Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 200443 CCA
Area Forecast Discussion...CORRECTED
National Weather Service Seattle WA
943 PM PDT Sun Mar 19 2017
Corrected the Watch/Warning/Advisory section.
.SYNOPSIS...High pressure aloft with low level offshore flow will
persist through Monday morning. Southerly flow aloft will strengthen
Monday afternoon, opening the door to a series of low pressure
systems from the south that will impact the area through midweek.
High clouds will continue streaming across the area overnight. These
clouds are associated with a low pressure system currently off the
After the upper ridge axis exits to our east late tonight, flow
aloft will back from westerly to southwesterly on Monday. The flow
will then become southerly on Monday night and strengthen on
Tuesday. On Monday PM, this will bring up plenty of mid-high level
moisture. Falling upper heights and shortwave energy coming up
from the south will eventually help precip spread up from the
south. Will take awhile for the rain from our south to overcome
the drying low-level easterly flow, but more organized precip
spreading up from the south on Monday night will eventually
overcome the dry low-level air. Even so, rain will be on the
lighter side. Snow levels will be up around 5000-5500 feet.
Following Monday night`s round of rain, may be some dry breaks on
Tuesday, but some disorganized dribs and drabs of rain will
continue in the moist southerly flow aloft. Precip will blossom
again on Tue night and peak on Wed morning prior to the passage of
an eastward-moving cold front. Showers will continue behind the
front on Wed PM. Haner/05
.LONG TERM from the previous discussion...
An upper ridge should briefly pump up over the area on Thursday,
bringing the best chance of a 12-24 hour period of dry weather from
Monday night through the rest of the week. The upper ridge will
quickly pass east late Thursday, with strengthening S-SW flow aloft
following right on its heels. A shortwave will ride up in southerly
flow on Friday, with the another good shot of rain at that time.
Looks like Friday`s system will start to split just as it reaches
the U.S. west coast, so mainly moderate precip amounts expected over
Western Washington. An upper trough axis will pass by next
Saturday, with the cooler air aloft and onshore flow supporting
showers. Models show an upper ridge briefly appearing next Sunday
before another turn to wetter SW flow aloft occurs. Haner
.AVIATION...An upper ridge over the area will shift inland Monday
with westerly flow aloft becoming southwest. At the surface,
offshore flow will prevail with high pressure inland and lower
pressure offshore. A frontal system will spread moisture into the
area from the south on Monday. The air mass is stable.
Mostly clear tonight. High clouds will increase across the area
Monday morning, then mid clouds will move up from the south in the
afternoon, followed by VFR low clouds and rain late in the
afternoon or in the evening.
KSEA...High clouds tonight. Clouds increasing and ceilings
lowering during the day Monday as moisture moves up from the
south. Northerly wind around 10 knots. Schneider
.MARINE...Offshore flow will produce small craft advisory easterly
winds at times for the Coastal Waters and western part of the Strait
tonight and Monday. A relatively weak frontal system will move up
from the south Monday afternoon and Monday night. Additional frontal
systems - around Tuesday night and Thursday night - will affect
the waters later in the week. Schneider
The flow on the White River at R Street is forecast to fluctuate
near 5500 cfs through Monday morning, resulting in minor flooding
near the low bank areas, near and downstream from the Stewart
Street Bridge, in Sumner. Aside from the White River, flooding is
not expected the next 7 days.
Meanwhile, the threat of landslides will remain elevated for at
least the next couple of days. There have already been a few
landslides in the CWA.
PZ...Small Craft Advisories are in effect for the coastal waters,
central/western Strait of Juan de Fuca, and Puget Sound/Hood
An illustrated version of this forecast discussion can be seen at