Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 061723 CCA
Area Forecast Discussion
National Weather Service Seattle WA
920 AM PST Tue Dec 6 2016
.SYNOPSIS...Cooler and drier air will be over Western Washington
through Wednesday and there will be Fraser River outflow in the
north. Easterly gap winds will increase Wednesday night. A front
will reach the area Thursday. Cool wet weather is likely through the
.SHORT TERM...A cooler drier air mass has arrived and that will give
Western Washington mostly clear weather through Wednesday. Fraser
outflow will keep breezy cold weather over Western Whatcom county
and into the San Juans. Easterly winds will increase Wednesday night
and Thursday--especially in the mountain foothills and passes. A low
pressure system and front will bring precip to Western Washington
starting around mid afternoon on Thursday with snow turning to rain
Thursday night--although the UW wrfgfs still has snow west of Port
Angeles out through Sekiu and Neah Bay, and areas of snow in Western
Whatcom county--with a good chance of some freezing rain for that
area. That is because the model still has breezy NE winds still
coming down the Fraser and easterlies out through the Strait. But
most of the coast by that time is in mild southwest flow behind the
front. Strangely, the model turns the precip from snow to rain
before the southerly wind breaks into the Puget Sound area, but you
can`t hang your hat on these sort of details two or three days away.
.LONG TERM...An active weather pattern is likely with a tricky
balance between the northern and southern streams--there will likely
be blocking around Alaska. This should make for a low snow level and
give the lowlands periodic flirtations with snow. Fraser outflow
could continue at times through the weekend but probably will not be
especially strong. Looking at the 12z GFS...there is a chance that a
weather system around Sunday night will be problematic for Western
Washington as the modified arctic front could push back south into
Western Washington at that time. The 00z ecmwf looks like there
could be light to moderate outflow through the weekend into the
Bellingham area, but keeps southerly gradients and milder weather
for most of Western Washington with no shift south. Then around 12z
Tuesday of next week the euro takes a southern stream low pressure
center and frontal system into Oregon--and Fraser outflow resumes as
the arctic front strengthens.
.AVIATION...Dry northerly flow aloft will continue over W WA today
as the upper level shortwave trough over E WA departs and an upper
level ridge offshore approaches the region. A surface high over B.C.
will maintain cold dry northerly low level flow. The air mass is
drying out with mostly clear skies in most areas. There is still
some local stratus hanging around for spotty IFR or MVFR conditions,
mainly from Tacoma southward. The stratus is expected to be gone by
20Z, leaving good VFR conditions throughout W WA.
KSEA...Clear skies today. The FEW-SCT003 stratus should be gone by
19Z. Surface winds will be N 5-10 KT today then become NE tonight.
.MARINE...A surface ridge over B.C. will gradually weaken through
tonight...allowing the northeasterly offshore flow over W WA to
gradually diminish. The gale warning for the north inland waters is
being replaced by a SCA this morning. Other SCA for the coast and
parts of the strait are set to expire later today as winds ease.
Models still indicate marginal SCA winds over the north inland
waters tonight north of the San Juans so that SCA will continue.
The big issue is the evolution of the low currently offshore W of
the N CA coast near 40N/145W. Models agree that the low will drift
slowly NE toward the Pacific Northwest over the next few days.
Models have been in disagreement and inconsistent as well. What
seems reasonably certain is that easterly offshore flow will
increase to gale levels Wednesday evening and continue through
Thursday night as the associated front moves NE across the area.
What is much less certain is the track and strength of the surface
low which will impact winds late Thursday and Friday. A gale watch
is in effect for Wednesday night through Thursday night for the
coast and parts of the strait. Kam
.HYDROLOGY...No river flooding is expected over the next week.
PZ...Small Craft advisory coast...west entrance...central strait
and north inland waters.
Small craft advisory for rough Grays Harbor bar.
Gale Watch coast...west entrance...central strait from
Wednesday evening through late Thursday night.
You can see an illustrated version of this discussion at