Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 211054
Area Forecast Discussion
National Weather Service Seattle WA
354 AM PDT Tue Mar 21 2017
.SYNOPSIS...A series of disturbances embedded in southwesterly
flow aloft will move through the area through Friday. Heaviest
precipitation is expected Thursday night into Friday. The best
chance for a break in the wet weather for the interior will be on
Thursday. Showers will diminish over the weekend as the upper
level flow becomes westerly.
.SHORT TERM...Satellite imagery shows a weak cold front rotating
around around an upper level low centered near 45n/134w moving
into western Oregon this morning. Rain out ahead of the front is
as far north as Pierce county at 3 am/10z. Temperatures at 3 am
were in the 40s.
Cold front moving through western Washington later this morning
into the midday hours. Rain will spread over the entire area this
morning with the precipitation type changing to showers behind the
front. The air mass will become somewhat unstable behind the front
this afternoon. The 00z model runs increased CAPE values up into
the 300-800 J/KG range this afternoon over the southwest interior.
Lifted index are still only around 0. The convective temperatures
off the mm5 are in the upper 50s, a couple of degrees above the
forecasted high temperatures. With the increasing CAPE values in
the models will add a chance of afternoon thunderstorms to the
Southwest Interior this afternoon. Model CAPE values fall off
sharply over the central Puget Sound area northward. In addition,
lifted indexes are higher and convective temperatures are even
warmer, in the lower 60s. Because of this will keep the afternoon
thunderstorm threat confined to mainly south of the Puget Sound.
Highs will be in the 50s.
Shower activity decreasing this evening before another somewhat
organized band spins out of the low offshore over western
Washington Wednesday morning into the early afternoon hours. Lows
tonight with plenty of cloud cover will be in the 40s with highs
on Wednesday in the lower to mid 50s.
Models continuing with the idea of a short break in the
precipitation beginning late Wednesday night and continuing
through much of the day on Thursday for the interior as the upper
level trough behind the next system digs south near 140w. Warm
front will brush the area Wednesday evening keeping a mention of
showers in the forecast. Confidence in the break starting to wane
as the model solutions are getting slightly faster with the next
system. Will keep in interior dry on Thursday with chance to
likely pops along the coast. If the model trends continue the dry
day on Thursday will disappear from the forecast. Lows Wednesday
night with a little bit of clearing will get down to as low as the
mid 30s. Lows near 40 will be common. Highs on Thursday will
remain in the 50s.
.LONG TERM...Extended models in good agreement with the system
late Thursday night into Friday being the wettest one of this
current series. With the upper level trough behind the front
continuing to dig the front will move slowly through the area on
Friday. Westerly flow aloft with just a chance of showers on
Saturday. Model solutions become inconsistent on Sunday with the
GFS taking the next system south into Oregon and Northern
California while the ECMWF brings the system right through western
Washington. Inconsistencies in the solutions continue on Monday
with the GFS building a temporary upper level ridge over western
Washington while the ECMWF brings yet another system into the
area. Given the lack of consistency in the model solutions will
trend the forecast toward climatology and have chance pops for
Sunday and Monday with temperatures near normal. Felton
.AVIATION...Southerly flow aloft over Western Washington early
this morning will continue today and tonight. as a weak weather
system moves rather slowly through the region. At the surface,
moderate southeast gradients will continue as a the frontal trough
approaches the area from the southwest. The air mass will be
generally moist and become weakly unstable this afternoon.
Conditions are VFR early this morning with ceilings 055-080. A
more solid band of rain will lift north into the area in the next
couple hours. Conditions should deteriorate gradually this
morning, probably becoming MVFR with ceilings 020-030 by late
morning. Some improvement in the showery and somewhat unstable
air mass behind the front is likely this afternoon and tonight.
KSEA...Northeast wind 7-12 kt early this morning, becoming south
to southeast 5-10 kt later this morning through tonight. KSEA will
probably have high-end MVFR ceilings for several hours, roughly 9
am to 1 pm. McDonnal
.MARINE...A weak front is lifting north through Western Washington
and its waters this morning, with winds turning to southeasterly
behind the front. Another frontal system will move across the
forecast area tonight and Wednesday morning, followed by onshore
flow. The result will be small craft advisory winds over most of
the zones, especially the coastal waters, through Wednesday or
Wednesday night. A more vigorous frontal system will affect the
waters around Thursday and Thursday night, with gale force winds
possible at the coast. Weak high pressure will probably build
over the region on Saturday. McDonnal
.HYDROLOGY...The White River at R Street near Auburn will remain
near flood stage into Wednesday morning. Elsewhere, river
flooding is not expected for the next 7 days. Felton
.CLIMATE...All climate stations for western Washington have
exceeded their normal monthly precipitation for March...and there
is more rain on the way for much of this week. Stations...their
normal monthly amounts for March and the precip they have received
so far for March 2017 through March 20th are as follows:
Sea-Tac -- Normal: 3.72 inches -- March 2017 so far: 5.39 inches
Olympia -- Normal: 5.29 inches -- March 2017 so far: 8.27 inches
Hoquiam -- Normal: 6.99 inches -- March 2017 so far: 11.32 inches
Quillayute -- Normal: 10.83 inches -- March 2017 so far: 12.95"
Bellingham -- Normal: 3.22 inches -- March 2017 so far: 3.90"
The combined rainfall for Sea-Tac for February and March through
the 20th is 14.24 inches. This is the 3rd wettest on record
including the Federal Building records which go back to the
1890`s. The only two years there has been more precipitation in
February and March, 2014 15.55 inches and 1972 with 14.85 inches.
There is a chance Seattle will break this record late this week.
PZ...Small Craft Advisory until 5 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 5 PM PDT this afternoon for East
Entrance U.S. Waters Strait Of Juan De Fuca-Northern Inland
Waters Including The San Juan Islands.
Small Craft Advisory until 11 AM PDT this morning for Central
U.S. Waters Strait Of Juan De Fuca-West Entrance U.S.
Waters Strait Of Juan De Fuca.
You can see an illustrated version of this discussion at