-- Discussion containing changed information from previous version are highlighted. --
870
FXUS66 KPQR 221751
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
1036 AM PDT Sun Sep 22 2024

.SYNOPSIS...A passing shortwave trough will bring cloud cover
and slightly cooler temperatures on Sunday. The trough is fast
moving so will exit the region by Monday. High pressure returns
on Tuesday with weak easterly flow through Wednesday morning.
Another stronger trough and associated front advects inland
Wednesday night through Thursday.

&&

.SHORT TERM...
-- Changed Discussion --
Now through Tuesday...The high pressure that
brought clear skies and light winds to the region will flatten
as a shortwave trough moves over the region. The jet stream in
this case is weakly "troughy" with more of a zonal flow. At the
mid-levels, the trough is a bit more apparent. Increased energy
with this system will be focused more to the north of Oregon so
precipitation chances (PoPs) are low and accumulation even
lower. There remains ample dry air near the surface which will
be difficult for this system to overcome. With northwesterly
winds associated with it though, that will bring breezier winds
to the Willamette Valley and along the coast.

The high pressure ridge begins to amplify once again on Monday.
The overall flow is promoting this ridge, but there is also
support from a tropical system along the southern California
coast. This ridge will be pervasive though and encompass much of
the Pacific Northwest. As high pressure increases it will
promote warm air to advect over the area. The bulk of the warm
air will be to the east of the Cascades but the thermal trough
that will form along the coast will encourage a weak easterly
offshore wind bringing in warmer air from eastern Oregon and
Washington. By Tuesday the ridge axis shifts directly over
central Oregon which will cause winds to shift to the south
aloft. This southerly flow at 850 mb (around 5500 ft) will
usher in warm air that is around 20 degrees C. This air will mix
down, especially up and over the Cascades, causing daytime
temperatures to rise significantly. Temperatures are forecast
to be around 10 degrees F above normal for late September which
is usually impacted by the decreasing sunlight. The lower inland
elevations will bump up into Moderate HeatRisk.

Probabilistically, the spread between the 10th-90th percentile
is only around 2-3 degrees F via the NBM. The deterministic
forecast is slightly higher than the NBM 50th percentile
though. One factor that could impact this forecast is that
aforementioned tropical system to the south. It is not uncommon
that these systems are not well resolved by models and can
"disrupt" them. If it weakens or shifts south, the amplification
of this ridge may be less and thus, less of a southerly push to
bring these unseasonably warm temperatures. -Muessle


.LONG TERM...Tuesday night through Saturday...This building
ridge is pretty quickly shifting east as an encroaching longwave
trough drops down from the Gulf of Alaska. The ridge will set
up over the Rockies by late Tuesday causing the easterly winds
to decrease and dampen the warm air intrusion. Unlike the
system on Sunday, this trough is much more robust and is easily
detected in all levels of the atmosphere. Looking at the jet
stream at 250 mb (~35,000 ft), the forecast area sits in the
right entrance region of the jet streak which is commonly
associated with instability. This instability will support more
rain on Wednesday. With the added instability, cooler air moving
over the warm airmass at the surface, and the addition of
moisture, chances for thunderstorms increase. Have capped this
probability around 15-20% as the storms will need a very
specific placement of the front to manifest.

Brewing in the northeast Pacific is a low pressure system which
will move eastward on Thursday and Friday. This incoming low
will cause the flow to become more zonal which can be observed
in the jet stream motion. The forecast area sits on the cool
side of the jet with a 140 kt jet streak just to the north along
the Washington-Canadian border. The low will enhance
precipitation on Thursday into Friday. Because this low is
moving inland to the north, we currently are sitting on the
southern portion of the front. But if that low shifts further
south, we could experience more rain than what is currently
forecast.

