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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS66 KSEW 221542
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
842 AM PDT Mon Apr 22 2024

.SYNOPSIS...High pressure will promote pleasant spring conditions
across western Washington today and Tuesday. A weak cold front
will pass across the region Tuesday night into Wednesday, bringing
scattered showers. Broad troughing will anchor over the Pacific
Northwest with a series of weak frontal systems passing over the
area with unsettled conditions featuring showers and mountain
snow.

&&

.SHORT TERM /TODAY THROUGH WEDNESDAY/...
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Forecast remains on track
this morning. Have allowed the frost advisory to expire with
temperatures warming. Previous discussion follows.

Upper-level ridging will build further eastward over the Pacific
Northwest today and Tuesday, with surface high pressure leading to
mild and pleasant spring weather over western Washington after.
Low clouds this morning will gradually dissipate through the
morning with sunny conditions across the region into the
afternoon. High clouds will begin filtering over the region late
this afternoon into this evening ahead of a weak frontal system.
Dry conditions today across all of western Washington. High
temperatures increasing a few degrees into the low 60s with mid to
upper 50s for water- adjacent areas.

A thermal trough will build northward along the coast as surface
high pressure shifts further to our east Tuesday, causing light
offshore flow to develop. This will lead another dry afternoon
with relative humidity values dropping into the 30 percent range
during the late morning and early afternoon. High clouds will
continue to filter overhead, though thinning into the afternoon
with plenty of sunshine still. Winds will shift back onshore late
Tuesday afternoon as the thermal trough weakens, causing
temperatures to steadily fall through the late afternoon and
evening and dew points/relative humidity values to rise. Overall,
a pleasant day across western Washington with highs in the upper
60s to near 70. Warmest temperatures will be across the Southwest
Interior and the river valleys through the Cascade Foothills
thanks to the offshore, downslope flow (around a 70% chance for
highs at or above 70 at Centralia and 20% at Seattle).

Increasing cloud cover and onshore flow featuring cooler
conditions Wednesday behind a weak cold front that will brush
across the area Tuesday night. Little in the way of precipitation
with this front aside from some light showers and snow showers in
the Olympics and North Cascades. Highs Wednesday in the upper 50s
to near 60.

.LONG TERM /THURSDAY THROUGH SUNDAY/...A progressive pattern
through the long term forecast period starting Thursday as a
stronger frontal system crosses the region with more widespread
lowland rain and mountain snow chances. Global deterministic and
ensemble members have trended a little quicker with the departure
of the first main longwave trough out of the western CONUS Friday
into Saturday with a progressive zonal flow pattern setting up
across the Pacific Northwest beneath a deep cutoff upper-level low
over the Gulf of Alaska that looks to slowly shift south and east
towards British Columbia. While disagreement exists on the exact
evolution and timing of the movement of this low, a cool and
unsettled period of weather looks likely with slightly below
normal highs in the mid to upper 50s and showers and light
mountain snow. Ensemble consensus keeps rainfall totals below
hydrologically impactful amounts and snowfall totals across the
Cascade passes below advisory thresholds at this time.

Davis
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&&

.AVIATION...
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Northwest flow aloft as an upper-ridge builds in. Low-
level onshore flow is relaxing and expected to turn northerly later
today. Mostly VFR across western Washington with a few lingering low
clouds throughout the Salish Sea with SCT to BKN MVFR CIGs at BLI,
CLM, and PAE. Low clouds should scatter out later this morning. VFR
conditions are in the forecast throughout much of the upcoming TAF
period as the aforementioned ridge dries things out. Northerly
surface winds are to increase in the afternoon between 6 to 12 kt
for most airfields with isolated gusts up to 20 kt for terminals
around Puget Sound. Surface winds should become light overnight
before picking up again into Tuesday.

KSEA...VFR throughout the upcoming TAF period. SCT low clouds at
around 2000-3000 ft but should scatter out later this morning.
Breezy ENE surface winds between 8 to 14 kt will turn more northerly
by 18-20z with gusts up to 20-25 possible kt. Winds should become
lighter 03z Tuesday onward before picking up again Tuesday
afternoon.

McMillian/LH
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&&

.MARINE...
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Surface ridge offshore will weaken today as a thermal
trough builds along the OR coast. This will result in northerly flow
that`ll become weakly offshore into tonight. No headlines are
expected but winds over the coastal waters could approach 20 kt in
the waters closer to the Oregon border. A push of westerlies down
the Strait of Juan de Fuca tomorrow evening into early Wednesday
looks to potentially reach Small Craft Advisory. The next
disturbance looks to arrive on Wednesday but again, winds are to
remain below headline thresholds. But a stronger frontal system may
pick up winds on Thursday with a better chance for additional
headlines.

Seas are expected to remain around 4 to 6 ft for much of the
upcoming week.

McMillian/LH
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&&

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

&&

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

$$