Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 200418

Area Forecast Discussion
National Weather Service Seattle WA
818 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...A weak 1021 mb low associated with a short wave
trough that extends from east-central British Columbia westward to
Haida Gwaii will drop SSE through the Washington offshore waters
Tuesday afternoon and evening. The 00Z NAM12 (consistent with the
18Z solution and the 18Z GFS) brings clouds eastward across the
Olympic Peninsula overnight and into the interior south of about
Everett Tuesday into Tuesday night as the system drops south-
southeast over the offshore waters. The heavier precipitation with
this system remains offshore. What is interesting is how the
models advect some moisture eastward into the southern half of
Western Washington Tuesday afternoon as another weak shortwave
trough drops rapidly southward from northern British Columbia.
Models generally show flurries along the northern edge of the
precipitation from Seattle to Tacoma, and accumulations up to 2
inches along the coast and from about Olympia southward per the
NAM. The air mass in the interior from Seattle northward will
likely be too dry to give more than some mid level cloudiness.
Precipitation will come to an end from the north overnight on
Tuesday. The next shift may need to consider issuing Winter
Weather Advisories for portions of the coast and the southwest
interior for later Tuesday if the rest of the 00Z guidance
remains consistent with the 00Z NAM.

Mid and high level clouds will be increasing overnight from the
northwest. Temperatures have already fallen into the mid 20s to
lower 30s across the area and will fall rapidly over the next
couple of hours. How far with the temperatures fall before clouds
fill in sufficiently to hold temperatures. Models are already a
bit warmer than current temperatures, so the current forecast lows
of upper teens to mid 20s will be maintained for now. With all of
the clouds on Tuesday, expect temperatures Tuesday afternoon to
struggle to reach forecast values. An update will be made to lower
afternoon temperatures Tuesday to a blend of raw model data that
have come in since the previous forecast was made earlier this

Another weak system will dig southward Late Wednesday into early
Thursday on a similar trajectory as Tuesday`s system (maybe a bit
farther east). Models indicate less moisture and precipitation
making it into Western Washington. Flurries or some spotty light
snow accumulations of an inch or less is possible with this
system. Temperatures will remain cold, with highs struggling to
reach 40 and lows still in the 20s. Albrecht

.LONG TERM...From the previous long term discussion: 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, a weak low will move by to the west of
the area Tuesday through Tuesday evening. Northeasterly flow will
become more southeasterly tonight and Tuesday as the low passes by
to the west, then will turn northerly again as the low moves into
central Oregon. The air mass is stable and dry. Mid and high level
moisture will increase overnight tonight. Lower to mid level
moisture will move onto the Olympic Peninsula Tuesday morning
then into the interior lowlands from about KSEA-KPWT southward on

VFR conditions will prevail through Tuesday morning, except
possibly on the central coast around KHQM late Tuesday morning.
On Tuesday afternoon, some MVFR ceilings 1-3 kft, are likely in
the interior from about KOLM southward along with some light snow.
Dry VFR conditions will likely persist from KPAE northward in the
interior. Albrecht

KSEA...VFR with increasing high and mid level moisture late
tonight into Tuesday morning. Expect some BKN-OVC050 to spread to
the terminal 18Z into the evening. It looks like it will remain
dry through Tuesday evening at the terminal, but there is a 20
percent chance that some flurries will reach the terminal during
the late afternoon and evening hours. Snow accumulations are not
likely. Light winds will become 12005kt overnight into Tuesday
then will shift to 02010kt Tuesday evening. Albrecht


.MARINE...A weak 1021 mb low west of Haida Gwaii will drop SSE
through the offshore waters on Tuesday then onto the central
Oregon coast Tuesday evening. Southeast winds of small craft
advisory strength are likely along 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 and mainly upper level system is forecast to move
southeast through Western Washington on Wednesday night, but this
system is not expected to have significant impact on the surface
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. Albrecht/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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