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

000
FXUS66 KSEW 161100
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
300 AM PST Mon Jan 16 2017

.SYNOPSIS...High pressure aloft will weaken and shift east of
the region today as a frontal system approaches the offshore waters.
A series of potent frontal systems will bring wet and windy
conditions to western Washington Tuesday and Wednesday along with
snowfall in the mountains. A cool upper level trough will settle over
the area for the second half of the week bringing cool and showery
weather.

&&

.SHORT TERM...
January may well and truly have two faces this year. The shift from
dry and cold weather toward a milder and more familiar unsettled
pattern is well underway. Clouds will thicken today but most of
western Washington is likely to stay dry until nightfall. A frontal
system pushing into the offshore waters will spread rain to the north
coast and north interior this evening then the precipitation will
spread inland and southward on Tuesday morning. The models appear to
be trending the timing of the precip arrival backward so this
morning`s forecast has pushed back the POPs today for just about
everywhere.

A 120 knot jet stream begins to take aim at the Pacific
Northwest by Tuesday morning and, with it, a ramping up of the
precipitation. The models are still at odds as to how wet it will be,
but the Canadian model seems to be on par with the operational GFS
with upwards of 6 to 8 inches of precip in the Olympics by Wednesday
evening and 5 to 7 inches in the North Cascades. The GFS
ensembles as well as the Euro continue to show lighter amounts. Much
of the lowlands outside of the Olympic rainshadow stand to receive 1
to 2 inches over the next 72 hours. It will also become windy at
times Tuesday through Wednesday and wind advisories may be needed
for the usual spots...i.e. the north interior and Admiralty Inlet
areas.

A winter storm watch for the Cascades from King County northward
remains in effect, but given the slower timing in the models
has been pushed back to begin early Tuesday and continue through the
evening. The precip will likely start off as snow in much of the
Cascades, especially in those areas prone to cold east winds. The
snow is expected to change over to freezing rain in the passes
(such as Snoqualmie Pass) Tuesday morning.

Heavy rainfall, with rising snow levels, may cause river flooding. A
Hydrologic Outlook has been issued. However, there is still some
uncertainty on the precip totals in the basins. The models are
showing a wide range and the GFS still has nearly double the amount
of precip in the mountains compared to the ECMWF. 27/05

.LONG TERM...
An upper level trough will settle over the region Wednesday night
into Friday for showery conditions and lower snow levels. A strong
jet is poised to remain well south of the region for the coming
weekend. It will remain unsettled, but significant weather systems
will remain to our south.

&&

.AVIATION...An upper ridge over Western Washington will move
inland today and tonight with westerly flow aloft. At the
surface, high pressure will be over Eastern Washington with lower
pressure offshore today. A front will reach the coast tonight.

The air mass is stable with mid and high level moisture. Ceilings
will gradually lower today to 6000 to 8000 feet today. MVFR low
clouds should eventually develop tonight or Tuesday morning as a
front moves onshore.

KSEA...Mid and high clouds today. Ceilings lowering to around
7000 feet this afternoon. MVFR low clouds will likely develop
Tuesday morning as a front arrives. Light wind becoming southeast
5-10 knots today. Schneider

&&

.MARINE...A series of fronts will move through the waters this
week. Gale force winds are likely at times for much of the waters
starting later today or tonight through Tuesday or Wednesday.
Winds will probably come down a bit most areas Thursday but then
increase again late in the week as another frontal system reaches
the area. Schneider

&&

.HYDROLOGY...
River flooding is not expected through Tuesday morning. A warm front
will move through Western Washington late tonight. Rain -- heavy
at times -- is expected Tuesday through Wednesday morning with snow
levels rising from near 4000 feet Tuesday morning to 7000 to 8000
feet early Wednesday. 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 early Tuesday through
Wednesday night will probably be 5 to 8 inches over the Olympics,
and around 4 to 6 inches over the Cascades.

A big uncertainty at this point is the fact that the operational GFS
run has consistently had the higher 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.

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. 27

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Storm Watch from Tuesday morning through Tuesday
     evening for Cascades from King County northward.


PZ...Small Craft Advisory from 10 AM this morning to 4 PM PST this
     afternoon for Coastal Waters From Point Grenville To Cape
     Shoalwater 10 To 60 Nm-Coastal Waters From Point Grenville
     To Cape Shoalwater Out 10 Nm.

     Gale Warning from 4 PM this afternoon to 4 PM PST Tuesday 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.

     Small Craft Advisory for rough bar from 4 PM this afternoon to
     midnight PST tonight for Grays Harbor Bar.

     Small Craft Advisory until 4 PM PST this afternoon 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.

     Gale Watch from this evening through Tuesday afternoon for East
     Entrance U.S. Waters Strait Of Juan De Fuca-Northern Inland
     Waters Including The San Juan Islands.

     Small Craft Advisory from noon today to 10 PM PST this evening
     for East Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from 4 PM this afternoon to 10 PM PST this
     evening for Northern Inland Waters Including The San Juan
     Islands.

     Small Craft Advisory from 10 AM this morning to 4 PM PST
     Tuesday for Admiralty Inlet-West Entrance U.S. Waters
     Strait Of Juan De Fuca.

&&

$$

www.weather.gov/seattle

An illustrated version of this forecast discussion can be seen 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.