Tornado in Washington, September 27, 2021

Area Forecast Discussion
National Weather Service Portland OR
327 PM PDT Mon Sep 27 2021

.SYNOPSIS...Typical fall weather is expected as the month of
September comes to a end. A more active weather pattern will result
in near to below average temperatures for much of the upcoming week.
Wet weather will be primarily confined through Tuesday and then
again on Thursday. Warmer and drier weather is likely for the
weekend.

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.SHORT TERM...THIS EVENING THROUGH WEDNESDAY NIGHT...Recent radar
scans reveal rain showers falling across parts of the Coast Range
and western Willamette Valley, while satellite imagery shows a break
in convective activity (aside from some cumulus) over the waters out
to beyond 100 nautical miles from the Oregon and Washington
shoreline. Most of the stronger showers are popping up along the
western Willamette Valley and near the Kelso-Longview area. Behind
this front is an area of deeper convection immediately ahead of an
eastward-advancing tropospheric fold. This should bring more
convective activity onshore in several hours. High resolution models
continue to suggest that the best chance for thunder in the
Willamette Valley and Cascade Foothills will be this evening when
surface-based instability may be realized by any breaks in cloud
cover that can happen. Still, the coldest temperatures aloft needed
for deep instability will hold off until tomorrow, when the axis of
strongest mid to upper-level winds - and hence highest vertical wind
shear - associated with the upper trough slides east of the area. In
addition, large-scale forcing for ascent will slide east of the
area, and in fact downward motion will be favored in anticyclonic
vorticity advection on the back side of this feature. Still, given
the deeper instability layer evident on model soundings, the slight
chance thunder mention that NBM is suggesting seems realistic enough
to maintain it in the forecast. Any showers/thundershowers that do
develop tomorrow afternoon should be east of the area (with
exception to the Cascades, where orographically enhanced uplift
could sustain cells for slightly longer) by around 5 p.m., leaving
at least a portion of the evening rain-free.

The next system, a weaker shortwave trough, approaches from the
northwest on Wednesday. This one should bring much lower (if any)
chances for thunder, and much lower precipitation totals across the
board. In fact, southern portions of our area may not even see any
rain from this system. The CMC brings rain only to the far northern
part of our county warning area, while the GFS and ECMWF bring
chances for rain all the way south to near Oakridge. For now, have
gone with a split between the three, with a slight hedge towards the
GFS and ECMWF (since the CMC is the outlier at this time), but if
model trends indicate otherwise then the chances for rain and
precipitation totals will be adjusted accordingly.  -Bumgardner

&&

.LONG TERM...THURSDAY THROUGH SUNDAY...The aforementioned shortwave
will bring a chance for rain to at least the northern half of the
region on Thursday, before rain probabilities diminish Thursday night
into Friday morning. Behind the front associated with this system,
the combination of cold air advection and onshore flow should keep
temperatures relatively cool again on Friday before a warming trend
begins on Saturday. As a ridge then expands north and west into
northwest Oregon and southwest Washington, temperatures are likely
to warm to above normal by Sunday. The NBM suggests the probability
for above normal (i.e., 70F) high temperatures at PDX on Sunday is
98%. Yet, evidence that supports the notion this will not be a
heatwave comes from the same model, as the NBM suggests the chances
for surpassing 80F are only 15%. Overall, the weekend should be dry
and pleasant.   -Bumgardner

&&

.AVIATION...Showery pattern continues through tonight as a cool
upper trough traverses west to east across the region. There is
enough instability for a chance of thunderstorms from the coastal
waters to the Cascades this evening, while coastal areas may see
thunder chances continue into Tuesday morning. 21Z satellite
loop shows a line of showers with embedded thunderstorms
extending southwest from Vancouver Island to off the Oregon
coast. Based on extrapolation, this feature should arrive at the
coast 03-05Z. Think the coastal TAF sites stand the best chance
for thunder so have added VCTS (vicinity thunderstorm) to the
KAST & KONP TAFs. Showers gradually decrease Tuesday morning as
high pressure build in from the west. Flight condition will
generally be a mix of VFR and MVFR but can expect occasional IFR
vis and cigs with heavier precipitation.

For detailed regional Pac NW aviation weather information, go
online to: https://weather.gov/zse

KPDX AND APPROACHES...Showers with a mix of VFR and MVFR
conditions. Cannot rule out brief IFR conditions through the
evening, but forecast confidence too low to add to TAFs at this
time. Heavier showers noted on radar now moving into SW
Washington and across the coast range and may continue to move
into the valley this afternoon, the more showers later in the
evening as the upper trough moves across. There is a 10-20%
chance of a brief thunderstorm impacting any given terminal
between through 06Z Tue. Winds will be from the south south-east
through about 00Z, the veer to the south this evening. /mh

&&

.MARINE...A cool and showery pattern continues across the waters
through Tuesday morning. Expect winds to remain breezy with
occasional gusts to 25 kt. Buoy 46005 reported significant wave
heights in the 23 to 26 feet range at 15 seconds between 2 and 5
AM. As of 2 pm buoy 46089 and 46243 jumped up to around 18 ft at
15 seconds. (Buoy 46029 unfortunately currently adrift and not
reporting reliably.) This is about the estimated height and time
of arrival based on the wave decay program. So generally expected
seas in the 15 to 18 ft range through tonight with low potential
for an hour or two near 20 ft. Seas substantially subside Tue
morning an is expected to be around 11 to 14 ft Tue afternoon.

The next storm system is on track to lift a warm front
northeastward across the waters Wednesday. This will bring
increasing southerly winds to the waters particularly off the
south Washington and north Oregon coastline. Winds appear likely
to end up Small Craft Advisory criteria with gusts of 25 to 30 kt
possible Wednesday into Wednesday night, particularly areas
farther north. But 18Z NAM model shows potential for gale gusts
on Wednesday for the northern zones, but other models are not in
alignment with this solution so will wait before making
significant changes to forecast.

High pressure will then shift over the waters late Thursday and
continue into next weekend across the waters. This should result
in winds largely remaining 20 kt or less. Seas look to remain
somewhat elevated though as a westerly swell around 10 ft pushes
across the waters Thursday into Friday. Seas then look to drop
more solidly into the single digits over the weekend. /mh /Neuman

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...Small Craft Advisory until 5 AM PDT Wednesday for coastal waters
from Cape Shoalwater WA to Florence OR out 60 nm.

     Small Craft Advisory until 9 PM PDT Tuesday for Columbia River
     Bar.

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