Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
300 PM PDT Wednesday Aug 27 2014
Synopsis...the marine layer will deepen tonight and a small upper
trough will move through area Thursday for cloudier cooler weather.
Another larger upper trough will bring a chance of showers Friday
through this weekend. Monday night and Tuesday could see brief
improvement before another upper trough moves through towards the
middle of next week.
Short term...moderate onshore flow at the surface will bring a
thicker marine layer inland tonight. The thickness of the marine
layer will bring areas of drizzle to the coast towards the
morning...and there will likely also be patchy fog on the coast. I
have kept the drizzle out of the interior in the forecast.
Thursday...a shortwave trough moving over the region and its
associated surface front will keep conditions much cooler...5 to 10
degrees cooler than they have been. I expect that we will only
partially break out of the marine layer late in the day. I have
added a chance of afternoon showers to the Olympics based on recent
high resolution model trends...and there is a slight chance of
showers for the north Cascades as well.
Friday a large upper level trough will approach the area. The marine
layer will keep the low levels moist. Both the nam12 and the uw
wrfgfs 4km are hinting at a Puget Sound sound convergence zone forming
Friday morning...and I have included a slight chance of showers in
the Puget Sound sound convergence zone area for the morning. Later in the
day on Friday the upper trough will begin moving through the
area...and I have kept a widespread chance of showers in the
forecast through Saturday. Jsmith
Long term...showers will begin to taper off Saturday night into
Sunday...but a series of small disturbances moving through the area
means that the threat of showers...mostly confined to the
mountains...will continue through Monday. In the most recent
runs...the European model (ecmwf) and GFS are both agreeing that there will be a
brief lull in the showers and possibly partly sunny skies Monday
night into Tuesday. However...they also suggest that another system
will move into the area Tuesday night or Wednesday...bringing
possible widespread showers. Jsmith
Aviation...westerly flow aloft with weak upper level ridge over the
area moving east tonight. Weak upper level trough reaching the coast
Thursday morning moving east of the Cascades Thursday afternoon. Low
level onshore flow pattern continuing through Thursday.
Satellite imagery shows stratus along the coast almost completely
dissipated at 22z/3pm. Onshore gradients increasing with the peak
heating of the day with kuil-kbli and khqm-ksea both near 1.5 mb.
Increasing onshore flow overnight and the approaching upper level
trough will enhance the marine layer Thursday morning over western
Washington. Expect stratus to reform along the coast tonight and
spread inland Thursday morning. Marine push does not look strong
enough at this point to get stratus up into Cascade valleys but it
does look strong enough to get the stratus to the most of the inland
terminals for a few hours Thursday morning. Ceilings with the
stratus look to be in the 1000-2000 foot range for the interior and
below 1000 feet along the coast. There is also a chance of drizzle
along the coast late tonight into Thursday morning with visibilities
1-3sm. Conditions improving Thursday afternoon with VFR over all but
the coastal areas.
Ksea...mostly clear skies into early Thursday morning .Stratus
arriving at the terminal around 12z with ceilings around 1000 feet.
Ceilings lifting during the morning hours with the stratus
scattering out Thursday afternoon. Southerly winds 4 to 8 knots will
try to come around to northwesterly tonight ending up in the 250-290
degree range for a few hours. Winds southwesterly 4 to 8 knots
Thursday morning. Felton
Marine...low level onshore flow pattern will continue through the
weekend. Highest winds and waves are expected in the central and
east Strait of Juan Delaware fuca during the late afternoon and evening
hours. A weak cold front will cross western Washington on Thursday...
followed by another weak system on Friday. Both of these systems are
not strong enough to disrupt the onshore flow pattern.
Pz...Small Craft Advisory central and eastern Strait of Juan Delaware
fuca...Admiralty Inlet...and northern inland waters.
You can see an illustrated version of the forecast discussion at