Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Portland or
203 PM PDT Monday Aug 31 2015
Synopsis...surface high pressure over Oregon Monday will weaken overnight.
A cold front will push southeast across the region Tuesday and Tuesday evening for
a chance of rain. An upper level trough of low pressure will move in Wednesday
and linger through the end of the week.
Short term...upper low seen in water vapor pictures over Southeast Alaska Monday
afternoon had a 130 knots jet at 300mb in the Gulf of Alaska associated
with it. Jet will aid in digging the trough...as well as translating
it east over the next couple of days. Models agree that this pushes a
weak cold front southeast through the forecast area Tuesday afternoon and
evening...before the main upper trough moves in Wednesday and Thursday.
Surface observation and radar have shown light showers mainly over the
Oregon coast and Coast Range and SW Washington...basically to the northwest of the
surface ridge where there is low level onshore flow. With a
continuation of a westerly flow aloft...expect little change to this
pattern overnight. As the upper low approaches Tuesday upper heights
begin to fall...and a weak cold front reaches the coast. Models in
good agreement in pushing the front inland midday Tuesday...and through
the southeast croner of the area Tuesday evening. With good low level moisture
available below 700mb...will carry likely or better probability of precipitation across the
north...and chance probability of precipitation across the S.
A change over to a more showery pattern is in store for Tuesday night
and continuing through Thursday as the upper trough follows and settles
in over the Pacific northwest. There is not great agreement amongst models
as to when the initial shortwave moves across...NAM and ec suggest
Wednesday evening. With cool air aloft moving in coupled with diurnal
heating best instability appears likely late in the day Wednesday. Will
carry highest probability of precipitation for the showery air mass Wednesday afternoon and
evening...and include a slight chance for thunderstorms across the north
part of the area where model soundings suggest deepest instability.
Will continue with chance probability of precipitation later Wednesday night into Thursday as
instability decrease some in response to cooling of lower levels.
Daytime temperatures will remain below normal next few days...even cooling
a few degrees Wednesday and Thursday behind the cold front.
Long term...Thursday night through Monday. Lower confidence than
normal in the evolution of the extended portion of the forecast
today given some key divergences in model guidance by the weekend.
Overall...a slightly more amplified and more slowly evolving pattern
expected as the upper trough remains over the Pacific northwest on Friday.
Deterministic 12z runs of GFS and European model (ecmwf) both slow the progress of
the upper trough..and have thus increased probability of precipitation Friday and Friday
night. By Saturday...model differences increasingly prominent...with
the European model (ecmwf) developing a closed upper low over western Oregon through
much of the weekend. The European model (ecmwf) ensemble mean also supports this
solution...but a non trivial number of members seem to be in line
with the GFS and slightly rising heights under weak shortwave
ridging. For this reason...kept the forecast dry and warmer with maximum
temperatures around seasonal normals for Sunday and Monday for now...but if
the European model (ecmwf) trend is correct...Sunday may be a cooler and perhaps
showery day. Cullen
Aviation...the warm front has had little impact to the aviation
weather and mostly VFR conditions prevail. The exception is the
north coast where ceilings have been bouncing between VFR and MVFR as
a weak cold front approaches from the north. The coast will likely
see IFR conditions late tonight into early Tuesday morning as well
as gusty south winds up to 25 knots. The coast should improve to MVFR
middle morning. Expect inland areas to remain cloudy with VFR
conditions through Tuesday morning.
Kpdx and approaches...cloudy with VFR conditions through Tuesday
Marine...coastal observations and bar reports suggests that the
south winds are easing below 25 knots this afternoon and will end the
Small Craft Advisory at 3 PM. There may be some localized small
craft gusts ahead of and with the next frontal passage late
tonight through Tuesday afternoon...but do not anticipate the higher
gusts will be widespread or consistent enough to warrant an
advisory. Seas will remain relatively steep today and tonight
with 7 to 8 feet heights at 10 to 11 seconds. A surface low over
the Gulf of Alaska...just offshore of Yakutat is generating a
northerly fetch that will bring a brief period of 10 feet swell to
the Oregon waters Wednesday.
Surface high pressure will build over the NE Pacific Wednesday night
and Thursday as a thermal low develops over northern California. This
will bring a return of northerly winds to the waters Thursday and
Friday. A low to the north will prevent a tight gradient and
expect below advisory winds Thursday and Friday. Tj
Pz...Small Craft Advisory for winds until 3 PM PDT this evening
for coastal waters from Cape Shoalwater Washington to Cascade Head or
out 60 nm.
Small Craft Advisory for rough Columbia River bar from 5 PM
this afternoon to 10 PM PDT this evening.
Small Craft Advisory for rough Columbia River bar from 4 am
to 9 am PDT Tuesday.
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This discussion is for northwest Oregon and southwest Washington
from the Cascade crest to 60 nautical miles offshore. The area is
commonly referred to as the forecast area.