Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 280418
Area Forecast Discussion
National Weather Service Seattle WA
917 PM PDT Thu Apr 27 2017
.SYNOPSIS...An upper level ridge will slowly cross Western
Washington late tonight through Saturday morning for a dry period
over the lowlands and a little more sunshine. An approaching cold
front will spread light rain across the area Saturday afternoon and
night. Northwest flow aloft and low level onshore flow will bring
scattered post-frontal showers on Sunday. A weak warm front may
bring a little light rain on Monday. A stronger upper level ridge
may move over the region Tuesday and Wednesday.
.SHORT TERM...Northwest flow aloft over W WA along with somewhat
moist low level onshore flow continue to produce scattered showers
over W WA this evening. Still looking for a convergence zone to form
this evening, but northerly low level flow through Puget Sound is
hampering that possibility. Can`t say it won`t happen but with the
shower activity expected to diminish overnight, time is running
short. Late day visible imagery shows showers just offshore
approaching the coast are trending weaker.
Models remain on course bringing the upper level ridge offshore
slowly eastward through Friday night with the axis moving over W WA
Saturday morning. The ridge axis was offshore along 140W this
evening and IR imagery shows a lot of cloud pushing a little E of
the ridge axis. So while Friday will be dry, there will probably be
high clouds over the area for filtered sunshine and temperatures
slightly below normal.
The dry period will extend through Saturday morning as an offshore
cold front approaches the coast. The GFS has migrated more towards
the faster GFS and NAM and brings light rain across just about all
of W WA by 00Z/5 PM. The main rain period will be Saturday night
with the front over the Cascades by sunrise Sunday morning.
Post-frontal NW flow aloft and low level onshore flow return on
Sunday for scattered showers and perhaps another convergence zone.
.LONG TERM...Previous discussion from the 319 AFD...Confidence in
this period was not very high, especially concerning the details. It
looks like the overall trend in the models was to gradually build an
upper level ridge over the Pacific Northwest. A warm front may bring
some precipitation to the CWA early in the period; otherwise, look
for warmer and drier conditions the middle of next week. Day 7 was a
question mark. It might end up being a transition day from warm, dry
conditions back to cool and unsettled weather.
.AVIATION...Moderate northwest flow aloft will weaken tonight and
become more northerly Friday. Moderate to strong low level onshore
flow will weaken later tonight and Friday. The air mass is moist and
weakly unstable with isolated showers ending. A convergence zone will
produce showers across central/south Puget Sound, then dissipate
after midnight. Areas of MVFR cigs will likely develop later tonight
as the air mass stabilizes. Cigs will lift to mostly VFR by late
Friday morning with some afternoon scattering. Northerly surface
winds this evening down to around Tacoma will shift back to
southerly later tonight.
KSEA...Northeasterly wind 6-8 kt will shift more easterly then
southeasterly after midnight. South-southwest winds 8-10 kt Friday
morning will shift to northerly again late Friday afternoon. Winds
should switch back to southwesterly by late evening. Showers with
the convergence zone should mostly dissipate after midnight. Period
of MVFR possible later tonight/Friday morning, other VFR. Scattering
of clouds possible late Friday morning/afternoon.
.MARINE...Moderately strong onshore flow produce gale westerlies in
the strait. The gale warning is already in effect. There is a small
craft advisory in effect everywhere else except Puget Sound. Winds
have mostly switched to northerly through Puget Sound but will likely
turn around to the south later tonight and remain below 20 kt. The
Puget Sound convergence zone over the central/south Sound will likely
dissipate later tonight.
Another bout of westerly small craft winds are possible in the
central/eastern Strait Friday evening. otherwise, high pressure over
the area will give light winds Friday into Saturday morning. A front
will bring increasing winds to the coastal waters Saturday afternoon
and to to the inland waters Saturday night. Strong onshore flow
Sunday will ease Monday. CHB
PZ...Small craft advisory all waters except Puget Sound.
Gale warning central and eastern Strait through midnight.
An illustrated version of this forecast discussion can be seen at