Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

Current Version | Previous Version | Text Only | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34

000
FXUS66 KSEW 151110
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
310 AM PST Sun Jan 15 2017

.SYNOPSIS...An upper ridge will bring sunshine to Western
Washington today with highs in the upper 30s to mid 40s. The ridge
will move inland tonight, and a warm front will spread rain
inland Monday and Monday evening. Rain, heavy at times, will
continue Monday night into Wednesday. An upper trough will bring
cooler showery weather Thursday through Saturday.

&&

.SHORT TERM...Northwest flow aloft continues over Western
Washington early this morning, ahead of an offshore upper ridge
along 130W. There is a band of high clouds over the area now,
associated with a dissipating upper shortwave, but the air mass is
otherwise dry. There is also a little freezing fog in the south
interior, mainly along the I-5 corridor from Olympia to Chehalis.

The offshore ridge will progress to the Pacific Northwest coast
late today. The band of cirrus over the region should move east
of the Cascades probably by daybreak, and dry northwest flow aloft
will prevail through the day. Surface high pressure over Eastern
Washington will provide light offshore flow. It will be a mostly
sunny day with highs again in the upper 30s to mid 40s.

The upper ridge will move slowly inland tonight, with high clouds
immediately upstream of the ridge axis moving over Western
Washington. That should moderate the overnight temperatures a bit,
keeping the lows in the upper 20s to mid 30s.

A warm front will spread rain inland across Western Washington
Monday -- Martin Luther King Jr Day -- through Monday evening.
Rain will probably begin in Seattle late afternoon or early
evening. Rain -- heavy at times -- will continue more or less
Monday night through Tuesday night as strong, moist southwest
flow stays aimed at Western Washington. The snow level will rise
to roughly 7000 to 8500 ft Monday night and remain there through
Tuesday night.

Precipitation for the 36-hour period from late Monday through
Tuesday night will probably be 4 to 7+ inches over the Olympics,
and around 3 to 5+ inches over the Cascades. The coast is likely
to receive 3 to 5 inches of rain, with something like 1 to 4
inches over the interior lowlands. It`s worth noting that the
operational GFS run has consistently had the highest QPF of all
the ensemble members; therefore, the GFS and UW WRF-GFS
precipitation amounts could be too high.

In addition, the front will bring windy conditions to the coast
and north interior Monday night -- southeast winds probably 20 to
30 mph with gusts to 40 mph. The winds could actually be a bit
stronger Tuesday or Tuesday night, at least at the coast and a
wind advisory might eventually be needed there. McDonnal

.LONG TERM...Models agree that the jet will shift south and take
aim at northern California on Wednesday. Western Washington will
have another wet day, but probably less wet with the snow level
lowering to around 6000 ft. And they agree fairly well that an
upper trough will move through Wednesday night and Thursday, for
showery weather with the snow level falling to around 3500 ft.
A larger upper trough will keep at least a chance of showers in
the forecast Friday and Saturday, with the snow level 2500 to 3000
ft. McDonnal

&&

.AVIATION...An upper ridge will move over Western Washington today
and then over Eastern Washington tonight with westerly flow aloft.
At the surface, light offshore flow will prevail with high
pressure east of the Cascades and lower pressure offshore. The
air mass is dry and stable.

There will be high clouds at times today. Patchy freezing fog is
possible in the usual favored valley locations - mainly in the
south interior - this morning. Mid level clouds will begin to
encroach from the west as a frontal system approaches.

KSEA...High clouds at times today. Increasing mid clouds tonight.
Light wind. Schneider

&&

.MARINE...High pressure will be inland with lower pressure
offshore through tonight. A series of fronts will move through
the waters Monday through Thursday. This will bring small craft
advisory winds at times to most waters with gales possible for the
Coastal Waters Monday and Tuesday. There could also be some west
swell approaching 20 feet around midweek. Schneider

&&

.HYDROLOGY...River flooding is not expected through Monday. A warm
front will move through Western Washington Monday through Monday
evening. Rain -- heavy at times -- will continue more or less
Monday night through Tuesday night with the snow level 7000 to
8500 ft. Southwest flow of 40 to 65 kt at 850 mb will prevail
during this period, which is a fairly classic sign of a heavy rain
event especially for the Olympic Mountains.

Precipitation for the 36-hour period from late Monday through
Tuesday night will probably be 4 to 7+ inches over the Olympics,
and around 3 to 5+ inches over the Cascades. There could be
another 1 to 2 inches on Wednesday, as the snow level falls to
around 6000 ft.

A big uncertainty at this point, as mentioned above, is the fact
that the operational GFS run has consistently had the highest QPF
of all the ensemble members; therefore, the GFS and UW WRF-GFS
precipitation amounts could be too high. It means that the NWRFC
guidance for the rivers could also be too high. Another
consideration with regard to the potential for flooding is that
rivers are running low, many of them at their lowest levels since
the end of the summer.

Bottom line is that we`re forecasting enough rain to produce some
flooding on at least the more flood-prone rivers in our forecast
area. A hydrologic outlook was issued Saturday, and we will update
it early this morning.

Note: Snow is not a significant contributor to flood flows on
Western Washington rivers. Even when there is snow on the ground,
river flooding is almost exclusively caused by runoff from heavy
rain. McDonnal

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Gale Watch from Monday morning through late Monday night for
     Coastal Waters From Cape Flattery To James Island 10 To 60
     Nm-Coastal Waters From Cape Flattery To James Island Out 10
     Nm-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.

&&

$$

www.weather.gov/seattle

You can see an illustrated version of this discussion at
www.weather.gov/seattle/gafd/latest_webafd.html



USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.