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Area forecast discussion 
National Weather Service Seattle Washington
300 PM PDT Wednesday Sep 19 2014

Synopsis...a strong high pressure ridge will build over the region
tonight and Saturday. Sunny skies will develop on Saturday after
some areas of morning low clouds and fog. Marine clouds and cooler
air will reach the coast on Sunday but the interior will remain
sunny and warmer. A weakening system on Monday and Tuesday will
bring in some clouds...a few showers...and moderating temperatures.
A large upper level trough will produce a wetter and cooler pattern
Wednesday through the end of next week.


Short term...latest radar loop this afternoon shows just a few
residual showers across the north interior north of Everett. The air mass is
beginning to stabilize so showers will come to an end by this
evening. Visible satellite trends show some partial clearing across
southern portions of the interior but the lower levels are still rather
moist across The Lowlands. High pressure aloft will rapidly build
over the region tonight through Saturday with large scale
subsidence. MOS and NAM time-heights/buffer sounds are showing a
subsidence inversion to develop overnight suppressing any residual
moisture within the lowest 1500-2000 feet. The gradients turn
northerly later tonight but the flow will probably not strong enough
to clear out moisture initially. However...thermally induced low
pressure will form just off the coast in the afternoon increasing
offshore flow conditions. This should clear out remaining moisture
and give sunny skies across the area Saturday afternoon. High
temperatures will warm several degrees well into the 70s but
probably will not hit potential highs inland because of the cloudy
start to the day. This will not be the case near the coast where
Saturday should be the warmest day...just ahead of the marine
stratus surge approaching from the S. Hoquiam and Forks could both
reach the low 80s.

The upper ridge shifts further inland on Sunday becoming centered
over the Great Basin/rockies. Thermally induced low pressure near
the coast in the morning will migrate inland over the interior
lowlands of western Washington. This means light surface flow but increasing southerly
850 mb flow which should aid in the warming process. Forecast highs
will remain a couple degrees above the warmest NAM MOS given the
overall pattern. Stratus will be nosing northward along the coast during
the day bringing cooler marine air to coastal spots. Highs there
will struggle to reach 70. Inland temperatures could reach the middle
or even upper 80s in places...but cooler near the water.

500 mb height falls continue Sunday night and Monday as the first in
a series of fronts try to push into the area. The main affect will
be to induce a marine push which will flood stratus and cooler air
inland. Highs on Monday will struggle to reach the low to middle 70s
assuming some spots around the SW interior/far S sound get some sun
breaks. The front itself will be rather weak but most models do show
some spotty light quantitative precipitation forecast around western Washington Monday afternoon/evening.

Long term...the GFS/European model (ecmwf) have trended toward a partial break
between weather systems on Tuesday. This is different from past runs
and completely removing precipitation does not yet seem prudent.
Probability of precipitation were removed for the interior Puget Sound sound region but kept a
chance of rain in the forecast mainly west of Puget Sound sound in case the
next system arrives a bit faster.

A pattern change will slowly unfold midweek as a large trough
develops over the NE Pacific sending a series of fronts into the
area. The pattern now looks more amplified compared to the moist westerly
flow pattern that was shown by runs a couple days ago.
Nevertheless...the upper low will send waves into western Washington through the
end of the week. Timing these is not really possible given the
progressive pattern so probability of precipitation are higher middle week then taper to chance
by Thursday and Friday just due to inherent uncertainty out to day
6/7. Mercer


Aviation...westerly flow aloft tonight will become SW as an upper
level ridge builds in to the southeast of the area. At the
surface...high pressure will build over southern British Columbia
late tonight and Saturday giving increasing northerly flow at the
low levels. The air mass is stable. Areas of low level moisture will
develop below a low level inversion late tonight into Saturday

Residual stratus and reduced visibility in drizzle and light fog
along the coast and over the north interior are decreasing rapidly
from the south this afternoon. Expect pressure gradients to become
light this afternoon then northerly tonight as high pressure
strengthens over British Columbia.

Increasing northeasterly flow from about 2500 to 5000 feet mean sea level
tonight combined with subsidence with building high pressure aloft over
the Pacific northwest will result in the development of a low level
inversion that will strengthen and lower 09z-18z Saturday. The
increased subsidence in conjunction with radiational cooling will
concentrate low level moisture in an increasingly shallow boundary
layer resulting in the rapid development of stratus and some fog
around 09z that will likely burn off around noon Saturday. Expect
areas of ceilings below 005 Saturday morning in the interior with some
dense fog in areas where the stratus intersects the ground. This is
not an uncommon occurrence during the early fall months and is shown
well in the latest nam12 solutions. The 12z uw wrfgfs 4 km
solution's column integrated cloud water product also shows this low
level moisture Saturday morning. Coarser large scale models were not
used as they tend to lose track of the low level moisture in these

Skies will rapidly clear midday on Saturday resulting in good VFR
conditions. Albrecht

Ksea...expect good VFR conditions through 09z. Low stratus with ceilings
003-005 are expected to develop around and to the north of the
terminal around 09z then spread into the terminal area before 12z as
a subsidence inversion develops over the area and trapped low level
moisture condenses with radiational cooling. There is about a 50
percent chance that visibilities at the terminal will drop to below
1/2sm fog if the stratus bases are slightly lower than currently
expected. The stratus will likely break up around 20z Sat. Southwest
winds 5-7 knots late this afternoon will become north 6-8 knots after 05z.


Marine...surface high pressure will develop over southern British
Columbia tonight then strengthen Saturday and Saturday night. This
will increase low level northerly flow late tonight through Saturday

Flow will become more easterly late Saturday night into Sunday as
the high over British Columbia moves southeastward. Small Craft
Advisory east winds are likely at the West Entrance to the Strait of
Juan Delaware fuca and the adjacent coastal zone late Saturday night into
Sunday morning.

A weakening front will approach the coastal waters late Sunday into
Sunday night. The approach of the front will likely induce a
southerly surge over the coastal waters Sunday into Sunday night.

A much stronger frontal system well offshore will be approaching
from the west Tuesday and Tuesday night and may result in some gale
force winds over portions of the coastal waters during the middle of
next week. Albrecht


Sew watches/warnings/advisories...




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