Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 201051
Area Forecast Discussion
National Weather Service Seattle WA
351 AM PDT Thu Apr 20 2017
.SYNOPSIS...An upper level trough will move through the area today
with showers and a slight chance of thunderstorms. An upper level
ridge will build offshore tonight and move over Western Washington
on Friday bringing sunny and warmer weather. The break in the wet
weather will be brief with weak systems arriving Saturday and
Sunday, and a stronger system around Monday. The weather will
remain wet and unsettled through the middle of next week.
.SHORT TERM...Satellite imagery shows cloudy skies over Western
Washington early this morning. A narrow band of showers is
approaching the coast at 10z/3am. Doppler radar has the max db
values in some of the showers in the 50-60 range. No lightning
strikes yet but these showers still could have a little hail in
them. Temperatures at 3 am were in the 40s.
Upper level trough offshore moving through Western Washington
today. The air mass will become more unstable later this morning
into the afternoon hours with some daytime heating. Lifted indexes
are around 0 or slightly negative all day. 500 mb temperatures
are near -30C and convective temperatures are near 50 in the
morning and rise only into the lower to mid 50s in the afternoon.
There is even a little bit of CAPE with values as high as 800 J/kg
in the middle of the day. Nothing spectacular in these convective
variables but enough to have a chance of thunderstorms in the
forecast for the entire day in addition to the showers developing
over the area. Highs today will be in the 50s.
Shower activity drying up quickly tonight with the end of the
daytime heating and an upper level ridge approaching the area.
The exception to this will be around Snohomish county where a
convergence zone will develop this evening and continue into the
early morning hours. Outside of the convergence zone skies will
begin to clear. With plenty of low level moisture around fog is a
possibility over the Southwest Interior late tonight. Lows will
mostly be in the 40s.
Friday still on track to be one of the warmest days of the year.
Upper level ridge moving over the area with the ridge axis right
over Western Washington at 00z Saturday. Winds up to 700 mb go
easterly early in the day and remain easterly all day. Surface
gradients going offshore but not enough to prevent northerly winds
near the Puget Sound in the afternoon. On Easter the surface
winds turned southeasterly in the afternoon which helped warm the
temperatures. With the afternoon northerlies will keep
temperatures near the Sound a couple of degrees cooler versus
Easter`s readings ( the warmest day of the year ). Still most
places will be in the 60s with the best chances for lower 70s near
the Cascade foothills and along the west slopes of the Olympics.
The dry weather will be short lived as the upper level ridge moves
east of the area Friday night. Upper level trough will set up shop
off the coast with the first of a pair of systems to spin out of
the trough moving over Western Washington on Saturday. The system
will be negatively tilted which will help slow it down a bit.
Highest pops on Saturday in the afternoon. Highs will lower back
down to the mid 50s to lower 60s.
.LONG TERM...Extended models continue to show good agreement in
the pattern with an upper level trough moving through Western
Washington on Sunday. Some small differences in the model
solutions on Monday with the GFS taking the next system inland a
little further south than the ECMWF. Both models have rain over
the area. The more direct hit on the ECMWF will result in higher
rainfall amounts. Another system arriving Tuesday into Wednesday
with the GFS slightly faster with this feature versus the ECMWF.
The main message for the extended period is the wet weather will
continue into the middle of next week. Felton
.AVIATION...An upper trough will move inland over the Pacific
Northwest today and surface high pressure will continue to build
into the area from the south. The flow aloft is southerly and will
become west to northwesterly. The air is moist and somewhat
unstable, with scattered showers increasing into the afternoon hours
as daytime heating destabilizes the air mass.
KSEA...Clouds should lift into the afternoon as the air becomes more
unstable. Southerly breeze continues.
.MARINE...High pressure will continue to build into the area from
the south today--giving a southerly breeze. The flow will become
light tonight, and then easterly/offshore on Friday in response to to
falling pressure over the offshore waters.
.CLIMATE...Lots of climate stats to choose from this morning.
Wednesday was day 140 of measurable precipitation in Seattle since
October 1st. The previous record for most precipitation days
between October 1st and April 30th in Seattle was 137 days in
2010-11 and 1998-99. The precipitation total for Seattle since
October 1st is now 44.05 inches. The record October through April
precipitation is 44.52 inches set last season (2015-2016). The
precipitation total in Seattle since February 1st is now 19.43
inches. In the 120 years plus of records in the Seattle area there
has never been 20 inches or more of precipitation for February
through April. Felton
You can see an illustrated version of this discussion at