Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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503
FXUS66 KSEW 240525
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
925 PM PST Mon Jan 23 2017

.SYNOPSIS...An upper ridge will bring dry weather to Western
Washington Tuesday. A weak frontal system will weaken further as
it moves into the ridge Wednesday and Thursday, for generally
cloudy skies and a chance of light rain. The ridge should
strengthen and bring dry weather again Friday and Saturday. A
front will probably bring some rain to the area Sunday.

&&

.SHORT TERM...Northerly flow aloft continues over Western
Washington this evening, between the high amplitude upper ridge
offshore near 135W and the upper low just offshore of northern
California. Light northerly flow prevails at the surface as well,
with mainly clear skies. Highs today were in the mid 40s to lower
50s; at 9 pm temperatures had fallen into the 30s to lower 40s,
with some spots at or below freezing and patchy fog starting to
form.

The pattern will change little tonight, with northerly flow aloft
and generally light surface winds. Lows will be in the mid 20s to
lower 30s, with fog and perhaps some low stratus becoming more
widespread overnight.

The offshore upper ridge will move slowly tonight and Tuesday.
Some high level moisture will probably move into the forecast area
Tuesday afternoon, and the day will start with areas of fog and
low cloud cover in the morning. A forecast for partly to mostly
sunny skies looks fine, and highs should be mainly in the mid to
upper 40s.

The upper ridge will weaken a bit as it moves to the coast
Tuesday night and then inland on Wednesday. The warm front of a
weak and decaying frontal system immediately upstream of the ridge
will increase the high -- and eventually mid -- level moisture
over Western Washington on Wednesday. Most of the models show
spotty light rain over Western Washington on Wednesday and
especially Wednesday night as the remains of the warm front move
inland more or less coincidental with the upper ridge axis. The
flow and the warm advection are quite weak, and our forecasts are
undercutting the precipitation chances and amounts shown by the
models -- at least for now.

On Thursday, the remains of the cold front should move into
Western Washington. It looks quite weak as well, as the upper
ridge will remain over the region as the cold front arrives. We
are forecasting low-end rain chance and light precipitation
amounts. McDonnal

.LONG TERM...Here is the long term section from the afternoon
forecast discussion -- A large scale upper ridge remains in place
over the region through about Saturday. This would maintain dry
weather and slightly above average temperatures. Offshore flow
would dry the air mass with only patchy fog in the most prone
sheltered spots.

Some models flatten the ridge by Sunday and bring a progressive
front through Western Washington with some rain. The 18z GFS run is
not as strong with the trough but still shows some light measurable
rain. Light rain is also shown by the 12z ECMWF, so the chance pops
remain in the forecast. Models differ on the details early next
week, but generally point toward split westerly flow and some weak
systems reaching the area. Expect a little more unsettled weather,
onshore flow, and only light precipitation. Mercer

&&

.AVIATION...An upper level ridge offshore will continue moving e.
Lightly nly flow aloft will strengthen some Tuesday morning. The
low level flow will be nearly flat. Expect areas of LIFR
CIGs/VSBYs to dvlp overnight and then persist until about 1900
UTC before lifting.

KSEA...Confidence in MVFR conditions developing over the terminal
later tonight was becoming lower. Conditions could probably end
up being worse (LIFR CIGs and LIFR VSBYs due to fog). In other
words, the majority of the models may be too optimistic. There are
a few solutions indicating LIFR CIGs/VSBYs over the terminal after
0900 UTC. VFR conditions should return by 2100 or 2200 UTC.

&&

.MARINE...
Light northerly flow will prevail overnight due to higher pressure
over British Columbia with lower pressure over Oregon. Meanwhile,
combined seas were in the 9 to 12 foot range and will continue
subsiding overnight.

High pressure will result in light winds Tuesday and Wednesday. A
weak trough will dissipate over the Coastal Waters Wednesday night
into Thursday. A warm front will clip the Coastal Waters on
Friday. 33/05

&&

.HYDROLOGY...River flooding is not expected for the next 7 days.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory for hazardous seas until 4 AM PST Tuesday
     for Coastal Waters From Cape Flattery To James Island 10 To
     60 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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