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fxus66 ksew 271034 
afdsew

Area forecast discussion
National Weather Service Seattle Washington
234 am PST Mon Feb 27 2017

Synopsis...showers have diminished overnight as the low that
brought precipitation to the area on Sunday moves shifts into
southwest Idaho. Another disturbance will move across the area
this afternoon through early Tuesday morning. This disturbance
will give some rain and snow showers to the area. Cool
northwesterly flow aloft will be a good snow producer for the
mountains Wednesday through Friday, while The Lowlands will see
rain at times and locally breezy to windy conditions. An upper
level trough will bring showers Saturday, which will taper off on
Sunday.

&&

Short term...northwest radars show showers in the central and
southern Washington Cascades while isolated showers are seen over
the coastal waters and on the olympic peninsula. A few showers can
pop up just about anytime/anywhere through midday, so chance pops
will be maintained.

Showers will increase again later this afternoon through Tuesday
morning as another disturbance, this one now over the Queen
Charlotte Sound of west-central British Columbia, moves southeast
into the area. This disturbance is more of an open wave in
strengthening northwest flow aloft, so shower activity will mainly
be focused on the west slopes of the mountains and in convergence
zones. Snow levels will remain rather low (below 500 feet across
most areas from Seattle northward Monday morning and again late
tonight and Tuesday morning - and around 1000 feet during the
afternoon and evening hours). But to complicate things, heavier
showers will locally lower the snow level and showers may contain
small hail, if not snow. Snow in The Lowlands, if it occurs, is
not expected to accumulate as near-surface temperatures during
the precipitation remains above freezing. But, heavier showers can
have a way of giving a local accumulation that is impossible to
pinpoint beyond an hour or two.

After some brief improvement midday Tuesday, expect showers to
increase again during the afternoon and overnight hours and
Wednesday morning as yet another disturbance moves in from the
northwest. This system will move inland as an open wave to the
north of the area resulting in increased onshore flow and snow
levels that gradually rise to around 2000 feet or so by Wednesday.

At least through Tuesday, temperatures will remain quite cool for
late February with lows in the upper 20s to mid 30s and highs in
the upper 30s to mid 40s. By Wednesday, temperatures in The
Lowlands will moderate so that lows will be around 40 and highs in
the mid to upper 40s. Albrecht

Long term...strong west to northwest flow Thursday through Friday
night will give slightly warmer conditions to the area and wet
conditions. This pattern supports copious snowfall in the
mountains above about 3000 feet and showers with breezy to windy
conditions in The Lowlands. The European model (ecmwf) remains a bit less wet than
the GFS, but it has trended a bit in the GFS direction over the
past couple of days. Highs will be in the mid 40s to around 50
with lows around 40. It will be breezy to windy at times in The
Lowlands as flow aloft interacts with the terrain.

Starting next weekend, a broad upper trough will develop over the
Pacific northwest with strong flow aloft moving more into Oregon
and rather low heights and cold conditions aloft. It will remain
showery over the area, especially over the mountains. Snow levels
may again get quite low again, below 1000 feet, next weekend.
Albrecht

&&

Aviation...an upper trough over the Pacific northwest is expected to
continue sliding southeast inland today. Cool unstable northwest flow aloft
over Washington. A weak shortwave in the flow is forecast to move
through the area today and generate additional showers - some mixed
or falling as snow with mainly MVFR conditions across western
Washington.

Mixed cloud cover conditions early this morning. A mainly broken mid
deck covered all but the northern third of western Washington early
this morning. Enough clearing to permit some shallow fog in spots
and even low stratus with IFR conditions. As the shortwave arrives,
mainly MVFR ceilings should prevail across the region by midday.

Ksea...the mid deck has dominated overnight, but a layer of low
stratus around the terminal may drift over the runways until about
16z. Then ceilings should primarily be MVFR with the incoming
shortwave and associated showers in the area. Southerly wind 5-10 kt
through the day today. Any brief showers as snow not expected to
produce any accumulation or impact operations. Buehner

&&

Marine...weak lower pressure is forecast to linger over western
Washington today. A weak Pacific frontal system is expected to move
ashore Tuesday evening with stronger low level onshore flow. The
next stronger frontal system is due to arrive Thursday and Thursday
night with another system to follow Friday. Buehner



&&

Hydrology...while it will be rather wet over the next week, snow
levels will generally remain below 3000 feet. Hence, no flooding
is expected during the next week.

&&

Sew watches/warnings/advisories...
Washington...none.
Pz...Small Craft Advisory until 2 am PST Tuesday for coastal waters
from Cape Flattery to James Island 10 to 60 nm-coastal
waters from James Island to Point Grenville 10 to 60 nm-
coastal waters from Point Grenville to Cape Shoalwater 10
to 60 nm.

Small Craft Advisory until 6 PM PST this evening for Grays
Harbor bar.

Small Craft Advisory for hazardous seas until 1 PM PST this
afternoon for coastal waters from Cape Flattery to James
Island out 10 nm-coastal waters from James Island to Point
Grenville out 10 nm-coastal waters from Point Grenville to
Cape Shoalwater out 10 nm.

&&

$$

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