Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
338 PM PDT Wednesday Aug 24 2016
Synopsis...a strong upper level ridge centered offshore combined
with low level offshore flow will bring sunny and very warm weather
through Friday. An approaching upper level trough and weak cold
front will trigger a cooling marine push Friday night before moving
across western Washington on Saturday. This will usher in cooler and
cloudier weather with a chance of showers this weekend. An upper
level trough will remain over the area next week for mostly cloudy
skies, a chance of showers, and temperatures near normal.
Short term...a strong upper level ridge is over the region this
afternoon with 500 mb heights over Seattle around 5850 meters. The
ridge axis is centered offshore along roughly 135w. Low level
offshore flow has developed today, mainly from a north-NE direction,
which has allowed a thermally induced surface trough to form along
the coast. Offshore flow will become a bit more east-NE later tonight
and Thursday morning, with another brief period late Thursday night
and Friday morning. The offshore flow, particularly from the east-NE,
will pump high temperatures up to the 80s to mid 90s Thursday and
Friday. The latest temperature guidance has come in little warmer
than previous, so Max temperatures both days have been nudged up
slightly. The warmest area will be the interior from Seattle and
Bremerton southward with highs in the lower to mid 90s. The rest of
West Washington will see highs mainly in the 80s, although a few spots could
see readings near 90. Relief from the heat will show up by late
evening with most locations eventually cooling off into the 50s.
Only the Puget Sound Metro areas will hang around in the lower 60s.
A heat advisory was issued earlier this morning for the warmest
areas Thursday and Friday afternoons.
An upper level trough and associated weak cold front moving southeast down
the b.C. Coast on Friday will trigger a marine push Friday night and
Saturday, flooding the West Washington interior with cool coastal marine air on
Saturday. Onshore flow on the coast will begin Friday afternoon, but
not soon enough to really affect temperatures over the interior.
Enough cooling will reach the interior on Saturday to knock Max
temps back to the 70s which is near normal.
With regard to cloud cover, timing-wise this looks like another
atypical marine push. The GFS and NAM agree on this, showing
boundary layer rh, and low clouds, only making it partway inland to
around Shelton Friday night, then pausing Saturday, before spreading
rapidly inland Saturday evening.
The main part of the upper level trough will move over southern b.C.
On Saturday. The associated weak cold front will reach the north
interior of West Washington Saturday afternoon then move southeast across the rest of
West Washington Saturday night. This looks like another skinny front, but it
appears to have a bit more moisture than the front from the previous
event. There will be a chance of showers as the front traverses the
Long term...the GFS, ECMWF, and Canadian models are now in good
agreement regarding the large scale changes in the pattern this
weekend and next week. However there is still lots of doubt
regarding the finer details like strength and timing of various
shortwaves and associated details like precip timing and amount.
The large scale pattern will be undergoing a pattern change late
this week with the upper level ridge that is currently along 135w
slowly retrograding back to around 150w-160w by Saturday. The ridge
will then remain there through next week. This shift will allow the
longwave upper level trough currently over the central U.S. To
retrograde west over the Pacific northwest, putting West Washington in a cooler
cloudier regime with a greater chance for showers next week. The 12z
GFS has come in quite a bit wetter than earlier runs and now shows
somewhat significant precip reaching West Washington at times through the
period, associated with about three upper level shortwave troughs.
The European model (ecmwf) and Canadian models seem to have similar ideas but there
are differences in wetness and timing. Forecast pops and
precipitation amounts have been nudged up early next week more or
less following the GFS solution. However confidence is still not
that high. What is more certain is that it will be cloudier and a
little cooler than normal. Kam
Aviation...an upper ridge offshore will provide northerly flow
aloft for western Washington tonight and Thursday. At the
surface, north to northeast offshore flow will prevail with a
thermal trough along the coast. The air mass is dry and stable.
There might be a few fog patches again Thursday morning in the
southwest interior valleys but any fog that forms should be very
Ksea...clear. Northerly wind 6-12 knots, easing to 4-8 knots
Marine...a thermal trough along the coast will provide offshore
flow through Friday morning. The thermal trough will shift inland
Friday afternoon and evening with increasing onshore flow. Small
Craft Advisory west winds should develop in the central and
eastern Strait of Juan de Fuca Friday evening and continue off and
on through Saturday night. Weaker onshore flow is expected
Sunday. A weak front will probably reach the area Monday. Schneider
Fire weather...it has now been about 16-17 days since the last
widespread measurable precipitation in western Washington, and
wildland fuels are likely to reach their Driest Point of the year
over the next 2-3 days.
A weak thermal pressure trough was located along the coast this
afternoon. It will re-emerge on Thursday and Friday mornings,
leading to warming and drying north-northeast wind flow.
Sustained winds over the Puget Sound lowlands and the northwest
olympic peninsula will exceed 10-12 mph this afternoon and again
on Thursday afternoon. However, we are focusing on Thursday
afternoon as the more critical period, since relative humidity
should be a bit lower tomorrow. So a red flag warning is in effect
for 4 fire weather zones where we are most likely to see 3-5 hours
of moderate north-northeast breezes combined with relative
humidity below 30 percent. On Friday, the breeze does not look
strong enough to reach The Crater for red flag conditions, but
it will be hot and dry on Friday nonetheless.
Any fires that get started in these conditions will have the
potential to exhibit rapid rates of spread.Haner
Washington...red flag warning from Thursday morning through Thursday
evening for Black Hills and southwest interior lowlands-
central and south Puget Sound lowlands-north coastal
lowlands-southeast Puget Sound lowlands generally below
Heat advisory from noon Thursday to 8 PM PDT Friday for Bellevue
and vicinity-Bremerton and vicinity-east Puget Sound
lowlands-Hood Canal area-lower Chehalis valley area-Seattle
and vicinity-southwest interior-Tacoma area.
You can see an illustrated version of this discussion at