FXUS66 KSEW 142218

Area Forecast Discussion
National Weather Service Seattle WA
318 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


.SHORT TERM /TONIGHT THROUGH MONDAY/...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

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 /TUESDAY THROUGH FRIDAY/...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.



.AVIATION...Southwest flow aloft over Western Washington today
will become westerly on Saturday as upper trough axis shifts
onshore. The air mass will become modestly unstable late today
through Saturday as cooler air aloft associated with the trough
moves over the region. It`s primarily VFR this afternoon with
scattered showers across the region. More widespread MVFR ceilings
expected Saturday morning with shower coverage increasing as upper
tough axis moves overhead. A convergence zone is expected to form
over Snohomish/King counties Saturday afternoon/evening. There may
sufficient instability to mention thunderstorms in Puget Sound
terminal TAFs Saturday afternoon/evening, but confidence is not
sufficiently high at this time.

KSEA...Mainly VFR this afternoon/evening with scattered showers in
the vicinity of the terminal. Ceilings are expected to lower to
MVFR early Saturday A.M. with increasing shower activity. Current
thunderstorm probabilities rise to 30 to 40 percent after 21Z
Saturday. If they remain this high, this may warrant the mention of
some tempo -TSRA in the TAF. Surface winds S/SW 8 to 13 knots.  27


.MARINE...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 somewhat...with likely small craft advisory westerlies
in the central/east strait Saturday afternoon and evening.

Looking ahead to early/mid next week, a more typical early summer
pattern takes shape with broad surface ridging over the
coastal/offshore waters and 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




NWS SEW Office Area Forecast Discussion