Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 261606
Area Forecast Discussion
National Weather Service Seattle WA
906 AM PDT Wed Apr 26 2017
.SYNOPSIS...Low pressure aloft will keep conditions unsettled
through Thursday. A high pressure system will move over the region
on Friday for a break from the wet weather. Another low pressure
system is anticipated by Sunday for more precipitation.
There were scattered showers over the CWA at this time. Expect the
activity to increase this afternoon.
The next shortwave currently offshore will move onshore and enhance
showers later today, particularly over the southern sections of the
forecast area. At the surface, a ridge of higher pressure off the
coast will increase onshore flow as well. This pattern will help
create a Puget Sound Convergence Zone later today that has the
potential to sag south from northern Kitsap and Snohomish Counties
south well into King County this evening thanks to the strong flow
to develop in the Strait of Juan de Fuca.
Another embedded shortwave is forecast to swing onshore Thursday to
keep the showers going, but mainly in the mountains and higher
terrain. High temperatures look to remain a bit below late April
Guidance strongly suggests that Friday may offer a break in the
precipitation with some sunshine as a ridge of higher pressure
builds off the coast. Have those sunglasses handy. Buehner/05
.LONG TERM FROM THE PREV DISCUSSION...The next question is will the
drier weather hang on into Saturday. The Canadian and ECMWF concur
with the ridge holding for another dry day while the GFS is quicker
in spreading precipitation onshore during the day. At this point,
have indicated a growing threat of rain for the Olympic Peninsula
and coast during the day Saturday and increasing clouds elsewhere.
Then this weather system should provide more rain Saturday night
with decreasing showers on Sunday.
As the month of May begins, guidance again is not consistent. The
ECMWF rebuilds the upper ridge while the Canadian and GFS swing yet
another shortwave onshore for additional showers. But beyond Monday,
restless natives eager for some sunshine may get some as it appears
higher pressure aloft will build over the region. Buehner
.AVIATION...Moderate westerly flow aloft today will become
northwest tonight as an upper trough moves east. At the surface,
strong onshore flow will develop today and continue tonight. The
air mass is moist and weakly unstable with showers. Showers will
decrease later tonight. The Puget Sound convergence zone is likely
to form KPAE-KSEA this afternoon and evening. A wind shift to the
northwest will occur over north Puget Sound this afternoon and
could cause southwest winds at KBFI/KSEA to turn westerly or switch
to northwest for a few hours late in the afternoon and early
evening. Mostly MVFR ceilings, with isolated IFR, this morning
becoming predominantly VFR this afternoon except lower in showers.
KSEA...Ceilings 1k to 1.5k ft this morning will become VFR this
afternoon 3-4k ft with lower scattered decks. Southwest wind 10-15
KT this morning will become northerly 5-10 KT around 00Z. CHB
.MARINE...Strong onshore flow will increase today, with westerly
gales forecast in the strait through tonight. Winds will ease late
tonight but will generally remain moderately onshore Thursday and
Friday. Small craft advisories are in effect over all waters, with
gale warnings in the central and east strait.
High pressure over the area will give light winds Saturday. A
front will reach the area Saturday night, with strong onshore flow
returning on Sunday. CHB
PZ...Gale Warning until 3 AM PDT Thursday for Central U.S. Waters
Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of
Juan De Fuca.
Small Craft Advisories are in effect for the remaining waters.
An illustrated version of this forecast discussion can be seen at