Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 261057
Area Forecast Discussion
National Weather Service Seattle WA
357 AM PDT TUE JUL 26 2016
.SYNOPSIS...Weak onshore flow today will give cooler temperatures
to the area. An upper level ridge over Western Washington will
strengthen Wednesday and Thursday with the warmest days of the
month expected Thursday and Friday. Temperatures will cool again
over the weekend as an upper level trough moves into the area.
.SHORT TERM...Satellite imagery shows stratus along the
Coast...down the Strait...through the lower Chehalis valley and
northeastward to near Sea-tac airport. Outside of the stratus
just a few mid level clouds. Not enough to block seeing the ISS
from the roof and to watch the lightning from the thunderstorms
over Okanogan county. Temperatures at 3 am/10z were in the mid 50s
to mid 60s.
Stratus will slowly push a little further inland this morning but
the onshore gradients are already weakening so do not expect the
low cloud cover to get all the way to the Cascade foothills. The
layer is pretty shallow with tops estimated around 1500 feet. The
combination of decreasing onshore gradients and the shallowness of
the marine layer will result in plenty of sunshine by afternoon
for all areas except right along the coast. With the marine push
fairly weak will only go for 5-10 degrees of cooling today versus
Monday for the interior...highs in the upper 60s and 70s. Highs on
the coast today in the mid 60s.
Warming trend kicks back in gear on Wednesday with the flat upper
level ridge strengthening over the area and the low level onshore
flow weakening. Temperatures aloft warm with model 850 mb
temperatures a couple of degrees C warmer than today. Highs most
places Wednesday around 5 degrees warmer pushing locations in the
interior back up into the mid 70s to mid 80s. Temperatures along
the coast will also warm up a bit...into the lower 70s.
Thursday looks to be the warmest day in the short term with 500 mb
heights in the upper 580 dms and light flow in the lower levels.
Warmest locations over the southwest interior will be pushing 90
on Thursday with mid 70s to mid 80s common for the interior and
mid 70s expected along the coast.
.LONG TERM...Differences in the extended model solutions for
Friday continue with the GFS faster with the approach of an upper
level trough resulting in lower 500 mb heights and the possibility
of cooler temperatures. The slower ECMWF solution keeps 500 mb
heights in the upper 580 dms on Friday with light flow at the
surface. GFS ensembles are a little stronger with the ridge
versus the operational run. Will stick with the idea on Friday
being the warmest day in the forecast period with highs in the mid
70s to lower 90s. Upper level trough dropping down from the north
on Saturday with increasing onshore flow during the day. High
temperatures will cool back down to near seasonal normals with
highs in the interior in the 70s and lower 80s with 60s along the
coast. More differences in the model solutions on Sunday with the
GFS deeper with the upper level trough versus the ECMWF. GFS
ensembles on again are weaker with the trough versus the
operational run. The operational GFS produces a little
precipitation over the area but given the weaker ensemble
solution...ECMWF solution and climatology...it has only rained 13
times on July 31st in Seattle in over 120 years of weather
records...will keep the forecast dry. Even with the weaker
solutions plenty of onshore flow so have added more cloud cover to
Sunday and taken a few degrees of the max temps with highs over
the interior mostly in the upper 60s to mid 70s. Weak troughiness
and onshore flow on Monday for a pretty normal day for the 1st of
.AVIATION...Morning low clouds over a good part of the lowlands will
give way to afternoon sun. The south part of Puget Sound had low
stratus at 3am and it should be into Seattle around 12z and burn off
by 18z. The coast will take til early afternoon.
KSEA...Morning low clouds are a good bet 12-18z, then sun.
.MARINE...High pressure is offshore with low pressure inland, the
westerly push in the Strait should not be as strong this evening and
a small craft advisory is in effect there.
PZ...Small Craft Advisory until 3 AM PDT Wednesday for Central U.S.
Waters Strait Of Juan De Fuca-East Entrance U.S. Waters
Strait Of Juan De Fuca.
You can see an illustrated version of this discussion at