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Area forecast discussion 
National Weather Service Portland or
858 am PDT Tue Aug 30 2016

Synopsis...low pressure deepening offshore will swing a compact
upper low across interior Oregon today. This low will bring periods
of drizzle areas near the coast and a few showers for the central
Oregon Cascades. A cold front will cross the region Wednesday with
widely varying rain amounts. Shower chances will continue for the
remainder of the week with the best chances in the north Oregon Coast
Range and Willapa Hills.

&&

Update...after collaboration with the fire weather forecaster, have
decided to remove thunder mention from the Lane County Cascades. 12z
Salem sounding has a less than impressive instability profile, and
even if any upright convection was able to develop aloft, a lack of
phase change would inhibit any lightning. In addition, bumped up
skycover this morning as almost every site west of the Cascades is
reporting overcast conditions this morning and no improvement is
expected until the shortwave swings through the area early this
afternoon.

Otherwise, made only minor tweaks to temperatures, including
increasing temps along the coast on Wednesday from the mid 50s into
the low 60s. /Bentley

Short term...(today through Thursday night)...broad upper low
continues to deepen off the British Columbia and pacnw coast today.
Currently, a compact mid-level low is centered near 38n, 130w and is
expected to push north-northeast today...crossing south central and
eastern Oregon as an open shortwave. Satellite and radar imagery
make this feature appear rather potent from broad isentropic lift
occurring to the north and northeast. A deeper analysis indicates
its proverbial bark is for worse than any actual bite at this hour.
Radar returns are keeping echos very high with lowest elevations
exceeding 20kft. Infrared cloud top temps are also quite cold, but judging
from the radar and hires model sounding data, cloud bases are no
lower than 15kft. Thus radar returns are likely ice crystals
falling from the cirrus and no liquid to ice phase change likely
occurring within the very high elevation updrafts. Additionally,
have not seen any lightning from these features despite the very
cold clouds. Continued the idea of matching neighbor offices with
regard to showers/possible thunderstorms across the central
Cascades for this afternoon and early evening as the low/open wave
is still progged to cross the region during the early side of peak
heating. Feel it will be much better to emphasize the 20% pops in
the inverse fashion meaning there is really an 80% chance no
showers, let alone thunderstorms, will occur. A look at time heights
within the most aggressive GFS model only show convective
parametrization saturated moisture spikes well above the freezing
level with very dry air below it at the mid-levels. Feel this must
be where the GFS is deceptively generating its precip amounts across
the Cascades.

With that as the main item of today's interest set aside, focus will
then turn to the primary trough offshore and its oft delayed
frontal crossing. To start, marine clouds have certainly penetrated
inland overnight. Cirrocumulus streaming overhead is obscuring the
satellite view of the lower clouds but ground observations are
showing some holes are out there. Feel The Breaks will be short
lived come daybreak with only sporadic view of the high clouds
happening at times. Periods of coastal drizzle appear they will
develop this morning through this afternoon as some of the lower
level broad lift wrings it out of the stratus layer.

Starting to see some of the rain associated with the frontal
boundary in the distance from the klgx radar. This band is oriented
nearly north to south which does not Bode well for a rapid approach
given a lack of upper jet support. In fact, the aforementioned
crossing shortwave will further Orient the upper flow meridional and
slow the frontal progression further. Now expect the frontal passage
may not reach the coast until mid-day Wednesday although the
stratiform rains associated with it will likely cross the coastline
at or around midnight tonight. Unfortunately, this slower timing
means the best deep moisture support and heavier rain amounts will
miss our land forecast area to the west and thus not result in any
notable rain amounts. The now somewhat limited frontal rains will
drift east across the County Warning Area Wednesday before rather unceremoniously
dissipating over the Cascades Wednesday evening.

Thursday may prove to be the more interesting day although models
continue to notably differ in the evolution. Both the GFS and the
European model (ecmwf) show a piece of energy breaking off and getting entrained in
the polar jet from an entirely separate upper low centered just
south of the far Aleutian chain reaches. This energy may get
entrained in our local upper low and energize enough to produce
upwards of an inch of rain somewhere within the greater pacnw
coastline. Models tend to not handle these fast moving features and
often create far more rain rain then is brought by reality. Will sit
tight on this somewhat newer development and give models more time
to latch on a consistent story. /Jbonk

Long term...no changes. Previous discussion follows...chance pops
remain for the remainder of the week and into the weekend as the long
wave trough axis waits until late in the weekend to shift east of the
Pacific northwest. Both the European model (ecmwf) and GFS agree on the arrival timing of the
upper low Thursday night, but they differ in intensity and speed. The
European model (ecmwf) remains more progressive and shifts east quickly while the GFS
slows down and cuts off an upper low over Washington. The latter
solution will definitely be more wet than the former, but until
models come into better agreement, have kept pops in the 30-50% range
during the period. /Bentley

&&

Aviation...widespread MVFR stratus deck across the region this
morning. These conditions are expected to continue today, but
expect some improvement for a mix of MVFR and VFR later this
afternoon, given latest satellite trends and model indications
that the lower layer around 2500 ft with will give way to VFR
this afternoon with cigs between 3500 and 4500 ft. As the front
approaches later today, this cloud deck will again become more
solid. This front will also bring a chance of showers to the
coast today and inland late tonight.

Kpdx and approaches...MVFR cigs around 2500 ft this morning
through 18z, lifting to 4000 feet by afternoon. May even see cigs
scatter for short time late afternoon before approaching front
brings reinforcing moisture to lower cigs back to around 3000 ft
later this evening or tonight. Cullen

&&

Marine...southerly winds will slowly increase especially over the
outer waters as a weakening cold front approaches. Winds may
occasionally gust in the 20 to 25 kt range beyond 30 nm today but
do not expect widespread or frequent enough to warrant an
advisory. Some of these localized gusts may move into the inner
waters later this evening. Winds will then vary between south and
southwest for the remainder of the week. The next front nears the
waters Thu. Then, stronger north winds return over the weekend as
high pressure rebuilds over the NE Pacific and a thermal low
builds up the Oregon coast.

Seas will remain generally 4 to 6 feet through much of the week,
largest during the periods of gustier winds. Seas are a mix of
long period (14 second) south swell and northwest swell at around
8 seconds. Cullen

&&

Pqr watches/warnings/advisories...
or...none.
Washington...none.
Pz...none.
&&

$$

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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.

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