Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
325 PM PDT Thursday Jul 30 2015
Synopsis...a strong upper level ridge will be over the Pacific
northwest through Saturday. The ridge will weaken and shift a little
east Sunday into Monday. An upper level trough will move over the
area by the middle of next week.
Short term...strong upper level ridging continues to build into the
area from the southeast today. 500 mb heights are nearing the 590
dam value in Seattle and little change in the pattern aloft is
expected on Friday. 850 mb temperatures are expected to be around
21c again Friday afternoon. The only minor change to the low level
pattern tonight into Friday will be the migration of the surface
thermal trough from the coast to a position from the Olympics into
the southern Cascades. This will provide some cooling to the coastal
zones on Friday with highs in the 70s on the beaches to the lower
80s at the base of the Olympics and east of Aberdeen. In the
interior...another hot day is expected east of Interstate 5 in the
north interior and elsewhere in the interior from Seattle southward.
SeaTac will once again approach the 95 degree mark on Friday...and
some locations in the south interior will approach 100 degrees.
Overnight lows tonight will fall into the middle to upper 50s in
outlying areas but will remain in the lower to middle 60s in the
central Puget Sound sound heat Island Area.
With the migration of the thermal trough a little to the east...we
will see some increase in westerly flow through the Strait of Juan
Delaware fuca and some influx of marine air into the Chehalis gap on
Friday night. This will allow the formation of some patchy stratus
on the coast Saturday morning and will provide further cooling over
the coastal zones and around the Strait of Juan Delaware fuca on Saturday.
Some minor cooling is expected away from the marine air in the
interior as 500 mb heights fall about 50m and 850 mb temperatures
drop to around 19c. Saturday will likely be yet another 90 degree
day for the year.
The heat advisory for the Seattle and Tacoma areas...the east Puget Sound
sound lowlands...and the southwest interior through Saturday evening
will be maintained.
Saturday night into Sunday will see 500 mb heights fall
further...down to around 580 dam as the ridging over the Pacific
northwest moves off to the East. Seattle will likely stop short of
the 90 degree mark Sunday afternoon...but dewpoint temperature rises
5 to 8 degrees will make it feel more humid. Albrecht
Long term...it seems that the operational 00z and 06z model
solutions over the past 2-3 days have favored delaying the arrival
of a rather significant upper level low from the northwest while the
12z and 18z solutions bring the bring it southeastward faster.
Latest GFS...ECMWF...and Canadian solutions favor a weak to moderate
marine push for Sunday night into Monday dropping highs in the
interior to the middle 70s to lower 80s on Monday...then increasing the
strength of the marine push Tuesday into Wednesday as the upper level
low off the alaskan Panhandle drops southeast to Vancouver Island Wednesday
night. The upper low will likely be around the area Thursday into
Friday of next week giving cooler than normal temperatures along
with clouds and a chance of showers. Albrecht
Aviation...an upper ridge will continue over the region. Continued westerly flow
aloft and low level northwest flow. Patchy LIFR ceilings/visibilities are anticipated
over the coast after 0900 UTC.
Ksea...VFR. Continued northerly winds.
Marine... persistent high pressure offshore with lower pressure east of the
Cascades will keep the flow northwest through Friday. Expect the flow
to become more westerly and strengthen this weekend... leading to
the possibility of gale force winds over the central and eastern
Strait of Juan Delaware fuca.
Fire weather...a critical period for fire weather is underway...
given how extremely dry the air mass is over western Washington.
Many locations are experiencing relative humidity down in the 20s
this afternoon...even locations that normally stay more humid such
as near the Strait of Juan Delaware fuca. When you get above a low-level
subsidence inversion near 2000 feet mean sea level...the air mass is also more
unstable. With that additional aggravating factor...will keep the
red flag warning going for elevations above 2000 feet. Looks like
little change in the character of the middle-level air mass until
Saturday evening...so have extended the red flag warning until 10 PM
Saturday. This is the kind of weather setup that helps small fires
grow to become large fires.Haner
Washington...heat advisory in effect until 9 PM PDT Saturday
for parts of the Puget Sound sound region and southwest interior.
Red flag warning in effect until 10 PM PDT Saturday for the
north and central Cascades and parts of the Olympic
Pz...Small Craft Advisory remains in effect until 11 PM tonight for
the coastal waters from James Island to Cape Shoalwater out
to 10 nm.
An illustrated version of this forecast discussion can be seen at