Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 272224

Area Forecast Discussion
National Weather Service Seattle WA
325 PM PDT Thu Oct 27 2016

.SYNOPSIS...An upper level trough lifting into British Columbia will
keep showers over the area through this evening. Weak upper level
ridging and high surface pressure east of the Cascades will help
conditions dry on Friday, especially over the interior. For the
weekend and early next week, a series of disturbances will keep
unsettled weather over the region, with brief periods of dry weather
between systems.


.SHORT TERM... A broad upper level trough over the Northeast Pacific
and Gulf of Alaska is helping nudge an upper level trough over the
offshore waters of Washington and Oregon into British Columbia this
afternoon and evening. Radar and satellite imagery this showing
scattered showers associated with the weak upper level trough,
mainly over the Coast, Olympic Peninsula, and North Interior.
Mesoscale models continue to show showers remaining liminted to the
Coast and North Interior with lower chances for showers over the
Southeast half of the forecast area.

The broad upper level trough over the Gulf of Alaska and Northeast
Pacific will dig south along 140W Friday, allowing upper level
heights to rise somewhat over Washington and Oregon. In addition,
weak high pressure at the surface will build over Eastern Washington
on Friday as another weak frontal system brushes the Olympic
Peninsula. This weak system will impact mainly the Coast, Olympics
with light offshore pressure gradients helping keep the interior
dry. Beyond this, models continue to show moisture working north
into the region Saturday as the offshore upper level low begins to
shift east. Expect rain along the coast early Saturday morning,
spreading into the Olympic Peninsula and interior through the day.
Upper level heights will rise Saturday night into Sunday as the
upper level trough shifts into Northern California and Oregon. This
will produce a short lived break between systems Saturday night into
Sunday morning.

.LONG TERM...Medium range models continue to show split flow
dominating along the west coast of the US from Sunday through the
middle of next week. This will allow a series of weak systems to
brush the area, briging periods of rain with drier weather between.
The challenge will be refining the timing, location and duration of
the rain as well the drier weather. The first upper level trough
moving into Oregon on Sunday continues to look like it will remain
mainly to the south of the area with Western Washington seeing
clouds and the best bet for rain south of Seattle. A weak system
embedded in the northern stream of the split flow will clip the area
Monday, resulting in a chance of rain Monday into early Tuesday. An
upper level ridge and low level offshore flow Tuesday will result in
the driest weather in the extended, followed by a shift in the large
scale pattern that will bring more consolidated flow into the
Western Washington and a return to more active weather.


.AVIATION...An upper trough embedded within moderate southwest
flow aloft will pass western Washington this evening. As a result,
shower activity will continue to expand early this evening, then
fade late tonight after the upper trough passes.
Expect MVFR cigs at times within shower activity. On Friday, a brief
period of upper ridging will build into the area, and drying low-
level easterly flow will strengthen during the day. This will lead
to more substantial thinning and scattering of low-mid level cloud
layers on Friday.

KSEA...The frequency of passing showers will peak in the 03z-08z
time frame tonight, as an upper disturbance scoots by. Ceilings
could drop briefly into the 020-030 range during heavier showers.
Showers will move out and dissipate overnight. Then the development
of drying easterly low-level winds on Friday morning will easily
clean out low clouds by mid-day, with mid-level clouds gone by late
afternoon. Haner


.MARINE...A weak 1008 mb low will move northward inside 130W on
Friday, bringing Small Craft winds to the coastal waters, and
possibly to the West Entrance. The low will further weaken over the
offshore waters on Friday night and Saturday. Low pressure will move
northeast through the Oregon waters on Sunday...then weaken as it
lifts NE through western Washington on Monday. Haner


.HYDROLOGY...River flooding is not anticipated over the next 7

PZ...Small Craft Advisory from 3 AM to 6 PM PDT Friday for Coastal
     Waters From Cape Flattery To James Island 10 To 60 Nm-
     Coastal Waters From Cape Flattery To James Island Out 10 Nm-
     Coastal Waters From James Island To Point Grenville 10 To
     60 Nm-Coastal Waters From James Island To Point Grenville
     Out 10 Nm-Coastal Waters From Point Grenville To Cape
     Shoalwater 10 To 60 Nm-Coastal Waters From Point Grenville
     To Cape Shoalwater Out 10 Nm.



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