419
FXUS66 KPQR 222213
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
312 PM PDT Sun Sep 22 2024

.SYNOPSIS...An upper trough will move across the area tonight with
high pressure quickly returning Monday. High pressure strenghtens
on Tuesday with weak offshore flow through Wednesday morning. Will
see quick warm-up Tuesday with inland highs 10 to 15 degrees above
seasonal normals.  A stronger trough and associated front moves
inland Wednesday night through Thursday for more seasonable weather.

&&

.SHORT TERM...Now through Tuesday night...Upper trough temporarily
depressing an upper ridge today, with main effect is increasing
marine clouds at the coast and porttion of the interior overnight.
The high pressure returns Monday with the surface ridge laying over
southwest WA, resulting in generally northerly winds over nw Oregon.
Northerly winds gusting 15 to 20 mph in the Willamette Valley Monday
afternoon.  The high pressure aloft pumps up further Tuesday to bring
unseasonably warm temperatures with inland high temperatures around
90 degrees. This is about 10-15 degrees above normal.  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.  By Tuesday afternoon the ridge axis shifts directly
over central Oregon which will cause winds to shift to the south
aloft. This will also induce a southwest marine push (surface high
pressure to the southwest and lower pressure over the Columbia
Basin) Tuesday night. This should bring a deeper marine layer inland
by Wednesday morning.

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...Wednesday through Sunday...A trough now over the Gulf of
Alaska will sag over the PacNW Wed. This system 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 (generally over the north
Willamette Valley into north Oregon and south Washington Cascades.

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./mh
  -Muessle

&&

.AVIATION...Weak upper level ridging has led to predominantly VFR
conditions across most of northwest Oregon and southwest
Washington this afternoon. A weak upper level trough will brush
the Pacific Northwest, which will help marine clouds currently
well offshore push onto the coast late this afternoon and evening.
Models suggest the marine clouds will most likely come in at high
end IFR to low end MVFR thresholds along the coast 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. The probability these marine
clouds reach the Portland metro taf sites between 10-14z Monday is
now 10-20%, but does increase to 30-40% around 16-18z Monday. to
30-40%. The probability of these marine clouds reaching as far
south as KEUG is below 10%.


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 inland towards the Portland metro. At
this point, there is a 10-20% chance that MVFR ceilings reach the
terminal between 11-14z Monday with probabilities increasing to
30-40% between 16-18z Monday. There is a low probability (10%
chance) that the marine clouds come in at high end IFR thresholds.


&&


.MARINE...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 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 gusty 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 (20-40% chance) 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 the north Vancouver Island/Haida Gwaii region during
this time. Variations in the models on the strength and track of
the low pressure are what are creating the uncertainty. The
majority of models suggest the low pressure 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, at
least the northern waters, and a 5-10% chance of storm force wind
gusts of 50-55 kt at some point late Thursday or Friday if the
stronger and closer track scenarios pan out.

Given timing uncertainty is smoothing the potential peak in
winds, the current forecast is still generally near to even
slightly below Small Craft Advisory level winds. Would expect the
official wind forecast to climb into at least Small Craft
Advisory thresholds, if not higher, over the course of the next
couple of days for the late Thursday and/or early Friday timeframe
as confidence in the exact timing of peak winds grows.


&&

.PQR WATCHES/WARNINGS/ADVISORIES...

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

&&

$$

www.weather.gov/portland

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