Forecast Discussion for SEW NWS Office
FXUS66 KSEW 081642

Area Forecast Discussion
National Weather Service Seattle WA
842 AM PST Sun Dec 8 2019

.UPDATE...An upper level ridge will build into the region today
and persist into early Tuesday. Radar indicates a few light
showers across portions of the Cascades this morning and satellite
shows plenty of low cloud cover across Western Washington. With
ample low level moisture still in place across the region, expect
clearing to be slow and for most areas to remain mostly cloudy
throughout the day-with the exception of some clearing in the
mountains. Afternoon highs will be in the upper 40s to near 50.
The previous discussion below applies, with updates made to the
marine and aviation sections. 14


.PREV DISCUSSION... /issued 330 AM PST Sun Dec 8 2019/

.SYNOPSIS...An upper ridge will build offshore today for dry
weather through early Tuesday with the best chances for sunshine
being on Monday. The next system should slide through later Tuesday
with unsettled conditions persisting thereafter.

.SHORT TERM /TODAY THROUGH TUESDAY/...Current satellite and radar
imagery show that system responsible for precip yesterday is out
of the area with W WA now falling under the jurisdiction of an
incoming upper level ridge still out over the offshore waters.
Current obs show sufficient low level moisture is in place to keep
low clouds over much of the area with areas of fog/reduced
visibilities in some locations.

This upper level ridge will keep conditions dry today...but any sort
of clearing is going to be slow work at best. Given the widespread
low clouds/fog this morning...expecting cloudiness to linger most of
the day...although the mountains may see some clearing as the day
progresses. This ridge will be the main weather factor throughout
the short term as dry conditions persist into Monday and early
Tuesday. Clouds should give way to some breaks of sunshine during
the day Monday...mostly for locations east of Puget Sound. The
western half of the CWA will start to see clouds associated with the
next weather system start to roll in which may limit the amount of
sun they get. The next system makes its way to the coast by mid to
late Tuesday morning but really doesn`t make its way inland until
Tuesday afternoon. Both ECMWF and GFS do not show this front being
of any particular note and the ensemble mean leans toward that
conclusion as well showing pretty minimal QPF values associated with
its passage. Neither model has the system holding together very well
and has a weak shortwave ridge in place over W WA by Tuesday night.

Lowland high temps throughout the short term will remain pretty
static...generally in the upper 40s. Overnight lows will not see
very much variation either...sitting in the upper 30s to around 40.

shortwave ridge for early Wednesday...although the ECMWF is more
progressive on kicking it out and getting the next system onto the
coast by late Wednesday morning...inland by Wednesday evening. The
GFS is slower with the system hitting the coast in the afternoon and
pushing inland overnight. Most of the ensemble members fall
somewhere in between causing any forecast solution to get lost in
the spaghetti.

Beyond this initial disagreement...models come back in line
regarding the active pattern for the remainder of the forecast
period as a broad upper level trough sets up over the Pac NW. This
will shuttle in a series of systems and while both deterministic and
ensemble models show some dry breaks...the overall pattern will be

Temps in the long term do not waver much from those in the short
term with the lowlands generally seeing high temps in the upper 40s
to around 50 while overnight lows will sit generally in the upper
30s to lower 40s.  SMR


.AVIATION...North northwesterly flow aloft into Monday with upper
level ridge offshore. Light flow in the lower levels with flat
surface gradients. The air is moist and cloudy across most of
Western Washington but some of the higher mountains are sticking up
out of the clouds and there are some breaks in the clouds so that
bodes well for some improvement today--cigs will set up and lower
again tonight however. There is likely to be areas of fog and rather
low cigs Monday morning.

KSEA...Improvement into the afternoon is likely--but then low
stratus will set up tonight and persist through midday Monday. Light
and variable winds.


.MARINE...Weak low pressure today will give way to high pressure
by Monday with light winds. A front will approach the area on
Tuesday and weaken over the area Tuesday night. Another stronger
front should reach the area Wednesday night and push inland on


.HYDROLOGY...River flooding is unlikely for the next 7 days.




NWS SEW Office Area Forecast Discussion