Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
900 am PDT Tuesday Sep 30 2014
Synopsis...moist onshore flow will bring some showers to western
Washington today...especially in the mountains. A weak system moving
through western Canada will brush the area on Wednesday for spotty
light showers in the north part. Thursday should be dry and partly
sunny but a front will sag south into the region on Friday. An upper
ridge will help limit rain to the coast and mountains this weekend.
Short term...moist northwest flow will prevail today as an upper trough
over the region shifts eastward. Radar shows some scattered showers
around the area and especially along the coast and mountains. With
continued flow through the Strait and southwesterlies through the
sound the convergence zone could redevelop today. The 00z WRF-GFS
was indicating this to occur north of the King/Snohomish County line
early this morning but there is little or no signature on radar yet.
The new 12z NAM has a couple areas of converging winds. One is over
King County and another further north over Snohomish/Skagit counties.
Quantitative precipitation forecast fields are less clear but imply some enhanced precipitation over
these areas today. Given the latest trends...convergence zone
formation could go either way today. Chance probability of precipitation in The Lowlands and
likely probability of precipitation in the mountains seem reasonable.
The region remains under cyclonic flow through the evening so there
is still some threat of showers. As higher pressure offshore starts
to build in later tonight the chance of showers will decrease. A
short wave ridge is over the area Wednesday so it should be mostly
dry with some morning low clouds and fog around. Models still show
spotty light quantitative precipitation forecast in the mountains so a few residual showers are
still possible. Highs will remain cool in the lower 60s.
Upper heights build on Thursday with light surface flow. Areas of
morning fog still seem likely unless high clouds from an offshore
system interfere with radiational cooling overnight. Most models
maintain dry weather on Thursday but the 06z GFS shows warm frontal
precipitation over western Washington just north of Puget Sound sound. Hopefully the new
12z GFS/European model (ecmwf) come into better agreement for the Thursday period and
beyond. The forecast for Thursday remains dry. Mercer
Long term...no changes to previous discussion...the warm front
discussed above and its trailing cold front should weaken and sag
south into western Washington on Friday. Model solutions differ
considerably for the weekend and early next week. The European model
keeps a stronger ridge over the region... with the frontal system
remaining mostly to our north. In the GFS solution the upper ridge
weakens and shifts east of the Cascades... allowing the frontal band
to move into western Washington unimpeded. And the GFS keeps some
precipitation over the region as another shortwave with another
shortwave arriving Monday. For now I will resist the temptation to
update the Saturday through Monday period...but it does look a bit
optimistic based on the latest model runs. Mcdonnal
Aviation...an upper trough over the pacnw will be replaced by a weak
upper ridge tonight. Another upper trough will move through b.C. And
Alberta Wednesday night and Thursday...this will keep west-northwest flow aloft over Washington.
The low level flow will remain westerly and the air over western Washington will
remain fairly moist with scattered showers...especially over the
Cascades and in the pscz at times.
Ksea...scattered showers and some clearing periods. A light SW
breeze...but if the radar shows a pscz develop again and move toward
Seattle the wind would become variable. There is a chance of that
happening later this afternoon and evening.
Marine...surface high pressure offshore with lower pressure inland
will result in westerly flow through Wednesday. A 1023mb high will work
east over the area Wednesday night and Thursday giving light gradients.
Pz...Small Craft Advisory central and eastern Strait of Juan Delaware fuca.
You can see an illustrated version of the forecast discussion at