Heavy Rain in Oregon, September 23 to 25, 2020

Area Forecast Discussion...CORRECTION
National Weather Service Portland OR
330 PM PDT Wed Sep 23 2020

Correction for long term forecast time frame.

.SYNOPSIS...A cold front continues to push inland today bringing
significant rainfall to areas west of the Cascades. In addition,
breezy south/southwesterly winds are increasing along the coast and
the Cascades. The front will weaken overnight becoming showery on
Thursday. A shortwave trough will move inland from the Pacific on
Friday bringing another shot of rain to the area. More settled
weather expected over the weekend into next week.

&&

.SHORT TERM...Tonight through Friday...The strong area of low
pressure continues to spin in the southeastern Gulf of Alaska.
Satellite imagery shows the cold front pushing inland today with a
wide swath of clouds that covers much of the Pacific NW. The cold
front is bringing in moisture from the south Pacific and is acting
like a pseudo-atmospheric river...although it is missing some of the
main attributes of this type of feature. The amount of moisture in
this system is quite substantial for this time of year with Total
Precipitable Water (TPW) content being about 150% of the normal TPW.
With a lot of moisture and support aloft, this system has been able
to tap into that upper level moisture bringing significant rainfall
to the area.

The front first started over the coast and has partially stalled over
the Coast Range but is continuing to push east this afternoon.
Precipitation amounts within the northern Willamette Valley and the
northern Cascades have just begun to increase unlike the coastal
regions where the accumulations have surpassed 1.5 inches or more
today. Rain is expected to persist through the evening with the
highest amounts staying generally north of Lane County, although
cannot rule out accumulations greater than 0.50 inches in the
southern portions of the forecast area. Will note that there is a
slight chance for thunderstorms along the north Oregon/south
Washington coastal areas and along the Washington/Oregon border west
of the Portland-Metro area. Breezy winds will continue through the
evening, easing after midnight.

Thursday will be fairly settled with some residual showers persisting
through the day. Overnight however, a short wave trough will push
over the eastern Pacific. This trough has not been well handled by
the models and it is trending more north vs. east over northwest
Oregon. Can still expect rain and this is a very wet system and
accumulations will likely be highest along the northern portions of
the forecast area. -Muessle

&&

.LONG TERM...Friday night through Tuesday...As the shortwave trough
exits the area to the northeast, residual showers will likely remain
through Saturday. There is a chance that this trough will slow which
would keep rain through Saturday. Again, much of the rain will fall
through the northern portions of the forecast area. Saturday night,
a high pressure ridge will begin it's initial stage of development
which will aid in pushing the showers north. This ridge will build
through the weekend into early next week keeping conditions dry and
settled. Some warmer air may filter in from the CONUS southwest on
Monday and Tuesday as the high pressure intensifies and a thermal
low develops at the surface. With that pattern taking hold, winds
are likely to turn easterly starting first along the central Oregon
Cascades and advancing northward through Tuesday. Likely these winds
will not be overly strong, but they will usher in drier air to the
area. At this time, no major concerns in the long term forecast as
the high pressure will remain in place through the week. There is
generally good confidence in the long term numerical model output
with this ridge taking hold, but there is less confidence in the
early onset of it. There is a chance that the ridge may be flattened
slightly on Saturday/Sunday which may damper it's overall impacts. -
Muessle

&&

.AVIATION...Cold front offshore is moving at a good pace and is
still expected to move onshore at 00-01Z. Coastal sites should
expect IFR to LIFR conditions ahead of the front but improvement
to MVFR in the post frontal airmass should be fairly quick.
Persistent showers and gusty conditions will continue through the
night and into tomorrow morning. There is a slight chance for
thunderstorms along the NW Oregon, SW Washington coast late
tonight. Southwesterly LLWS up to 50 kt around 020 ft is expected
along the coast until the front passes. Expect the front to be
east of the Coast Range by 01Z Thursday.

Inland sites to expect MVFR conditions with gusty winds through
03Z. The dominant trend will be VFR after 04Z with some
lingering low level clouds moving across the area, so intermittent
periods of MVFR are possible until 10Z. Front is expected to be
east of the Cascade Range by 09Z Thursday.

Refer to http://weather.gov/zse for detailed regional aviation
weather and hazard information.

KPDX AND APPROACHES...MVFR conditions are expected through 04Z
Thursday as a cold front approaches the area. Expect gusty winds
and showers in the post frontal airmass. Conditions to return to
VFR after 09Z -BPhillips

&&

.MARINE...A strong cold front associated with a low in the Gulf
of Alaska is moving at a good pace offshore. Current observations
are continuing to show gale force winds across all waters and
wind waves near 10 ft. Winds are starting to parallel the coast
ahead of the cold front that is on track to arrive around 6 PM
this evening. Expect strong southerly winds until this time. The
Gale Warning will remain in effect through 9 PM tonight based on
the high likelihood of gale force gusts continuing across the
waters within the cold frontal airmass.

Waters will continue to be hazardous through tonight as the
westerly swell that was produced by the strong Gulf of Alaska
low moves towards the waters. Wind waves will dissipate to below
7 ft over night but seas will build to 14 ft, 15 sec early
Thursday morning. Wind gusts will continue to be at or above 21
kt through Thursday evening.

There is a second disturbance that will move across the waters
early Friday that has the potential to produce another round of
gales. Models are guidance has been unreliable with this
disturbance and so changes to timing and intensity are likely. As
a precaution a Gale Watch has been issued for the northern
waters between midnight Thursday and 11AM Friday morning. High
pressure to reestablish over the NE Pacific over the weekend.
-BPhillips

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...Gale Watch from late Thursday night through Friday morning for
     coastal waters from Cape Shoalwater WA to Cascade Head OR
     out 60 NM.

     Gale Warning until 9 PM PDT this evening for coastal waters
     from Cape Shoalwater WA to Florence OR out 60 NM.

     Small Craft Advisory until 2 AM PDT Friday for Columbia River
     Bar.

&&

Interact with us via social media:
www.facebook.com/NWSPortland
www.twitter.com/NWSPortland