Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 192356

Area Forecast Discussion
National Weather Service Seattle WA
330 PM PST Mon Feb 19 2018

.SYNOPSIS...Dry and cold northerly flow aloft will prevail over
the region through tonight. A weak disturbance will bring some light
snow to the southwest part of the state Tuesday and Tuesday evening.
Another system will move southward off the coast late Wednesday into
Thursday with a slight chance of snow showers. Another stronger
front will bring rain or snow to the region Friday into the weekend.


.SHORT TERM...IR satellite imagery shows a batch of high clouds
racing southward along the B.C. coastline toward WA. These clouds
should pass the area by later this evening giving a window of
partial clear skies later tonight and early Tuesday. Low
temperatures should fall into the low to mid 20s across the
lowlands, with mid/upper 20s right along the water. Colder valleys
or low lying areas could reach the upper teens. If clouds linger
over the area later than expected, then forecast lows could end up a
few degrees too cold.

An upper level trough embedded in northerly flow will split as it
drives southward off the Washington coast Tuesday. Most models keep
the bulk of precipitation offshore with the stronger trough and
associated surface low just inside 130 W. Some models show a
secondary trough moving south out of B.C. over the area by late
afternoon. There could be just enough moisture to generate some
spotty light snow or flurries from around King county southward.
Accumulating snow will probably stay confined to the southwest
interior and possibly the central coast. The 18z NAM-12 is the most
significant outlier of all the models. It holds back surface low
pressure along the south WA coast into Tuesday evening with a band
significant precipitation over Puget Sound and southward. QPF
amounts over a half inch liquid seem way overdone with the cold dry
air mass currently in place. The Canadian is the other model
spreading moisture much further north with light snow accumulation
as far north as Seattle. All other models including the ECWMF and
WRF keep most snowfall south of around Tacoma. Ensemble means
indicate around an inch or locally 2 inches over the southwest
interior from Grays Harbor inland to Thurston and Lewis counties.
Since most snow will fall during the afternoon and early evening
will hold of on issuing an advisory. 00z model guidance might agree
better on timing, location, and amounts of snowfall.

Another weak system will dig southward Late Wednesday into early
Thursday on a similar trajectory as Tuesday`s system. Models
indicate less moisture and precipitation making it into Western
Washington. Flurries or some spotty light snow is possible but
little or no accumulation expected. Temperatures will remain cold,
with highs struggling to reach 40 and lows still in the 20s.

.LONG TERM...Most models now advertise a strong front arriving from
the north on Friday. The flow will go onshore with southerly flow
increasing. Models also show cold air aloft driving southward which
may keep snow levels rather low. At sea-level, a rain/snow mix is
possible and hilltops could potentially see some snow accumulation.
This is a ways out, so stay tuned.

A cold and showery air mass holds over the area into the weekend
with some models showing additional shots of cold air and enhanced
precipitation bands. The mountains should get appreciable amounts of
snow in this pattern. Snow levels will stay low, possibly not far
above sea-level over the weekend. Mercer


.AVIATION...Northerly flow aloft will continue tonight and Tuesday.
At the surface, northerly and offshore gradients will continue to
ease this evening and turn southerly later tonight. A low center
will move south well off the coast on Tuesday. The air mass is
stable and dry except for some high clouds and a cumulus field over
the southwest interior. The cumulus will evaporate after sunset but
the high clouds will increase this evening, then probably diminish
late tonight.

VFR conditions will prevail through tonight. On Tuesday afternoon,
as the low center moves south off the coast, some MVFR ceilings 2-3k
ft are possible over the southwest interior, along with a chance of
snow. Elsewhere there may be ceilings, but they will be VFR and
conditions should remain dry.

KSEA...VFR with no ceiling or a cirrus ceiling. Northerly wind 10-12
kt will fall below 10 kt by 03Z and become light southerly late
tonight. Some mid and high level cloudiness is likely Tuesday
afternoon. CHB


.MARINE...A low center will move southeast well off the coast on
Tuesday. Southeast winds of small craft advisory strength are likely
on the coast Tuesday and a small craft advisory is now in effect.
After the low moves south, some 20 kt northeast winds are likely
over the inland waters, and some advisories may be necessary then.

A weaker system is forecast to move southeast through Western
Washington on Wednesday night or so, but this is not forecast to
bring any advisory level winds. A stronger system will arrive
Friday. This system is likely to bring at least small craft advisory
strength winds to all waters, and could bring gale southerlies or
westerlies to some waters. CHB


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


PZ...Small Craft Advisory from 4 AM 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.


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