Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 122315

Area Forecast Discussion
National Weather Service Seattle WA
315 PM PST Sun Feb 12 2017

.SYNOPSIS...An upper level ridge over the Pacific Northwest will
bring dry and mostly sunny weather to Western Washington through
Tuesday morning. Nights will be cool with patchy fog. The ridge will
move east by Tuesday allowing moist southwest flow aloft to bring
rain from a frontal system to the area late Tuesday. A couple of
frontal systems will bring wet and windy weather to the area Tuesday
night through Thursday. A shift to weaker west flow aloft will bring
cooler showery weather to the area Friday through next weekend.


.SHORT TERM...The broad upper level ridge over W WA this
afternoon will build and gain a little amplitude tonight and Monday.
The air mass will remain dry with only some patchy fog expected late
tonight and Monday morning over the S Puget Sound region and the SW
interior. High clouds over the area earlier this morning have moved
east so skies should remain mostly clear through Monday night. The
dry air mass and mostly clear skies will allow low temperatures
Monday and Tuesday mornings to drop into the upper 20s and 30s. More
sunshine combined with low level offshore flow will help highs on
Monday to reach the lower to mid 50s, which is a little above normal.

The upper level ridge will slide over the ID Panhandle Monday night
with SW flow aloft developing over W WA. This will open the door for
weather systems crossing the NE Pacific to move inland over W WA.
The first system will approach the area on Tuesday. Models have
trended a little slower with spread of precip over the area and now
only have the coast with a little precip up until 00Z. Cloud cover
will also be slower to spread inland so Tuesday morning and perhaps
a bit of Tuesday afternoon should be mostly sunny. This should allow
another day of mild temperatures on Tuesday with highs in the lower
to mid 50s.

Rain will spread slowly eastward Tuesday night, probably not making
it east of Puget Sound until after 06Z/10 PM. Although the models
are in pretty good agreement with the onset of precip through
Tuesday night, the GFS and ECMWF are still showing inconsistency
regarding the structure of the approaching frontal systems. Models
agree well in general that Wednesday and Thursday will be quite wet
as well as windy at times over the coast and N interior. The air
mass will be warm with snow levels rising to 7000 to 9000 feet
Tuesday night through Wednesday night.

The inconsistent details in the models make it harder to figure out
precip amounts, especially over the mountains, and how strong the S-
SE winds will be and where. Since confidence is low on total QPF
over the area the forecast has gone with high end amounts to better
gauge potential impacts on area rivers. At this point 48 hour precip
from 00Z Wednesday through 00Z Friday should be 2-5+ inches over the
coast and Olympics, 2-4 inches over the Cascades, highest north, and
1-2 inches over the interior lowlands. It`s a little too soon to try
to pin down more specific speeds in the winds but at this point it
is looking like advisory level winds at worst and possibly a little
weaker. Kam

.LONG TERM...For the period Thursday night through next weekend the
models agree in general that the moist SW flow aloft over the NE
Pacific will shift to weaker, cooler, and somewhat moist west flow
aloft. Beyond this there is little confidence in details of specific
systems and the model solutions will probably be inconsistent for a
few more days. This looks like it will be a cooler period with
scattered showers at times. Kam


.AVIATION...An upper level ridge of high pressure over the Pacific
Northwest will gradually migrate eastward by late Monday. South-
southeasterly flow aloft. At the surface, light to moderate offshore
flow will prevail. The air mass is dry and stable except for high
clouds. Residual surface based moisture will produce some local fog
late tonight and Monday morning, mainly around the southwest
interior affecting KOLM.

KSEA...Conditions will likely remain VFR with just high clouds.
North wind to 5 KT will become northeast after 03-04z this evening.


.MARINE...Offshore flow persists through Tuesday as surface high
pressure remains centered east of the Cascades. Small craft advisory
easterly winds are possible by Monday or Tuesday on the coast and at
the west entrance. Stronger southeast winds arrive ahead of a low by
late Tuesday and Wednesday with gales likely over the coastal waters
and solid small craft advisory strength winds elsewhere. Another low
may bring another bout of stronger winds on Thursday but confidence
is very low on the track of the system. By Thursday, west to
southwesterly swell could reach 20 feet in the coastal waters
depending on the track of the incoming system. dtm


.HYDROLOGY...The lower Chehalis River remains above flood stage in
Grays Harbor County. It will continue receding slowly, and the
Chehalis at Porter should fall below flood stage tonight.

As discussed in the short term section above, the frontal systems
that move through Western Washington Tuesday night through Thursday
will bring 48-hr precipitation totals (00Z Wednesday through 00Z
Friday) of 2 to 5+ inches for the coast and Olympics, 2 to 4 inches
over the Cascades, and 1-2 inches over the interior lowlands. Snow
levels will rise to 7000 to 9000 during the period, but could begin
falling on Thursday. The most likely scenario is that only the
Skokomish River will flood, but there is at least a threat for other
rivers flowing out of the Olympics and possibly the Cascades.

In addition, this morning`s USGS landslide guidance puts our current
soil saturation at or just above the threshold at which landslides
typically occur. The next couple days will be dry which will allow
saturation levels to drop below the threshold, but the rainfall
midweek could raise the potential for landslides again. Kam





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