Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
329 am PDT Thursday Aug 28 2014
Synopsis...a weak low pressure system aloft will move across the
area today for cooler weather. A stronger system will impact the
region Friday and Saturday for unsettled conditions and below
normal daytime temperatures.
stratus /along with some fog/ continued to gradually expand across
The Lowlands this morning. There was also a middle-level cloud deck
present over the northeast part of the County Warning Area. Expect low clouds to
blanket much of The Lowlands by midmorning.
Meanwhile...a weak upper trough will move across the region today.
The main impact this system will have on our weather will be
cooler temperatures...with readings at most places this afternoon
predicted to be near normal for this time of the year. There will
also be a risk of light precipitation on the coast...Olympics...and north
Another...but stronger...upper trough will approach the Pacific
northwest tonight and this will serve to significantly deepen the
marine layer. The upper trough will move over the Pacific northwest
on Friday...leading to a threat of showers...especially over the
Coast...Mountains...and within the Puget Sound sound convergence zone
/pscz/. The presence of an upper trough...plenty of clouds...and
strong onshore flow will result in below normal temperatures on Friday.
The upper trough will slowly move east during the day Saturday but
500 mb heights will remain below 570 dam over the County Warning Area. Overall...
Saturday is not expected to be much different than Friday. Thus
anticipate unsettled conditions with daytime temperatures remaining 5 to
10 degrees below normal at most locations.
500 mb heights will rebound early in the period...particularly on
Labor Day. If the medium range solutions are correct...daytime
temperatures on Monday could end up about normal. Thus it is possible that
we might be slightly on the cool-side for Monday.
Beyond Monday...it appeared that the weather pattern over the Pacific
northwest will return to cool and unsettled. If the European model (ecmwf) and GFS
end up being correct...maximum temperatures across The Lowlands will only be
in the 50s next Wednesday. However...the Canadian model was warmer for
day 7 and indicated highs in the 60s. All three models had an
upper trough over the region...so it is possible that our forecast
might be on the optimistic side for the latter part of the
extended period. Will need to see if the medium range solutions
continue with the idea of an upper trough over the County Warning Area late in the
period. Left the inherited forecast unchanged for this reason.
Aviation...clouds will continue to increase across western Washington this
morning as a weak cold front pushes through western Washington. The flow
aloft is westerly. Stronger onshore flow is bringing the marine
layer into the interior and most of The Lowlands will be in MVFR to
IFR range this morning. This front may also produce areas of fog and
drizzle along the coast...with isolated showers over the mountains.
A pscz may form over the King/Snohomish border this afternoon for
isolated showers as well. Otherwise low level clouds will lift this
afternoon with VFR conditions expected. 33
Ksea...a stronger onshore push this morning will bring marine clouds
to the terminal with MVFR to IFR conditions expected. Surface winds
are light southerly. Low clouds will gradually lift this afternoon for VFR
Marine...a weak cold front will cross western Washington this morning.
Another weak system will reach the area on Friday. Highest winds and
waves are expected in the central and east Strait of Juan Delaware fuca.
Kuil-kbli pressure gradient is -2.4 mb this morning...down from -3.3 mb
last evening. Gales verified last night but winds will ease by middle
morning as the pressure gradient eases further. Expect another round of
Small Craft Advisory winds through the Strait tonight with
Post-frontal onshore flow. High pressure over the NE Pacific will maintain
onshore flow across western Washington through the weekend. 33
Pz...Small Craft Advisory in effect until 5 am Friday for the central
and eastern Strait of Juan Delaware fuca.
You can see an illustrated version of this forecast discussion at