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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS66 KPQR 231030
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
329 AM PDT Sat Sep 23 2023

.SYNOPSIS...
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The first signs of a significant change toward fall
weather have arrived, just in time for the first full day of
astronomical autumn. The first in a potentially long series of
frontal systems will brush SW Washington and NW Oregon today with
generally light rain. A stronger system will begin to spread rain
into the region Sunday, along with gusty winds and building seas
along the coast. The frontal zone will stall nearby, with waves of
low pressure bringing occasional rain and gusty winds through Tuesday
morning. The front then moves east of the Cascades, with cool and
unsettled weather to follow for the remainder of the week.
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&&

.DISCUSSION...
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Today through next Friday...GOES-West infrared imagery
shows plenty of mid to high level clouds streaming into WA/OR early
this morning. These clouds are the first signs of the anticipated
shift to early fall weather, as an unseasonably strong early season
Pacific jet stream extends across the north Pacific and aims toward
the Pac NW. The jet stream is largely zonal between 45N-55N, with a
maximum of 160+ kt in its core near the western Aleutians.

Shortwave energy at the leading edge of the jet stream began
dribbling into the region Friday evening, but the low levels are
starting out dry. This is making it difficult for any precipitation
to reach the ground initially, but the air mass will gradually
moisten over the next 12-24 hours. By midday today, HREF mean total
precipitable water values are near 1 inch, allowing for increasing
showers as additional shortwave energy arrives this afternoon and
evening.

The crux of the jet energy noses into the NE Pacific tonight and
Sunday, causing rapid cyclogenesis near 45N/140W. The 00z GFS shows
the low center strengthening from around 1000 mb at 23/21z to 966 mb
at 24/21z. This low will push an occluding frontal system toward the
coast Sunday night, and this front will likely have additional waves
of low pressure developing along it. With southerly 850 mb winds on
the order of 55-65 kt, strong winds will be in play along the coast
as the front moves onshore. However MSLP gradients are very easterly,
which would likely limit the strongest winds to beaches and
headlands. Latest NBM/HREF probabilistic guidance shows very low
chances (generally less than 20%) of winds exceeding 55 mph even for
the beaches and headlands, so we will refrain from issuing any High
Wind Watches for the time being. Inland areas will largely miss out
on the wind Sun/Mon due to the easterly MSLP gradients. Winds will
largely be limited to exposed ridges, but waves of low pressure may
briefly mix gusts of 20-30 mph down to lower elevations.

In addition to the wind, the forecast area can expect rain to
increase Sunday, with occasional rain continuing through Monday. A
moderate to low-end strong atmospheric river (AR; based on
CW3E IVT guidance) is still slated to impact the PacNW Sunday
night through Monday. Models show the jet streak peaking around
130-150 kts pushing through the region during this time period
along with a significant amount of upper level moisture.
Ensembles indicate some uncertainty on when the exact period of
heaviest rain will be, but they are zeroing in on sometime
Monday morning through Monday evening as the surface front moves
through with rain showers continuing into Tuesday. For the 24
hour period from 11pm Sunday - 11pm Monday, NBM v4.1 indicates a
70-95% probability of 1 inch of rain along the coast and 60-80%
east of the Coast Range. For the 48 hour period from 5am Monday
to 5am Wednesday, NBM v4.1 indicates a 50-90% probability of 2
inch of rain along the coast, highest probabilities north of
Newport, and the S Washington Cascades, and a 20-40% elsewhere.
Deterministic precipitation forecast indicates 1-3 inches along
the coast, Coast Range, and S Washington Cascades and 0.75-1.5
inch elsewhere for Sunday night through Monday night.

