Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
830 PM PDT Thursday Jul 2 2015
..correction in the 4th paragraph...
Synopsis...strong high pressure aloft will remain over the Pacific
northwest into at least early next week for continued sunny and
unseasonably warm weather.
Short term...strong upper level ridge remains over the western U.S.
And the Pacific northwest. Satellite imagery shows some cirrus clouds
moving onshore in the light westerly flow aloft...the only blemish
is the clear skies for tonight into Friday.
The upper low that was in the Gulf of Alaska on Tuesday has been kicked
out by another upper low moving east along the Aleutians. The former
Gulf of Alaska low is riding over the ridge way up in the southern
It is the Aleutian low that pumps up upper ridge near 140w and
results in more northwest flow aloft and sagging 500 mb heights as the
former Gulf of Alaska low works it way inland across northern
Canada. The 00z forecasts in hand thus far continue this trend that
results in chipping a few degrees off the record or near record warm
temperatures today...for fridays highs. This trend continues into
Sat as heights slide a bit more then.
No change in the heat advisory for the I-5 corridor from central
Puget Sound sound southward and the warm Special Weather Statement
elsewhere at this point. A subtle increase in low level onshore flow
may permit some marine cloudiness to reach the coastline Saturday
morning....perhaps in a few spots Friday morning.
In the wake of the now Canadian low heading further inland over the
Holiday weekend...surface high pressure builds in b.C. Setting up
low level offshore flow by Sunday morning. The east-NE winds...normally
cold in the winter season...will serve up warmer temperatures on
Sunday with some spots reaching their warmest of the year thus far -
well into the 90s.
And an interesting climate stat...SeaTac Airport averages 3 days of
90 degrees or better per year. Todays high of 93 was the third day
of 90 plus this year...and it is only July 2nd. Social media is
eating that stat up. Stay cool - buehner
Long term...from the previous discussion...a somewhat broad weak
upper level trough with northwest flow aloft will be over the pacnw Monday
through Thursday. Mostly sunny weather will continue but the air
mass will be a little cooler with 500 mb heights down to 5820 to
5840 meters. Onshore flow will remain generally westerly and weak
for limited stratus. Maximum temperatures on Monday will be in the 80s to lower
90s...a step down from sundays hot readings. Slightly more air mass
cooling should drop the warmest spots into the 80s Tuesday through
Thursday. The GFS has some precipitation brushing the far north Cascades
Tuesday and Wednesday caused by weak shortwave troughs moving across
southern b.C. European model (ecmwf) shows about the same on Tuesday...but there is
still enough uncertainty to hold off on showers for a little longer.
Aviation...a flat upper ridge will remain over the region through
Friday...with westerly flow aloft over western Washington and light
onshore flow at low levels. The air mass will remain dry except for
shallow surface-based marine moisture near the coast.
Skies will be generally clear through Friday...except for areas of
late night and morning stratus along the coast.
Ksea...north wind 4-10 knots...becoming northwest midday Friday. Mcdonnal
Marine...weak onshore flow will continue through Friday night...
with high pressure well offshore and low pressure over eastern
Washington. Small Craft Advisory northwest winds will continue over
the outer coastal waters...with Small Craft Advisory westerlies in
the central Strait of Juan Delaware fuca and eastern Strait during the
Low pressure will develop along the Washington coast this weekend
and high pressure will move into southwest Canada. The result will
be offshore flow and winds should remain 20 knots or less. Weak onshore
flow should resume Monday. Mcdonnal
Washington...heat advisory for Seattle...Bellevue...Tacoma...and east Puget Sound
sound lowlands zones.
Pz...Small Craft Advisory outer coastal waters...central and
east Strait of Juan Delaware fuca.
You can see an illustrated version of this forecast discussion at
www.Weather.Gov/Seattle/gafd/latest_webafd.Html - temporarily out
of service during a software update period.