Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Portland or
832 PM PDT Tuesday Sep 16 2014
Synopsis...a low pressure system currently well off the Oregon coast
will move towards southern Oregon and bring a chance of showers late
Wednesday into Thursday across northwest Oregon and southwest
Washington. Onshore flow will keep most of the smoke from the 36
pit and deception complex fires over the Cascades and points east.
However...high pressure and offshore flow looks increasingly likely
to build over the region towards the weekend so expect smokey
conditions and temperatures possibly approaching 90f again across the
Short term...upper level moisture associated with a large shortwave
trough will continue to lift north tonight...already seeing some
echoes on radar imagery this evening but its still most likely virga.
Models continue to show the best threat for any measurable
precipitation hitting the SW Washington and northwest Oregon coast...but it
looks like it may reach the coast early Wednesday morning. Will also
see patchy low stratus along the coast overnight...particularly along
the central coast near Newport. Otherwise just expect increasing middle
level clouds into early Wednesday morning as the surface low spreads
moisture northward. /27
The 36 pit and deception complex fires continue to produce plenty of
smoke. Continue to check with your state air quality agency for the
latest air quality observations. Expect valley locations immediately
down river of these fires like Estacada and Oakridge to experience
problems with smoke each night and early morning this week.
Fortunately...west winds each afternoon and evening should disperse
smoke...especially when the fires are generally most active....to the
For Wednesday...models agree the upper level trough and surface low
pressure currently well off the California coast will shift
northeastward towards the California/Oregon state line through
Wednesday evening...before splitting and nose diving into Southern
California late Thursday. This rather messy pattern will result in
waves of mainly middle to upper level clouds spreading across the area
through Wednesday evening. Aside from a few sprinkles from time to
time...suspect most locations will remain dry based on model
soundings. Kept a slight chance mention of thunder across our
southeastern zones Wednesday afternoon and evening where perhaps
enough middle level instability could generate a lightning strike or
two...but confidence in this scenario panning out is very low.
For Thursday...the NAM and GFS suggest a low level surge of
isentropic lift ahead of the trough axis late Wednesday night into
Thursday morning may saturate the atmospheric column and result in
light precipitation across much of the area Thursday morning. Low
level westerly flow should also aid in wringing out light rain
across the orographically favorable Coast Range and Cascades. As a
result...probability of precipitation were nudged upwards during this period into the low
end likely category along the coast and across the topography...and
high end chance category for valley locations. Models suggest cloud
cover will be ample on Thursday...but if we do get any breaks in the
clouds...it will not take much heating to produce thunderstorms. At
this point...cloud cover looks pretty solid Thursday so left the
mention of thunder out of the forecast...but this will be Worth
monitoring over the next 48 hours.
Overall...continue to remain nervous that models are too aggressive
with the moisture and precipitation chances late Wednesday into
Thursday given the splitting nature of the upper level trough. In
addition...this will be the first fall like storm system to try and
break down the longwave ridge...which also does not Bode well.
For Friday...shortwave ridging moving into the area late Thursday
into Friday should allow the area to dry out considerably.
Strengthening thermal low pressure over California on Friday will
turn winds more northerly...and the area will begin another warming
Long term...no changes. Previous discussion follows...Saturday
through Tuesday...GFS...dgex...and European model (ecmwf) all show a return to
easterly flow on Saturday which will warm temperatures across the
area. This warm up will only last through Sunday...as winds shift
back to the west from an upper-level trough and associated cyclone
deepening over the Gulf of Alaska. This will cool temperatures and
bring the marine layer back in at night Monday and Tuesday. -McCoy
Aviation...rather tranquil with light winds and variable middle and
high clouds. Onshore gradient is quite light...so not expecting
any fog or low clouds...though can not completely eliminate a few
patches of fog in bays and Coast Range valleys tonight. Will see
increasing middle level clouds on Wednesday...along with a shower or two.
Smoke from the 36-pit fire continues to drift to the east and NE.
Will see smoke continue in far East Columbia gorge...Upper Hood
River Valley and around Montana Hood...with some smoke trapped settling
in valleys to east and north of the fire. With S winds aloft over next
few days...smoke from fires over SW Oregon and northwest calif will drift
north to NE...affecting slantwise visibility from Cascades eastward.
Kpdx and approaches...VFR with variable high clouds tonight...and
increasing middle level clouds on Wednesday. Rockey.
Marine...fairly benign conditions persist over the waters for
the next few days as weak low pressure will maintain S to southeast wind
less than 15 knots. There will be a mix of wave trains with varying
periods...predominately from the west at around 15 second. Overall seas
running 5 to 7 feet...with highest further offshore.
High pressure will rebuild over the waters Friday and Sat...with thermal
low pressure over the coast and coastal waters. This will bring
another round of east to NE winds to the waters this weekend with
winds possibly gusting to 25 knots on Sat and Sat night. Rockey.
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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 as the forecast area.