Tornado in Oregon, November 10, 2020

Area Forecast Discussion
National Weather Service Portland OR
943 AM PST Tue Nov 10 2020

.SYNOPSIS...An upper level trough of low pressure will support
widespread rain and Cascade snow showers through this evening.
Showers will linger through Wednesday but will be less frequent and
intense. A frontal system will produce another round of rain and
Cascade snow Thursday afternoon and night. There is the potential
for a high impact weather system late Friday into Saturday.
Additional fronts follow Sunday and Monday, but details remain
uncertain

&&

.SHORT TERM...Today through Thursday night...No changes. Previous
discussion follows. An upper level trough will support cool weather
with showers through this evening with snow levels between 3000 and
3500 feet. The showers will increase in frequency and intensity this
afternoon as a shortwave moves over the region coupled with much
cooler temperatures aloft. This increased instability will provide a
chance of small hail with the rain showers, and possibly trigger
isolated thunderstorms. The best chance for these more convective
showers are across the coast and coastal mountains of extreme SW
Washington and extreme NW Oregon. Snow rates will be greatest this
afternoon, and expect total snow accumulations of 6 to 12 inches
above 3000 feet which supports the current winter weather advisory.
The showers decrease Wednesday as a transitory ridge moves over the
area.

If skies clear Wednesday evening, then fog will likely develop in the
valleys Wednesday night. Have added patchy fog to the forecast late
Wednesday night, but the forecast confidence is low in where there
will be enough clearing of the clouds to support fog. If high clouds
ahead of the next front moves in sooner than the previous forecast,
then the chance for fog diminishes. Models are in fair agreement that
a warm front will return rain to the coast late Thursday morning
spreading inland Thursday afternoon and evening.

Snow levels will be around 3000 feet when the precipitation reaches
the Cascades late Thursday afternoon, then rise to 3500 to 4000 feet
before the heavier rain with the cold front reaches the Cascades
Thursday evening. The exception is the south WA Cascades, where snow
levels may remain around 3000 feet or less. Snow totals will flirt
with advisory thresholds (~ 6 inches) for the higher elevations of
the Oregon Cascades and for the south Washington Cascades including
the main Cascade passes (Willamette, Santiam, McKenzie and Government
Camp). The highest rainfall will be for the coast and coast range
where 0.75 to 1 inch of rain are possible Thursday morning through
Thursday night. Similar rain totals are possible for the south WA
Cascade foothills as well. Generally expect 0.25 to 0.5 inch of rain
elsewhere. Precipitation becomes showery behind the front Thursday
night, and lingers into Friday morning. ~TJ

.LONG TERM...Friday through Monday night...No changes. Previous
discussion follows. Models are coming into better agreement on a
potentially high impact low pressure system expected Friday into
Saturday. Although there are slight model difference on the forecast
strength and track of the surface low, all of the guidance indicates
the potential for strong winds, heavy rain & Cascade snow, and
hazardous surf. The Canadian deterministic 0z model is forecasting
the low to bomb to around 968 mb as it moves over the British
Columbia coast Friday afternoon. This is the strongest and furthest
north solution. The ECMWF and GFS forecast the low will move inland
near the Puget Sound at 975 and 980 mb. Models are notorious for
under forecasting winds for these type of lows, and pressure
gradients are a more reliable indicator on the potential strength of
the expected winds.

These varying model forecasts suggest surface pressure gradients of 5
to 10 mb are possible between Astoria and Newport Friday afternoon,
and pressure gradients of 6 to 12 mb from Portland to Eugene. For
past events, the higher end of these gradients usually produce high
winds for the coast and Willamette Valley that exceed the minimum
thresholds for warning. Besides the winds over the land, the winds
circulating around the low as it moves across the Pacific will
generate very large seas over the coastal waters leading to hazardous
surf and sneaker waves for the beaches.  We are also approaching a
period of high tides, that when coupled with the large seas may
result in coastal flooding. The front will bring a shot of rain and
snow too. Lastly, heavy rain and mountain snow are also possible with
this system. It is too soon to forecast rain and snow amounts with
any confidence, but generally expect snow levels to be around 4000
feet.

Post-frontal showers are expected to continue Saturday night before
another low and frontal system impacts the area Sunday and Sunday
night. Latest models are forecast a secondary low riding on the the
tail end of the front that may slow and stall it. As this occurs,
models show a deep moisture plume developing from the coast into the
sub tropics hinting at the possibility of an atmospheric river type
of event early next week. ~TJ

&&

.AVIATION...Widespread VFR conditions today will occasionally
dip into MVFR category with showers throughout northwest Oregon and
southwest Washington. Brief IFR conditions in heavier showers are
possible at the northern TAF sites through about 01Z. Along the
north coastal areas and into the lower Columbia Valley, there is a
slight chance of thunderstorms today through 01Z, with small hail
also possible. After 01Z-05Z, expect shower activity to decrease,
leaving primarily VFR conditions overnight and into Wednesday
morning. There will still however be a chance for temporary MVFR
conditions with the scattered showers.

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

KPDX AND APPROACHES...VFR conditions predominantly through the next
24 hours. However, expect occasional MVFR conditions mainly
associated with showers through 03Z-05Z this evening, then still a
chance of temporary MVFR conditions with remaining showers after
05Z.

&&

.MARINE...No changes. Previous discussion follows. West to northwest
winds will slowly diminish Tuesday and Tuesday night. Seas from 10
to 12 ft in zones PZZ275 and PZZ255, along with 12 to 15 ft in zones
PZZ270 and PZZ250 through Tuesday.

Relatively benign conditions Wednesday, then the weather becomes
potentially much more active during the late part of the week.
Another system brings what is likely another round of small
craft winds Thursday. Confidence has increased in the forecast
for Friday and Saturday, as the GFS, ECWMF and UKMET models are
in much better overall agreement. Guidance is showing a well
defined surface surface low around 975 mb, will approach the
Vancouver Island/ Olympic Peninsula area during the latter part
of the week. It appears likely that sustained gale force winds
can be expected, with the possibility of storm force wind gusts
Friday and Saturday. In addition to the expected high winds,
large seas will follow. Seas are expected to build into the 25 to
30 ft range on Friday, peaking in the range of 35 to 45 ft
Friday night into Saturday morning. Saturday afternoon seas will
likely subside into the 17 to 20 ft range and continue falling
on Sunday. Expect a similar time frame for winds however, there
is the chance for a system on Sunday to bring gale force gusts
back into the waters. /42

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory until 10 AM PST Wednesday for Cascades
     in Lane County-Northern Oregon Cascades.

WA...Winter Weather Advisory until 10 AM PST Wednesday for South
     Washington Cascades.

PZ...Small Craft Advisory until 10 AM PST Wednesday for coastal
     waters from Cape Shoalwater WA to Florence OR out 60 NM.

     Small Craft Advisory from 10 AM this morning to 2 PM PST this
     afternoon for Columbia River Bar.

     Small Craft Advisory from 10 PM this evening to 2 AM PST
     Wednesday for Columbia River Bar.

&&

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