Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 261736 AAA
Area Forecast Discussion
National Weather Service Seattle WA
1030 AM PDT TUE JUL 26 2016
UPDATED MARINE AND ADVISORY SECTIONS.
.SYNOPSIS...A very weak low pressure system over the area will
move into eastern Washington late today or early this evening.
High pressure aloft will prevail tonight through Friday. Expect
onshore flow to persist. Temperatures will be near normal today
and above normal Wednesday through Friday.
A very weak upper level trof over the area this morning will move
into eastern WA late today or early this evening. There is enough
moisture and instability with this feature for a risk of
thunderstorms over the north Cascades near the crest. Thus, the
forecast was updated to mention a slight chance of thunderstorms
over the aforementioned area late this afternoon through early
this evening. Otherwise, anticipate sunny skies to return to much
of the CWA this afternoon.
High pressure aloft will prevail over the region tonight through
Thursday. Onshore flow will persist; therefore, expect late night
and morning low clouds, mainly over the coast. Heights/thicknesses
will rise, so temperatures will be above normal, especially with
a weaker onshore pressure gradient. There may be a slight risk of
thunderstorms over the north Cascade crest again late Wednesday
.LONG TERM FROM THE PREV DISCUSSION...
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
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 August. Felton
.AVIATION...Expansive area of low clouds across the Puget Sound
Lowlands and along the coast this morning. IFR cigs present this
morning and the challenge will be forecasting burnoff of the
clouds. Tops are 2400 ft per latest reports so the deck may
persist until around 19Z to 21Z time frame, lingering the longest
along the coast. Then VFR with sun this afternoon. Indications are
for potential MVFR cigs along the coast again tonight with a
weaker return of stratus expected. West/southwest winds this
morning becoming northwest this afternoon. Wind speeds 5-10 knots.
KSEA...Low clouds will persist likely closer to 19-20Z time frame
with southwest winds 5-10 knots during that time. Clearing after 20Z
with winds shifting northwest and remaining 5-10 knots. VFR
conditions after 20Z and overnight, will monitor potential for MVFR
cigs around 12Z Wed with some low cigs expected to return but not as
strongly as this morning.
High pressure is offshore with lower pressure inland. While the
westerly push through the Strait of Juan de Fuca is not expected to
be as strong tonight as it was last night...winds are still near 30
knots in the central portions of the strait this morning and are
expected to rise to around 30 knots there tonight. The forecast
winds were increased to accomodate current observations and latest
Scatterometer winds over the offshore waters early this morning and
observations near the west coast of Vancouver Island agree with
short term models that imply northwest winds 15 to 25 knots over the
outer portions of the coastal waters. A small craft advisory was
issued effected immediately for the coastal waters beyond 10 nm
through Wednesday afternoon to account for this low level jet.
A period of lighter winds is expected through the strait before
strengthening speeds expected later in the week. Small craft
advisory conditions will likely persist at most times through the
end of the week over portions of the coastal waters in northwest
flow around high pressure well offshore. Albrecht
PZ...Small Craft Advisory in effect until 3 AM Wednesday for the
central and eastern Strait Of Juan De Fuca.
Small Craft Advisory in effect until 3 PM Wednesday for the
Washington coastal waters from Cape Flattery to Cape Shoalwater
beyond 10 nm from the coastline.
An illustrated version of this forecast discussion can be seen at