Area forecast discussion
National Weather Service Seattle Washington
915 PM PDT Tuesday Jun 18 2013
Synopsis...an upper low just off the coast will continue to bring
scattered showers and a few thunderstorms to the region through
Wednesday. An associated trough rotating westward to the north of
the low will likely bring a period of rain to much of western
Washington Wednesday night and Thursday. There could be locally
heavy rain across the north Cascades from this system. Weak high
pressure will build into the area Friday night and Saturday. Another
wet weather pattern is likely to develop early next week with
several fronts moving across the area.
&&
Short term...the upper low just off the coast is gradually
weakening and ever so slowly beginning its drift southeastward. Over
the next couple of days the low will track eastward across northern
Oregon and then northeast to northern Idaho by Thursday. Showers and
thunderstorms this evening will gradually dissipate overnight with
another round of showers and isolated thunderstorms tomorrow with
daytime heating.
The models... while not agreeing on all of the details... continue
to indicate a vorticity maximum will rotate westward across the north
Cascades and possibly the Olympics bringing a period of rain...
potentially heavy... to the north Cascades and northern lowlands.
Some models are indicating over 4 inches of rain in the vicinity of
Montana Baker and north Cascades in a 36 hour period ending Thursday
night.
Friday into Saturday brings a bit of a break to the showery weather
with warmer and drier conditions as the closed low moves eastward. A
new trough embedded in westerly flow will bring a chance of rain
back to the area by Saturday night. Colman
&&
Long term...from previous discussion...temporary high pressure will
build into the area Friday night and Saturday. A front may reach
western Washington Sat night and Sunday morning. There is a difference
between the GFS and the European model (ecmwf) this weekend...if the 12z GFS is
correct it will not rain much...mainly just on the coast and in the
Olympics and that model showed no rain at all for a good portion of
the Seattle Everett area. The Euro on the other hand gives much of
western Washington a good soaking rain with most of western Washington getting at
least an inch of rain. After that...rain is likely at times Sun night
through next Tuesday as a new upper trough moves into the NE Pacific.
&&
Aviation...upper level low off the coast Tuesday evening will move
southeast into northwest Oregon Wednesday morning. South
southeasterly flow aloft becoming more southeasterly Wednesday
morning. Air mass stabilizing somewhat after 06z.
Doppler radar shows shower activity pretty much confined to the
mountains at 04z. Scattered to broken deck at or above 10000 feet over the
area. These conditions will continue into Wednesday afternoon.
Surface gradients onshore overnight into Wednesday morning which
brings up the possibility of a stratus deck around 2000 feet forming
after sunrise. What stratus that does form will not be widespread
and will only last until late morning at the latest. Air mass
becoming more unstable Wednesday afternoon for another round of
scattered showers.
Ksea...mostly middle level cloud cover overnight into Wednesday.
Some stratus forming over the sound Wednesday morning could float
over the Airport for a couple of hours in the morning. Confidence
not real high in this scenario. Will go with a tempo group with 2000
broken for a few hours after sunrise. Felton
&&
Marine...a 1012 mb surface low will shift east and diminish over
the coastal waters tonight. High pressure over the offshore waters and
low pressure over interior b.C. Will maintain onshore flow across
western Washington Wednesday through Friday. The central and east
Strait of Juan Delaware fuca may see gale force westerly winds Wednesday
evening. Winds will gradually ease through the Strait Thursday
morning. A front will approach the coastal waters on
Saturday...moving inland Sat night. 33/Felton
&&
Sew watches/warnings/advisories...
Washington...none.
Pz...none.
&&
$$
You can see an illustrated version of the forecast discussion at
www.Weather.Gov/Seattle/gafd/latest_webafd.Html
