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FXUS66 KSEW 060329
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
829 PM PDT Fri Jun 5 2020

.UPDATE...Only changes to the ongoing forecast were to nudge POPs
down a bit overnight, aside from both northern sections of the
area and along the Cascades and add thunder to the lowlands
around Puget Sound. The remainder of the forecast is on track.

Water vapor loop this evening depicts a picturesque closed low centered
around 140W and 50N, with broadscale influence stretching from
the Gulf of Alaska SE to the WA/OR/N California Coasts. Shorter
wave perturbations exist in abundance within this broader scale
rotation, however, one of note is moving into western Oregon.
Forcing for ascent supplied by the shortwave in conjunction with
peak daytime heating allowed for the development of showers and a
few thunderstorms earlier this afternoon, which are continuing
into this evening. For western Washington, lightning activity has
been largely confined to the eastern Olympic Peninsula, where
torrential rainfall within the strongest cells produced up to 3/4
inch of rain in isolated spots within a span of 2 hours. This
activity has weakened over the last hour, tho could still pose
some travel headaches. Within the writing of this update, also
began to notice lightning in the cell between Tacoma and Renton,
now moving eastward and expanding in coverage from S Lake
Washington to Orting. In general, expect isolated cells to
produce lightning (and locally heavy rain and breezy winds) through
sunset, with both intensity and coverage beginning to decrease
into the overnight hours as loss of daytime heating hinders
instability. Perhaps the exception will be continued coverage of
rain showers within an area of convergence in the eastern
Strait/Northern Sound vicinity.

After a lull in activity outside of a convergence zone Saturday
morning, shower coverage will increase quickly from the SW as the
closed low digs south and another shortwave ejects towards the OR
Coast. A better chance for thunderstorms is expected Saturday
afternoon and evening, with cooler mid level temps providing for
better instability.

Previous discussion can be referenced below with an updated marine
and aviation section.

Kovacik


&&

.PREV DISCUSSION... /issued 322 PM PDT Fri Jun 5 2020/

.SYNOPSIS...An upper level trough will bring unsettled
conditions to the region this weekend. After a break brief on
Monday, another system will arrive on Tuesday for a return to
cool and unsettled weather for much of next week.

&&

.SHORT TERM /TONIGHT THROUGH MONDAY/...High and mid level clouds
continue to stream across the region this afternoon as an upper
level trough digs southward over the offshore waters. Little in
the way of precip is expected through this evening apart from a
few showers near the coast and over the mountains. Increasing low
level onshore flow this afternoon and evening is expected to
generate a convergence zone over Skagit County late tonight with
some shower activity spilling over into neighboring counties.

The upper trough will be over the coastal waters by Saturday
afternoon with temperatures aloft dipping to around -26 to -28 C
at 500 millibars. Combined with some daytime heating from the
strong June sun, this should produce a broad area of modest
instability over Western Washington for widespread showers and
perhaps an isolated thunderstorm. Unlike last Saturday, any
thunderstorms that form will be more of the weaker garden variety
with perhaps some small hail and a brief rumble of thunder. High
temperatures will be on the cooler side...probably a degree or two
either side of 60. Unsettled, cool conditions continue into
Sunday as the upper trough moves onshore over northwest Oregon.

Shower activity should diminish by Sunday night as the upper
trough moves well east of the Cascades. Some drying is possible on
Monday as weak ridging noses into the region, but still expecting
considerable cloud cover to hang around with a few lingering
showers.

.LONG TERM /TUESDAY THROUGH FRIDAY/...Another system will reach
the area on Tuesday. It is expected to weaken as it moves onshore,
but will likely provide Western Washington with another damp day.
Deterministic model runs fall out of agreement with the details
by the middle of next week. Though temperatures will get a step
closer to seasonal norms, model ensemble anomalies keep a longwave
upper trough along the west coast into the end of next week for a
probable continuation of unsettled conditions. 27

&&

.AVIATION...VFR ceilings continue at the TAF sites this evening
and will prevail through the overnight hours. Convective shower
activity developed earlier this evening and has been threatening a
few of the TAF sites around the Sound. Have added a tempo group to
cover this activity. Some lightning strikes occurred in the
vicinity of KSEA and KBFI but are moving east of the airfields.
Coverage should decrease overnight, tho a convergence zone could
bring rain to KBLI and perhaps KPAE overnight. Rain will then
increase again Saturday morning and afternoon with a better chance
for thunderstorms. Ceilings may also flip flop between MVFR and
VFR for a portion of the day. Northerly winds should shift to the
S/SW overnight if not observed to have done so yet, with more of a
westerly component at KCLM and KHQM. Speeds generally 5-10kts.

KSEA...VFR cigs overnight, flip flopping between MVFR and VFR
Saturday. Lightning in the vicinity this evening will move east
and diminish after sunset. Light northerly winds will transition
more southerly at 5 to 10 knots late this evening or overnight.

Kovacik

&&

.MARINE...No changes were made to the ongoing marine forecast.
The small craft advisory for the central & eastern Strait and
adjacent Northern Inland Waters looks good for the overnight
hours, as recent observations suggest winds right around 21kts.
These conditions will subside near daybreak Saturday. Will need to
monitor potential for another round of small crafts Saturday
evening for the Strait, which will not be as strong as the push
that is expected Sunday evening into Monday. Previous discussion
provided below:

The next system will approach the area this weekend,
allowing for onshore flow to continue over the next few days.
Another push of small craft westerlies down the Strait is expected
through early Saturday. Small craft southwesterly winds could
bleed into the adjacent northern inland waters this evening as
result. A stronger push of westerlies is expected to occur along
the Strait Sunday night into Monday, with gradients and models
hinting at the possibility of gales. This will continue to be
monitored in the upcoming shifts. Otherwise, onshore flow will
continue into early next week as additional systems move through
the Pacific Northwest, likely yielding additional headlines for
area waters. Seas 5-7 feet along the coastal waters will subside
to 4-6 feet by Saturday.

Kovacik/SB

&&

.HYDROLOGY...The daily hydrology discussion has ended until the
start of the next rainy season; it will only be updated as needed
until then.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 5 AM PDT Saturday for 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.

&&

$$

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