Heavy Snow in Oregon, March 1 to 5, 2024

Area Forecast Discussion...UPDATED
National Weather Service Portland OR
858 PM PST Thu Feb 29 2024

Updated Aviation/Marine Discussion...

.SYNOPSIS...A large upper level trough over the western CONUS will
maintain cool and showery conditions across northwest OR and
southwest WA over the next several days, coldest this weekend when
the trough axis moves overhead. With cold temperatures aloft in
place, snow levels will likely hover between 500-1500 feet through
Monday morning, occasionally lowering all the way to the valley
floor during heavier showers. However, southerly surface winds and
above freezing temperatures below 500 feet will limit impacts, as any
snow that does fall will tend to melt as soon as it hits the ground.
Travel impacts are NOT expected below 500 feet, but impacts will
occur above 500-1000 feet where accumulating snow is occurring.
Heavier showers will also have the potential to produce small graupel
or small hail, and possibly a rumble of thunder or two.

&&

.SHORT TERM...THURSDAY NIGHT THROUGH SATURDAY NIGHT...The short term
forecast is highlighted by cool and showery conditions each day as a
broad upper level trough remains in place over the western CONUS. The
airmass aloft is rather cold with this trough, resulting in lower
than normal snow levels for this time of year. The observed 12z Salem
sounding from Thursday morning showed 850 mb temps near -2.5 degrees
Celsius, however this was shortly before a cold front moved through.
850 mb temps have likely fallen to around -4 degrees Celsius by now,
according to the latest suite of model data.

So far on Thursday, webcam/surface weather/spotter reports revealed
mainly accumulating snow for elevations above 1000 feet, mainly a
rain/snow mix with a dusting of accumulation on grassy surfaces
between 500-1000 feet, and mainly rain below 500 feet (aside from
brief periods of very wet snow or a rain/snow mix with heavier
showers passing by). As expected, snow has not been able to
accumulate beyond a dusting on grassy surfaces for elevations below
500 feet due to southerly surface winds/above freezing air temps and
ground temps. Therefore, travel impacts have only been occurring in
the Cascades and Coast Range mountains where accumulating snow
continues. Anyone with travel plans over the Cascades and/or Coast
Range should be prepared for winter driving conditions over the next
several days. It's also worth mentioning that a few isolated
thunderstorms were observed along the immediate Oregon coast Thursday
morning/early afternoon, some of which likely produced small graupel
or small hail as the hail growth zone is saturated (however we have
not received any reports to confirm this speculation). Thunderstorm
chances remain in the forecast through the upcoming weekend along and
near the coast (15-35% chance according to the NBM).

For Thursday night/Friday morning, suspect snow levels will fall even
lower than 500 feet between midnight and Noon as 850 mb temps cooler
further to around -6 degrees Celsius. This should be cold enough for
heavier showers to almost certainly lower snow levels down to the
valley floor, and the HREF backs this up well by showing brief
periods of snow as the dominant precip type. However, similar to what
was observed Thursday morning, southerly surface winds and above
freezing air/ground temps should be enough to prevent snow from being
able to accumulate much beyond a dusting to half an inch on grassy
surfaces for elevations below 500 feet. This will limit travel
impacts. The opposite will hold true for elevations above 500-1000
feet where temps will remain cold enough to support additional
snowfall accumulation. The Winter Storm Warning for elevations above
1500 feet in the Cascades/Upper Hood River Valley has been extended
through most of Friday morning to account for this. The Winter
Weather Advisory for elevations above 1500 feet in the Coast Range
has also been extended through most of Friday morning.

The pattern remains very similar Friday night into Saturday, as well
as Saturday night into Sunday. It will still be cool and showery with
abnormally low snow levels, and periodic chances for wet snow down to
the valley floor during the overnight/morning hours each day.
However, snow will likely be unable to accumulate below 500 feet
(except on grassy surfaces) as temps are simply too warm and the
precipitation is too showery.

Finally, one area of uncertainty pertaining to the forecast we
should definitely highlight Friday and Saturday is the potential
for the development of compact surface/meso lows rotating around
the larger parent trough feature to our west. Should one develop
and track along the coast or into the Willamette valley, it
would provide a burst of increased precipitation, a narrow swath
of heavy wet snowfall as low as 100-500ft, and breezy to locally
strong winds(35-55mph gusts). Unfortunately, confidence in the
exact placement of these smaller scale features is low (10-20%)
at this time as evident by the large spread in both
deterministic and ensemble system member surface low locations
at this time-frame. At least we can begin to narrow in on the
timing of such a feature developing/arriving as it would likely
coincide the upper-level shortwave expected to swing towards the
region the second half of Friday into Saturday morning shown by
most guidance. The exact amplitude and placement of this upper-
level shortwave and any additional thereafter will largely
dedicate the development of one of these impactful features.
Hopefully a greater agreement is reached in future model runs.
-TK/Schuldt

