Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 221047
Area Forecast Discussion
National Weather Service Seattle WA
247 AM PST Wed Feb 22 2017
.SYNOPSIS...A series of upper lows will move south across the
offshore waters today through Friday, producing cool weather for
Western Washington with scattered rain and snow showers. An upper
ridge will bring cool sunny weather Saturday, with a few showers
in the mountains. Another upper trough will arrive Sunday, and the
chance of showers will continue early next week.
.SHORT TERM...An upper trough extends south from west central
British Columbia across the offshore waters of the Pacific
Northwest along 130W. The air mass over Western Washington
remains cool, moist, and weakly unstable early this morning with
scattered showers -- locally falling as graupel (small ice
pellets) or wet snow. A Puget Sound convergence zone formed
Tuesday evening and continued at 2 am, extending eastward from the
north Puget Sound across Snohomish County. Temperatures were in
the 30s to lower 40s.
The upper trough will move slowly east over the offshore waters
today, while a small shortwave digging south from Haida Gwaii
carves out an upper low along the trough axis over our offshore
waters. Moisture, weak instability, and even the limited daytime
warming will cause scattered showers to increase this afternoon.
In addition the convergence zone could continue for much of the
day. The temperature at 850 mb (roughly 4500 ft) will be -6C to
-7C, and the snow level will be around 1000 ft. So some showers
could fall as wet snow over the higher terrain of the Western
Washington lowlands or contain graupel. There`s also a slight
chance of thunderstorms over the coastal waters and at the coast.
Precipitation amounts will be fairly light -- mostly 0.10 inch of
liquid equivalent or less, and accumulation should be zero or
near zero. Highs will be in the lower to mid 40s, pretty much
matching today`s convective temperature.
The upper trough will move inland tonight and cross the Cascades
late tonight, while the shortwave digs southeast into Oregon.
Showers should pretty much end in the evening as the air mass
stabilizes. Overnight lows will be in the lower to mid 30s.
North to northwest flow aloft will follow the trough Thursday and
Thursday night. Moisture should be more limited, but the air mass
will be similarly unstable and slightly cooler. Light scattered
rain or snow showers should develop with daytime heating, in most
cases amounting to less than 0.10 inch liquid equivalent. The
snow level will be around 400 ft Thursday morning, rise to 1000 ft
or so during the afternoon, and fall again overnight. Snow
accumulation should be zero or near zero.
A more vigorous shortwave will dig south across central or north
Vancouver Island late Thursday night, carving out another upper
low over our offshore waters on Friday. Most of the shower
activity will be near the coast where the snow level will be
around 1000 ft. Over the interior lowlands, light northeast
surface winds will keep the snow level generally in the 300 to 700
ft range, but precipitation amounts will again be mainly less than
0.10 inch. So in the end it should be another cool day, with a few
rain and snow showers, and zero or near zero accumulation.
For the mountains -- there will be light scattered showers today
through Friday night, and the snow level will be at the surface
the entire time. Precipitation amounts will be light; three-day
snow totals in the Cascades should be in the 2 to 5 inch range,
with around 3 to 8 inches in the Olympics. McDonnal
.LONG TERM...An upper ridge will progress quickly across the
Pacific Northwest on Saturday. It should be a cool day with
sunshine for the lowlands, but a chance of light showers over the
mountains. Another upper trough will move across the area from the
northwest on Sunday. The GFS keeps the lowlands basically dry,
while the ECMWF has more widespread precipitation. I added at
least a slight chance of rain or snow showers into the Sunday
forecast. Model solutions separate a bit more after Sunday, but a
chance of precipitation should continue Monday and Tuesday with
the air mass probably beginning to warm slightly. McDonnal
.AVIATION...An upper trough will settle into the Pac NW today.
Moderate west flow aloft will weaken this afternoon. Cooling aloft
will allow for the air mass over western Washington to become more
unstable today. This combined with some large-scale lift resulting
from the incoming trough as well as the air mass being fairly
moist will support scattered convective showers into this evening.
A mix of VFR and MVFR cigs in place over much of the area with MVFR
cigs focused mainly over the central Puget Sound area...affecting
terminals KSEA and KBFI some but mostly KPAE where a Puget Sound
Convergence Zone is still generating showers this early morning. As
stated above...atmospheric conditions will allow for scattered
showers through this afternoon...potentially allowing for brief
reductions of visibility and cigs dipping down into MVFR before
lifting once more.
KSEA...Cigs will walk the line between MVFR and VFR this morning as
PSCZ to the north continues. As chances for showers increase this
afternoon...cigs likely to remain MVFR before starting to lift as
activity starts to taper off this evening. Winds generally from the
south at 4 to 8 kts. SMR
.MARINE...Remaining SCAs for central and eastern strait look good to
expire with issuance of morning forecast package as obs in that
area have fallen below thresholds. This weaker onshore flow will
continue today as a weak surface low moves south through the
Washington coastal waters tonight and Thu as it dissipates. Although
generally weak...models are hinting at the prospect of borderline
SCA winds tonight and Thu morning for the outer coastal waters. Have
opted to lean forward on this and will be putting out SCA with the
morning package. Moderate offshore flow is expected on Fri in
response to a strengthening low that will move southward along 129W.
Offshore flow will weaken on Fri night and Sat morning. Weak flow
will prevail later Saturday into Sunday. Haner/SMR
.HYDROLOGY...No flooding is expected during the next week. USGS
landslide guidance remains above the threshold at which landslides
typically occur. The guidance depends on rainfall amounts during
the past 3 days and the past 15 days; the three- day amounts are
fairly low, but the total for the last 15 days is still high.
PZ...Small Craft Advisory until 3 AM PST early this morning for
Central U.S. Waters Strait Of Juan De Fuca-East Entrance
U.S. Waters Strait Of Juan De Fuca.
Small Craft Advisory from 10 PM this evening to 10 AM PST
Thursday for Coastal Waters From Cape Flattery To James
Island 10 To 60 Nm-Coastal Waters From James Island To
Point Grenville 10 To 60 Nm-Coastal Waters From Point
Grenville To Cape Shoalwater 10 To 60 Nm.
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