Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Portland Oregon
428 am PDT sun Oct 23 2016
Synopsis...showers associated with a weakening cold front will
decrease this morning, leaving a mostly dry day for much of SW
Washington and northwest Oregon. Another surface low digging south from the
Gulf of Alaska will develop into a stronger and more complex low a
few hundred miles off the Oregon coast today. Disturbances moving
around the low will keep a chance of rain at times through Tuesday,
with the strongest bringing some windy conditions along the coast
Monday night. The low will probably also bring some heavy surf to the
coast. Another low is expected to bring more rain and gusty coastal
winds Wednesday. Forecast models suggest another large low pressure
system will set up shop a couple hundred miles off the Oregon coast
Friday then linger, keeping a chance of rain at times through next
Short term...today through Tuesday...complex forecast over the next
72 hours with a lot of potential sources of error in regard to timing
and strength of wind and rain.
Models show a good level of agreement in the overall pattern, with
deep and complex low pressure rapidly developing 300-400 miles off
the Oregon coast today. This low is expected to go from the current
nascent system to around 975 mb or possibly deeper by Monday morning.
This would be of obvious concern if it were expected to continue
toward the pac northwest coast, but there is a high level of confidence that
the low will stay near or west of 130w...limiting the wind impacts
for much of our forecast area.
A loosely defined baroclinic or frontal zone remains draped across
western WA/OR. A few showers are still occurring across the district,
but should dissipate today as warmer air aloft moves in ahead of the
developing low offshore and increasingly caps any instability. With
higher pressure east of the Cascades, some weak offshore flow will
develop, which will in turn dry out the lower levels of the
atmosphere a bit...further deterring shower activity this afternoon.
As a result today should be mild and mostly dry with temperatures
reaching into the 60s for most of The Lowlands. Some high clouds will
keep it mostly cloudy at times through the day.
As the low blossoms offshore, the baroclinic zone will likely become
energized and infused with an increasing amount of moisture from the
S-SW. This will have to battle a bit with the offshore flow to really
spread rain across our forecast area, so we kept pops fairly low
until midday Monday. The better chances will be near the coast until
then. Pressure gradients remain offshore through Monday morning,
likely keeping most of the wind associated with this system off the
With the baroclinic zone draped north-S near the Oregon coast, a strong
low offshore, and a 70-110 kt S-SW jet stream lurking in the area at
times, there will be the potential for secondary cyclogenesis closer
to the Oregon coast at any time late tonight through Tue morning.
However models do not seem to be latching on to this idea at this
time. Therefore any strong winds will probably remain confined to the
coastal and offshore waters...and possibly the or/Washington beaches and
headlands. This is due to the orientation of the pressure gradients,
with isobars mainly parallel to the coast. If an unexpected secondary
low develops - which is very possible and probably would not be
well-initialized nor well-modeled - pressure gradients could turn
more southerly, opening the door for more widespread winds and/or
Otherwise, conditions will be fairly benign with some light to
moderate rain at times and a bit of wind on the beaches and headlands
through Tuesday...and this is what is currently reflected in the
Long term...Tuesday night through Saturday...a surface low offshore
will weaken Tuesday evening and another low will approach from the
south. The models vary on the strength of this low, but it has the
potential to support strong gusty winds along the immediate coast as
it moves south to north offshore of the Oregon and Washington coast
on Wed. Modeled precipitable water values show that there could be
quite a bit of moisture available with this front and the Portland
forecast area may get a decent shot of rain with the warm front
Tuesday night into Wed morning, and again with the cold front
Wednesday night. There may be a break in wet weather Thursday before
another complex low sets up offshore Friday and Saturday. Tj
Aviation...a weakening front will bring periods of light rain to
the forecast area this morning. Expect primarily VFR conditions
today and tonight with cigs generally above 4000 ft. However,
brief periods of MVFR cigs near 2000 ft are possible this morning.
Expect mid and high clouds to increase later today and tonight as
a cold front approaches the region. Could see some patchy fog or
low stratus develop in the interior late tonight.
Kpdx and approaches...primarily VFR today and tonight with brief
periods of light rain possible through 18z. Patchy fog or low
stratus possible late tonight. /64
Marine...winds and seas are expected to fall below Small Craft
Advisory criteria early this morning as a weak front pushes
further inland. However, this break will be shortlived as a
developing low near 130w swings a cold front across the waters
later today and tonight. Gales are expected to develop over our
southern outer waters by this afternoon and spread north tonight
so will go ahead and upgrade the gale watch to a warning for the
outer waters. The active weather pattern looks to swing a slightly
stronger front across the waters on Monday as another low develops
offshore. This system will spread gales into the inner waters on
Monday so will also issue a Gale Warning for the inner waters.
There remains the potential for some low-end storm force winds
with monday's system, but confidence is low. Gales look to persist
over all the waters through Tuesday.
Seas look to subside below 10 ft this morning as the winds weaken,
with seas remaining below advisory criteria through tonight. The
active weather over the waters will allow seas to climb into the
mid teens by late tonight, with seas likely peaking near 25 ft
with the strong winds expected on Monday night. Seas subside back
into the mid to upper teens on Tuesday. /64
Pz...Small Craft Advisory for winds until 5 am PDT early this
morning for coastal waters from Cape Shoalwater Washington to
Florence or out 60 nm.
Small Craft Advisory for winds from 3 PM this afternoon to 9 PM
PDT this evening for waters from Cape Shoalwater Washington to
Cascade Head or from 10 to 60 nm.
Gale Warning from 9 PM this evening to noon PDT Tuesday for
waters from Cape Shoalwater Washington to Cascade Head or from 10
to 60 nm.
Small Craft Advisory for hazardous seas until 5 am PDT early
this morning for coastal waters from Cape Shoalwater Washington to
Florence or out 60 nm.
Small Craft Advisory for winds from 9 PM this evening to noon
PDT Monday for coastal waters from Cape Shoalwater Washington to
Florence or out 10 nm.
Gale Warning from noon Monday to noon PDT Tuesday for coastal
waters from Cape Shoalwater Washington to Florence or out 10 nm.
Gale Warning from 3 PM this afternoon to noon PDT Tuesday for
waters from Cascade Head to Florence or from 10 to 60 nm.
Small Craft Advisory for rough Columbia River bar until 5 am
PDT early this morning.
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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.