The slightly lower probability of 2 inches in 48 hours for
inland locations is in part due to a timing shift in the next
shortwave in the ensembles. Previous runs were extremely
uncertain on whether this wave would start Tuesday night or
Wednesday. Uncertainty still remains on the exact timing, but
latest guidance has pushed it closer to Wednesday morning.
Ensembles continue to indicate this third wave will be less
intense than the Monday AR, but periods widespread moderate to
heavy rain are still expected. For the 24 hour period from 5pm
Tuesday - 5pm Wednesday, NBM v4.1 indicates a 60-90%
probability of 1 inch of rain along the coast and Cascades north
of Lane County and 20-50% east of the Coast Range. Much more
uncertainty in the ensembles beyond Wednesday, but the general
consensus is showery and cooler pattern continuing through at
least the end of next week.  Weagle/HEC
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&&

.AVIATION...
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12Z TAFs: MVFR stratus increasing on the coast ahead
of a weakening front. But the two TAF sites remain VFR as of 10Z.
HREF guidance seems a bit overdone, but does show the mores spotty
nature of the MVFR clouds at the coast. Expect occasional MVFR
through the day at the coast with rain increasing this morning
then tapering off tonight. Inland ares expected to remain VFR with
cigs 4000-6000 ft developing. May see lower cigs with the rain
however, HREF indicating a 5 to 15 percent chance for 3000 ft
cigs around terminals late this afternoon and evening. Will also
see increasing mountain obscuration as clouds and rain develops
this afternoon.


KPDX AND APPROACHES...VFR through the next 24-hrs. There around a
a 10% chance of MVFR CIGs around the Terminal later this afternoon
and evening. /mh
-Muessle
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&&

.MARINE...
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A cold front associated with a small disturbance spinning
out of a low in the northeast Pacific will swing through the
waters today. This is expected to bring winds gusting around 20-25
kt. The front moves ashore this afternoon. Model guidance
suggests strongest winds will be north of Lincoln City in the morning
and into the early afternoon. Small Craft Advisory continues for
areas north of Cape Foulweather out to 60nm through this afternoon
as guidance is showing weaker winds now.

This will be followed by a strong trans Pacific jet stream that will
dip south towards the Pacific NW, deepening the upper level
trough in the Gulf of Alaska toward the northeast Pacific. It
will form into a deep cutoff low Sunday into Monday bringing the
first Gale headline of the season. High confidence (>80% chance)
the low will produce Gale Force wind gusts starting Sunday
afternoon with the potential for sustained Gales Sunday night into
Monday.

Moderate confidence that there will be a 6-9 hour period Sunday
night which could produce Storm Force wind gusts between 40-60nm.
Model guidance seems to be picking up on multiple disturbances
that will move northeast along the cold front which will tighten
gradients across this area. A Storm Watch continues Sunday
afternoon through late Sunday night. Gales will continue across
the waters into Monday afternoon and possibly the evening. Waters
between the coast and 20 NM stand a 30-60% chance for Gale Force
winds during this time and have therefore issued a Gale Watch.

A dynamic wind sea will develop during this time across
the waters bringing seas into the mid, possibly upper teens with a
dominant period around 10 seconds. The dynamic fetch setup in the
SW quadrant of the low will generate seas into the upper 20s
around 45N/135W. This will generate a westerly swell that will
move into the waters Monday night which will maintain seas in the
mid teens through late Tuesday morning. GEFSwave ensemble shows
about a 60-70% probability of seas exceeding 20 ft primarily
beyond 20 NM, with a 30-50% probability in the nearshore zones
for the Monday/Monday night timeframe.

The low will occlude offshore and a secondary weakening cold
front will move across the waters late Monday which likely to
bring gales across much of the water. Winds and seas will
continue to diminish into the middle of the week. /mh
-BMuhlestein
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&&

.PQR WATCHES/WARNINGS/ADVISORIES...

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OR...None.
WA...None.
PZ...Small Craft Advisory from 9 AM this morning to 5 PM PDT this
     afternoon for Coastal waters from Cape Shoalwater WA to
     Cape Foulweather OR out 10 NM. Waters from Cape Falcon to
     Cape Foulweather OR from 10 to 60 NM.

     Gale Watch from Sunday afternoon through Monday evening for
     Coastal waters from Cape Shoalwater WA to Florence OR out
     10 NM.

     Small Craft Advisory until 5 PM PDT this afternoon for Waters
     from Cape Shoalwater WA to Cape Falcon OR from 10 to 60 NM.

     Storm Watch from Sunday afternoon through late Sunday night for
     Coastal waters from Cape Shoalwater WA to Florence OR from
     10 to 60 NM.
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&&

$$

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