Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 200451
Area Forecast Discussion
National Weather Service Seattle WA
951 PM PDT FRI AUG 19 2016
.SYNOPSIS...A strong upper level ridge combined with low level
offshore flow will bring hot weather to Western Washington through
Saturday. The ridge will begin weakening this weekend, and a shift
to onshore flow Saturday evening will break the hot spell by
bringing cooler marine air inland. An upper level trough will move
across the area Sunday night for more cooling. Another upper level
ridge will gradually build over the Pacific Northwest next week with
weak onshore flow turning more offshore around mid week.
.SHORT TERM...Satellite imagery shows just a few high clouds off
to the west this evening. After a record setting hot day across
the area temperatures at 04z/9 pm where still warm...in the mid
60s to lower 80s.
No change to the forecast this evening with another record setting
day on tap for most of the interior on Saturday. Offshore surface
gradients are weakening tonight but the air mass over the area is
a little warmer than last night. The weaker offshore flow will
result in lower low temperatures for locations that were exposed
to the offshore flow last night...like Bellingham and
Quillayute...while the central sound will see lows tonight similar
to last night...in the 60s. Not good sleeping weather after a hot
Changes in the pattern taking place on Saturday. The offshore
surface gradients will be gone Saturday morning with the
gradients turning onshore in the afternoon. This will cool the
coast down considerably from Fridays highs where both Quillayute
and Hoquiam were at least 95 degrees for only the second time in
the record books. Look for around 20 degrees of cooling along the
coast Saturday. The transition to onshore flow will also cool down
locations near the water along the Strait and over the North
Interior by up to 10 degrees. From the Puget sound area southward
the transition to onshore gradients will not occur until the
afternoon. The air mass overhead will be slightly warmer on
Saturday with model 850 mb temperatures plus 20-21C. This will
counter the late shift to onshore flow with the end result being
another hot day with high temperatures similar to Friday...in the
90s. The best chance for cooling over the interior from Seattle
south will be in the lower Chehalis valley and the southern
portion of the Hood Canal but even there we are only talking about
highs in the mid and upper 90s lowering to the lower to mid 90s.
The heat is over Saturday night as the low level onshore flow
increases. Models have the KOTH-KSEA gradients peaking between 4
and 5 mb and the KHQM-KSEA gradient peaking around plus 3 mb
Saturday evening. This is definitely strong enough to push the
marine air into the interior overnight. Lows will return to closer
to the normals...50s and lower 60s.
Low level onshore flow continuing into Sunday with an upper level
trough approaching the area by 00z Monday. 500 mb heights...near
590 dms tonight will be down in the mid 570 dms Sunday afternoon.
With the low level onshore flow continuing all day highs will only
be in the upper 60s to mid 70s over the interior with 60s along
Upper level trough moving through the area Sunday night. The
trough is not strong enough to produce any precipitation but will
deepen the marine layer over Western Washington. The deeper marine
layer on Monday will lead to less sunshine in the afternoon with
daytime highs in the mid 60s to lower to mid 70s across the entire
With the cooler temperatures forecast have allowed the heat
advisory for the Coast...Strait and Northern Interior to expire.
The heat advisory will continue for the Everett area through
Saturday evening. The excessive heat warning for the interior
from Seattle southward will continue through Saturday evening as
The red flag warnings for the Olympics and North and Central
Cascades will also remain in effect through Saturday evening.
.LONG TERM...Previous discussion follows. After the upper level
shortwave trough moves E on Monday, another upper level ridge will
begin building just offshore along 135W. This ridge looks like it
will have higher amplitude than the current ridge over the area.
The low level flow shifts to a N-NE offshore direction Tuesday
night then stays that way for a day or so. By Thursday or Friday
the low level flow, especially at 925 mb, shifts to more of an
E-NE direction, which would allow temperatures to rise back into
the 90s. The current forecast has highs in the 80s on Thursday
with the SW interior poking up into the 90s on Friday. Kam
.AVIATION...Northerly flow aloft over Western Washington this
evening will gradually back to northwest late tonight and
Saturday, as the offshore upper ridge weakens ahead of an
approaching upper trough. At the surface, northerly flow will turn
onshore late Saturday. The air mass will remain dry with no
significant cloud cover below 12000 ft, until marine stratus moves
onto the coast late Saturday.
KSEA...North to northeast wind 6-12 kt, becoming light and
variable late tonight, then northwest 4-10 kt Saturday afternoon.
.MARINE...Northerly flow will continue through Saturday, with
marginal small craft advisory winds over the central and southern
part of the coastal waters this evening.
The flow will turn onshore Saturday afternoon, with increasing
onshore flow through Sunday evening. Gale westerlies are likely in
the central and eastern Strait of Juan de Fuca late Sunday
afternoon or evening, but the could occur Saturday evening as
well. A gale watch is in effect. Small craft advisory winds
should also occur over parts of the coastal waters, Admiralty
Inlet, and the northern inland waters as well as onshore flow
develops and strengthens. Weaker onshore flow will continue
Monday, and the flow will probably trend offshore on Tuesday.
WA...Excessive Heat Warning until 9 PM PDT Saturday for Bellevue and
Vicinity-Bremerton and Vicinity-East Puget Sound Lowlands-
Hood Canal Area-Lower Chehalis Valley Area-Seattle and
Vicinity-Southwest Interior-Tacoma Area.
Red Flag Warning until 10 PM PDT this evening for Black Hills
and Southwest Interior Lowlands-Central and South Puget
Sound Lowlands-North Coastal Lowlands-Southeast Puget Sound
Lowlands Generally Below 1500 Feet.
Red Flag Warning until 10 PM PDT Saturday for East Portion of
the Olympic Mountains-West Portion of the Olympic Mountains-
West Slopes of the Central Cascades Generally above 1500
Feet-West Slopes of the North Cascades Generally above 1500
Heat Advisory until 9 PM PDT Saturday for Everett and Vicinity.
Red Flag Warning from 8 AM to 10 PM PDT Saturday for East
Portion of North Cascades National Park/Lake Chelan
National Recreational Area.
PZ...Small Craft Advisory until 11 PM PDT this evening for 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.
Gale Watch from Saturday evening through late Sunday night 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