Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 200950

Area Forecast Discussion
National Weather Service Seattle WA
300 AM PDT Tue Mar 20 2018

.SYNOPSIS...Today will be mostly dry, with weak high pressure over
the area. Moist southwest flow will develop on Wednesday, and rain
will develop by Wednesday night. A deep upper level low will make
the air mass cool and unstable Thursday through Saturday. The snow
level will fall to around 1000 feet Friday and Saturday, and
isolated thunderstorms are possible Friday.


.SHORT TERM...Western Washington is between weather systems today.
Most of the area will be mostly sunny. The northwest interior will
see some cloudiness as a weak system moves through southern British
Columbia. There is a slight chance of showers on the north coast,
over the Olympics, and in the north Cascades. But the rest of the
area will have sun, with high temperatures in the mid 50s. The
vernal equinox will occur today, at 915 AM PDT.

Wet weather will return Wednesday and Thursday as a deep upper
trough forms over the northeast Pacific, setting up moist southwest
flow over Washington. Rain will spread from the south, with a chance
of rain over southern areas Wednesday afternoon, then rain becoming
likely over all areas Wednesday night. Warm advection will boost
afternoon high temperatures a bit, into the mid and upper 50s over
most areas.

The upper trough off the coast will move closer on Thursday, keeping
moisture flowing into the area from the southwest. The precipitation
will be more showery, but pops will still be likely or higher. Thick
clouds and lower heights will keep high temperatures on Thursday in
the mid and upper 40s. Burke

.LONG TERM...The upper trough gets really close on Friday, and 500
mb heights will fall into the 520s. That is cold for early spring.
The snow level will fall to around 1000 feet Friday and remain there
through Saturday. Showers are likely both days, and there could be
snow flurries at times. The cold air aloft will make the atmosphere
unstable, so have introduced a slight chance of thunderstorms into
the forecast for Friday. Highs on both Friday and Saturday will be
in the mid to upper 40s.

The upper trough will move east of the area Saturday night, and by
Sunday heights will build a bit. Weak impulses moving through the
northwest flow aloft will keep a chance of showers in the forecast.
High temperatures will rebound a bit, into the low 50s. Burke


.AVIATION...Broad upper level ridge well offshore with light
northwest flow aloft over western Washington this morning. The flow
aloft will back to southwesterly by this evening as an upper level
trough approaches from the northwest. The air mass is generally dry
and stable except for some mid level moisture across the northern
half of the area and some areas of low stratus around Puget Sound.
The stratus will clear later this morning, but high and level
moisture will gradually increase over the next 24 hours as the
region comes under the influence of the approaching trough. Mainly
VFR expected today except some areas of low MVFR and IFR ceilings
around the Puget Sound terminals this morning.

KSEA...Areas of stratus have formed over north Puget Sound early
this morning. With weak northerly surface gradients, some of this
might get into Sea-Tac for a short period this morning. Will likely
include some tempo ceilings of 007-010 through 16Z or 17Z. Apart
from that, VFR for the remainder of the day. Light and variable wind
this morning becoming N-NW less than 8 knots this afternoon.    27


.MARINE...Weak high pressure will give way to a low pressure system
that will drop down over the region after the middle of the week.
Stronger southerly winds are likely to develop starting on Thursday,
with small craft advisory winds likely at times into the weekend. It
is still a bit early to count on the details, but the quiet weather
of the past several days is pretty likely to give way to a more
blustery weather pattern typical of spring toward the end of the


.HYDROLOGY...River flooding is not expected in the next 7 days.



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