Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 212207
Area Forecast Discussion
National Weather Service Seattle WA
307 PM PDT Tue Mar 21 2017
.SYNOPSIS...A frontal system is moving northeast across the area
this afternoon. Another front will bring precipitation to the
area late tonight and Wednesday morning. It will become locally
breezy to windy across the area with the front on Wednesday.
Thursday appears that it will be dry in the interior. Then another
front will bring wet conditions to the area Thursday night into
Friday. Showers will diminish on Saturday, then another system
will move into the area next Sunday.
.SHORT TERM...Satellite imagery and area radars show a warm
front lifting northward across Western Washington this afternoon.
Showers are seen mainly along the coast and from the East Puget
Sound Lowlands into the Cascades. Isolated thunderstorms so far
have been limited to the Oregon Cascades and points eastward.
There is still some chance that portions of the south interior can
see a thunderstorm late this afternoon as convection that appears
to be increasing over northwestern Oregon lifts northeastward.
Showers will increase later this evening or during the late night
hours and continue into Wednesday morning as the main body of the
surface and upper level trough that is now over the outer portions
of the offshore waters approaches. The trough will move east
through the interior of Western Washington midday Wednesday. The
air mass will destabilize somewhat behind the trough Wednesday
afternoon and showers will become increasingly tied to the
terrain. At the same time, the flow aloft will interact with the
terrain producing a mesocyclone to the northeast of the Olympics.
Latest mesoscale models, increase the inflow through the Strait of
Juan de Fuca and the Chehalis Gap as southerly pressure gradients
increase in the central Puget Sound area. So while it will become
breezy to locally windy Wednesday afternoon, winds may not be
quite as strong as previously expected and will remain well below
Showers will taper off Wednesday night, then the next front will
approach the Washington coast Thursday evening. It appears that
the interior zones will remain dry all day Thursday as the
approaching front induces some offshore flow. Despite the clouds,
portions of the interior should warm nicely into the mid and upper
50s (well, that is warmer than it has been!)
The front will move onto the coast late Thursday night and will
linger over the area into Friday as flow aloft remains
southwesterly with the next upper trough stuck out beyond 130W.
Moderate amounts of precipitation can be expected with this
somewhat slow-moving front.
Rather high snow levels around 5000 feet this afternoon and
tonight will fall to 4500 feet on Wednesday and 3000-3500 feet
Wednesday night onward. Temperatures over the next few days will
range from lows in the 40s to highs in the 50s. Albrecht
.LONG TERM...Precipitation will become showery and decrease Friday
night into Saturday as flow aloft becomes more westerly. Saturday
will probably be the better day of the upcoming weekend. Models
now generally agree that another front will move inland on Sunday.
Fronts will then follow every 36-48 hours. Temperatures will
average a few degrees below normal for late March. Albrecht
.AVIATION...An upper level trough just off the Pacific Northwest
coast will shift inland across the region Wednesday. Moderate south-
southwest flow aloft. The air mass is weakly unstable and moist.
There will be slight risk of isolated thunderstorms south of KOLM
until around 01z. Generally VFR conditions with showers through the
period. A surface trough will move through Puget Sound around midday
Wednesday bringing a return to onshore flow. Expect increased
southwest winds with gusts to 35 knots Wednesday afternoon around
greater Puget, mainly from KSEA/KBFI north to KPAE and at HQM on
KSEA...Mostly VFR cigs with scattered showers. South-southeasterly 5-
9 kt, then southwest 12-16 kt w/ gusts up to 35 kt after 20z
.MARINE...Surface low pressure remains offshore through tonight with
southeasterly small craft winds developing over most waters except
Puget Sound/Central Strait. Winds in the western Strait may stay
easterly near 20-25 kt before easing this evening. The surface
trough will reach the coastal waters late tonight and lift across
the inland waters by late Wednesday morning. Expect a switch to
strong onshore flow Wednesday afternoon and westerlies picking up in
the Strait with gales expected in central and eastern portions.
Small craft winds most other waters with the system on Wednesday
including Puget Sound. dtm
.HYDROLOGY...The White River at R Street near Auburn will remain
near flood stage into Wednesday morning, primarily due to releases
from Mud Mountain Dam. Elsewhere, river flooding is not expected
for the next 7 days. Albrecht
PZ...Small Craft Advisory until 11 PM PDT Wednesday 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 11 AM PDT Wednesday for East Entrance
U.S. Waters Strait Of Juan De Fuca.
Gale Warning from 11 AM to 11 PM PDT Wednesday for Central U.S.
Waters Strait Of Juan De Fuca-East Entrance U.S. Waters
Strait Of Juan De Fuca.
Small Craft Advisory from 11 PM this evening to 11 PM PDT
Wednesday for Admiralty Inlet-Northern Inland Waters
Including The San Juan Islands.
Small Craft Advisory until 7 PM PDT this evening for West
Entrance U.S. Waters Strait Of Juan De Fuca.
Small Craft Advisory from 7 AM to 11 PM PDT Wednesday for Puget
Sound and Hood Canal.
You can see an illustrated version of this discussion at