NWS Medford, OR wrote its last area forecast discussion before the first event at Feb 28, 8:24 PM PST, 3 hours 36 minutes earlier.
Area forecast discussion, Feb 28, 8:24 PM PST, 3 hours 36 minutes before
Area Forecast Discussion
National Weather Service Medford OR
824 PM PST Wed Feb 28 2018
.DISCUSSION...A major winter storm will be affecting the forecast
area through Saturday. Winter Storm Warnings and Winter Weather
Advisories are in effect for most of the forecast area. The
impacts will come in three fairly distinct rounds. First, tonight
a strong cold front is nearing the coast this evening, and it's
bringing heavy rain and gusty winds to the coast and snow to areas
generally above 3000 feet (lowering to 2500 feet by morning) over
Oregon. Snow is affecting almost everywhere in Northern
California's Siskiyou and Modoc Counties (snow noted in Dunsmuir
on latest cam imagery this evening), and increasing winds to
foothills/high terrain and areas east of the Cascades.
Snowfall rates will intensify and become very heavy over Northern
California tonight, and 1-2 inches per hour is likely in the Mount
Shasta area (all the way down to Dunsmuir) between midnight and
around 5am. High-resolution models are in good agreement on this,
and so far, observations back up these high precipitation amounts.
We only went with a fraction of the actual high-resolution
precipitation, but this brought up the already-high snow amounts
by enough to make some changes for this small but important area
of the forecast area...along I-5.
Snow levels look on track so far this evening, so have not made any
changes to these except to bring snow level down to Dunsmuir
through the night.
Tomorrow, a very cold (about the coldest air mass one can get this
time of year per SPC Medford upper air sounding climatology) upper
trough will follow the front. This will bring snow levels down
sharply, generate vigorous showers and thunderstorms over the waters
that will progress inland in the late afternoon and evening
(thunderstorms likely fading once inland), and this will bring
significant snow impacts down to around 1000 feet elevation. It's
likely the most significant lower elevation snow from this will
occur in the Illinois Valley area (especially Hayes Hill) as very
unstable, cold, west-southwest flow favors this outcome. In
summary, Thursday afternoon/evening will likely feature
widespread travel impacts as this upper trough pushes inland, and
these impacts will likely increase as one moves closer to the
coast.
Lastly, another low pressure system will push into the coast Friday
afternoon, and this could bring strong winds to the coast and
another round of winds to the Shasta Valley and East Side (enhancing
the already-blowing/drifting snow situation there).
Please see the detailed previous forecast discussion below for
more info on this major winter storm, and if traveling, please
read and heed the forecast, stay tuned to road conditions, and
try to alter plans accordingly.
&&
.AVIATION...01/00Z TAF CYCLE...Areas of MVFR and local IFR
cigs/vsbys in low clouds and rain/snow occurring with terrain
obscuration. Southerly winds will be strong overnight with strongest
winds at the coast, east of the Cascades, in the Shasta Valley, and
over higher terrain. Strong winds aloft will lead to LLWS concerns
for valley locations west of the Cascades this afternoon and evening
as a strong cold front moves inland. Areas with heavy snow will
see vsbys locally decrease to IFR levels. -Schaaf
&&
.MARINE...Updated 800 PM PST Wednesday 28 Feb 2018...A cold front
will move through overnight and bring strong south winds, heavy seas
and heavy rain to the area. Winds will diminish late tonight after
the front moves onshore, but a steep and high northwest swell will
follow Thursday into Friday. Swell dominated seas will peak Friday
at 18 to 19 feet with a 15 second period. In addition, a low will
move into the waters Friday, which will bring in another round of
gusty south winds reaching gale force over parts of the area.
Conditions will improve Friday night into Saturday, but it will
remain cold and unsettled into next week. /FB
&&
.PREV DISCUSSION... /Issued 323 PM PST Wed Feb 28 2018/
DISCUSSION...
Short Term...Tonight Saturday Night...The snow is starting.
We are seeing some flakes start to fall in some area web cameras,
and RADAR shows a wide swath of returns moving in overhead.
Meanwhile, satellite imagery shows a textbook cloud shield
forming offshore, typical of mature systems, and this is just what
we are expecting. Snow levels have come up today, resulting in
rain in the valleys west of the Cascades, but as the cold front
sweeps through the area tonight into tomorrow, widespread snow is
expected down to roughly 2000 feet through Thursday afternoon.
