Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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662
FXUS66 KSEW 130529
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
929 PM PST Fri Jan 12 2018

.SYNOPSIS...A weak warm front will lift northeast across the area
tonight into Saturday morning for light rain at times. Sunday
will be dry and mild as an upper ridge moves across the area. A
cold front will move up from the southwest on Monday, bringing the
return of rain. Additional fronts will move through the region
next week for a return to wet weather.

&&

.SHORT TERM...A building upper ridge axis off the coast is
spreading a warm front through Western Washington tonight. Warm
advection lift and rainfall should peak later this evening and
overnight. Even so, QPF amounts will be fairly modest, with
amounts through Saturday morning exceeding one inch only over
windward slopes of the northwest Olympic Peninsula and North
Cascades.

The air mass is in the process of warming quite a bit. 850 mb
temps will warm from near 0C to about 7C over SEA by mid-day
Saturday, and freezing levels will climb from about 4500 feet to
9000 feet. Surface temperatures will hold steady or slowly rise
overnight. A cold but light northeast wind continues near BLI with
temps in the mid 30s, but the BLI-CYWL gradient has been
weakening this evening. So the reinforcement of colder air into
Western Whatcom County will weaken.

During the day Saturday, the offshore upper ridge axis will
amplify, sharpen, and then move across Western Washington during
the afternoon. This will bring a sudden end to precip during the
late morning and early afternoon, with some sunshine peaking out
in the hour or two before sunset. With a decently mixed and warm
air mass on Saturday, along with an hour or two of sun near the
end of the day should bring temperatures well into the 50s for
most of the lowlands except up north near BLI. Should be the
warmest day so far this year for much of the lowlands.

Low-level offshore gradients will quickly develop on Saturday
night. A dry air mass above the boundary layer and clear skies
will certainly lead to lowland fog development by Sunday morning,
especially with surface dewpoints up in the 40s on Saturday
evening. Fog and low clouds will have a tough time lifting and
scattering from the Southwest Interior and South Sound regions on
Sunday, keeping the air mass inverted and surface temps cool there.
Meanwhile, east winds in the foothills will really boost
temperatures over the eastern lowlands.

A negatively tilted cold front will arrive on Monday. Rain should
spread up along the coast late Monday morning, then spread into
the interior I-5 corridor in the afternoon. The SEA-EAT pressure
gradient is forecast to strengthen to near -13 mb on Monday
morning, so will need to watch the wind forecast near the Cascade
foothills of eastern King and Snohomish Counties. Haner

.LONG TERM...From Previous Discussion: A series of fronts and
upper troughs will bring wet weather Tuesday through Friday.
Models are in good agreement with this general idea but struggling
a bit with the details and run to run continuity. One way or
another, it looks wet and breezy to windy at times. The
wettest/windiest period will probably be Wednesday night/Thursday.
Schneider

&&

.AVIATION...A warm front will brush Western Washington tonight.
Moderate westerly flow aloft tonight will ease Saturday as a flat
upper ridge builds over the area. At the surface, moderate southerly
flow will continue tonight, then become northeasterly Saturday
afternoon.

Light rain will continue over Western Washington tonight, then taper
Saturday morning. Ceilings are mostly MVFR; a few are flirting with
IFR. That is likely to remain the case tonight and Saturday morning.
Some improvement is likely Saturday afternoon.

KSEA...Ceiling will probably be around 2k ft all night, with light
rain at times. Southerly wind 6-10 kt. Probably good VFR developing
21Z-00Z with wind becoming light northerly. CHB

&&

.MARINE...A warm front will brush the area overnight bringing an
increase in winds, to small craft advisory strength, across the
coastal waters. Seas have subsided slightly but may hover around 10
ft. A stronger system will arrive around Monday, and another one
will arrive around Wednesday. CHB

&&

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

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 10 AM PST 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-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 until 4 AM PST Saturday for
     Grays Harbor Bar.

&&

$$

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