding is expected through the next 7
days. While periods of heavy precipitation are expected in the
next few days, snow levels will remain generally around 1,500
feet, which should preclude major hydrologic concerns. However,
rises are expected on many rivers over the next few days.


Sew watches/warnings/advisories...
Washington...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 8 PM PDT this evening through late
Monday night for Olympics.

Pz...gale watch from 7 PM PDT 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.

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 2 PM PDT this afternoon for Admiralty
Inlet-Puget Sound and Hood Canal.

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