Forecast Discussion for SEW NWS Office
000
FXUS66 KSEW 232241
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
341 PM PDT Wed Oct 23 2019

.SYNOPSIS...Tranquil weather will continue through Thursday, with
the next weak frontal system and chance for rain expected on
Friday. Dry conditions then return by the middle of the weekend,
and look to remain in place through at least the middle of next
week.

&&

.SHORT TERM /TONIGHT THROUGH SATURDAY/...Looping water vapor
imagery this afternoon reveals an expansive area of high pressure
over the far eastern Pacific, with influence extending into the
Pacific Northwest. Both upstream and downstream of this feature
lie two long wave troughs, the former of which reveals a river of
atmospheric moisture pushing into BC, with source of moisture the
tropical Pacific. At the sfc, atmospheric river is characterized
by a frontal system, with high pressure set up over the Pacific
Northwest and northern Rockies.

Morning fog (at least in lowland locations near Puget Sound) have
given way nicely to a much appreciated mostly sunny sky. Some high
level cloud cover is noted both on satellite and outside the
office window, and these clouds are associated with the moist
system moving into BC. Rest of the day will be seasonably mild,
with mid and high clouds continuing to increase from the north.

Mid and high level cloud cover will continue to increase from the
north overnight. In addition, another round of fog appears likely
in the SW interior and across the lowlands near Puget Sound
(though enhanced cloud cover could end up limiting coverage to
some extent). Similar to today, any fog will erode through the
morning hours, this time giving way to more higher level cloud
cover. Thursday is expected to remain dry, with persistent stream
of Pacific moisture remaining in BC.

By Friday, a shortwave trough, initially embedded within the
longer wave trough upstream of the eastern Pacific ridge will
progress eastward and begin to break down the ridge. It will carry
with it the aforementioned sfc frontal system with it enroute to
the Pacific Northwest. Luckily, this system will become cutoff
from the main feed of tropical Pacific moisture, allowing for a
quick moving chance for showers through the day across western
Washington. As the ridge attempts to build back over the eastern
Pacific into Saturday, another shortwave will drop across
Washington from the north. This may allow for lingering light
showers over eastern portions of the area the first half of the
day, along with enhanced cloud cover. This looks to be mainly
confined to the Cascades and adjacent foothills.

Kovacik


.LONG TERM /SUNDAY THROUGH WEDNESDAY/...A dry forecast is in store
for western Washington. A treat for us on Halloween week, if you
will. Ensemble guidance is strongly suggestive of a ridge
influence in the upper levels for Sunday and Monday. With high
pressure also at the sfc, this will likely make for pleasant
conditions.

Going farther in the long term period towards midweek, it becomes
a question of how much ridge influence continues to build across
the area and how long the ridge can maintain its strength. Thus
far, it appears ridging will continue to influence the region with
dry conditions continuing through Wednesday.

Kovacik

&&

.AVIATION...Northwest flow aloft with an upper level ridge over
the area. Surface flow is generally light offshore and northerly.
The air mass is generally dry and stable. VFR cigs through early
tonight with high clouds around. Fog/low clouds are expected to
develop tonight, especially across the Southwest Interior, with
patchy LIFR visibilities into Thursday morning across the Puget
Sound. Winds will generally be light northerly. JD

KSEA...VFR cigs into early tonight with high clouds around. LIFR
visibilities possible Thursday morning, especially from 12z-18z
with patchy fog development across the Puget Sound. Winds will
generally be northerly 4 to 8 knots through Thursday morning. JD

&&

.MARINE...High pressure over the area with light offshore flow
through Thursday. A system will move through the waters Friday
and Friday night with increased winds. A Gale Watch has been
issued for the central and eastern portions of the Strait of Juan
de Fuca on Friday for a strong westerly push and elevated
gradients. SCA level winds expected elsewhere for most waters
Friday. High pressure will build back in across the area following
the system. JD

&&

.HYDROLOGY...The final remaining river flood warning for the
Snoqualmie River near Carnation has been cancelled this afternoon.
Hydrograph data shows the river has fallen over a foot below flood
stage, with no additional rises forecast. No hydrologic headlines
are in place for this event anymore. The next chance for rain will
be on Friday, but given the progressive nature of the weather
system bringing the rainfall, additional flooding impacts are not
expected. Dry conditions then prevail this weekend into the middle
of next week. Therefore, no river flooding is expected over the
next 7 days.

Kovacik

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Gale Watch from Friday morning through Friday evening for
     Central U.S. Waters Strait Of Juan De Fuca-East Entrance
     U.S. Waters Strait Of Juan De Fuca.

&&

$$

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NWS SEW Office Area Forecast Discussion