Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
930 PM PDT sun Jul 5 2015
Synopsis...a weak marine push will allow some clouds and cooler air
to filter onto the coast late tonight and into the Chehalis gap and
south Puget Sound sound Monday morning. In the interior it will remain
quite warm through midweek as an upper level ridge combines with
thermally induced low pressure at the surface. More seasonable
weather will arrive late in the week as an upper trough nears the
area and deeper marine air pushes into the interior.
Short term...large wildfires east of Whistler and northwest of the
Fraser valley is producing a smoke plume that is drifting into
western Washington. The smoke layer is thickest from Vancouver
Island into the northwestern olympic peninsula...but a thin layer
extends into the entire area. Believe it or not...this smoke layer
held temperatures down 2-3 degrees below what they could have been
this afternoon. It will also likely hold minimum temperatuers
up a degree or two tonight.
Onshore pressure gradients are higher this evening than what we were
expecting over the past couple of days. Koth-ksea is over 4 mb and
the pressure gradient through the Strait is over 2 mb. The nam12 has
picked up on the stronger onshore flow and the 12z European model (ecmwf) seems to
have picked up on it as well. The stronger gradients will allow
stratus to fill in along the coast then to spread inland into the
south Puget Sound sound to just about SeaTac Airport for a few hours
Monday morning. A quick update to the cloud forecast for Monday
morning and for slightly cooler temperatures from about Seattle
southwestward through the Chehalis gap will be made this evening.
SeaTac may stay just below 90 on Monday...and if it does-it will end
the all time streak of 90 degrees at the record tying 5 consecutive
days. With dewpoint temperatures in the middle and upper 50s on
Monday...it will feel a bit muggy and a bit more uncomfortable. The
heat advisory for the central Puget Sound sound urban corridor will be
maintained through Monday evening.
The 00z nam12 shows a Finger Ridge over the coastal waters Monday
afternoon and evening. With heating over the Cascades and interior
valleys during the afternoon...expect onshore pressure gradients to
be a bit stronger Monday evening. This will likely result in a
few degrees cooling at lower elevations in the interior on Tuesday.
Will let the next shift make the change to the Tuesday forecast
after looking at the incoming GFS and European model (ecmwf) solutions.
Short wave ridging aloft will shift in to western Washington on
Wednesday resulting in weaker onshore pressure gradients and a few
degrees warming. Albrecht
Long term...from the previous long term forecast...night and
morning low clouds and fog may trend toward an all day marine layer
for the coast by the end of the week. Temperatures should start to fall off
a bit across the interior of western Washington on Thursday and then dramatic
cooling should occur on Friday with a deep marine push. An upper
trough will be nearing the area and another upper trough will kick
out of northern calif and across the inland portion of the pacnw...that
will increase low level onshore flow into western Washington and the air aloft
will cool as well.
Aviation...smoke emanating from wildfires will continue to drift
across the northwest portion of Washington overnight...possibly through Monday morning.
The smoke layer was in the 5-7k feet range in some areas and the
smoke has also locally reduced visibilities down into the 2-4sm range...
particularly near the Canadian border. This was being complicated by
fog on the north coast. The smoke will primarily impact the following
taf sites...kbli and kclm. Based on the steering flow...the densest
smoke layer should not drift much farther S...however a thin smoke
layer will be present over kpae...kbfi...and ksea.
Meanwhile...westerly flow aloft and weak low level onshore flow will
prevail through Monday. Anticipate areas of stratus and fog /LIFR ceilings and
visibilities/ to gradually spread inland overnight. Areas of LIFR ceilings
and/or visibilities are possible over the far southern Puget Sound sound region by
Ksea...VFR. Anticipate a thin layer of smoke aloft to remain over
the area overnight. Light northerly winds will become southerly overnight
Marine...smoke from wildfires in British Columbia will continue to
impact the northern Pacific coastal waters zones...the Strait of
Juan Delaware fuca...and parts of the northern inland waters through
Meanwhile...high pressure along the coast and lower pressure east of the
Cascades will result in weak onshore flow through Tuesday. The onshore
flow will strengthen some Wednesday through Friday.
Fire weather...while conditions in the mountains will remain very
dry and hot...winds are expected to be lighter through at least
Monday as thermally induced low pressure sits over the interior.
A shallow 1000 feet thick layer of marine air is pushing onto the
coast...into the Chehalis gap...and into the southern Puget Sound sound
lowlands tonight through early Monday. The marine layer will have no
effect on the weather in the mountains.
The next fire weather concern is turning to lightning
potential over the north Cascades on Tuesday afternoon. Looks like
there will be enough instability at the tail end of an upper
disturbance diving southeast through b.C. To trigger a thunderstorm or two.
Isolated thunderstorms were introduced to the north Cascades on
Tuesday aftn/evng. There is a potential for scattered coverage. Given
how dry fuels are...if later forecasts increase coverage to
scattered...then a Fire Weather Watch or red flag warning would be
needed for zone 658 (north cascades) and 662 (north Cascades
National Park east of the crest). However...even isolated lightning
can start fires with the historically dry fuels. Haner/albrecht
Washington...heat advisory in effect for parts of the Puget Sound sound region
until 9 PM Monday.
Pz...Small Craft Advisory in effect for the central/eastern Strait
of Juan Delaware fuca until 5 am PDT Monday.
The illustrated version of the discussion is unavailable at this