Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 251030
Area Forecast Discussion
National Weather Service Seattle WA
330 AM PDT WED MAY 25 2016
.SYNOPSIS...Mostly cloudy skies will continue today with low level
onshore flow. An upper trough will bring an increasing chance of
showers Thursday through Saturday. Another upper level trough will
reach the area Sunday. An upper level ridge will begin to build
off the coast Memorial day.
.SHORT TERM...Satellite imagery shows cloudy skies over most of
the area this morning with some breaks in the cloud cover over the
Strait of Juan de Fuca...San Juans and the northwest interior.
With the breaks in the cloud cover the coolest location at 3 am
was Friday Harbor with a temperature of 45 degrees. Most of the
remainder of the area reporting temperatures in the upper 40s to
mid 50s at 3 am.
Little change in the weather pattern today and tonight with light
low level onshore flow and light northwesterly flow aloft. Onshore
gradients increase slightly later this afternoon into tonight.
The light flow in the lower levels and the increase in the onshore
flow will make it hard to dissipate the cloud cover. Mostly cloudy
forecast looks good. Highs will be in the upper 50s to mid 60s...a
little below normal for this time of year. Lows tonight will be
Some changes in the pattern begin to take place on Thursday as an
upper level low makes its way down the British Columbia coast. Low
level onshore flow increasing in response to the approaching low.
Shower chances are not that great with the low center still north
of Vancouver Island at 00z Friday but with plenty of low level
moisture around even a little lift can trigger a shower. Main
story on Thursday will be a continuation of the weather today
...mostly cloudy with temperatures a little below normal.
Low moving slowly south Thursday night and elongating by 12z
Friday into a large trough extending from the northern portion of
Washington back up into central British Columbia. With the trough
overhead a chance of showers will continue for the entire area.
Good convergence zone pattern at the surface with the best chances
for showers in the convergence between Mount Vernon to north of
Seattle. Lows will be in the mid to upper 40s.
Friday looks to be the wettest day in the short term with the
trough overhead and low level convergence continuing all day. Have
increased the pops into the likely category on Friday.
Temperatures aloft cooling with the snow level down to around 4500
feet so the higher passes...North Cascades and White...as well as
Paradise on Mount Rainier could see a couple inches of wet snow.
Highs will only top out in the lower 60s on Friday.
.LONG TERM...Extended models in pretty good agreement on the 00z
run with one upper level trough kicking out on Saturday being
replaced by another upper level trough moving down from the
northwest Saturday night into Sunday. This pattern will keep
showers in the forecast over the weekend. Some trofiness
remaining Memorial day morning but at this point it looks like the
trough will move east leaving a dry afternoon. Models showing good
agreement with a ridge building offshore Memorial day afternoon
into Tuesday so have gone ahead and dried out the forecast and
raised the max temperatures by a few degrees on Tuesday...into the
70s from Seattle south to Lewis county. Felton
.AVIATION...A long wave trough continues to linger over the West
as the associated upper level low spins over the MT/Saskatchewan
border. The air mass over western WA is stable. The flow aloft is
north to northwesterly with low level onshore flow at the surface.
Low level stratus in place over the area...much like nights
previous...although cigs remain generally VFR...with the exception
of along the coast where cigs have dropped into MVFR conditions. As
has been the pattern...should see cigs lift and cloud cover break up
some by Wed afternoon. SMR
KSEA...VFR conditions likely to persist this early morning into the
afternoon...although brief periods of MVFR cigs not out of the
question this morning. Light and variable winds expected to shift to
S...picking up speed in the afternoon to around 8-12 kts. Cigs will
once again reach their lowest point in the morning hours before
lifting by late morning/early afternoon with cloud cover starting to
thin as well. Clouds expected to return late tonight. SMR
.MARINE...Onshore flow will prevail this week with high pressure
over the NE Pacific. Inherited Small Craft Advisory looks to be in
need of extending for the outer coastal waters. Have also issued an
SCA for central and east Strait...as winds will re-emerge there by
this evening. Will allow Northern Inland Waters to expire although
whether or not they will be included in the SCA for this evening
will be left up to day shift. Given the lingering nature of the
weather pattern...can expect to see Small Craft Advisory winds in
the Central and East Strait of Juan de Fuca during the afternoon and
evening hours for the remainder of the week. SMR
PZ...Small Craft Advisory until 11 AM PDT Thursday for Coastal Waters
From Cape Flattery To James Island 10 To 60 Nm-Coastal
Waters From James Island To Point Grenville 10 To 60 Nm-
Coastal Waters From Point Grenville To Cape Shoalwater 10
To 60 Nm.
Small Craft Advisory until 5 AM PDT early this morning for
Central U.S. Waters Strait Of Juan De Fuca-East Entrance
U.S. Waters Strait Of Juan De Fuca-Northern Inland Waters
Including The San Juan Islands.
Small Craft Advisory from 5 PM this afternoon to 5 AM PDT
Thursday 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