Will mention that these incoming cooler temperatures at higher
elevations plus incoming moisture equals one thing...termination
dust! It is possible that the peaks of the volcanos could see a
few snow flurries at times late this week which may accumulate
up to 0.5 inch or so. That termination dust generally is the
initial sign for an end to summer.
  -Muessle
-- End Changed Discussion --

&&

.AVIATION...
-- Changed Discussion --
Weak upper level ridging has led to predominantly VFR
conditions across most of northwest Oregon and southwest
Washington. The main exception to this is along the coast where
light winds and raditional cooling led to LIFR fog forming in the
coastal river valleys. Expect these conditions to trend to VFR
rather abruptly over the next 1-2 hours.

A weak upper level trough will brush the Pacific Northwest later
today which will help to push marine clouds currently well
offshore onto the coast late this afternoon and evening. Models
suggest the stratus will most likely come in at high end IFR to
low end MVFR thresholds with probability for IFR conditions at any
given hour peaking around 40-50% along the central Oregon coast
and 50-60% for the north Oregon coast after 00-03z Monday. There
is a 30-40% chance that this stratus pushes reaches the Portland
metro taf sites between 10-14z Monday while the probability of
this occurring farther south drops to less than 10% at KEUG.


PDX APPROACHES...Weak upper level ridging and VFR conditions today
will give way to weak upper level troughing overnight. This will
allow marine clouds to push into portions of northwest Oregon and
southwest Washington overnight. At this point there is a 30%
chance that MVFR stratus reaches the terminal between 11-14z
Monday with probabilities increasing to at least 50% by 17z
Monday. There is a low probability (10-20% chance) that the marine
clouds come in at high end IFR thresholds.
-- End Changed Discussion --


&&


.MARINE...
-- Changed Discussion --
A weak front sliding across the waters will briefly turn
the winds more southwesterly across the northern waters
overnight. However, there is a high probability (>90% chance) winds
remain well below Small Craft Advisory thresholds.

High pressure over the northeast Pacific and thermally induced
lower pressure over the southern Oregon coast will re-establish
itself and produce northerly winds across the waters on Monday.
There is a 90% chance that wind gusts climb into Small Craft
Advsiroy thresholds of 21 kt or more across the waters off the
central coast of Oregon. The main uncertainty lies around how far
north these winds will extend, but it appears there is a low to
moderate probability that they will reach northward to somewhere
between Cape Foulweather and Cape Falcon.

High pressure nudges closer to the coast Tuesday, which should
result in northerly winds weakening substantially. An approaching
front will turn the winds southerly late Tuesday night into
Wednesday. There is a 50% chance that winds climb into the 20-25
kt range across the waters at least briefly ahead of the front
with the highest probabilities (>75% chance) across the inner
waters in the vicinity of the Columbia River.

Uncertainty in forecast details grow Thursday into Friday. This is
due to a developing surface low pressure that is most likely to
track towards Vancouver Island/Haida Gwaii during this time.
Depending on the strength of this developing low pressure and how
close the Pacific Northwest will determine winds over the waters
initially as well as a following swell. The majority of models
suggest it will remain weak enough or far enough away to only
bring Small Craft Advisory level wind gusts to the waters.
However, there is a 25% chance of Gale Force southerly winds of 35-40
kt spreading across the waters and a 5-10% chance of storm force wind
gusts of 50-55 kt at some point late Thursday or Friday with the
closest and strongest scenarios.

Given timing uncertainty is smoothing the potential peak in
winds, the current forecast is still generally below Small Craft
Advisory level winds. Would expect the official wind forecast to
climb into at least Small Craft Advisory thresholds at some point
late Thursday and/or Friday as confidence in the exact timing of
peak winds grows over the next 1-2 days.
-- End Changed Discussion --


&&


.PQR WATCHES/WARNINGS/ADVISORIES...

OR...None.
WA...None.
PZ...None.

&&


$$

www.weather.gov/portland

Interact with us via social media:
www.facebook.com/NWSPortland
x.com/NWSPortland