Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 200341
Area Forecast Discussion
National Weather Service Seattle WA
840 PM PDT TUE JUL 19 2016
.SYNOPSIS...An offshore upper low will move slowly toward Western
Washington through Thursday for partly sunny skies and a chance of
showers, mainly over the mountains during the afternoon and
evening hours. Thunderstorms are possible over the Cascades this
evening and again Thursday. A weak upper ridge should bring
sunnier weather Saturday through Tuesday.
.SHORT TERM...The upper trough will weaken Wednesday, with upper
heights over Western Washington rising a little as the large
high over the central United States bulges northwest. The upper
flow over the forecast area will veer slightly from south to
southwest, and the air mass will be somewhat drier and less
unstable. The chance of showers Wednesday should be mainly limited
to the Olympics, North Cascades and Mount Rainier with morning
clouds over the interior lowlands giving way to partly to mostly
sunny skies. Highs will be in the mid 60s to lower 70s at the
coast, and in the 70s across the interior lowlands.
The offshore upper trough will move into the coastal waters late
Thursday and then begin to make its way inland Thursday evening.
As this happens, the flow over Western Washington will again back
to southerly and destabilize somewhat. So the chance of showers
will increase again, and we have decided to add a slight chance of
thunderstorms near the Cascades crest during the afternoon and
The upper trough will move inland Thursday night, which will cause
onshore flow to increase. Upper heights will begin to rise again
Friday, but weakly cyclonic westerly flow aloft will continue. Showers
will probably linger along the west slopes of the Cascades, and
possibly in a weak Puget Sound convergence zone and at the coast,
into Friday morning. Friday should also begin with a decent amount
of marine layer cloudiness across the forecast area, but give way
to partly or mostly sunny skies over the area in the afternoon.
.LONG TERM...A rather nondescript pattern will prevail during the
extended forecast period Saturday through Tuesday with light zonal
flow aloft, weak onshore flow at the surface, and 500 mb heights
around 5820 meters. This should be a stretch of seasonal weather
with mostly sunny weather, except night and morning marine stratus
mainly at the coast, with highs in the mid 60s to lower 70s at the
coast and 70s to lower 80s inland. McDonnal
.AVIATION...A broad upper low over the coastal waters will remain
stationary through Wednesday night giving light southwest flow
aloft. A weak ridge of high pressure will remain over the coastal
waters through Wednesday. The air mass will be somewhat moist and mostly
stable through Wednesday.
VFR conditions prevail over most of Western Washington. The coastal
areas have some cigs around BKN060. Expect isolated showers over the
region through Wednesday, mainly during the afternoon and evening
hours and over the higher terrain. Stratus with bases around 020-025
will develop in the interior about 12Z tonight, then will scatter
out 17Z-19Z Wednesday.
KSEA...Clouds FEW-SCT060. BKN025 is possible 13Z-18Z Wednesday.
Southwest to south winds 4-8 knots. Albrecht/13
.MARINE...A weak ridge of high pressure over the coastal waters
will give light southerly flow through tonight. Light southwest
flow is expected over the inland waters. Inflow through the Strait
of Juan de Fuca is expected to remain 20 knots or less through
The surface ridge will move into the offshore waters later Wednesday
through Saturday resulting in a more west to northwest flow over the
coastal waters. There is a chance that gale force westerly winds
will occur for a time Thursday evening and small craft advisory
conditions are possible in portions of the Northern Inland Waters.
Otherwise, small craft advisory conditions are expected through
portions of the Strait during the late afternoon and evening hours.
You can see an illustrated version of this discussion at