Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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589
FXUS66 KSEW 200345
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
845 PM PDT Mon Jun 19 2017

.SYNOPSIS...A cold front will reach the area Tuesday morning.
A Puget Sound Convergence Zone could keep some showers going
in the afternoon between Seattle and Everett. An upper ridge
will build offshore and then it will shift closer to the area
for warmer weather by the weekend.

&&

.SHORT TERM...Clouds will continue to fill in over Western
Washington tonight and a cold front will reach the area Tuesday.
Strong onshore flow will develop in the wake of the cold front and
scattered showers might linger into the afternoon in the PSCZ.
Northwest flow aloft and low level onshore flow will persist on
Wednesday--but surface onshore pressure gradients will be weaker.
Morning stratus will give way to mostly sunny skies in the afternoon.
Strong surface high pressure just offshore Thursday and thermal low
pressure over Northern California will turn the flow more northerly,
so there should be less low cloud cover Thursday morning. Temps
will be near average through midweek and then Thursday should
start the warming trend into the end of the week.

.LONG TERM...Here is the discussion from the afternoon AFD:
The GFS solution was more aggressive in building the offshore
ridge into the area on Friday than the ECMWF and Canadian. The
ECMWF and Canadian solutions indicated a vigorous, fast-moving
upper level low dropping down the back side of the offshore ridge
and across WA during the day Friday. Not sure which model to believe
since none of them have been consistent lately during the extended
period. Decided to compromise between the warmer GFS solution and
cooler ECMWF/Canadian solutions for Friday.

At least the medium range solutions agree (for the time being!)
that there will be an upper level ridge over the region on
Saturday. This combined with low level offshore flow will
translate to a rather warm day across the CWA, with a few places
in the southwest interior possibly reaching 90 degrees.

Unsurprisingly, beyond Saturday, the models diverge. The ECMWF and
Canadian solutions hang onto the ridge longer than the GFS;
therefore, the GFS was quicker in cooling temps down. For now,
chose to compromise by indicating a gradual cool-down. At any
rate, temps are forecast to be above normal Days 4-7.

&&

.AVIATION...A weakening cold front will approach the
region tonight from the offshore waters and move through Western
Washington Tuesday morning. The air mass will remain stable and dry
at low levels this evening with VFR conditions.  Low stratus will
move into the area late tonight into early Tuesday morning with
isolated showers. Expect IFR to low MVFR conditions to settle into
the coast by late evening, followed by the strait and interior by
around 11Z. IFR and MVFR conditions will improve to VFR levels
behind the front around midday Tuesday. The development of a Puget
Sound Convergence Zone behind the front may help lower conditions
persist in portions of the central Puget Sound area into early
Tuesday afternoon.

KSEA...VFR conditions this evening will gradually lower early
Tuesday morning as stratus at 800-1100 AGL develops over the
interior around 10/11Z until around 19Z Tue. There is some chance
that lower ceilings and showers will persist into the early
afternoon hours behind the front with the development of a
convergence zone. Northerly winds 4-7 knots will becoming variable
this evening, switch to south 7-10 knots after 06Z.

&&

.MARINE...Offshore high pressure is driving onshore flow this
evening which will receive a boost with developing ridging over the
coastal waters ahead of an offshore cold front. A small craft
advisory remains in effect for the central and eastern portions of
the Strait of Juan de Fuca for tonight.

A weak front will move through the coastal waters Tuesday morning
with increasing onshore flow behind the front Tuesday afternoon and
evening. A gale warning is in effect for the eastern two thirds of
the Strait of Juan de Fuca for Tuesday afternoon and evening. Small
craft advisory westerly winds are expected over most of the
remainder of the waters late Tuesday into Tuesday night.

Moderate onshore flow will continue Wednesday, weakening on Thursday
then becoming northerly on Friday with high pressure over southern
British Columbia. Over the coastal and offshore waters, persistent
low level northwesterlies will result in the potential development
of a steep short period swell.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory from 6 PM this evening to 2 PM PDT Tuesday
     for Central U.S. Waters Strait Of Juan De Fuca-East
     Entrance U.S. Waters Strait Of Juan De Fuca.

     Gale Warning from 2 PM Tuesday to 3 AM PDT Wednesday for Central
     U.S. Waters Strait Of Juan De Fuca-East Entrance U.S.
     Waters Strait Of Juan De Fuca.

&&

$$

www.weather.gov/seattle

You can see an illustrated version of this discussion at
www.wrh.noaa.gov/sew/gafd/latest_webafd.html



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