289
FXUS66 KSEW 010324
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
823 PM PDT Tue Apr 30 2024

.SYNOPSIS...Shower activity will taper off overnight with
unusually cool temperatures and areas of frost by Wednesday
morning. Another weak system will slide to our south on late
Wednesday into early Thursday. Drier and slightly warmer
conditions are expected Thursday and Friday before more
precipitation arrives late Friday and Saturday.

&&

.SHORT TERM /TONIGHT THROUGH FRIDAY/...Current radar is not terribly
impressive although there are some scattered showers out there.
Cells are mainly focused on eastern Grays Harbor county as well as
along the Cascade foothills. Radar loop indicates that these cells
are uniformly moving south and weakening as they do so. Switching
gears to visible satellite, can definitely see some pretty
impressive convection which would imply more activity that what
radar is conveying. The fact that there have been no lightning
strikes in some time would certainly seem to side more with the
story that radar is telling as opposed to satellite...but will
continue to monitor. While will opt to not rehash forecast in this
discussion, it is worth noting that aside from those two areas of
showers, W WA is seeing clouds clear out of much of the CWA...which
would put things on track for the cold conditions expected tonight.
Current obs reporting dewpoints in the 30s also help to reaffirm
this assessment.

As such, no evening updates expected as inherited forecast looks
good. For forecast details, please refer to the Previous Discussion
section below.

From Previous Discussion...A low pressure system continues to
progress east near Portland this afternoon. Isolated to scattered
showers continue to wrap around this system and will gradually taper
off through this evening. A stray thunderstorm or two still cannot
be ruled out this afternoon, with the highest chances closer to
Portland. Clearing skies and a cooler airmass will result in areas
with temperatures reaching into the low to mid 30s tonight allowing
for areas of frost development. A Frost Advisory remains in effect
for tonight.

A weak system will slide mainly south of the area late Wednesday
into early Thursday bringing another window of possible shower
activity, mainly south and west of Seattle. This will be followed
by transient ridging Thursday into early Friday allowing
temperatures to warm into the 60s Thursday and Friday. The next
system will begin to spread precipitation across the area Friday
afternoon and evening.

.LONG TERM /SATURDAY THROUGH TUESDAY/...From Previous Discussion...A
broad upper level low will make its way to the coast Friday night.
Cluster analysis continues to indicate disagreement in the evolution
of this low.
Roughly half of the members take the bulk of the low and
associated precipitation to the south while the other half keep a
more progressive pattern moving the system across Washington
fairly quickly but with heavier rainfall. In either case, the
system will be associated with at least some precipitation and wet
conditions are likely late Friday through Saturday.

Beyond the early weekend system, zonal flow and at least semi-
active weather is expected to continue. This will keep
temperatures in check and a chance of precipitation in the
forecast.
-Wolcott-

&&

.AVIATION...An upper level trough will continue to shift eastward
into Wednesday, with light northerly flow aloft. Low-level flow
remains northerly this evening, but returns to onshore again late
tonight. Some scattered showers mostly near the Cascades and few
downwind of the Olympics this evening, but these should fade away by
06z. Enough surface wind overnight may preclude widespread poor
visibility, but expect patchy restrictions after 12z in the wind
sheltered locations both northwest and southwest interior regions.
Otherwise, streaming mid level clouds likely to increase after 15z
Wednesday morning.

KSEA...Ceilings near 4000 feet this evening with skies clearing
overnight. Increasing middle level clouds after 14z, but VFR
conditions likely (greater than 70% chance) to continue. North
surface wind around 7 kt this evening becoming southerly by 08z and
continuing so overnight and through the period.    Cullen

&&

.MARINE...Low level flow becoming onshore tonight with surface
high pressure building over the coastal waters. System moving
southeast through the coastal waters Wednesday night moving into
Northern Oregon early Thursday morning. Weak high pressure over
the waters Thursday night into Friday. Next system splitting as it
moves into the waters Friday night into Saturday. Another system
arriving late in the weekend.

Small craft advisory westerlies in the Central and Eastern Strait
late tonight into early Wednesday morning.

Seas near 10 feet tonight subsiding to 6 or 7 feet early Wednesday
morning. Felton

&&

.HYDROLOGY...No river flooding is expected over the next 7 days.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Frost Advisory until 10 AM PDT Wednesday for East Puget Sound
     Lowlands-Everett and Vicinity-Hood Canal Area-Lower
     Chehalis Valley Area-Southwest Interior-Tacoma Area.

PZ...Small Craft Advisory from 11 PM this evening to 5 AM PDT
     Wednesday for Central U.S. Waters Strait Of Juan De Fuca-
     East Entrance U.S. Waters Strait Of Juan De Fuca.

&&

$$