Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
300 PM PDT Wednesday may 6 2015
Synopsis...upper level low pressure will move south of the region
tonight. High pressure offshore will build offshore providing dry
weather and a warming trend Thursday and Friday. The ridge
strengthens over the Pacific northwest this weekend with
temperatures rising well above average. Unsettled weather will
return Monday through middle week.
Short term...latest radar loop indicates scattered showers over
parts of western Washington moving southward. The bulk of shower activity is over the
Cascades and foothills as expected but some isolated showers have
also developed across The Lowlands. Only a couple lightning strikes
noted so far east of the crest but lifted indice's over the western slopes/foothills
are around -1 to -2c...so a mention of isolated thunder will stay in
the forecast this evening. Any isolated showers over The Lowlands
should diminish quickly by Sun Down with loss of diurnal instability.
An upper level ridge will amplify off the Pacific northwest coast Thursday and
Friday. The low level flow will be light and northerly which will may
slightly hold back afternoon temperatures in locations near the
water. It will be dry with just patchy morning clouds...mainly near
the coast. Highs will be in the 60s Thursday rising into the low and
middle 70s by Friday.
Long term...the high amplitude ridge offshore will shift eastward with
the ridge axis directly over the area providing maximum
compressional warming. 500 mb heights build over 5700m with weak
thermal ridging and light flow. High temperatures should reach the
maximum potential so forecast temperatures follow the warmest MOS
guidance. Expect widespread 70s with a couple 80s possible around
the SW interior.
An upper level low will approach the region on Monday. The latest
12z European model (ecmwf) is now in line with the GFS showing the upper low cutting
off and diving southward offshore. Previous runs have shown the low moving
inland much closer to our area which would produce an unstable
showery pattern. Although models have trended drier...will maintain
the chance probability of precipitation for next week as models continue to evolve. Highs
will cool off into the low to middle 60s. Mercer
Aviation...an upper level low over northern Oregon will drop south
into California on Thursday. At the surface...a ridge of high pressure
will develop over the offshore waters along 134w. The air mass is
somewhat moist and unstable...it will dry and stabilize tonight as
the upper low moves away from the area. Northerly flow aloft.
Scattered showers and isolated thunderstorms can be expected near the
Cascade crest...they will weaken as they move south-southwest through the
Cascade foothills and the southwest interior. Some cumulus buildups
are seen elsewhere over land this afternoon. All of the clouds will
dissipate around sunset as the air mass dries and stabilizes. VFR
conditions will prevail across the region through about 10z.
Skies will clear around 04z with loss of insolation...allowing fog
to form 10z-17z Thursday in favored river valleys from Arlington and east of
ksea and kshn-kolm-ktdo. Mainly clear conditions will dominate
midday Thursday Onward. Albrecht
Ksea...any shower or thunderstorm activity this afternoon is expected to
remain east of the terminal and will die off about 02z. The scattered layer
around 030 and the scattered-broken layer at 060 will dissipate around 04z
leaving clear skies that will continue through Thursday. Expect north wind
10-13 knots to ease to NE 4-7 knots after 04z...become light after
10z...then resume at 35010-14 knots about 19z Thursday. Albrecht
Marine...a building ridge along 134w will give light to moderate
onshore flow through Thursday. The ridge is now expected to bridge NE
into central British Columbia Friday into Sat allowing a weak thermally
induced trough of low pressure to develop over the interior of Oregon
and Washington. Flow over the inland waters will become light northerly
Friday into the weekend with the thermally induced trough in place.
Small Craft Advisory northwest winds are likely at times over
portions of the coastal waters...especially Thursday evening...then
again over the weekend as the ridge shifts west to around 140w.
Pz...Small Craft Advisory central and eastern Strait of Juan Delaware
fuca from until 1 am PDT Thursday.
You can see an illustrated version of this forecast discussion at