-- Discussion containing changed information from previous version are highlighted. --
657
FXUS66 KPQR 231657
AFDPQR

Area Forecast Discussion...UPDATE
National Weather Service Portland OR
956 AM PDT Mon Sep 23 2024

Updated Aviation section

.SYNOPSIS...The upper trough is slowly moving over the northern
portions of the forecast area which is increasing onshore flow
and cloud cover. Rain chances have decreased. High pressure
returns tonight through Wednesday morning. Warm temperatures on
Tuesday. Another front arrives on Wednesday night which brings
higher probability for rainfall through the remainder of the
week.

&&

.SHORT TERM...Now through Thursday...A bit more of a dynamic
pattern for the next few days with a less confident than desired
forecast. High pressure rebuilds today pushing the shortwave
trough north and into Washington. High pressure returns today
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 this afternoon.
Windiest locations will be from Salem southward where the HREF
shows greater than a 60% chance of gusts exceeding 20 mph. There
is around a 10% chance that gusts will exceed 25 mph from
Corvalis southward after 6 PM tonight.

The high pressure intensifies 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. With the intrusion of
warmer air, high resolution models are suggest around a 60%
chance of highs greater than 90 degrees F but less than a 3%
chance of exceeding 95 degrees F. Have trended towards the 90th
percentile of the NBM for daytime temperatures on Tuesday. By
Tuesday afternoon the ridge axis shifts directly over central
Oregon which will cause winds to shift to the south. 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.

On Wednesday yet another change is in store as a stronger system
shifts southward from the Gulf of Alaska. This low pressure
system is decaying as it nears the area which is supported by
the jet stream. Recent modeled wind speeds have showed this jet
shifting a bit further northward in comparison to previous
rounds. This means that we could see less precipitation that
previously expected. Have trended with the NBM for now, but will
see how conditions shift when high resolution models come into
play. The dry air at the surface from the previous ridge may
also make it difficult for significant rain fall until the
atmosphere becomes saturated. Because we could be sitting on
the divergent side of the trough, and with the southerly flow,
there is around a 15-20% chance for thunderstorms along the
south Washington Cascades in the afternoon. Wednesday blurs into
Thursday with lingering showers. -Muessle


.LONG TERM...Thursday night through Monday...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. Will note that similar to the jet on Wednesday, this
upper air pattern too has shifted northward which will result
on lower probability for precipitation Friday night into
Saturday. High pressure returns on Sunday into Monday. -Muessle

&&

.AVIATION...
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High pressure with northwest to northerly flow aloft
today will maintain predominately VFR conditions with variable
high clouds. Shallow marine stratus layer over the coastal waters
with likely IFR conditions, however slight offshore flow appears
to be keeping the stratus just offshore as of 16Z. Later this
afternoon into evening hours sea breeze should kick in to pull the
IFR CIGs back to the coast. Deeper stratus near KAST with CIGs
now around 1100 ft should gradually improve to VFR 20-23Z.
VFR prevailing inland with CIGs around 4500-6000 ft mainly KSLE
northward this morning.

PDX APPROACHES...Predominately VFR with cigs around 4500 ft
through about 18Z then variable high clouds. Light northwest
winds expected to increase to around 6-8 kt this afternoon.
/ mh /DH
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&&

.MARINE...
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High pressure over the NE Pacific and thermally induced
lower pressure over the southern Oregon coast re-establishes today
producing gusty northerly winds. Small Craft Advisory wind gusts
of 21 kt or more are expected across the coastal waters off the
central coast of Oregon by this afternoon and continuing into
early Tuesday morning. High pressure weakens on Tuesday while
thermal troughing shifts inland, allowing for northerly winds to
decrease substantially through the day. An approaching front will
turn the winds southerly late Tuesday night into Wednesday. There
is a 60-70% chance that winds climb into the 20-25 kt range across
the waters, at least briefly, ahead of the front with highest
probabilities (>75% chance) across the inner waters in the
vicinity of the Columbia River.

Still uncertainty in forecast details late this week as a stronger
low pressure system is expected to develop and rapidly intensify
across the far NE Pacific, but there is much better agreement
among the latest model runs. The surface low is very likely (>90%
chance) to take the northerly track toward Haida Gwaii on
Thursday. The majority of models suggest the associated front only
brings Small craft Advisory level wind gusts to the coastal
waters. However, there is a 10-30% chance of Gale Force southerly
winds of 35-40 kt spreading across at least the northern waters.

Seas around 5 to 7 ft today 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 through Thursday. Another west to
northwest swell is expected to move in behind the front on Friday
with seas likely building over 10 ft, with only a 10% chance of
seas exceeding 15 ft. /DH
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&&

.PQR WATCHES/WARNINGS/ADVISORIES...

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OR...None.
WA...None.
PZ...Small Craft Advisory from 2 PM this afternoon to 11 PM PDT this
     evening for PZZ253.

     Small Craft Advisory from 5 PM this afternoon to 5 AM PDT
     Tuesday for PZZ272.

     Small Craft Advisory from 2 PM this afternoon to 5 AM PDT
     Tuesday for PZZ273.
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&&

$$

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