228
FXUS66 KPQR 220954
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
254 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...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

&&

.AVIATION...Northwest flow aloft as weak upper trough moves across
the region this morning. High pressure with light winds at the
coast may allow low stratus or fog to develop, with LIFR
conditions possible from KTMK to KONP through 18z Sunday, and
30-40% chance of MVFR at KAST between 14-18z Sunday. Otherwise,
expect VFR with increasing high clouds by this afternoon. A weak
front will approach the northern coast later today with MVFR
stratus likely (60-80%) pushing onshore after 00z Monday.

PDX APPROACHES...VFR through the period, with increasing high
clouds later Sunday. Light northwest winds expected to increase
to 5-7 kt this afternoon. -DH

&&

.MARINE...High pressure remains over the waters through early next
week while a thermal trough of lower pressure persists along the
northern CA/southern OR coast. This will maintain general north to
northwest winds across the coastal waters, except across the
northern coastal waters later Sunday into Monday as a very weak
front moves toward the Washington coast and brings light west to
southwest winds. Pressure gradients expected to increase Monday
night with strengthening northerly winds and gusts to around 15-20
kt through Tue morning. Seas expected to remain around 5 to 7 ft
through Monday night. Then, a fetch of northwesterly swell is
likely to build seas to around 7 to 9 ft on Tuesday.

A stronger front will approach the coastal waters by mid-week.
Expect a southerly wind reversal to begin late Tuesday into
Wednesday. There is around a 50-70% chance that winds gust up to
25 kt on Wednesday. Late in the week there is possibility of a
stronger low developing in the NE Pacific. Latest guidance shows a
20-40% chance of a larger northwest swell exceeding 15 ft on
Friday, and around a 10-20% chance of wind gusts exceeding 45 kt.
Most likely the low will move NE toward British Columbia. -DH

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...None.
&&

$$

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