Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 192229
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
315 PM PDT Wed Apr 19 2017

.SYNOPSIS...An upper level trough will bring showers and a slight
chance of thunderstorms on Thursday. An upper level ridge will bring
sunny weather Friday. The break in the weather will be brief with
weak systems arriving Saturday and Sunday, and a stronger system
around Monday. The weather will remain wet and unsettled next week.

&&

.SHORT TERM...Radar shows an organized rain band has changed to
scattered showers after a front moved through the region this
afternoon. There have been a few thunderstorms well off the coast
this afternoon, so have introduced a slight chance of thunderstorms
to the coastal waters tonight. Some breaks in the cloud cover early
Thursday morning will allow lows to drop back into the 40s.

An upper level trough offshore tonight will move inland Thursday.
Convective variables are similar to Tuesday`s numbers with lifted
indexes near 0, 500 mb temperatures near -30C and convective
temperatures in the lower 50s in the afternoon. This will keep a
slight chance of thunderstorms in the forecast for Thursday
afternoon in addition to the shower activity. Some thunderstorms may
contain small hail. Highs will be in the 50s.

An upper level ridge building offshore will shove the upper level
trough east of the area Thursday night. Shower activity will dry up
quickly in the evening with the possible exception of over Snohomish
and north King counties with a convergence zone. The upper level
ridge will move over Western Washington on Friday with 500 mb
heights rising to around 570 dm. Temperatures aloft warm but are not
very impressive with 850 mb temperatures by 00z Saturday around plus
5C. Model soundings do show easterly winds right off the ground all
the way up to 700 mb. This offshore flow will help warm temperatures
up into the 60s Friday afternoon with the warmer locations near 70
degrees.

Another upper level low will develop offshore Saturday. This will
spin a weak system into the area Saturday afternoon. Light rain is
likely over the southwest half of the area, with a chance of rain
elsewhere. Rain or showers are likely everywhere Saturday night.
Western Washington will still be in the warm sector so highs will be
mostly in the 60s. Burke

.LONG TERM..The upper level low will move through Western Washington
Sunday, keeping showers likely. A more consolidated front will reach
the area Monday or Monday night. A broad upper level trough will
move over the area on Tuesday, with moist westerly flow aloft
Wednesday. Both these patterns will keep a chance of showers in the
forecast. Highs will be mostly in the 50s Sunday through Wednesday.
Models have been unusually consistent about this sequence of events.
Burke

&&

.AVIATION...An upper level trof of low pres offshore will move
inland Thu morning. Strong sly flow aloft over the region will
weaken overnight. A surface trof off the coast will move across the
area this evening. Expect areas of MVFR CIGs, becmg more wdsprd
late tonight. The low level flow will generally be sly. The mtns
will be mostly obscd.

Meanwhile, LAPS (Local Analysis and Prediction System) indicated
that the air mass was destabilizing over the area this afternoon. At
this time, it appeared that the best chance of tstms will be from
the coast wwd, where the highest instability is located.

KSEA...Still expecting VFR conditions to prevail thru this evening;
although, there may be a brief period of MVFR CIGs after 0300 UTC (8
PM PDT). However, still anticipating MVFR CIGs to become predominant
later tonight. VFR conditions should return late Thu morning. Winds
will generally be sly 5-10 kt.

&&

.MARINE...
A surface trough off the WA coast will move across the area this
evening. Expect southerly flow to prevail tonight and Friday. The
flow will become flat Thursday night, and then easterly or offshore
on Friday in response to to falling pressure over the offshore
waters.

Meanwhile, a few of the stronger thunderstorms over the coastal
waters this afternoon through tonight could produce brief heavy
rain (reducing visibilities into the 1-3 NM range), gusts near 30
knots, and locally higher waves.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisories are in effect for all waters except the
 western/central Strait of Juan de Fuca and the Puget Sound/Hood
 Canal.

&&

$$

www.weather.gov/seattle

An illustrated version of this forecast discussion can be seen at
www.weather.gov/seattle/gafd/latest_webafd.html



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