663
FXUS66 KSEW 191621
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
921 AM PDT Fri Jul 19 2024

.SYNOPSIS...Western Washington on the back side of an upper level
ridge centered over the Rockies. An approaching upper level trough
will build the ridge over the area Saturday. The trough will nick
the northwest portion of Western Washington Sunday. A change in
the weather pattern beginning Monday with an upper level trough
moving into the area. The trough will remain over Western
Washington through Thursday resulting in cooler weather next week.

&&

.SHORT TERM /TODAY THROUGH SUNDAY/...Satellite imagery shows
stratus has made it into portions of the Sound this morning, but
has already started to dissipate. Expect stratus to continue to
scatter and retreat back to the coast through the remainder of
the morning. The forecast remains largely on track. The remainder
of the previous discussion follows below, with updates made to
the aviation and fire weather sections. 14

Light onshore flow continuing today. Ridge building slightly over
the area with upper level trough digging offshore. 500 mb heights
this morning in the lower 580 dms rising to the upper 580 dms
this afternoon. With the light onshore flow highs today just a
couple of degrees warmer than Thursday with mid 70s to mid 80s
inland and mid 60s to mid 70s along the coast.

Upper level ridge continuing to build tonight into Saturday with
500 mb heights in the lower to mid 590 dms and the flow aloft
becoming southerly by Saturday afternoon. Temperatures aloft
warming with 850 mb temperatures rising from plus 14C to 16C
tonight to plus 16C to 20C Saturday afternoon. Low level flow
remaining light onshore. The warming temperatures aloft combined
with the marine layer being pretty shallow will result in less
morning cloud cover and warmer high temperatures. Highs Saturday
in the upper 70s to near 90 inland and in the 70s along the coast.
Lows tonight in the 50s and lower 60s.

Upper level ridge slowly shifting east Saturday night into Sunday
as an upper level trough approaches the area. Onshore flow
increasing Sunday with the marine push overnight into Sunday
morning resulting in cooler high temperatures by 5 degrees or so
Sunday afternoon for the interior and 10 degrees or so along the
coast. Weak shortwave moving by to the west could produce a shower
or two along the coast Sunday morning. Highs in the mid 70s to
mid 80s for the interior and mid 60s along the coast. Lows Sunday
morning in the 50s and lower 60s.

.LONG TERM /MONDAY THROUGH THURSDAY/...Models in good agreement
with a pattern change beginning Monday with an upper level trough
moving over the area. The trough will remain over Western
Washington through the week with stronger onshore flow in the
lower levels. This combination will push high temperatures down
near normal. The last time Seattle had back to back days with
below normal high temperatures was June 26 and 27. Morning clouds
with afternoon sunshine will be the rule through the period with
the possibility of the clouds hanging in all day along the coast.
Highs for the interior in the upper 60s to upper 70s with mostly
60s along the coast. Felton

&&

.AVIATION...Western Washington is positioned between an upper
trough offshore and ridging aloft over the interior West for
west to southwest flow aloft. The low level flow is weak
onshore. IFR stratus will spread locally inland from the coast
through the Chehalis Gap and Strait of Juan de Fuca before burning
back to the coast by around 18Z. LIFR/IFR ceilings in stratus and
fog will return to the coast tonight, but will likely make little
progress inland as onshore flow will be weaker.

KSEA...Current IFR cigs at the terminal. However, latest satellite
trends are showing marine stratus dissipating. VFR for the remainder
of the day. Surface winds southwesterly 4 to 8 knots veering to
northwest this afternoon then northerly by late this evening.
27/McMillian

&&

.MARINE...Surface high pressure offshore and lower pressure inland
will continue to produce varying degrees of onshore into the
weekend. Onshore flow will increase Saturday night into Sunday
as thermally induced low pressure over the interior shifts
eastward. This will likely result in gales for the central/east
strait and small craft advisory winds for adjacent waters and
portions of the coastal waters.

A broad upper trough will settle over British Columbia early next
week as a series of weather systems rotate around it. This will
keep onshore flow in place with additional headlines
expected...especially in the Strait of Juan de Fuca and across
portions of the coastal waters. 27

&&

.FIRE WEATHER...Warm and dry conditions through Sunday. Afternoon
minimum RHs again dipping close to critical thresholds today and
Saturday for the lowlands and through Sunday for the Cascades.
Instability may increase enough to keep a mention of elevated
concern for the Cascades especially Saturday afternoon and evening.
Another disturbance may bring a renewed threat for thunderstorms
Sunday across some portion of the mountains, high resolution
ensembles including the NBM showing a 10-15% chance at this time.  A
cooler more humid pattern develops Monday as the pattern switches to
be dominated by an upper level trough over the region.

&&

.CLIMATE...Seattle tied the record for consecutive days with highs
80 degree plus with Thursday being the 15th day in a row.
Forecast highs continue the streak through the weekend before it
comes to an end Monday. Moving down a notch, the record for
consecutive days with highs 75 degrees plus in Seattle is 23 days
from June 17th through July 9th in 2015. There have only been 3
streaks of 20 plus days in 80 years of records. The other two 20
day plus streaks, 21 days July 29th through August 18th, 1977 and
20 days July 18th through August 6th, 1994. Felton

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory from 5 PM this afternoon to 3 AM PDT
     Saturday for Central U.S. Waters Strait Of Juan De Fuca-
     East Entrance U.S. Waters Strait Of Juan De Fuca.

&&

$$