Scientific Forecaster Discussion
fxus66 ksew 201130
Area forecast discussion
National Weather Service Seattle Washington
330 am PST Mon Feb 20 2017
Synopsis...a frontal system will move northeast across western
Washington today, bringing rain with snow in the mountains.
Another system will brush the area Tuesday as it moves by to our
south. An upper trough will be over the region Wednesday through
Friday for cool weather, a few sun breaks, and a chance of
showers each day. Sunny and cool weather is likely Saturday.
Short term...southwest flow aloft continues over western
Washington, while the next frontal system is making its way into
western Oregon and California. The air mass over the area remains
moist and slightly unstable, with areas of light precipitation. At
3 am temperatures were in the upper 30s to mid 40s.
The frontal system will move northeast into the region across the
region today and this evening. Its surface low -- around 992 mb
and about 70 miles off the southern Oregon coast early this
morning -- will weaken as it moves across northwest Oregon late
this morning and into eastern Washington late this afternoon. The
system should bring widespread precipitation to western Washington
today. Showers will decrease fairly quickly tonight as the system
moves off to the northeast.
There will be a Winter Weather Advisory for snow in the Cascades
of Pierce and Lewis counties -- including Mount Rainier National
Park -- through this evening, where 5 to 10 inches of new snow is
likely. We can expect 3 to 6 inches of snow at Snoqualmie and
Stevens passes, with only 1 to 3 inches in the north Cascades. The
snow level will be around 2500 to 3500 ft.
Another frontal system will begin developing offshore of northern
California late today, moving inland across northern California
and Oregon tonight and Tuesday. The north side of this system
should bring some precipitation to mainly the southeast corner of
our forecast area. Precipitation amounts will be relatively light,
and much of the area north and west of Seattle could have dry
weather all day.
An upper trough will move over the region from the west Tuesday
night, and then deepen over the area as it slowly moves east
through Wednesday night. It will bring generally light, scattered
showers and probably a weak Puget Sound convergence zone. The
snow level will fall to 1000 to 2000 ft, with several more inches
of snow in the mountains.
Highs across the western Washington lowlands will be in the mid
to upper 40s today and Tuesday, and then in the lower to mid 40s
Wednesday. Lows should be in the mid 30s to lower 40s tonight,
cooling into the lower to mid 30s Wednesday night. Mcdonnal
Long term...northwest flow aloft will develop over western
Washington Thursday, as the trough discussed above moves east.
Another trough will be carved out in the northerly flow aloft on
Friday. Both days should be partly sunny to mostly cloudy, with
spotty light showers, and highs in the lower to mid 40s. The snow
level will be only 500 to 1000 ft. The trough will dig into
California by Saturday, leaving dry northwest flow aloft over our
forecast area; it should be sunny and cool. Models agree fairly
well on all of this, and that another upper trough will dig over
the region from the northwest Saturday night or Sunday. That could
renew showers across the area, with the snow level only around
500 ft. Mcdonnal
Aviation...SW flow aloft will continue over West Washington through
tonight. The air mass is stable. The air mass will be
moist...especially from a line from kuil-knuw southward.
A 990 mb low will move by to the south of the area over
northwestern Oregon midday today. Another low will follow on a
similar track on Tuesday. Clouds and a rain shield will move north
of the low through central and south portions of the forecast area
today as the first low moves by to the south, then precipitation
will taper off to showers tonight between lows. Expect mainly VFR
conditions across the area this morning to become MVFR south of
kuil-knuw this afternoon and tonight as flow becomes onshore with
the low passing off to the southeast and east of the area. The
showers and onshore flow will rapidly diminish tonight with
conditions returning to mainly VFR after 05z. Albrecht
Ksea...rain will spread north into the terminal area early this
morning. With northerly flow this morning, expect conditions to
remain VFR with ceilings around 035-050. Conditions will
deteriorate to MVFR with cigs dropping to around 015 and vis
dropping to 3-5sm -ra this afternoon as the low moves by to the
south and southeast of the area. Cigs will remain MVFR late this
afternoon and early this evening in a period of convergent
southwest flow. Expect improvement back to VFR after 05z this
evening. Northeast winds 4-8 kt will back to north this afternoon
then turn to the southwest then south this evening. Albrecht
Marine...a low off the southern Oregon coast early this morning
will deepen to near 990 mb as it moves onshore over the northern
Oregon coast midday today. A second low, this one with a central
pressure of about 1000 mb, will move inland on the southern Oregon
coast Tuesday morning.
Winds will turn to northeasterly this morning then northerly
midday as the first low moves by to the south. Flow will then turn
westerly or southwesterly briefly this evening as the first low
moves off to the east of the area. Winds will turn back to
northerly again late tonight into Tuesday as the second low moves
by well to the south of the area. Winds are expected to remain
below 20 knots through Tuesday over the coastal and inland waters.
A Small Craft Advisory for hazardous seas remains in effect for
the coastal waters through noon today for a 11 foot westerly
swell. Seas will slowly subside this afternoon.
A surface ridge will build offshore from western Washington later
Tuesday into early Wednesday. This ridge will produce moderate
northwesterly flow over the coastal waters. Small Craft Advisory
conditions are likely over the coastal waters and through the
Strait of Juan de Fuca Tuesday afternoon and evening. Winds will
ease Wednesday into Thursday as a weak low forms around Cape
Flattery then drifts southward through the coastal waters. Albrecht
Hydrology...no flooding is expected during the next week. 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.
Washington...Winter Weather Advisory until 10 PM PST this evening for
Cascades of Pierce and Lewis counties.
Pz...Small Craft Advisory for hazardous seas until noon PST today 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 this afternoon
for Grays Harbor bar.