Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 172201
Area Forecast Discussion
National Weather Service Seattle WA
300 PM PDT Fri Mar 17 2017
.SYNOPSIS...A frontal system will bring rain to the area tonight
into Saturday morning. Showers Saturday afternoon will decrease
through Saturday evening as cooler drier air arrives. Dry weather
is likely Sunday and Monday before a front reaches the area Monday
night or Tuesday. Unsettled weather will persist next week.
.SHORT TERM...Rain will fall through tonight and the snow level
will rise above pass level, and then fall back to the passes
Saturday afternoon as the precip turns to showers. Heavy rain in
the Olympics should push the Skokomish River in Mason county back
above flood stage by Saturday. After the cold front moves inland,
a Puget Sound convergence zone is likely to develop in the afternoon
then peter out in the evening. High pressure will build Sunday and
the air mass should be fairly dry through Monday for perhaps two
partly to mostly sunny days. Clouds will probably be increasing
Monday afternoon and the 12z gfs does show some precip creeping
back into Southwest Wa by late afternoon, but maybe the rain
will hold off til Monday night.
.LONG TERM...The forecast is just a broadbrush chance of showers for
now with southwest flow aloft, 500mb heights in the 543-553dm range,
and 850mb temps that don`t stray too far from 0c for a snow level
around pass level. The euro did not come in today for us, but the 12z
GFS showed that Thursday looks like the best chance for a dry day,
and although the front on Friday looks like the strongest system of
the week, with the deepest low passing through the offshore waters,
it also looks like it will splits somewhat--with the heavier rain
moving into California.
.AVIATION...Southwest flow aloft will continue through Saturday.
A low pressure system in the central Oregon offshore waters will
move northeast into the interior of Western Washington late
tonight then into southern British Columbia Saturday morning.
Onshore flow will develop behind the low. The air mass is stable
and moist and will become somewhat unstable behind the front on
Rain has moved into much of the area this afternoon and will
continue to increase in intensity through tonight. Precipitation
will turn showery behind the low by about 12Z Saturday and flow
will turn west to southwest at lower levels. With the increase in
precipitation tonight, conditions will deteriorate to lower end
MVFR with areas of IFR visibilities and ceilings around.
Conditions will improve somewhat behind the low on Saturday
morning as precipitation becomes increasingly tied to terrain and
convergence zones and the lower levels destabilize somewhat.
KSEA...Light east to northeast winds 4 to 8 kt through tonight
will turn south to southwest 15g25 knots about 12Z Saturday.
Ceilings and visibilities with rain will continue to deteriorate
through tonight as the offshore low approaches the area, then will
improve gradually early Saturday as the low passes by to the north
and northeast. There is about a 70 percent chance that convergence
zone activity will affect areas on the north side of the terminal
after 15Z Saturday. Albrecht
.MARINE...Midday polar-orbiter scatterometer data, GOES IR data,
and observations show a 1000 mb sitting near 43N 130W with a
secondary low near 38N 137W. The lead low is moving off to the
northeast and short term models bring it inland somewhere between
Grays Harbor and Cape Elizabeth between 09z and 12Z tonight. The
low will then track northeastward through the Northern Inland
Waters and southern portions of British Columbia Saturday morning.
The lead low will have a difficult time developing with the
secondary feature off to the south. This appears to be handled
well by the various short term models.
Winds are backing around to easterly late this afternoon and will
become more northeast to north over the coastal waters tonight
ahead of the low. Winds will then back to northwest or west behind
the low. Small craft advisory conditions are expected ahead of the
front over the southern 2/3 of the coastal waters tonight, then
across the coastal waters and Puget Sound and Hood Canal behind
the front. A surge of westerly winds behind the low will likely
give gale conditions to the central and eastern portions of the
Strait of Juan de Fuca starting between 12Z and 15Z Saturday, with
small craft advisory conditions in the Northern Inland Waters
south of the San Juan Islands and in Admiralty Inlet. Warnings and
advisories are up for this system.
Moderate to strong onshore flow will weaken Saturday night. High
pressure will develop over the area on Sunday and move into
southeastern British Columbia later Sunday into Monday giving
moderate offshore flow.
Another front will reach the area Monday night. This front will be
followed by additional frontal systems from the southwest Tuesday
.HYDROLOGY...A Flood Warning remains in effect for the White River
at R Street (affecting Sumner) and a Flood Watch remains in effect
for the Skokomish River in Mason County. More heavy rain tonight
and Saturday could drive the Skokomish River above flood stage again.
Additional rainfall on already saturated soils will keep the threat
of landslides elevated across much of the CWA the rest of this week.
WA...Flood Watch from late tonight through Saturday evening for
PZ...Small Craft Advisory from 11 PM this evening to 9 PM PDT
Saturday for Coastal Waters From James Island To Point
Grenville 10 To 60 Nm-Coastal Waters From James Island To
Point Grenville Out 10 Nm-Coastal Waters From Point
Grenville To Cape Shoalwater 10 To 60 Nm-Coastal Waters
From Point Grenville To Cape Shoalwater Out 10 Nm.
Small Craft Advisory for rough bar from 6 AM to 6 PM PDT
Saturday for Grays Harbor Bar.
Small Craft Advisory from 6 AM to 9 PM PDT Saturday for Coastal
Waters From Cape Flattery To James Island 10 To 60 Nm-
Coastal Waters From Cape Flattery To James Island Out 10 Nm.
Gale Warning from 6 AM to 6 PM PDT Saturday for Central U.S.
Waters Strait Of Juan De Fuca-East Entrance U.S. Waters
Strait Of Juan De Fuca.
Small Craft Advisory from 6 AM to 6 PM PDT Saturday for
Admiralty Inlet-Northern Inland Waters Including The San
Small Craft Advisory from 6 AM to 3 PM PDT Saturday for Puget
Sound and Hood Canal.
You can see an illustrated version of this discussion at