.LONG TERM..SUNDAY THROUGH THURSDAY...Looking into the upcoming week,
an active pattern continues bringing cold air and moisture. This will
result in the probability for seeing more snow for the Cascades and
Coast ranges along with a showery, rain/snow mix for elevations below
1000 ft through each evening and into the following morning. There is
the possibility that very little, if any snow accumulations could be
observed, but any snow that does accumulate will melt quickly as the
ground remains above freezing and daytime high temps for the lower
elevations are expected to be in the mid to upper 40s. This pattern
that we are in, is very typical as we slowly transition into spring.
Looking at the WPC 500 mb cluster analysis, most are in very good
agreement for the majority of the upcoming week, as a series of
Alaskan lows will bring cool, moist air into the region. Towards the
latter part of the upcoming week, there is noticeable difference in
the clusters with around 40% developing a moderate/strong ridge, 30%
with a very weak ridge and 30% a moderate trough. Needless to say,
there remains some uncertainty for conditions through the latter part
of the upcoming week.

Also, it should also be noted that with relatively cold 850 mb
temperatures, that conditions are getting more favorable for
thunderstorms. Some thunderstorms will likely produce small hail or
graupel.

&&

.AVIATION...S/SW flow aloft will persist through the TAF period,
as a broad low pressure system moves southeast towards our region.
Expect rain showers from the southwest to persist through the
period, occasionally dropping vis to IFR (10-20% chance) at any
given hour. Currently VFR along the coast and MVFR inland, expect
conditions to improve to widespread VFR conditions around 08Z
Friday, persisting through the period. However, chances for MVFR
conditions remain, with a 30-50% chance of MVFR conditions at any
inland terminal for any given hour. Otherwise, inland terminals
will maintain S/SW winds between 10-15 kts with gusts up to 20-25
kts and coastal terminals 15-20 kts with gusts up to 30 kts
through most of the TAF period.

*NOTE: The observation out of KONP is generally limited to wind
 and pressure at this time. Periodically cloud cover, vis, and
 present wx are reported but are unreliable.

PDX AND APPROACHES...Currently MVFR, expected to improve to
predominantly VFR beginning around 08Z Friday. Guidance suggests
a 20-40% chance of cigs dropping to MVFR through the TAF period.
Southerly winds between 10-15 kts with gusts up to 25 kts through
the period, gradually weakening to below 10 kts by 00Z Saturday.
-JH

&&

.MARINE...UPDATE...Gale Warning for central and southern waters
as frequent gale force winds are expected to persist through late
Friday morning.

Hazardous Seas Warning in the northern waters will
spread to all waters by Friday afternoon, with steep, short
period seas continuing to build into the 15-18 ft range. A
westerly swell will also be increasing through the night to around
16 ft which will produce occasional combined seas of 18-20 ft
tonight through early Saturday morning.

In addition to large and hazardous seas, expect unsettled weather
conditions to persist into the weekend as a deep trough of low
pressure dominates the NE Pacific, maintaining strong southwest
breezes along with numerous showers and a chance of thunderstorms
which will be capable of producing locally stronger and erratic
wind gusts. Models continue struggle resolving a surface low set
to track in from the southwest late Friday into Saturday. The
upper low at the center of the large trough is progged to drop
southeast toward the PNW while a shortwave rotates within the
larger trough. This will a compact surface low developing
southwest of the marine zones. Models vary widely on how deep this
low gets and therefore how strong the winds end up becoming late
Friday into early Saturday. However, there is decent agreement on
the track of this low which will produce a period of offshore
winds from 00-12Z Saturday. Mariners should closely monitor the
forecast for the possible addition of another round of gales late
Friday as models converge on a solution. -JH/CB

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...Winter Storm Warning until 7 AM PST Saturday for Central Coast
     Range of Western Oregon-Coast Range of Northwest Oregon.

     Winter Storm Warning until 10 AM PST Friday for Cascade
     Foothills in Lane County-Cascades in Lane County-Northern
     Oregon Cascade Foothills-Northern Oregon Cascades-Upper
     Hood River Valley.

WA...Winter Storm Warning until 7 AM PST Saturday for Willapa Hills.

     Winter Storm Warning until 10 AM PST Friday for South
     Washington Cascade Foothills-South Washington Cascades.

PZ...Hazardous Seas Warning until 4 PM PST Friday for Columbia River
     Bar.

     Hazardous Seas Warning until 10 AM PST Saturday for Coastal
     waters from Cape Shoalwater WA to Cape Falcon OR out 60 NM.

     Gale Warning until 10 AM PST Friday for Coastal waters from
     Cape Foulweather OR to Florence OR out 10 NM. Coastal waters from
     Cape Foulweather OR to Florence OR from 10 to 60 NM.

     Hazardous Seas Warning from 10 AM Friday to 10 AM PST Saturday
     for Coastal waters from Cape Foulweather OR to Florence OR
     out 10 NM. Coastal waters from Cape Foulweather OR to
     Florence OR from 10 to 60 NM.
&&

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