FXUS66 KSEW 042225

Area Forecast Discussion
National Weather Service Seattle WA
225 PM PST Mon Mar 4 2024

.SYNOPSIS...An upper level trough over Washington will move east
tonight into Tuesday. A surface trough will continue to produce
showers this afternoon along the coast, and into Puget Sound. Some
of the strongest showers may produce mixed precipitation,
including rain, snow, and graupel. Heavy snow over the Cascades
will diminish this afternoon as well. A dry pattern will takeover
for the middle of the week, with a mix of clouds and sun.
Temperatures however will remain below average until the end of
the week, with highs in the mid 40s, and lows in the upper 20s to
low 30s for much of the lowlands.


shows a 300/500 mb trough still over Washington, with a split-flow
pattern with two main shortwave troughs, the lower one sitting
over the Pacific Ocean off the California coast, and another one
over the B.C. Canada/Washington border. A pocket of cold air
remains aloft with the trough over the immediate region, with 500
mb temperatures still around -35 degrees celsius. Flow at 700-850
mb has become more westerly, with a small pocket of moisture out
ahead of the trough, and drier air now beginning to follow behind
with a northerly flow pattern.

A surface trough (that has been virtually parked off the
Washington coast this past weekend) will begin to move eastward
today. This boundary has been the focus point for mixed
precipitation showers over the past couple days (and this morning
with snow accumulations in Whatcom/Skagit/Island counties).
Monday will be the last day of seeing the chance for mixed
precipitation showers with this setup. CAPE values of around 200
J/kg has elevated some of the afternoon showers into pulse-thunderstorms
(with a clear cumulus field on satellite and popcorn showers on
radar in Puget Sound). More widespread shower activity has been
ongoing all morning and early this afternoon along the coast. The
heaviest showers have been located between Cape Elizabeth and
North Cove, with graupel mixing in with the rain. The warmer
surface temperatures this afternoon may make it hard for some of
the snow to make it to the ground, but will not rule out mixed
precipitation with any of the heavier showers that do develop.
Shower activity is expected to last through Monday evening.
Additionally, snow is beginning to subside over the north
Cascades. Will allow the advisory in place expire at 4 PM this

The upper level trough will depart tonight/Tuesday, and will be
replaced with weak upper level ridging, bringing in a northerly
flow from Canada into the region. With the northerly flow,
temperatures are not expected to warm up until the end of the
week. Low morning temperatures the next couple of mornings may
drop into the upper 20s and low 30s. This may cause wet roads that
are not dry to freeze overnight, creating slick spots. There may
also be some patchy fog in isolated areas with the cooler
temperatures/partial clearing that will occur in the overnight
hours. These conditions are expected to last through Thursday,
with a slight chance of snow showers in the mountains Thursday
afternoon. Winds in this period will transition to become
northerly and light at around 5 mph.

.LONG TERM /FRIDAY THROUGH MONDAY/...The upper-level ridge is
expected to be replaced with a trough coming from Alaska Friday
into the weekend. This will return the chances for precipitation
into the region during this time period. This system is expected
to bring warmer air with it, which will cause temperatures to
return closer to normal with highs in the lowlands returning into
the low to mid 50s, and low temperatures only dropping down to 40
(as early as Friday). This will produce primarily rain for areas
below There is a potential for heavier QPF accumulations with the
system moving through Sunday into Monday, but confidence is low at
this time. Gusty winds will also be possible Saturday into Sunday,
but again, low confidence in exact strength and duration.



.AVIATION...Additional showers this evening, especially along the
coast, with a cool upper level trough and trough axis. Showers
will decrease in coverage after 06z as this system starts to exit.
The flow aloft is SW to W. MVFR conditions are likely tonight
with a moist low level air mass in place. Tuesday is drier with VFR
conditions expected during the afternoon. 33

KSEA...A light, north to northeast, wind shift is forecast
between 00-04z and will persist through Tuesday. Most shower
activity remaining west and north of the terminal. MVFR ceilings
expected overnight into Tuesday morning. 33


.MARINE...Weak low pressure for generally light winds over the
western WA waters tonight. The flow will turn light offshore
Tuesday and Wednesday. A weak system will dissipate over the
Coastal Waters Wednesday night. South winds will increase over the
Coastal Waters on Friday ahead of the next incoming system. This
system will slowly track inland and weaken on Saturday. 33


.HYDROLOGY...Strong, moist, S/SW flow will bring periods of heavy
rain along the Olympics and Cascades early next week with rising
rivers. Sharp rises are possible on the Skokomish River in Mason
County with flooding possible. 33


WA...Winter Weather Advisory until 4 PM PST this afternoon for West
     Slopes North Cascades and Passes.



NWS SEW Office Area Forecast Discussion