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282
FXUS66 KSEW 150234
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
734 PM PDT Fri Jun 14 2024

.SYNOPSIS...A cool upper-low will influence conditions here
throughout the weekend with showers and possible thunderstorms. Weak
upper-level troughing aloft into midweek and possibly beyond as
well. Below-normal temperatures are expected to warm into next
week.

&&

.SHORT TERM /TONIGHT THROUGH TUESDAY/...
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Showers passing through
western Washington this evening, and the threat for thunderstorms
this evening remains low to none. Otherwise, the rest of the
forecast remains intact and the previous discussion can be found
below. An update to the aviation section has been provided.

A broad upper-level low currently exists off the British Columbia
coast. Downstream, upper-ridging is prevalent over the Great
Plains into the Canadian Prairies. Here locally, a weak frontal
boundary associated with the aforementioned upper-low is crossing
into western Washington with light shower activity. An isolated
chance for a strike of lightning or two exists this evening with
low-level lapse rates around 7.0-7.5 C/km. The best chance will be
over the Cascades and higher elevations. Showers are on tap to
remain overnight with low temperatures ranging between the mid to
upper 40s.

The upper-low continues eastward, centering over the PNW on
Saturday with showers and convergence zone activity. Cold air
aloft will bring snow levels down to 4,000-5,000 ft with higher
passes (Washington, Chinook, etc.) seeing flakes in the air but
road accumulation is unlikely. Along with that, hi-res guidance
indicates mid-level lapse rates of 7.0-8.0 C/km and CAPE values
around 500-600 J/kg present in the environment, increasing
confidence of thunderstorm potential for the Puget Sound region
and western Washington. Severe weather isn`t expected but
localized downpours, lightning, small hail, and gusty winds are
possible within these features. Burn scars could also be affected
as a flash flood watch for the potential for debris flows caused
excessive rainfall is posted over the Bolt Creek burn scar along
US-2 Saturday afternoon through Sunday evening. Highs are to top
out in the mid 50s to lower 60s, around 10 degrees below average
with lows in the 40s.

Troughing remains throughout the short term as cooler weather and
shower chances continue but to a lesser extent by Monday.
Northerly flow aloft as we`ll be on the backside of the trough,
helping to dry things out but lingering showers are possible,
especially over the Cascades. Highs will gradually warm but still
below average to the lower to mid 60s with overnight lows
remaining largely in the 40s.

.LONG TERM /WEDNESDAY THROUGH SATURDAY/...The long term pattern is a
slightly nebulous as models aren`t in the best agreement.
Generally weak troughing appears to be on the horizon in the first
half of the week with brief transitory ridging in the cards also
as things progress. Confidence is there however, regarding
temperatures as they`re expected to warm with highs returning to
the 70s by Tuesday-Wednesday.

McMillian
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&&

.AVIATION...
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Southwest flow aloft over Western Washington today
will become westerly on Saturday as upper trough axis shifts
onshore. The air mass will continue to destabilize tonight through
Saturday as cooler air aloft associated with the trough moves in the
region. VFR CIGs continue with showers moving across Puget Sound
(which may bring CIGs down briefly to MVFR). More widespread MVFR
ceilings are expected Saturday morning with shower coverage
increasing. A convergence zone is still expected to form over
Snohomish/King counties Saturday afternoon/evening. This will
introduce a slight chance of thunder in the afternoon in Puget Sound
terminals. There remains a chance of thunder at remaining terminals
in the afternoon, but coverage is not expected to be as widespread
as the convergence zone. The strongest storms will be capable of
producing small hail, gusty winds (with some erratic shifts
possible), lightning, and heavy rain. Winds outside of the
thunderstorms will transition from southwest 6 to 12 kt, to westerly
at 8 to 12 kt Saturday.

KSEA...VFR CIGs with vicinity showers (a brief MVFR or visibility
reduction is possible if a shower tracks over the terminal). The
ceilings will lower to MVFR Saturday morning with additional showers
overhead. Confidence remains the same for thunderstorms in the
afternoon (30-40 percent chance via a convergence zone after 21Z
Saturday). Lightning, gusty winds (with erratic shifts), and small
hail are all possible. Winds will remain southwest 8 to 12 kt this
evening, decreasing to 4 to 8 kt overnight, then becoming westerly
Saturday afternoon.

27/HPR
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&&

.MARINE...
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Broad surface troughing will develop across the
waters through midday Saturday. Surface ridging will gradually
rebuild over the coastal waters late Saturday into Sunday with
lower pressure remaining over the interior. This will increase
onshore flow with gusty winds exceeding 20-25 kt Saturday afternoon
and evening.

By early/mid next week, broad surface ridging will develop over the
coastal/offshore waters, with lower pressure east of the Cascades.
This will maintain onshore flow with westerlies reaching small craft
advisory strength in the central/east strait daily in the late
afternoon and evening. Otherwise, fairly quiet conditions are
expected across area waters.

27/HPR
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&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...None.
&&

$$