Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 231605
Area Forecast Discussion
National Weather Service Seattle WA
905 AM PDT THU JUN 23 2016
An upper low just off Vancouver Island today will continue to
slowly drift onshore. By this evening it will settle across the
Olympic Peninsula. This will bring damp and cloudy weather for
Western Washington today. Temperatures will remain below normal
with scattered rain showers and an isolated rumble of thunder
as the low pushes over the peninsula. The low should move move
fair rapidly east and by Friday will exit east out of Washington.
Lingering showers will end by Saturday as high pressure builds
into the area and temperatures warm across the area to near and
above normal by the weekend and next week.
The satellite shows a cloudy day in progress west of the Cascade
Crest in Western Washington. The upper low just off Vancouver
Island will make a slow and steady eastward progression today.
Radar shows scattered bands of showers and amounts haven`t been
significant yet but off and on showers should persist most of the
day. No real changes to our forecast today. Look for temps
remaining a good 5-7 degrees below normal today with the clouds
and onshore flow and enhanced moisture.
Regarding any thunderstorm potential today. Looking at 12Z UIL
sounding, looks like surface based instability will be hard to
come by given the expected thick cloud cover. The overall
probability looks low, however will look for possible isolated
strikes in the northeast Olympic Peninsula and then in the typical
Convergence Zone area, and then Southwest areas nearer the center
of the Upper Low after 00Z this evening. Lifted Index Values show
up negative in those three areas. With lack of surface based
instability, believe those are the three possible areas because of
potential for assist from elevated heating in the Olympics. The
convergence zone may provide additional lift to help force
parcels up that will overcome lack of surface based instability.
And upper level cooling and dynamics associated with the upper
low will help to produce enhanced instability in the southwest
this evening. Will just go with the current weather grid
indicating a rather broadbrushed low end slight chance of thunder
as the overall probability is low but not a zero chance, and one
worth mentioning overall. Johnson
.LONG TERM...Previous Discussion. The aforementioned ridging looks
to remain in place Saturday...for the remainder of the
weekend...and into at least the first half of next week. This will
allow for temperatures to warm with low to mid 70s in the interior
lowlands on Saturday and into the upper 70s Sunday and into next
week. Upper level heights associated with this ridge look to
continually creep up...but fortunately onshore flow during the
forecast period looks to offset some of this heating...keeping
temps from crossing into the 80s. One last item of old
business...nice to see GFS has opted to join the remaining models
regarding its solution for Sunday. Previous runs had a shortwave-
ish sort of feature with some shower potential over the Cascades.
While it retains the feature...it has dropped the potential for
rain. Have altered the forecast accordingly...resulting in a
solidly dry forecast from Saturday to the end of the forecast
.AVIATION...An upper level low pressure system will reach the coast
late this afternoon, and track eastward over the inland waters
tonight. Light southwest flow aloft becoming westerly tonight, then
northwesterly Friday. Air mass moist, especially in the lower
levels. Air mass becoming weakly unstable this afternoon and
evening. A few isolated thunderstorms are possible but the
probability is too low to include in TAFs.
KSEA...Skies mostly cloudy today. Widespread MVFR conditions through
early afternoon with some spotty IFR. Cigs should improve a bit
during the afternoon with mostly VFR cigs, though dropping to MVFR
at times with showers. There is a small threat of a thunderstorm
this afternoon and early evening but this is too low to mention in
the TAF. South-southwest wind increasing to 8-12 g22 knots today.
MVFR cigs likely to develop again later tonight as the air mass
becomes more stable. DTM
.MARINE...Onshore flow behind a front will produce Small Craft
winds across most the interior waters this afternoon and evening.
The strongest westerly winds will be in the strait, possibly up to
30 kt in the central/east portion. Winds easing after midnight.
A typical summer-time onshore flow pattern will prevail Friday
through Monday with high pressure offshore and lower pressure
inland. Expect small craft advisory west winds for the Central and
Eastern Strait each evening in this pattern. DTM/Schneider
PZ...Small Craft Advisory until 3 AM PDT Friday for East Entrance
U.S. Waters Strait Of Juan De Fuca.
Small Craft Advisory from 5 PM this afternoon to midnight PDT
tonight for Admiralty Inlet-Northern Inland Waters
Including The San Juan Islands.
Small Craft Advisory from 3 PM this afternoon to 3 AM PDT Friday
for Central U.S. Waters Strait Of Juan De Fuca.
Small Craft Advisory from noon today to midnight PDT tonight for
Puget Sound and Hood Canal.
You can see an illustrated version of this discussion at