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fxus66 ksew 200516 
afdsew

Area forecast discussion
National Weather Service Seattle Washington
900 PM PST sun Feb 19 2017

Synopsis...rain at times will continue through Monday evening, with
snow in the mountains. An upper trough will bring showers Tuesday,
and it will remain over the region through Friday for cool weather,
a few sun breaks, and a chance of showers each day. Clearing is
possible over the weekend.

&&

Short term...a 995-1000 mb surface low is filling and lifting north-
northeast across southern British Columbia this evening. Moist
onshore flow will support showers the rest of tonight but
generally decreasing as the main low feature moves further away from
the area. Snow levels are down near 2500 feet and snow is falling in
the mountains with another couple inches possible tonight even at
pass levels. Models hold onto more precipitation across the north
Cascades overnight so a few more inches could fall at Mount Baker.
Will plan to extend the advisory through most of tonight for the
northern Cascades.

The next system seen on infrared imagery nearing 130 W will track
northeast into the Pacific northwest late tonight through Monday.
Models show the surface low filling and tracking across southern WA
Monday afternoon. This means less flow and precipitation all around.
However, snow levels will remain low and models show enough quantitative precipitation forecast
for advisory level snow amounts from about Snohomish County south
to Lewis County Monday into Monday evening. Since snow is still
falling in these areas this evening will plan on extending the Winter
Weather Advisory for this area through Monday evening. Another 6 to
11 inches seems likely.

Showers will decrease Monday night with a few scattered showers around
on Tuesday. Models were showing more gradient across wrn WA but newer
runs today take the next low well south of our area. Precipitation
on Tuesday will generally be light. Another trough will bring showers
and even cooler temperatures by Wednesday. Snow levels maybe down
near 1000 to 1500 feet. A convergence zone could also form which
could drag the snow level down a bit lower but lowland snow
accumulations are not expected. Just cool and showery weather with
highs in the 40s. Mercer

Long term...previous discussion...the upper trough will dig offshore
Thursday and Friday. On Thursday the trough will be close enough for
a decent chance of showers, but by Friday it may be far enough away
for the shower chance to diminish. Highs will be in the lower to mid
40s Thursday, and a few degrees warmer on Friday. Showers could still
have snow at times both days.

The upper trough is forecast to be well to the south on Saturday,
and under a cool dry northerly upper flow, the day is forecast to be
sunny. Highs will be in the 40s. The next system arrives from the
northwest in both the Euro and GFS models on Sunday. Burke

&&

Hydrology...no flooding is expected during the next week. There
will be light rain at times with low snow levels. USGS
landslide guidance remains above the threshold at which landslides
typically occur. This means there will continue to be an elevated
threat of landslides for at least the next few days. Burke

&&

Aviation...moist SW flow aloft will continue over West Washington through
Monday. The air mass is slightly unstable this evening; it will
become increasingly stable tonight through Monday. A filling 996
mb surface low over central Vancouver Island will dissipate
overnight. A 990-993 mb low will move by just to the south of the
area midday Monday. Low level southerly flow will weaken overnight
then turn northerly early Monday morning as the next low upstream
approaches northern Oregon. Conditions across the region will be
mainly in the VFR category through early Monday afternoon, then
will become mostly MVFR Monday afternoon as the low moves off to
the east of the area and low level flow becomes onshore. Albrecht

Ksea...occasional showers will continue overnight, but conditions
will remain mainly VFR with cigs falling to around bkn035 in
the light shower activity. Ceilings later tonight into Saturday
morning will mainly be around ovc050 with the approach of the next
low. Ceilings will likely fall to around bkn010-020 Monday afternoon
as the low passes. Southerly winds 9-12 knots this evening will
become light around 12z then become northerly 4-8 knots Monday
morning. Winds will likely shift to southwest 7-10 knots after 22z
Monday. Albrecht

&&

Marine...a 996 mb low over central Vancouver Island will
dissipate as it moves northeasterly this evening. A second low now
centered near 41n 130w will move NE onto the northwestern Oregon
coast midday Monday. A third low, this one with a central pressure
of about 1000 mb, will move inland on the southern Oregon coast
Tuesday morning.

Winds associated with the first low are now weakening as the low
dissipates. Residual small craft advisories are in place for most
of the inland waters zones, except the central and western
portions of the Strait of Juan de Fuca, until midnight. Small
craft advisories for hazardous seas are in effect for the coastal
waters through Monday as a 10-11 ft westerly swell decays into the
waters.

The second and third lows will be moving inland to the south of
the Washington waters. Hence, winds will generally be in the 20 kt
or less category, with the potential exception to the waters
around Destruction Island on the central Washington coast Monday
afternoon.

A surface ridge will build offshore from western Washington later
Tuesday into Wednesday. This ridge will produce moderate
northwesterly flow over the coastal waters. Small Craft Advisory
conditions are also likely through the Strait Tuesday afternoon
and evening. Albrecht

&&

Sew watches/warnings/advisories...
Washington...Winter Weather Advisory until 3 am PST Monday morning for
the north Cascades.
Winter Weather Advisory through 10 PM PST Monday night for
the Cascades from Lewis County north to Snohomish County.

Pz...Small Craft Advisory for hazardous seas until noon PST Monday
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 PM PST Monday for
Grays Harbor bar.

Small Craft Advisory until midnight PST tonight for Admiralty
Inlet-East Entrance U.S. Waters Strait of Juan de Fuca-
northern inland waters including the San Juan Islands-Puget
Sound and Hood Canal.

&&

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