Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 151658
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
858 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...High pressure and offshore flow will continue
today for more cool and dry weather across Western Washington.
Expect highs in the upper 30s to lower 40s with mostly sunny
skies.

A pattern change will bring wetter, warmer and windier weather
back to Western Washington. This change will take place on Monday
as a strong jet lines up over WA. Strong, moist westerly flow
will continue through Wednesday and models are ramping up the
precip totals in the mountains. 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. It will be windy in the lowlands over this
period as well, especially the coast and north interior with Wind
Advisories possible. The warmer air mass will cause temps to spike
into the lower 50s both Tuesday and Wednesday...with lows in the
40s. 33

.LONG TERM...Previous discussion...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 fog is possible
this morning in the usual favored valley locations...mainly in the
south interior...and is expected to dissipate by noon today.
Although at the time of this writing...cigs in the area look mainly
VFR. 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/SMR

&&

.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. Will take a look at inherited
Gale Watch...but would expect an upgrade to that with the afternoon
forecast package. There could also be some west swell approaching 20
feet around midweek. Schneider/SMR

&&

.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.

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



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