Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 211628
Area Forecast Discussion
National Weather Service Seattle WA
930 AM PDT SAT MAY 21 2016
.SYNOPSIS...An area of rain over the Seattle metro area this
morning will probably shift south this afternoon and taper off
to scattered showers. An upper trough will keep the weather
showery and cool through the weekend. The best chance for some
sunshine will be Monday through Wednesday as a weak upper ridge
crosses the area. Another upper trough will likely drop down
over the region for Thursday through Saturday of next week.
.SHORT TERM...The area of showers that developed overnight in
Eastern Washington moved westward and it is a wet morning in the
Seattle metro area. For the afternoon, there is a good chance
that the precip will be mainly hung up over southwest WA and
the the showers will taper off elsewhere. The easterly mid level
flow overnight that steered the showers over the Cascades should
turn to the northwest tonight and then most of the precip will
be pushed back into the Cascades again--and it might very well
be wet in the Cascades into Sunday as well. Showers will taper
off Monday and Tuesday and there is a good chance for sunnier
weather with high temps warming a notch--highs may struggle to
get out of the upper 50s this weekend but should warm back into
the mid 60s Monday and Tuesday with more afternoon sun.
.LONG TERM...There could be a break in the showers Monday through
Wednesday--and Tuesday especially looks like the best chance of a
sunnier and dry day. The big upper trough over the Western U.S.
will weaken and shift to the four corners area by about next
Thursday--just as another upper trough drops in to the Pacific
Northwest. Both the 00z ECMWF and the 00z GFS have that similar
.AVIATION...A large upper level low centered over southeastern OR
will maintain easterly flow aloft over W WA through 03Z. The upper
level low center will shift over the ID panhandle tonight, which
will allow northerly flow aloft to develop over W WA. A weak upper
level shortwave trough over the Cascades that is rotating around the
main low has brought a fairly moist air mass to W WA this morning.
Low and mid level moisture from the shortwave trough has brought
areas of light rain to the W WA interior. In addition, weak onshore
flow has brought areas of IFR stratus from the central coast into
central and southern Puget Sound. The moisture from the shortwave
will persist over the area for a mix of cigs from mainly 2000 to
5000 feet. Cigs will probably be higher over the north interior. The
IFR cigs over Puget sound should mostly dissipate by 21Z. The light
rain over the area should slowly taper off after 19Z.
KSEA...IFR cigs are expected to persist through 18z then lift to
MVFR cigs through around 23Z. VFR cigs from BKN-OVC030-050 are
expected to continue after 23Z. Surface winds will remain SW 7-14
.MARINE...A weak surface low over Vancouver island will maintain
weak to moderate W-SW onshore flow through this weekend. The onshore
flow will rise to SCA levels in the central and east strait by late
this afternoon...then probably peak in the 20-30 kt range tonight.
The SCA for the strait starts at 1 pm which gives a decent cushion
before the winds increase. Mesoscale models also show SW winds
rising to SCA levels across Puget sound early this evening. Those
winds should be more spotty with the highest winds likely in the
main Puget sound channel.
The surface low over B.C. is expected to shift slightly NE early
next week, but this will not really impact the onshore flow pattern
very much. Onshore flow should continue through mid week with SCA
winds mainly in the evenings. Kam
PZ...Small craft advisory central and east strait this
afternoon and tonight.
Small craft advisory for Puget Sound tonight.
An illustrated version of this forecast discussion can be seen at