Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 201616
Area Forecast Discussion
National Weather Service Seattle WA
916 AM PDT WED JUL 20 2016
.SYNOPSIS...An offshore upper low will move slowly toward
Western Washington through Thursday. The area will have partly to
mostly sunny skies, but there is a chance of showers mainly over
the mountains during the afternoon and evening hours.
Thunderstorms are possible over the Cascades Thursday. A weak
upper ridge should bring sunnier weather Saturday through Tuesday.
.SHORT TERM...Light southwest flow aloft continues over Western
Washington this morning, ahead of the broad upper trough offshore.
Moisture is rather limited across the forecast area this morning.
Temperatures were in the mid 50s to mid 60s at 8 am with partly to
mostly sunny skies.
A 5700 meter upper low over Washington`s northern coastal waters
will weaken a bit today, and upper heights over Western Washington
will rise slightly as the large upper high dominating much of the
United States bulges northwest. The air mass will remain somewhat
unstable but it will dry, and the chance for showers should be
limited to the Olympics, the Mount Baker area and Mount Rainier.
The interior lowlands will have a mostly sunny day, with highs in
the 70s. The coast will be partly sunny to mostly cloudy, and
highs will be in the mid 60s to lower 70s.
The upper trough will move slowly eastward toward the region on
Thursday, probably move into Western Washington Thursday night and
east of the Cascades Friday morning. It will bring a chance of
showers Thursday afternoon and evening, mainly to the mountains
and adjacent lowlands where instability will be higher. Models
differ somewhat on the details of this feature, mainly making for
lots of uncertainty with regard to the threat of thunderstorms in
the Cascades. For now we will keep them in the forecast near the
A weak Puget Sound convergence zone could form Thursday night as
the trough axis arrives, and that could linger into Friday
morning. Stronger onshore flow associated with the upper trough
should deepen the marine layer and make for a generally cloud
start to Friday.
By Friday afternoon, the upper trough will be east of Washington
and northwest flow aloft will develop over our forecast area ahead
of a weak upper trough building offshore. The morning cloud cover
should give way to partly sunny skies Friday afternoon, and the
threat of precipitation should pretty much be over. McDonnal
.LONG TERM...Here is the long term section from the early morning
discussion -- The aforementioned minor ridge does look to amplify
for the remainder of the weekend...warming up temperatures in the
interior lowlands into the mid to upper 70s and answering the many
who have wondered where summer has been hiding this year. Once
that ridge moves on at the start of next week...an alternating
pattern of minor ridges and troughs will not make all that much
impact in the local weather...ultimately proving to be an extended
dry period with fluctuations in high temperatures generally
ranging 2 to 5 degrees and mainly staying within the 70s for
interior lowland locations into Tuesday of next week. SMR
.AVIATION...An upper level low off the Pacific Northwest coast will
shift inland across Washington Thursday night. Light southwest flow
aloft becoming southerly on Thursday. Low level onshore flow will
strengthen Thursday afternoon and evening. The air mass will be
stable and mostly dry except somewhat moist in the lower levels.
Increasing moisture at all levels Thursday afternoon and night and
becoming unstable with scattered showers. There will be a chance of
thunderstorms over the Cascades late Thursday afternoon and evening.
KSEA...VFR skies with few-bkn around 7k ft. West wind 4-7 kt
becoming northwest after 19z...then northeast 3-5 kt after 04z this
.MARINE...Surface high pressure over the coastal waters and lower
pressure inland will give onshore flow. Small craft winds to 25 kt
are likely this evening in the Central/E Strait with an advisory in
affect. Lower pressure will develop east of the Cascades Thursday
with stronger westerly onshore flow. A gale watch was issued for
the Central/E Strait Thursday night for winds to 35 kt.
Varying degrees of west to northwest onshore flow is expected
Saturday through early next week. &&
PZ...Gale Watch from Thursday afternoon through late Thursday night
for Central U.S. Waters Strait Of Juan De Fuca-East
Entrance U.S. Waters Strait Of Juan De Fuca.
Small Craft Advisory from 3 PM this afternoon to 1 AM PDT
Thursday for Central U.S. Waters Strait Of Juan De Fuca-
East Entrance U.S. Waters Strait Of Juan De Fuca.
You can see an illustrated version of this discussion at