Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 202253
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
353 PM PDT Tue Jun 20 2017

.SYNOPSIS...Strong westerly flow aloft will prevail tonight. High
pressure aloft will prevail Wednesday through Friday. Low level
onshore flow will weaken and become more northerly with time.
Temperatures will be near normal on Wednesday and above normal
Thursday and Friday.

&&

.SHORT TERM...
The shallow or low-topped Puget Sound Convergence Zone (PSCZ) was
over northern King County at this time and will continue moving
south, reaching KSEA later this afternoon. Expect only drizzle
and fog from the PSCZ since it is so shallow. It will dissipate by
late this evening.

Meanwhile, strong zonal flow aloft will prevail over the region
tonight. The surface onshore pressure gradient continued to
tighten this afternoon. The KUIL-KBLI pressure gradient was +3.9
mb at 3 PM PDT, and is forecast by the GFS model to peak near +5
mb later this afternoon. The surface pressure gradient will
gradually relax tonight but will remain strong. The NAM continued
to advertise stratus for much of the lowlands tomorrow morning
whereas the GFS continued to less cloud cover. Decided to
compromise between the two extremes.

The flow aloft will become northwest on Wednesday as the upper
level ridge of high pressure offshore undergoes some
amplification. Expect any morning stratus to burn off in the
afternoon. Max temps will be near normal.

An upper level ridge will prevail over the region Wednesday night
through Friday. The low level flow will have a northerly component
to it, so expect less cloud cover Thursday morning and even less
Friday morning. Temps Thursday and, especially, Friday will be
above normal.

.LONG TERM...
The medium range solutions were in better agreement for this
weekend. Expect an upper level ridge to dominate the Pacific
Northwest. The low level flow will become offshore as the
thermally induced trough of low pressure expands north into the
area and then shifts on or just off the coast. Temps will climb
well above normal, possibly flirting with daily max temp records
in some places. The forecast for KSEA for Sunday is 89 degrees.
The record for the date is 88, set in 2006. Some places in the
southwest interior are forecast to climb into the lower to
mid 90s on Sunday.

Beyond Sunday, the models differed on how quickly the upper ridge
axis and thermally induced trough of low pressure will shift
east. It`s possible that the interior could experience additional
warming on Monday. The current forecast indicates a cooling trend
for the entire CWA, beginning Monday. Temps are forecast to
return to near normal on Tuesday, thanks to onshore flow.

&&

.AVIATION...Moderate westerly flow aloft. Strong low level onshore
flow tonight will weaken Wednesday morning. A Puget Sound
Convergence Zone over the central Puget Sound area will dissipate
this evening. The air mass is moist and stable. The air mass will
dry midday Wednesday.

at 22Z, MVFR ceilings were seen over the coastal zones while
conditions in the interior were mainly VFR. A Puget Sound
Convergence Zone is producing LIFR to IFR conditions in low
ceilings, drizzle and fog around KPAE. MVFR to local IFR
conditions will continue around the convergence zone as it drops
southward toward KBFI and KSEA late this afternoon and early this
evening. The convergence zone is expected to dissipate after 03Z.
Stratus will reform at most locations 12Z-19Z Wednesday at the
top of a deepening marine layer. Cloud bases will average 020 MSL.
Good VFR conditions are expected to develop midday Wednesday as
the air mass dries. Albrecht

KSEA...The convergence zone from northern portions of Seattle to
KPAE will drop southward into KSEA before 00Z. At this time it
appears that VIS will stay 6SM or higher and CIGS will drop to
about 020 with the convergence zone. Southwest winds 8-12 kt at
the terminal with NW flow at KBFI will turn to all Nly by about
00Z. Lower cigs are expected to scatter out after about 03Z then
reform at around BKN015 about 12Z Wednesday. Albrecht

&&

.MARINE...Strong onshore flow will continue through tonight. A
gale warning remains is in effect for the eastern two-thirds of
the Strait of Juan de Fuca through tonight, and small craft
advisories for westerly to northwesterly winds are in effect for
the other waters (with exception to Puget Sound and Hood Canal).

Moderate onshore flow will continue Wednesday, weakening on
Thursday then becoming northerly on Friday as a ridge of high
pressure builds over British Columbia. Small craft advisory
conditions are expected to continue over the coastal waters
through much of the upcoming week as strong high pressure remains
offshore. A large fetch of persistent northwesterly winds from
the coastal waters to off of Vancouver Island will result in the
development of steep short period swells that will be difficult to
distinguish from locally generated wind waves. Small craft
advisory inflow is also expected to develop over the eastern two
thirds of the strait late Wednesday afternoon and Wednesday
evening.

A thermally induced trough of low pressure will develop northward
along the Oregon and Washington coast Saturday through Sunday.
While not yet in the forecast, there is the potential that
northerly flow in Puget Sound and Hood Canal could reach Small
Craft Advisory criteria Friday or Saturday with the thermal trough
to the south developing northward. Albrecht

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Gale Warning in effect until 3 AM Wednesday for the central
 and eastern Strait of Juan de Fuca.

 Small Craft Advisories are in effect for the remaining
 waters except the Puget Sound/Hood Canal.

&&

$$

www.weather.gov/seattle

An illustrated version of this forecast discussion can be seen at
www.wrh.noaa.gov/sew/gafd/latest_webafd.html



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