Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 251042
Area Forecast Discussion
National Weather Service Seattle WA
342 AM PDT SUN SEP 25 2016
.SYNOPSIS...A weak front over British Columbia will keep clouds
over Western Washington this morning with spotty very light rain
in the north. A strong upper ridge will build later today for
sunny warmer weather into Monday. A weak front will move through
Monday night. Generally dry weather is expected Tuesday through
Thursday. An upper trough will bring a chance of showers Friday
.SHORT TERM...Satellite imagery shows upper level ridge offshore
extending into the area. Weak warm front over Southern British
Columbia with cloudy skies over Western Washington. Temperatures
at 3 am/10z were in the 50s and lower 60s.
Upper level ridge continuing to build over the area today with 500
mb heights near 590 dms by 00z Monday. The ridge building will
push the warm front to the north. This along with some subsidence
with the ridge building will dissipate the cloud cover over the
area by the afternoon. Warm front still close enough this morning
to keep just a chance of light rain north of a line from about
Forks to Mount Baker. Warming trend in the temperatures aloft
continuing today. Sandpoint profiler already shows temperatures at
about 1000 feet 3 or 4 degrees c warmer than yesterday and
temperatures up near 850 mb 2 or 3 degrees c warmer. Surface
gradients are light and will remain light into the afternoon
hours. Temperatures will warm quickly once the sun comes out with
highs in the upper 60s in the north to near 80 in the south.
Upper level ridge shifting east tonight with the low level flow
turning lightly offshore. Warm front to the north dissipating. With
the clear skies some patchy fog could form late in the southwest
interior. Lows will be in the 40s to mid 50s.
Upper level ridge continuing to move east on Monday. Temperatures
in the lower levels peak on Monday with model 850 mb temperatures
in the plus 14 to 18c range. Surface gradients turning onshore
in the afternoon but not early enough to prevent Monday from
being the warmest day in the forecast for the interior. Record
highs on Monday are the coolest of any day of the month. Forecast
of 78 degrees at Sea-Tac would tie the record. The record high at
Olympia of 81 degrees ( set in 2006 ) is also within reach. With
the transition to onshore flow the coast will be a little cooler
compared to today with highs near 70. For the interior highs in
the 70s and lower 80s.
Marine push on Monday night with the upper level ridge well to the
east...low level onshore flow and a weak shortwave moving into
Southern British Columbia. Shortwave close enough for a chance of
showers along the coast and over the northern portion of the area.
Lows will be in the 50s.
Onshore gradients decreasing on Tuesday with shortwave east of the
area by 18z. Model cross sections show low levels of the air mass
drying out Tuesday afternoon. Will mention of some sunshine later
Tuesday. Highs will be 10-15 degrees cooler than on Monday...in
the 60s for the most part.
.LONG TERM...Extended models in good agreement during the first
part of the extended with an upper level trough digging offshore
on Wednesday and Thursday. Run to run consistency not very good
with the models yo-yoing around on when the trough will be close
enough to introduce some precip[pitation into the forecast. 00z
runs...like they did yesterday...have backed off on the timing
pushing the initial precipitation back into Friday. Current
forecast has chance pops in the forecast for Thursday night. With
the lack of consistency will keep the chance pops in the forecast
for Thursday night at this point. Models in pretty good agreement
that when the trough moves into the area later Friday into
Saturday the air mass will be one of the cooler ones to move into
the area in awhile with the snow levels lowering down to near
5000 feet. Paradise on Mount Rainier...and maybe the higher
mountain passes like the North Cascades could see a little snow
next weekend. Felton
.AVIATION...Clouds will scatter out midday as a strong upper ridge
builds over the region. The air will be dry and stable this
afternoon through tonight with fog patches a good bet around
daybreak Monday in the normal spots and valleys.
KSEA...The clouds should scatter out by 18z, light nly wind this
afternoon, clear tonight and Monday.
.MARINE...A surface high over western WA will keep the flow light
today. Light northerly flow will develop this afternoon before
turning more onshore or westerly on Monday.
The onshore flow will strengthen Monday night in response to the
passage of a cold front. This will lead to the possibility of
gale force westerlies over parts of the Strait of Juan de Fuca.
The onshore gradient will weaken on Tuesday.
.CLIMATE...Monday ( September 26th ) is the only day in the month
of September where there has not been an 80 degree day at
Seattle-Tacoma airport. The last 80 degree plus day during the
last week of September and October for Seattle was 10 years ago
80 degrees on September 27th, 2006. Felton
You can see an illustrated version of this discussion at