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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS66 KSEW 211021
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
321 AM PDT Tue Mar 21 2023

.SYNOPSIS...
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High pressure aloft will make for mostly dry and mild
conditions through mid-week before the next seasonally strong
frontal system approaches Thursday. Widespread rain and some
thunder is expected Thursday, with the passage of this system
marking the return to cooler and wetter conditions. Showers are
likely into the weekend with low snow levels. Drier weather
possible for the weekend with cool temperatures before the next
system approaches for early next week.
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&&

.SHORT TERM /TODAY THROUGH THURSDAY/...
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The upper level pattern
this morning features a large area of closed low pressure offshore
the CA Coast, with weak high pressure noted to its north over
eastern BC and into the NW Territories. Downstream from these
features, weak troughing is noted over the northern Rockies, with
quasi-zonal flow to its south, while upstream weak ridging is
noted over the Pacific with cyclonic flow over the Bering Sea.
Near the surface, weak troughing continues along the WA/OR
Coasts, with a much more robust cyclone depicted off the central
CA Coast. Elsewhere, a newly developed cyclone was noted around
the OK panhandle, with high pressure over the N Great Plains/Great
Lakes and across much of the south.

Based on the pattern described above, W WA is becoming
increasingly influenced by the area of high pressure over BC while
much of the active weather remains to the south over CA. MRMS
radar imagery depicts some lingering shower activity north of
Puget Sound and around the Central and Eastern Strait of Juan de
Fuca. This is likely sustained by weak convergence as west winds
continue down the Strait and meet the weak southerly flow across
Puget Sound. As upper level high pressure builds south, the
pressure gradient will weaken, allowing most of the activity to
diminish between 12-15z. Upper level high pressure influence will
help scatter out cloud cover through the day, resulting in partly
sunny skies today and highs in the upper 50s. Inverted surface
troughing building northward from the CA sfc cyclone will yield
N/NE winds around 10 mph (slightly higher over water). This trough
could supply enough lift to generate a few showers later today between
Puget Sound and the Cascades as it slides eastward and daytime
heating provides a bit of instability. Coverage appears isolated
should it initiate given the weak ridging aloft.

Surface troughing will weaken overnight with high pressure
continuing in the upper levels. A dry and calm night will be in
store tonight with some patchy fog developing in the usual areas,
particularly around Southern Puget Sound and the Hood Canal/Kitsap
Peninsula. Upper level high pressure will then skirt east through
the day Wednesday giving way to a temporary split-flow pattern,
with enough mid level energy to generate a few showers by the
afternoon and evening. Will see one more mild afternoon with high
temps near 60.

Changes abound Wednesday night and Thursday as a much more
organized frontal system approaches the area from the Pacific.
Rain will begin to approach the Coast Thursday morning and as it
does so, the upper level parent trough will take on a slight
negative-tilt. This could aid in some lightning activity across
the southern coastal waters and around the central coast. Rain
will then continue to spread eastward through the day with the
potential for isolated lighting, mostly from southern Puget Sound
southward and westward in the closest proximity to the negatively
tilted upper level trough and after the front passes.

Rain will then eventually taper to showers later Thursday into
Friday with snow levels crashing down to around 1000ft. A
convergence zone is also likely to initiate, at the very end of
the short term or right into the long term.

Kovacik


.LONG TERM /FRIDAY THROUGH MONDAY/...The upper level pattern on
Friday will feature a rather strong trough over the region in the
upper levels, with the low levels featuring an unstable, post-
frontal environment. Post-frontal showers will be ongoing into
Friday along with a convergence zone given a strong pressure
gradient down the Strait. Snow levels will also be dropping to
between ~700-1000ft and given any convective enhancement to the
precip, could be looking at a rain/snow mix in the heavier
activity, especially in the convergence zone. Still a ways out to
determine the likelihood of this potential and any associated
impacts (which given the time of year are likely to be minor and
temporary if anything), but the potential for a rain/snow mix
appears possible.

Additional shortwave energy will then likely enhance shower
activity across the area the second half of Friday and Friday
night. Snow levels creep back up to 1000-1500ft thru the day with
highs unlikely to break 50. As this system dives to the south
Friday night into Saturday, it will drag snow levels down even
further, likely around or just below 500ft. As this occurs,
moisture will also be scoured and pulled south but some lingering
activity will have the potential to fall as a rain/snow mix or
light snow, especially south of Puget Sound. Low temps to start
off Saturday will be in the low-mid 30s.

Most of the weekend looks dry and chilly with highs struggling to
get to 50 with lows in the low-mid 30s. The next system looks to
approach late Sunday and into Monday, with some discrepancy as to
how much moisture will make its way into the area. In general,
active weather and below normal temps look to continue through
early next week.

Kovacik
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&&

.AVIATION...
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High pressure aloft will build into western Washington
through the day today. Southwesterly flow aloft will switch to
northerly by the mid-late morning. Latest radar shows scattered
showers still persisting across the north and central Sound.
Expect these to taper for an overall drying trend through the
morning hours. Elsewhere, some minimal clearing has allowed for
the development of fog at some area terminals overnight.
Conditions remain a mixed bag across the area terminals as a
result- primarily MVFR to IFR in showers and fog. Expect a more
widespread stratus deck to hang around through the morning hours
for continued MVFR conditions. Conditions will then gradually improve
through the morning to VFR by this afternoon. Southerly winds
generally persisting at 4-8 kt will switch to northerly by mid to
late morning, generally persisting at 3-7 kt.

KSEA...Stratus deck currently rendering MVFR conditions at the
terminal. Expect MVFR conditions to persist through the morning,
but expect a gradual improvement towards VFR towards the afternoon.
Southwesterly winds 4-8 kt will transition to northerly between
14-16Z and look to persist at 3-7 kt through the remainder of the
day. 14
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&&

.MARINE...
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Weak high pressure will remain situated across the area
waters today into Wednesday. Northerly winds will pick up tonight,
becoming breezy across the coastal waters. Expect sustained winds
to generally persist at 10-20 kt, with some gusts to 25 kt possible.
Will need to monitor the situation to see if issuance of a small
craft is necessary should wind gusts higher than 20 kt be frequent.
A disturbance will pass through the region Wednesday for a strong
push of westerly winds down the Strait of Juan de Fuca. A stronger
frontal system then looks to arrive Thursday, bringing gusty
southerly winds to the area waters ahead of it- and likely
yielding area headlines.

Seas across the coastal waters will generally hover between 3-6
ft into Thursday, before building to 15-17 ft late Thursday into
Friday. Seas then look to remain elevated above 10 ft through
Saturday, before gradually subsiding Sunday. 14
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&&

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

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

$$