Once the front passes, the upper level trough will dig south, and
a potent shortwave will rotate through overhead. Snow levels are
expected to drop further as this occurs Thursday afternoon and
evening, resulting in snowfall down to below 1000 feet. The large
scale trough will set in directly overhead, and showers are
forecast to continue into Saturday. Snow levels will fluctuate,
rising during the day and dropping overnight, while showers
continue to stream through the region. Have extended the winter
weather headlines out through Saturday, and lowered the west side
advisory elevations down to 1000 feet for Thursday afternoon and
beyond. This will be an extended snow event lasting for
approximately three days, and snow is expected to pile up
significantly in the coastal ranges, the Cascades, and in the
Mount Shasta area where upslope will be strong and ongoing.
Meanwhile, gusty winds are expected over high terrain, the Eats
Side, and in the Shasta valley. The combination of wind and snow
will produce very low visibilities, with white-out conditions
possible at times. Those traveling in any of these areas,
including along Interstate 5, anytime between now and Saturday
should reconsider their plans, or alter the schedule for travel
after the heavy snow and wind has cleared the area. For more
information and detail on snow amounts and timing for what will
likely be the biggest storm of the season, please see the winter
weather message at WSWMFR.
An interesting aspect of this event is the surface low pressure
center, which is likely to intensify as it swings onshore Friday
afternoon. If this low deepens, it will produce strong winds along
the coast and into the coastal mountains, and produce another
strong shot of precipitation. Model solutions depict this low
gaining similar characteristics to a Polar Low, a type of system
usually seen at more northerly latitudes, but we do not expect
such a strong system will actually form.
As mentioned before, snow showers will continue through Saturday,
with the heaviest snow, other than along the front itself
tonight, expected Thursday afternoon and evening, and again Friday
afternoon. Meanwhile, upslope snows will continue for southern
and western Siskiyou county and for the Kalmiopsis Wilderness, at
almost a constant rate. By the time snow tapers off and the trough
exits late Saturday, some higher peaks in these areas could
receive 3 to 4 feet of fresh snow. Only light snow showers are
expected to continue into the extended period. -BPN
Long Term...Sunday through Wednesday night...The long term
forecast has changed a little bit from yesterday. The biggest
change will be on Sunday where a few lingering light showers will
continue through the day. Snow levels will continue their pinball
trend with snow levels raising to 2000 to 2500 feet during the day
and around 500 feet by Sunday night. That being said, the
majority of precipitation will end on Sunday night with only a
Dusting to a quarter of an inch possible in the Cascades and near
Mt. Shasta City.
On Monday, conditions begin to dry out with partly cloudy skies
expected. High temperatures will rebound into the upper 40s to low
50s for many of the west side valleys and into the 30s east of the
Cascades. Lows, will start off similar to what they were on Saturday
night and slowly increase each subsequent night...becoming near
normal or slightly above normal by the end of the forecast period.
The next system is poised to arrive on the west coast around Tuesday
into Wednesday. Snow levels will start off around 4000 to 5000 feet,
and it could bring some moderate precipitation to the area. That
being said, the GFS is the more progressive and stronger system
versus the ECMWF which is slower and allows the precipitation to
largely dissipate before reaching the coast. Overall, this system
could be a weak to moderate atmospheric river-type system. This
forecast will favor the GFS as have previous forecasts, but did add
a little bit of the ECMWF into the mix as solutions this far out
tend to be somewhere in between. -Schaaf
&&
.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory until 4 AM PST Friday for ORZ029>031.
Winter Weather Advisory until 7 AM PST Saturday above 2000 feet
in the for ORZ021>026.
Beach Hazards Statement through late Thursday night for
ORZ021-022.
Winter Storm Warning until 7 AM PST Saturday for ORZ024.
Winter Storm Warning until 1 PM PST Thursday for ORZ027.
Winter Weather Advisory until 7 AM PST Saturday for ORZ027-028.
CA...Winter Weather Advisory until 4 AM PST Friday for CAZ084-085.
Winter Storm Warning until 7 AM PST Saturday for CAZ082-083.
Winter Weather Advisory until 7 AM PST Saturday for
CAZ080-081-083.
Winter Storm Warning until 7 AM PST Saturday for CAZ080.
Pacific Coastal Waters...Small Craft Advisory for hazardous seas from 1 AM to 10 PM PST
Thursday for PZZ350-356-370-376.
Gale Warning until 1 AM PST Thursday for PZZ350-356-370-376.
Gale Warning from 10 PM Thursday to 10 PM PST Friday for
PZZ350-356-370-376.
Hazardous Seas Warning until 1 AM PST Thursday for
PZZ350-356-370-376.
Hazardous Seas Warning from 10 PM Thursday to 10 PM PST Friday
for PZZ350-356-370-376.
NSK/BMS/FJB