Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

Current Version | Previous Version | Text Only | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

944
FXUS66 KSEW 202124
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
230 PM PDT Sun May 20 2017

.SYNOPSIS...An upper level ridge will build over the region through
early next week. An upper trough will reach the Pacific Northwest
Tuesday night and Wednesday. The trough will weaken but linger to
the end of the week.

&&

.SHORT TERM...A couple of sunny warm days are on tap for the area,
but not as warm as earlier solutions when it looked like the 850mb
temp might be up around 20c by Tuesday. Now the temp aloft looks
to peak Monday night around 16c and the thermal trough makes much
less of an appearance this far north--but it is over the Willamette
on Monday. Areas of marine clouds are probably going to stay out
over the coastal waters as well, and gradients wont go offshore,
but it will turn northerly and that is pretty good. The peak warm
day will probably be Monday, the warmth could linger into Tuesday
but it is more likely that a strong marine push will be going by
afternoon, so temps should range from around 60 on the coast to
the mid and upper 70s in the warmest areas.

.LONG TERM...With marine air pushing into the area through Tuesday
night and an upper trough sliding in through Wednesday, the forecast
returns to mostly cloudy with a chance of showers. The trough will
probably weaken through the end of the week, but it it is too early
to say with any confidence which days might still have a good chance
of showers and which are likely to be dry--perhaps showers will be
limited to the Cascades and the convergence zone area.

&&

.AVIATION...An upper level ridge over the offshore waters will
slowly build eastward into the area as it amplifies Sunday through
Monday. At the surface, a ridge of high pressure at 130W will
maintain light onshore flow through Monday. Onshore flow will
increase somewhat during the late afternoon and evening hours with
sea breezes. The air mass will be mainly dry and stable, with some
low level moisture reaching the immediate Washington coast and
into the Strait of Juan de Fuca during the late night and morning
hours.

Satellite imagery shows some fair weather cumulus that has formed
with daytime heating in the interior. This will dissipate during
the late afternoon hours. Stratus can be seen over the coastal
waters. This will push back to the east onto the immediate
coastal waters and eastward through the Strait of Juan de Fuca
late tonight into Sunday morning with the diurnal push inland of
marine air. Expect good VFR conditions to dominate across the
area through Sunday, with the exception to the immediate coastline
and areas adjacent to the immediate Strait of Juan de Fuca where
conditions could drop to IFR or LIFR later tonight into early
Sunday. Albrecht

KSEA...VFR conditions through Sunday. North to northwest wind 8 to
12 knots easing to light northeasterly 05Z through 19Z. Albrecht

&&

.MARINE...High pressure along 130W combined with lower pressure to
the east of the Cascades will give northwesterly onshore flow
through Monday night. The onshore flow will be enhanced somewhat
during the late afternoon and overnight highs as typically occurs
due to daytime heating. A Small Craft Advisory is in effect for
the central and eastern portions of the Strait of Juan de Fuca
through much of tonight, and may be required again for Sunday
evening. Heating in the interior on Monday may result in small
craft advisory conditions occurring again across portions of the
Strait of Juan de Fuca, and along the immediate coast during the
evening hours.

Big changes will start to occur on Tuesday as a strong upper level
disturbance drops southeastward from the Gulf of Alaska into
Eastern Washington. Forecast models have trended faster and
stronger with onshore flow that develops on Tuesday and continues
into early Wednesday. Gale conditions appear likely across
portions of the Strait of Juan de Fuca starting Tuesday, while
Small Craft Advisory conditions are likely over the remaining
waters.

Onshore flow will continue Wednesday onward, but will relax
somewhat as temperatures in the interior cool. Albrecht

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory from 6 PM this evening to 5 AM PDT Sunday
     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



USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.