FXUS66 KSEW 170507 CCA
Area Forecast Discussion...CORRECTED
National Weather Service Seattle WA
907 PM PST Fri Nov 16 2018
Corrected to add paragraph about wind shear in the KSEA aviation
.SYNOPSIS...A dry weather pattern with areas if night and morning
fog will persist into early next week. Overnight temperatures
will be on the cooler side, dropping into the 30s and even upper
20s by Sunday morning at some locations. A weather system will
bring wet weather back by mid to late next week.
Clouds still lingered over mainly the southern part of the CWA
at this time. The forecast was updated to slow down the rate of
clearing. Otherwise, no major changes to the previous thinking
The upper level ridge along 135W will continue to build over the
coastal waters tonight. Clearer skies will help low temperatures
dip into the 30s tonight, with near freezing temperatures in the
far north. Patchy morning fog - with some freezing fog - Saturday
will burn off to sunny skies and near normal temperatures in the
low 50s. Overnight temperatures Saturday night into Sunday morning
at some locations will dip into the upper 20s. Morning fog will
return again Sunday with clearing with high temperatures rising a
bit over Saturdays temperatures, especially through the eastern
interior lowlands. The upper level ridge will shift onshore
Sunday, with low level flow remaining offshore.
.LONG TERM...Medium range models remain in general agreement
through midweek. The upper level ridge will linger over the region
Monday and Tuesday, keeping conditions dry. Monday will be mostly
sunny while a weak weather system tries and fails to make much
headway against the ridge. Increasing clouds Tuesday, but dry with
high temperatures in the low to mid 50s. Models continue to show a
weather system affecting the area Wednesday, with rain reaching the
coast early Wednesday morning, followed by a second impulse during
the day Thursday followed by a showery pattern into Friday.
.AVIATION...A strong upper level ridge offshore will provide the
region with strong nly flow aloft overnight. The low level flow
will also be nly. Areas of MVFR CIGs will lower into the IFR
category overnight. Expect areas of LIFR VSBYs, due to ground-
based fog, to dvlp in the areas that are cloud-free.
KSEA...MVFR CIGs (1-3k ft) should continue thru at least 0600 UTC
(10 PM PST), perhaps as late as 1200 UTC (4 AM PST). The short
range ensemble forecasts indicated that the clouds will clear out
overnight but the models' timing has been too quick. This is the
reason the clouds are being held onto a little longer than what
the models indicated. There is a possibility of IFR CIGs (500-900
ft) early Saturday, hence the TEMPO group. Confidence was not
high, though. Otherwise, expect northerly winds near 10 knots to
persist thru daytime Saturday.
It looks like there will be a period of low level wind shear
Saturday morning due to winds just above the surface being out of
the east near 15 knots while the surface winds remain out of the
north. This may persist into Saturday afternoon.
.MARINE...High pressure over the Pacific will build east
overnight and into Saturday for generally benign weather
conditions through early next week. The northerly flow this
evening was a little stronger; therefore, wind speeds were a bit
higher over the Puget Sound and Admiralty Inlet. The models
indicated that the northerly flow will strengthen further on
Saturday, which means that a Small Craft Advisory will probably be
needed for the Puget Sound.
Next weather system will approach by Wed-Thu and turn winds more
southerly again and increase the speeds. Johnson/05
.HYDROLOGY...Models are indicating a rainy period to start up
midweek through the end of next week. This would drive rivers up
sharply. But so far, no river flooding looks likely through the
next seven days.
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