Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
922 am 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.
Strong upper level ridging over Texas and Oklahoma continues to
expand and retrograde toward New Mexico and the four courners region
of the Desert Southwest today. The latest GFS solution continues to
raise 500 mb heights to around 590 dam this afternoon in response to
the synoptic scale pattern shift. At the surface...the thermal
trough that was previously expected to develop from the Olympics
down to the Willamette Valley has actually formed just off the
Washington coast. In response...cross Cascade gradients are
none...Bellingham to Williams Lake has lowered to around -4 mb...and
kuil-kbli is -1.4 mb. Northerly pressure gradients through the Puget Sound
sound area is about the same as yesterday at this time and
temperatures are running about 4 degree f hotter than this time
yesterday. At 850 mb...temperatures will rise to around 21c this
aternoon and again Friday afternoon. So...we appear to be on track
for a high at SeaTac today around 95 degree. The GFS MOS guidance for
Shelton is an eye popping 100 this afternoon. It appears that Friday
will be equally hot. Low temperatures tonight will cool into the 50s
in outlying areas...but hilltops and urban areas will have a hard
time dropping below 65 degree. If there is one piece of good
news...dewpoint temperatures are in the low to middle 50s this
morning...and will fall into the middle to upper 40s this afternoon as
very dry air aloft gets mixed downward with some low level
instability and some afternoon breezes.
We may see some slight cooling on Saturday as 850 mb temperatures
fall to around 18c and heights aloft fall into the middle 580 dam
range. But it appears that Saturday will make 4 90-plus degree days
in a row at SeaTac and will add to the annual record. The heat
advisory that is in effect through Saturday evening for the Seattle-
Bellevue-Tacoma metropolitan areas...the east Puget Sound sound lowlands...and
the southwest interior will obviously be maintained.
Current forecasts look like they are in good shape. No morning
update is planned. Albrecht
Long term...from the previous long term discussion...most models
weaken the high pressure ridge Sunday and Monday. 500 mb heights
continue to trend lower and each day will have increasing onshore
gradients. Areas near the foothills and parts of the SW interior may
still reach close to 90 on Sunday...but most spots will fall back
into the 80s...with 70s along the coast...Strait...and north interior. A
stronger onshore push is likely Sunday evening which should result
in further cooling. By Tuesday...highs should be closer to average
for this time of year as a marine air mass prevails. A trough off
the coast will approach on Wednesday with a chance of
showers...especially west of Puget Sound sound and across the north. Mercer
Climate...Seattle is on the verge of setting the record for the all-
time warmest month. This includes the federal building records which
go back to the 1890s. The warmest month on record for Seattle is
August 1967 with an average temperature of 71.1 degrees. Through the
29th the average temperature for July 2015 is 70.7 degrees. Seattle
needs to record an average temperature of at least 76.5 degrees the
next two days to break the record.
Seattle also has a chance to break the record for the warmest
average monthly low temperature. The current record is 59.9 degrees
set in August 2013. Through the 29th the average low temperature for
July 2015 is 59.7 degrees.
It will not be a record for the warmest average monthly high
temperature. The record is 83.7 degrees set in August 1967. Through
the 29th the average high temperature for July 2015 is 81.8 degrees.
Even with a couple of hot days ahead...the average high temperature
for the month will only increase .8 to 1 degree. July 2015 will end
being the second warmest month on record in terms of average high
Today will be the 10th day this year with a high of 90 degrees or
more. This will break the record for the most number of 90 degree
plus days in a year. The old record was 9 set in 1958.
Today will be the 8th day this month with a high of 90 degrees or
more. This will break the record for most 90 degree plus days in a
month. The old record of 7 was set in July 1958. Felton
Aviation...an upper ridge will continue over the region. Continued westerly
flow aloft and low level northwest flow. VFR conditions will prevail
across the entire area.
Ksea...VFR. Continued northerly winds.
high pressure offshore with lower pressure east of the Cascades will keep
the flow onshore through Friday. The flow will become briefly
northerly or offshore early Friday due to a thermally induced area of
low pressure trying to set up along the Washington coast.
Onshore flow will strengthen this weekend...leading to the
possibility of gale force winds over the central and eastern
Strait of Juan Delaware fuca on Sunday. It looks like the threat of gale
force winds over these areas will be higher on Monday.
Fire weather...looking at a few days of critical fire weather ahead
for nearly all of western Washington. First...a very dry air mass
has settled over western Washington and will remain in place through
Saturday. This raises concern for both The Lowlands and the
mountains. The dry air mass is deep...with 850-700 mb layer average
relative humidity will be running as low 10 percent between now and
Saturday. Middle-level Haines index will reach a value of 6 at times
over SW Washington and from about I-90 on south over the Cascades.
This indicates not only a dry air mass...but also the added effect
of middle-level instability. With the added concern of elevated
mountain zones rising into the more unstable air aloft...a red flag
warning for dry and unstable conditions will remain in effect over
the mountain zones through Friday evening...with the possibility of
extending the warning into Saturday. This kind of air mass supports
active fire behavior and rapid growth on existing fires. Haner
Washington...heat advisory in effect from noon Thursday until 9 PM Saturday
for parts of the Puget Sound sound region and southwest interior.
Red flag warning in effect until 10 PM PDT Friday for the
north and central Cascades and parts of the Olympic
Pz...Small Craft Advisory in effect from 5 PM this afternoon 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