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fxus66 ksew 221708 
afdsew

Area forecast discussion
National Weather Service Seattle Washington
908 am PST Wed Feb 22 2017

Synopsis...a series of upper lows will move south across the
offshore waters today through Friday, producing cool weather for
western Washington with scattered rain and snow showers. An upper
ridge will bring cool sunny weather Saturday, with a few showers
in the mountains. Another upper trough will arrive Sunday, and the
chance of showers will continue early next week.

&&

Short term...an upper trough will move slowly southeast over the
offshore waters today. Moisture, weak instability, and even the
limited daytime warming will cause scattered showers to increase
this afternoon. The temperature at 850 mb (roughly 4500 ft) will
be -6c to -7c, and the snow level will be around 1000 ft. So some
showers could fall as wet snow over the higher terrain of the
western Washington lowlands or contain graupel. There's also a
slight chance of thunderstorms over the coastal waters and at the
coast. Precipitation amounts will be fairly light -- mostly 0.10
inch of liquid equivalent or less, and accumulation should be zero
or near zero. Highs will be in the lower to mid 40s, pretty much
matching today's convective temperature.

The upper trough will move inland over Oregon tonight and cross
the Cascades late tonight. Showers should pretty much end in the
evening as the air mass stabilizes. Overnight lows will be in the
lower to mid 30s.

North to northwest flow aloft will follow the trough Thursday and
Thursday night. Moisture should be more limited, but the air mass
will be similarly unstable and slightly cooler. Light scattered
rain or snow showers should develop with daytime heating, in most
cases amounting to less than 0.10 inch liquid equivalent. The
snow level will be around 400 ft Thursday morning, rise to 1000 ft
or so during the afternoon, and fall again overnight. Snow
accumulation should be zero or near zero.

A more vigorous shortwave will dig south across central or North
Vancouver Island late Thursday night, carving out another upper
low over our offshore waters on Friday. Most of the shower
activity will be near the coast where the snow level will be
around 1000 ft. Over the interior lowlands, light northeast
surface winds will keep the snow level generally in the 300 to 700
ft range, but precipitation amounts will again be mainly less than
0.10 inch. So in the end it should be another cool day, with a few
rain and snow showers, and zero or near zero accumulation.

For the mountains -- there will be light scattered showers today
through Friday night, and the snow level will be at the surface
the entire time. Precipitation amounts will be light; three-day
snow totals in the Cascades should be in the 2 to 5 inch range,
with around 3 to 8 inches in the Olympics. Smith/mcdonnal

Long term...an upper ridge will progress quickly across the
Pacific northwest on Saturday. It should be a cool day with
sunshine for The Lowlands, but a chance of light showers over the
mountains. Another upper trough will move across the area from the
northwest on Sunday. The GFS keeps The Lowlands mostly dry, while
the European model (ecmwf) has more widespread precipitation. I added at least a
slight chance of rain or snow showers into the Sunday forecast.
Model solutions separate a bit more after Sunday, but a chance of
precipitation should continue Monday and Tuesday with the air mass
probably beginning to warm slightly. Mcdonnal/Smith

&&

Aviation...an upper trough axis will move southeast down the b.C.
Coast today and pass across western Washington tonight. Within the
upper trough is some very cold air aloft. The resulting
instability will lead to plenty of convective shower activity
through this evening, with the possibility of isolated lightning
near the coast. A Puget Sound convergence zone is expected to sink
southward this afternoon and early evening, helping to focus some
showers there. Otherwise, the unstable air mass will help the
ongoing fog at places like pwt, shn, olm, and cls to disperse
this morning. Other than the areas of interior fog this morning,
an unstable air mass commonly leads to MVFR cigs in the morning
and VFR in the afternoon. Cigs generally coming back down into the
015-025 range tonight, with some areas of fog again.

Ksea...cigs will gradually lift into the 025-035 range by mid-
afternoon with light southerly winds. A convergence zone is
expected to slip south of the terminal around 00z (4 pm), causing
a wind shift out of the north. Speeds may exceed 6 knots for 1-2
hours before settling down to less than 6 knots. Winds for the
evening rush should still favor a northerly direction but will
become light and variable by 06z (10 pm). Passage of the
convergence zone will be accompanied by a few showers. Looking
forward to Thu morning, moisture will be fairly shallow, and light
winds may support some fog in the vicinity of the terminal. Haner

&&

Marine...fairly light onshore flow will continue today. A weak
surface low will move south through the Washington coastal waters
tonight and Thu as it dissipates. Although generally weak...models
are hinting at the Prospect of borderline Small Craft Advisory winds tonight and
Thu morning for the outer coastal waters. Moderate offshore flow
is expected on Fri in response to a strengthening low that will
move southward along 129w. Offshore flow will weaken on Fri night
and Sat morning. Weak flow will prevail later Saturday into
Sunday. Haner

&&

Hydrology...no flooding is expected during the next week. USGS
landslide guidance remains above the threshold at which landslides
typically occur. The guidance depends on rainfall amounts during
the past 3 days and the past 15 days; the three- day amounts are
fairly low, but the total for the last 15 days is still high.

&&

Sew watches/warnings/advisories...
Washington...none.
Pz...Small Craft Advisory from 10 PM this evening to 10 am PST
Thursday 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.

&&

$$

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