High Wind in Washington, February 28, 2018

Area Forecast Discussion
National Weather Service Seattle WA
228 PM PST Wed Feb 28 2018

.SYNOPSIS...
A large upper level low with a center over the northeast Pacific
will steadily drop south and slide east over the next couple of
days. This will provide cloudy, and showery weather across Western
Washington with temperatures remaining below normal for late
February and early March. There will be some snowflakes mixing in
for the lowlands early Friday morning but temperatures are
marginal for accumulation. The upper level low pressure will
meander inland through the weekend and keep temperatures below
normal, with some lower chances of showers continuing. Ridging
looks to build in by early next week and should allow for
moderating temperatures and drier conditions.

&&

.SHORT TERM...
Rain continues to overspread the area this afternoon ahead of the
approaching frontal system and upper low over the northeast
pacific. Winds have increased on the along the northern areas with
gusts as much as 35-40 MPH. But elsewhere winds remain much
lighter. Snow levels have risen to 2500-3000 ft this afternoon but
snow continues to fall above that elevation and the winter weather
advisory in the Cascades will be continued for several more inches
of snow. Through the day on Thursday, the upper level low will
drop southeast towards the area and mid level heights will drop as
colder air aloft moves in, causing snow levels to fall below 2000
feet again. Rain will tend to become more showery by Thursday
morning and scattered showers will continue throughout the day
with instability increasing due to the upper low and cold air
aloft. Have included an isolated thunder mention over the extreme
southwest and adjacent Pacific waters which is consistent with the
SPC Day 2 general thunder outlook in that area.

Overnight Thursday the upper low will push closer to the region.
The surface feature will push towards the coast well south of the
area. A very marginal lowland snow situation looks to set up for
the late overnight and early morning hours, especially in the 4-8
AM time frame. There will be some moisture present along with
marginally cold temperatures.

At present the forecast reflects a snow level down to 500 ft or
less for much of the area, and a mention of a rain snow mixture
for the lowlands, but no accumulation. The marginal temperatures,
along with marginal dew point values which will not support strong
wet bulb cooling are not favorable for accumulation, but are cool
enough that there should be some flakes in the air. Additionally
the amounts of precip are light and so intense rates are not
expected, a factor which will also work against accumulation.
Finally, ground temperatures will be warm. After highs today and
Thursday in the mid-upper 40s along with lows in the mid-upper 30s
that night, the ground will be above freezing. All this leads to
the current thinking that while snow will fall and mix in with the
rain, travel impacts should be little to none. Any accumulations
would be primarily on grassy surfaces, and mainly above 500 feet
elevation. All of this is of course subject to change, but it is
the present thinking based on the current guidance.

Showers will continue on Friday with temperatures staying only in
the low-mid 40s, as snow levels linger in the 1000-2000 ft range
through the day. The upper low begins to weaken and will steadily
drift into southwest Oregon by late overnight Friday. This will
bring a slow drying and warming trend into the weekend although
temperatures will remain below normal for highs.

.LONG TERM...
As the upper low fills and moves east over the weekend, the area
will remain in a broad upper level trough. This will help
conditions to remain cooler than normal. However both the GFS and
ECMWF show that the chances of showers will decrease Saturday,
there is an indication of an enhancement and increase in rain
chances on Sunday as a quick moving upper level system sweeps
through the area and provides more lift to couple with the present
instability of the upper troughs upper level cold pocket. While a
few showers will linger early next week, there are indications
that the overall pattern is trending drier and warmer with ridging
trying to develop from the south. Current forecasts do reach the
50s for most area by Tuesday. This will be close to the normal
which is 52 degrees for SeaTac on Tuesday. It has not reached 50
degrees since February 8 for Seattle. However after the slight
warming trend early next week, troughing appears to return. The
8-14 outlook through mid March shows below normal temperatures
returning.

&&

.AVIATION...Strong southerly flow aloft will continue through the
night as a vigorous front moves ashore. The air mass is moist
and stable with areas of MVFR cigs and vsby and the mountains and
higher terrain is obscured. Blustery southerly or southeast surface
winds are generally confined to the typical areas that are prone to
southeast winds--the coastal waters and the northern inland waters
and around the San Juans and Whidbey Island. The frontal passage
will reach the coast early this evening and the I-5 corridor by the
middle of the night--with rain turning to showers overnight.

KSEA...Rainy, a breezy southerly wind is likely tonight, TAF is
in the ballpark. Conditions will improve Thursday behind the
front.

&&

.MARINE...A strong Pacific front will move ashore through this
evening--fropa is probably at the coast around 6pm, then inland
overnight. Gale warnings are in effect for the normally windier
areas and winds will peak late this afternoon or evening and then
ease behind the front. A low center near the south tip of Haida
Gwaii will drift south Thursday and it may spin out little troughs
that will probably bring the wind up to southeast 25kt at times--
although probably not for Puget Sound. The flow backs from SE to NE
through the end of the week and a little Fraser outflow ought to
start up by daybreak Saturday--probably high end small craft
advisory strength, but possibly gales in the north.

&&

.HYDROLOGY...River flooding is not expected in the next 7 days.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Weather Advisory until noon PST Thursday for Cascades of
     Pierce and Lewis Counties-Cascades of Whatcom and Skagit
     Counties.

PZ...Gale Warning until 7 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-West Entrance U.S. Waters Strait Of
     Juan De Fuca.

     Small Craft Advisory for rough bar from 10 PM this evening to 4
     PM PST Thursday for Grays Harbor Bar.

     Gale Warning until 10 PM PST this evening for East Entrance U.S.
     Waters Strait Of Juan De Fuca-Northern Inland Waters
     Including The San Juan Islands.

     Small Craft Advisory until 4 AM PST Thursday for Admiralty Inlet-
     Central U.S. Waters Strait Of Juan De Fuca-Puget Sound and
     Hood Canal.

&&

www.weather.gov/seattle