Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 212227
Area Forecast Discussion
National Weather Service Seattle WA
327 PM PDT THU JUL 21 2016
.SYNOPSIS...An upper trough over the coastal waters will move into
Western Washington tonight, bringing partly to mostly cloudy
skies and some showers. Thunderstorms are possible over the
Olympics, Cascades, and south interior this evening. Scattered
showers will decrease Friday. A weak upper ridge should bring
sunny weather Saturday through Thursday.
.SHORT TERM...Southerly flow aloft prevails over Western
Washington this afternoon, ahead of an upper low moving northeast
over the Oregon offshore waters. Skies are mostly to partly sunny,
with some altocumulus clouds spreading north into the forecast
area. Convection has developed over the Cascades crest from about
Stampede Pass southward during the last 90 minutes or so, with a
few lightning strikes just east of Mount Rainier.
The upper low will move into the north Oregon coastal waters this
evening, then continue across the south part of Western Washington
tonight. Models show favorable conditions for showers and possible
thunderstorms to continue developing and becoming somewhat more
widespread this evening: southerly flow aloft, upper level
diffluence and instability, and a 90 kt jet streak along the
southeast side of the upper low leaving us in its left front
region. We will keep the mention of thunderstorms over the
Cascades and Olympics and parts of the south interior lowlands.
The upper low will continue northeast from the south interior of
Western Washington across the Cascades early Friday afternoon. This
timing is a bit slower, and that has been a fairly clear trend
over the past several runs. Scattered showers should continue
tonight and Friday morning, then probably become limited to the
mountains and a weak Puget Sound convergence Friday afternoon and
evening. The upper trough should also result in stronger onshore
low level flow tonight, with marine stratus pushing inland
overnight. Friday should start generally cloudy, with the marine
layer clouds probably giving way to partly sunny skies over most
of the interior lowlands in the afternoon.
Drier northwest flow aloft will develop Saturday, ahead of a weak
upper ridge over the offshore waters. However low level onshore
flow will maintain a moist marine layer below about 5000 ft. So we
can expect some marine stratus again Friday night and Saturday
morning, and then a mostly sunny afternoon. The ridge should move
inland on Sunday, and it should be another sunny warm day. McDonnal
.LONG TERM...A fairly typical pattern for late July will persist
next week with light zonal flow aloft and 500 mb heights generally
between 5820 and 5900 meters. Light onshore flow will continue at
low levels. The result will be sunny day and mostly clear nights,
except some late night and morning marine stratus mainly at the
coast. Heights will probably climb and onshore flow weaken during
the week for a gradual warming trend. Some parts of the lowlands
should reach 90 degrees Wednesday and especially Thursday.
.AVIATION...Upper level low pressure off the Oregon coast will
track northeast across Western Washington later tonight. Moderate
south to southwest flow aloft becoming light northwest flow aloft by
Friday morning. Moisture increasing in the mid levels through this
evening, then becoming moist at all levels overnight. The air mass
will become increasingly unstable with scattered showers developing
and a chance of thunderstorms, mainly over the Cascades. Some
isolated thunderstorms could drift into the interior lowlands but
the chance of thunder at any given terminal is less than 15 percent.
Low level onshore flow will strengthen tonight, causing marine
stratus to fill in across the interior lowlands later tonight and
Friday morning with mostly MVFR and areas of IFR, especially near
the coast. Showers will linger into Friday morning then decrease.
KSEA...VFR skies with increasing clouds near 8-10K ft. N-NW wind 8-
12 kt...then switching to S-SW wind 5-8 kt late this evening. A few
showers will develop later this evening and into Friday morning. The
chance of an isolated thunderstorm within 5sm of the terminal is
less than 15 percent. Marine stratus with 1-2K ft cigs will invade
the terminal during the pre-dawn hours Friday and persist through
the morning. IFR cigs are possible at times. Cigs will lift into a 3-
4K ft stratus deck Friday afternoon with a continued chance for a
few showers. DTM
.MARINE...Onshore flow will strengthen overnight. Gale force winds
are still possible in the Central and Eastern Strait with small
craft winds in areas near the East Entrance to the Strait.
Otherwise, light to moderate onshore flow will continue through the
middle of next week. DTM
PZ...Gale Warning from 5 PM this afternoon to 5 AM PDT Friday for
Central U.S. Waters Strait Of Juan De Fuca-East Entrance
U.S. Waters Strait Of Juan De Fuca.
Small Craft Advisory from 5 PM this afternoon to 9 AM PDT Friday
for Northern Inland Waters Including The San Juan Islands.
Small Craft Advisory from 5 PM this afternoon to 1 AM PDT Friday
for Admiralty Inlet.
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