Funnel Cloud in Oregon, October 30, 2024

Area Forecast Discussion
National Weather Service Portland OR
308 PM PDT Wed Oct 30 2024

.SYNOPSIS...A series of storm systems will bring valley rain
and mountain snow through Friday, with additional systems brushing
the region over the weekend and into next week. Heavy snow is
expected above 4500 ft in the Cascades through Friday, especially
for Linn and Lane Counties. There is a low (10%) possibility of high
winds along the coast early Friday. Temperatures remain seasonably
cool through early next week.

&&

.SHORT TERM...(through Friday)...Water vapor satellite imagery this
afternoon shows a shortwave trough digging toward the southern
Oregon coast, while pushing an occluding cold front across the
Pacific Northwest. Rainfall rates with the initial frontal band have
generally been around a tenth of an inch, except at the coast where
around a half inch of rain fell in a 2-hour period. Snow level
appear to be sitting around 4500 ft this afternoon, but are expected
to fall tonight to around 3500 ft behind the frontal passage. Web
cams do show snow falling at the higher Cascade passes, but not
sticking much as temperatures are currently hovering around 33-35
degrees. Expect temperatures to continue to cool with light snow
showers sticking to surfaces and roads overnight. Another upper
level shortwave is then expected to enhance snowfall rates Thursday
through Thursday night, while surface low pressure drops toward the
region. This will result in winter-like travel conditions across the
Cascade passes through Friday. Have upgraded the Winter Storm
Warning to include the Linn County Cascades (including the Santiam
Pass), along with the Lane County Cascade warning already in place,
as both deterministic and probabilistic guidance suggests snow fall
rates trending upwards of 1 inch per hour Thursday afternoon through
evening south of Mount Jefferson.

The lowlands will see rain increase across the area on Thursday,
while those out on Halloween evening should expect chilly and moist
conditions, with frequent showers through Thu night. Thunderstorm
chances will persist along the coast through at least Thursday
night. CAPE remains rather limited (less than 500 J/kg) as the
colder air aloft remains farther north, but will maintain rather
strong shear through the lower atmosphere. In general, expect 48-
hour rainfall amounts of around 1.50-2.50 inches along the coast and
1.00-2.00 inches across the interior lowlands through Friday
afternoon.

A potential wrinkle in the forecast appears early Friday as a number
of ensemble members (~40%), as well as the deterministic GFS, shows a
rapidly developing low near the coast. There is high uncertainty
with this feature, but it does bring a low (10%), but potentially
impactful scenario of strong to high winds to the central and
northern Oregon coast Friday morning. Will need to continue to
monitor.

Depending on what exactly occurs, will likely see some version of
weak low pressure lingering near the Washington coast on Friday with
valley rain showers and mountain snow likely (>90%) continuing
through at least Friday evening.  /DH

.LONG TERM...Saturday through Wednesday...The long term forecast
continues to be highlighted by uncertainty, but without any real
signals of impactful weather. Light rain showers appear likely on
Saturday as the upper level trough shifts east toward the Rockies
and a northwesterly jet remains over the Pacific NW. Good agreement
among clusters of upper level ridging building over the Pacific
later in the weekend, but does not appear to be very strong as most
clusters show upper level troughing flattening out the ridge by
early next week. With such uncertainty, which is typical with early
fall patterns, will maintain elevated chances for precipitation
through early Tuesday. Ensembles and the clusters are in fairly
decent agreement of a period of drier weather toward the middle of
next week. /DH

&&

.AVIATION...A front is generally bringing MVFR conditions to most
taf sites early this afternoon. As the front moves east of the
Cascades over the next few hours and we enter a more unstable
airmass, expect conditions to trend into predominantly VFR
thresholds. However, brief dips into MVFR/IFR thresholds will more
than likely still occur between 00-12z Thursday, but will be
difficult to predict with any skill more than minutes in advance.
The probability of these conditions manifesting themselves at
any given hour at any taf site is around 20%. There is increasing
consensus that another wave of showers will spread into the
region Thursday morning with the probability for MVFR conditions
increasing to >50% at any given hour after ~15z Thursday.

PDX APPROACHES...A front is beginning to exit the area. A more
unstable environment will spread across the region behind the
front and bring periods of showers. Expect predominantly VFR
conditions through 12z Thursday. However, brief dips into
MVFR/IFR thresholds that will be difficult to predict more than
minutes in advance will almost certainly come to fruition at some
point between 00-12z Thursday. The probability of these conditions
manifesting themselves at any given hour is around 20% during
this time. Another wave of showers will spread into the region
Thursday morning with the probability for MVFR conditions
increasing to >60% at any given hour after ~15z Thursday.

&&

.MARINE...A strong front crossed the waters this morning and is
now pushing inland. This brought widespread Gale Force wind gusts
of 40 kt to the waters this morning. Seas responded accordingly
and have since climbed into the low teens. Winds have turned
westerly and backed off considerably. This should result in
significant wave heights temporarily dropping late this afternoon
back into the single digits. However, an incoming westerly swell
will allow seas to climb back into the low to mid teens overnight
into Thursday.

A surface low pressure currently located northwest of Haida Gwaii
will drop southeastward towards the waters off the Washington
coast over the next 24-48 hours. This should largely maintain a
showery weather pattern with occasional wind gusts of 25-30 kt
near and under stronger showers. In addition, there is at least a
20-30% chance of a thunderstorm impacting a given location through
Thursday night.

It should be noted that a small number of models suggest a developing
surface low pressure will spin up and move towards the Oregon or
south Washington coast Friday. This scenario would result in Gale
to Storm Force wind gusts of 40-50 kt developing across the
waters on Friday. The official forecast does not reflect these
values at this time given confidence in this scenario panning out
is low. The probability for this scenario happening on Friday is
around 10% at this time.

High pressure then slowly builds over the northeast Pacific this
weekend into early next week, but remains far enough offshore,
that the Pacific Northwest will remain susceptible to fast moving
fronts crossing the waters.

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...Winter Weather Advisory until 5 PM PDT Friday for ORZ126.

     Winter Storm Warning until 5 PM PDT Friday for ORZ127-128.

WA...Winter Weather Advisory until 5 PM PDT Friday for WAZ211.

PZ...Small Craft Advisory until 5 PM PDT Thursday for PZZ210.

     Small Craft Advisory until 5 AM PDT Friday for PZZ251-271.

     Small Craft Advisory until 8 AM PDT Thursday for PZZ252-253-272-
     273.

     Hazardous Seas Warning from 8 AM Thursday to 5 AM PDT Friday for
     PZZ252-253-272-273.
&&

www.weather.gov/portland

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