Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 180400
Area Forecast Discussion
National Weather Service Seattle WA
900 PM PDT SUN JUL 17 2016
.SYNOPSIS...An upper low just off the coast helped to touch off
scattered showers and thunderstorms today. The showers were mainly
over the Cascades. Afternoon showers and thunderstorms are a good
bet again Tuesday--and once again, most should be over the Cascades.
It is likely that the upper low will weaken a bit and that some
upper level ridging will push up over Washington by Thursday.
.SHORT TERM...The showers and tstms were quick to end late this
afternoon and early evening. There is still some activity over
Eastern Washington, and also up over southwest British Columbia--
including Vancouver Island. Looking at the 00z nam, Tuesday looks
like a repeat as a little shortwave trough rotates around the
low and moves up from Oregon. The nam shows precip spreading
north through the Cascades by midday and into the north Cascades
by mid afternoon. Much of the precip in the nam is just east
of the Cascade crest. After that, showers look hit and miss and
are pretty spotty after the main convection dies off Tuesday night
and Wednesday is dry in the model, even though the upper low is
still sitting just offshore. The 00z gfs is similar except that
there is less precip in the North Cascades and some light rain,
perhaps a quarter of an inch, spreads westward out of the Cascades
in the evening across the counties north of Seattle.
The GFS solution was faster in kicking the upper level trof
eastward on Thursday. Decided to go with the slower ECMWF solution
and kept a risk of showers in the forecast.
Beyond Wednesday, high pressure aloft will prevail over the
region for dry weather. The low level flow will remain onshore, so
expect late night and morning low clouds, especially over the
coast. Temperatures will be near normal for late July.
.AVIATION...A broad upper low will remain over the offshore waters
tonight and Monday with weak southwesterly flow aloft. At the
surface, onshore flow will prevail with high pressure offshore and
lower pressure inland.
Satellite imagery shows just patches of sc over Western Washington
this evening. The sc deck is shallow with bases around 5000 feet
msl. These clouds will dissipate overnight. With the low level
onshore flow expect MVFR low clouds to develop along the coast early
Monday morning then spread inland in the 12z-15z Monday morning. The
MVFR ceilings will burn off over the interior midday.
KSEA...Scattered sc deck around 4500 feet overnight. MVFR low clouds
will likely develop at the terminal Monday morning shortly after
sunrise...around 15z. The low cloud cover will burn off midday.
Northwest wind 4-8 knots will ease and become more southerly
early Monday morning. Felton
.MARINE...Onshore flow will prevail for the next several days
with high pressure offshore and lower pressure inland. Small craft
advisory west winds are forecast tonight and again Monday evening in
the Central and Eastern Strait of Juan De Fuca. Onshore flow will
increase a bit Monday evening and there is a small chance gale force
west winds could develop in the Central and Eastern Strait.
Weaker onshore flow is expected Tuesday and Wednesday with
no headlines expected, then onshore flow will increase again
Thursday and Friday for more small craft advisory westerlies late
in the day in the Central and Eastern Strait. Felton
PZ...Small Craft Advisory until 5 AM PDT Monday 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