Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 181046
Area Forecast Discussion
National Weather Service Seattle WA
346 AM PDT Tue Oct 18 2016
.SYNOPSIS...Weak disturbances will move across the area today and
keep conditions unsettled. High pressure aloft will move over the
region tonight before moving east on Wednesday. Expect another low
pressure system to bring wet and locally windy conditions to the
area Wednesday night. An occluded front will move across the
region Thursday morning. Expect showery weather behind the front.
There were quite a few showers present over the CWA at this time.
An upper level trof of low pressure was just off the coast early
this morning and will move across the area this morning, and will
be followed by a much weaker trof this afternoon. Therefore,
expect scattered to numerous showers over the area today. The best
chance (albeit slight) of thunderstorms will be over the coastal
waters but that does not mean that an isolated random lightning
strike cannot occur elsewhere during the afternoon.
Concerning tidal overflow today: Astronomical high tides will be
about the same today as they were on Monday, but atmospheric
pressures will be running about 10 mb higher. Therefore, while
tidal levels will come close to the tidal overflow value, they are
not expected to exceed it.
Weak high pressure aloft will prevail over the region tonight for
a drying trend. The upper level ridge will shift east during the
day Wednesday, allowing an upper level trof to approach from the
west. It appeared that Wednesday morning will be dry for much of
the CWA. Expect rain to overspread the area from the west during
the afternoon; although, the northeast part of the CWA will
probably stay dry during the day.
Expect wet and locally windy conditions Wednesday night. The
occluded front will move across the area during the late Wednesday
night through Thursday morning time frame. Look for post-frontal
showers Thursday afternoon. Albrecht/05
The medium range solutions appeared to be more or less in good
agreement for Thursday night and Friday. With that being said,
expect a threat of showers Thursday night. Another, but weaker,
occluded front will bring a chance of steady precipitation to the
CWA during the day Friday. Anticipate showers Friday night.
Saturday and beyond: Confidence in the forecast was not high due
to disagreement in the model solutions. However, it does appear
that the medium range models were in some agreement that the
weather pattern will be a bit active. Did not feel comfortable
with a dry forecast for any particular day during the extended
period for this reason.
.AVIATION...A surface and upper level trough extending from the
Gulf of Alaska into the Washington and Oregon coastal waters is
giving moist and unstable southwest flow aloft to the region. The
trough will lift northeastward across the region this morning,
giving a drier and more stable westerly flow aloft later this
afternoon and tonight.
Conditions continue to be generally VFR. Areas of MVFR conditions
are being seen in heavier or more persistent showers. Ceilings
will gradually fill in to become broken at 2000-2500 feet this
morning through midday. Low level moisture along with showers and
isolated thunderstorms will gradually decrease this afternoon and
KSEA...Southeast to south wind 5 to 8 kt. An area of showers
moving northeast across the region will taper off mid morning.
Ceilings will generally be between 2000 and 3000 ft across the
area until midday. Any thunderstorm activity that develops today
is expected to stay away from the terminal and approaches.
.MARINE...A weak surface trough of low pressure over the coastal
waters will lift northeast through the inland waters this morning.
A ridge of high pressure will build along the coast late this
afternoon into tonight. A small craft advisory for hazardous seas
continue across the coastal waters early this morning, and will be
coming to an end in the near shore coastal waters by 6 am. 10 foot
seas will continue in the outer coastal waters through tonight.
A moderate to strong frontal system associated with a low moving
from the offshore waters into northern Vancouver Island will move
onshore late Wednesday night or Thursday morning. Expect south to
southeast winds to rise to 20 to 30 knots over the coastal waters
Wednesday afternoon, then rise to near gale force later Wednesday
night. Gales are also possible ahead of this incoming front late
Wednesday night or early Thursday at the Entrances to the Strait
of Juan de Fuca and in the Northern Inland Waters. Models have
trended downward with the expected winds later Wednesday night and
Thursday morning, so will not issue a gale watch at this time.
A weaker front will move through the waters late Thursday night or
early Friday. Albrecht
The Skokomish River has fallen below flood stage. Will allow the
flood wave to reach the mouth of the river before ending the
warning. Renewed flooding is possible on this river on Thursday.
Another 2.5 to 3 inches of rain is forecast to fall on the basin
during the 24-hr period ending at 5 PM PDT, Thursday. This should
be enough to drive the river close to or slightly above flood
stage (17.0 feet).
Apart from the uniquely flood-prone Skokomish River, flooding is
not anticipated on other area rivers the next 7 days.
PZ...Small Craft Advisory for hazardous seas are in effect for the
Small Craft Advisory for rough bar in effect until 6 AM PDT
Wednesday for the Grays Harbor Bar.
An illustrated version of this forecast discussion can be seen at