240
FXUS66 KPQR 241032
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
332 AM PDT Tue Sep 24 2024

.SYNOPSIS...High pressure today with temperatures around 10
degrees F above normal. Cooler wet front moves in on Wednesday
bringing widespread rain. Showers persist through Friday
morning, then ridging persists through early next week.

&&

.SHORT TERM...Now through Friday...Observations this morning
show marine stratus along the coast with decreased visibility in
fog. Satellite shows clouds moving in along the Columbia River
towards Portland but no fog is expected due to a dry lower
atmosphere. Clouds will burn off today as high pressure builds
through the day. The ridge axis is setting up along the coast
which will promote a very weak easterly flow which will
transition to the south as the thermal trough moves inland over
eastern Oregon. Temperatures are expected to rise considerably
today with highs inland in the 90s, with the coast fairly mild
in the 70s. Now, if the cloud cover becomes more pervasive or
doesn`t erode all the way, this could inhibit the daytime
heating and thus temperatures will be lower. Based on high-
resolution ensembles there is more of a spread than what we have
previously seen in the longer range outputs. In the greater
Portland-Metro area the spread from the 10th to the 90th
percentile ranges from 85-92 degrees F. Similarly the NBM shows
a range of the 10th-90th percentile of 84-91 degrees F. The
deterministic outputs are trending on the warmer side of this
spread but, as previously stated, if the ridge moves/decays
faster or if cloud cover persists, temperatures will be on the
lower end side.

This ridge is just one component of a very broad Omega blocking
pattern combined with several low pressure systems that are
rotating around one another. This is known as the Fujiwhara
effect. One low is positioned over Mississippi (not associated
with the incoming system in the Atlantic), and the backside low
over the northeast Pacific near the Alaska Panhandle. Through
the day today, the low in the Pacific will drop down causing the
ridge to flatten and shift eastward through Wednesday morning.
This will set the stage for a cooler front to advect over the
Pacific Northwest. The incoming low is cool air wrapped with
temperatures at 850 mb around 3 degrees C which will mix down
causing highs to drop significantly. We could see as much as a
25 degree F swing between today`s and Wednesday high
temperatures.

In addition to the colder air, precipitation is expected. Now
the low is weakening as it nears the coastline. This rain will
persist through Friday as yet another shortwave trails behind
this first front on Thursday night into Friday morning.
Accumulations will vary greatly based on the location and
terrain. Counties north of Marion- Polk will see the bulk of the
rain - especially along the coasts and ranges. A bit of a rain
shadow will keep accumulations lower in the Willamette Valley.
Accumulation will range from 0.05-0.75 inch through Friday.
-Muessle


.LONG TERM...Friday Night through Monday...Little to say in the
long-term forecast as yet another ridge builds in. The jet
stream will become zonal on Friday night and shift well to the
north of the region. This will allow for the ridge to build, and
conditions to dry and clear. Temperatures will stay seasonable
with northerly winds in the afternoon. Now something to note
though is that on Saturday night into Sunday morning, there is a
shortwave trough that is moving through at the mid-levels. At
this point, not quite sure at what the impact will be, but it`s
not uncommon to see breezier winds and a more unstable
atmosphere. This trough is most likely associated with a weak
tropical storm that is over the California coast. Dry and
seasonable conditions persist through Monday. -Muessle

&&

.AVIATION...Upper level high pressure will continue to shift
inland today with increasing south to southwesterly flow aloft.
Light offshore flow in the lower levels this morning will likely
maintain VFR conditions across the area, except for marine stratus
along the northern Oregon coast slowly pushing up the lower
Columbia River. LIFR CIGs and fog at KAST likely (40-60%)
persists through 16z this morning. As of 10z, lower MVFR stratus
around FL015 has pushed into KKLS. Expect the stratus to lift and
scatter out by this afternoon, improving to VFR briefly. As flow
turns more onshore this evening, expect stratus offshore to push
onto the coast by 00-01z Wednesday. High probability (60-80%) for
conditions to deteriorate at KONP and KAST to IFR or LIFR.
Prevailing VFR inland with variable high clouds. Low chances
(10-20%) for MVFR CIGs between 12-18Z this morning.

PDX APPROACHES...VFR with variable high clouds expected through
the period. Low chances (10-20%) for low-end MVFR stratus between
12-18z this morning. Winds expected to remain light. -DH

&&

.MARINE...Thermal trough along the Oregon coast will maintain
breezy north winds with gusts up to 20 kt across the coastal
waters through this afternoon. Thermal troughing shifts inland
later today as high pressure weakens over the waters, allowing for
winds to decrease and turn onshore tonight. An approaching front
will turn the winds southerly late Tuesday night into Wednesday.
Guidance continues to show a 60-70% chance that winds climb into
the 20-25 kt range briefly ahead of the front, with highest
probabilities across the inner waters between 8 AM and Noon on
Wednesday. Winds are expected to abruptly shift to the northwest
with the frontal passage by Wed afternoon.

Winds turn southerly again on Thursday ahead of another frontal
system. Still expecting a low pressure system to rapidly
intensify across the far NE Pacific and move toward Haida Gwaii on
Thursday. The trailing cold front associated with this low will
likely approach the coastal waters later Thursday. While the
majority of models suggest Small Craft Advisory winds, there is
still a 20-40% chance of Gale Force southerly wind gusts to 35 kt
across the northern coastal waters.

Seas continue build to around 8 to 9 ft on Tuesday as a fetch of
northwest swell moves into the coastal waters. Seas likely linger
around 7 to 9 ft through Thursday. Another west to northwest
swell is expected to move in behind the front on Friday with seas
likely building up to around 12 to 14 ft. -DH

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...Small Craft Advisory until 5 AM PDT early this morning for
     PZZ272-273.
&&

$$

www.weather.gov/portland

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