Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 212359
Area Forecast Discussion
National Weather Service Seattle WA
359 PM PST Tue Feb 21 2017
.SYNOPSIS...A chance of showers continues tonight. Wednesday
through Friday a number of upper lows will move south over the
outer waters with showers and cooler weather in the forecast.
Saturday through Monday look to be slightly drier but still
.SHORT TERM...Rain showers over the South Sound and southern
Cascade foothills will taper off this evening, but showers may pop
up elsewhere in the CWA through the night. Have decreased POPs for
tonight over much of the area but left a chance of showers in. In
addition, a Puget Sound Convergence Zone could set up between
Seattle and Tacoma and extend eastward into the Cascades.
Wednesday an upper level low will move south-southeast over the
coastal waters, keeping showers in the forecast. At the same
time, 850 MB temps over Western Washington will decrease to -6 to
-8 degrees C and remain that way through early next week.
During the afternoons and evenings, diurnal heating will be
strong enough to keep any precipitation in liquid form, but
nights and early mornings could see snow showers or a rain and
snow mix. With a lack of significant moisture in the atmosphere
and overnight lows hovering around freezing, accumulations are
unlikely. Any snow that accumulates locally on the higher hills
will melt away with high temperatures in the 40s each day.
Thursday morning will probably be the coldest of the next several
days and thus has the highest chance for snowfall, however
accumulations are still unlikely in most areas. A couple upper
level lows will move south through the area well west of the
Washington coast on Thursday and Friday, with the chance of
showers continuing. JSmith
.LONG TERM...The forecast looks drier for Saturday through Monday,
with a weak trough moving through the area on Sunday for a
chance of showers in the mountains. Models diverge on Tuesday with
the GFS showing a wetter solution and the ECMWF holding off for a
.AVIATION...An upper trough over the Alaska Panhandle this
afternoon will settle down the B.C. coast tonight and into the Pac
NW on Wed. Moderate west flow aloft will become light on Wed PM. As
cooling aloft occurs, the air mass over western Washington will
become more unstable on Wed, and some large-scale lift resulting
from the incoming trough, along with a moist air mass will
support scattered convective showers.
KSEA...A diffuse Puget Sound Convergence Zone (PSCZ) in the
vicinity of the terminal early this evening will lead to light
and variable surface wind, until a southerly breeze more
decisively develops late this evening. PSCZ will support a few
light showers in the vicinity of the terminal through this
evening. Prevailing cigs during the evening rush will be in the
030-040 range, but a shower could temporarily drop cigs to below
030. Overnight and on Wed morning, MVFR cigs of 020-030 will
prevail. A PSCZ will re-develop near Seattle on Wed afternoon,
bringing a few more showers and the possibility of some variable
winds at the terminal late in the afternoon. Haner
.MARINE...Onshore flow will peak this evening, with SCA-strength
westerlies through the Strait and into Admiralty Inlet. Weaker
onshore flow will continue on Wed to the south of a weak surface
low that will slide down the west coast of Vancouver Island. That
surface low will move south through the Washington coastal waters
on Wed night and Thu as it dissipates. 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.
.HYDROLOGY...No flooding is expected during the next week and no
significant precipitation is in the forecast. However, USGS
landslide guidance remains above the threshold at which landslides
PZ...Small Craft Advisory until 6 PM PST this evening for Coastal
Waters From Cape Flattery To James Island 10 To 60 Nm-
Coastal Waters From Cape Flattery To James Island Out 10 Nm-
Coastal Waters From James Island To Point Grenville 10 To
60 Nm-Coastal Waters From James Island To Point Grenville
Out 10 Nm-Coastal Waters From Point Grenville To Cape
Shoalwater 10 To 60 Nm-Coastal Waters From Point Grenville
To Cape Shoalwater Out 10 Nm.
Small Craft Advisory until 3 AM PST Wednesday for Central U.S.
Waters Strait Of Juan De Fuca-East Entrance U.S. Waters
Strait Of Juan De Fuca.
Small Craft Advisory until 9 PM PST this evening for West
Entrance U.S. Waters Strait Of Juan De Fuca.
Small Craft Advisory until midnight PST tonight for Admiralty
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