Scientific Forecaster Discussion
Area forecast discussion...corrected
National Weather Service Seattle Washington
416 am PDT sun Aug 31 2014
Corrected a couple of typos in the short term segment.
Synopsis...weak disturbances embedded in the strong westerly flow
aloft over the area will keep the weather somewhat unsettled
today. A low pressure system passing to our north will result in
a chance of light rain tonight. High pressure aloft will prevail
over the region on Labor Day. Look for a stronger and colder
system to impact the area on Tuesday.
an upper jet will remain draped over the region today. Poorly
defined and fast-moving disturbances embedded in the flow will
result in some instability showers over the County Warning Area...hence the chance
probability of precipitation.
The models were in agreement in taking the vigorous short wave
trough...presently near the Gulf of Alaska...across British Columbia
tonight. The upper jet will lift northward tonight...allowing
heights/thicknesses to build over the area. Hesitated to completely
remove probability of precipitation from the tonight forecast since it is conceivable that
there could be some spotty light precipitation due to warm air advection.
There might be some spotty light rain or drizzle over the northwest
part of the County Warning Area Monday morning...otherwise Labor Day will be a dry day
with near normal temperatures under high pressure aloft.
A stronger and colder low pressure system will impact the region
on Sunday. The best chance of precipitation will be across the northern
zones. Daytime temperatures on Tuesday should be 5-10 degrees cooler than
Monday. Although the models did not indicate it...would not be
surprised if it becomes locally breezy in the afternoon.
The snow level is expected to fall to near 5500 feet late Sunday
night in the north Cascades...thus do not be surprised to see a
rain/snow mix at this elevation late Sunday night.
an upper ridge will build over the region on Wednesday for the
establishment of dry...northerly flow. Wednesday night will likely be
the chilliest morning this week...with the min temperature at many
lowland sites predicted to be in the 40s.
Expect an upper ridge to dominate the weather pattern over the
area during much of this period. However the medium range
solutions differed on the strength of the ridge. Beyond Friday...
confidence in the forecast was low due to differences in the
medium range solutions...that is an upper trough versus an upper
ridge. The only changes made were to blend better with
Aviation...light northwest flow aloft today between weather systems. A
weak weather system will dissipate as it moves into the area on
Monday. Moist and weakly unstable atmosphere. Isolated showers.
Areas of MVFR ceilings expected this morning. Clouds should lift and
scatter by midday.
Ksea...MVFR ceilings this morning...lifting above 3k feet and scattering
by around 18z. S-SW wind 4-8 knots becoming northerly for a few hours this
evening. A Puget Sound sound convergence zone may reform and produce
additional showers late this afternoon and evening...otherwise just
isolated shower activity.
Marine...a weak decaying trough will pass north of the area tonight
enhancing onshore flow this evening. Westerly winds may reach
low-end small craft to 25 knots for a few hours this evening in the
central/east Strait. Another stronger surface trough will bring
increased onshore flow Tuesday and Wednesday. Small craft winds are
again likely at times in the central/east Strait.
You can see an illustrated version of this forecast discussion at