Winter Storm Warning 17, NWS Medford, OR

Area Forecast Discussion
National Weather Service Medford OR
853 AM PST Sun Nov 24 2019

.SHORT TERM...Expect a shift towards active weather early next
week as a strong jet and upper trough moves into the area Monday
followed by a strong low pressure system on Tuesday. Today,
conditions are relatively mild with only a very weak front moving
into the area, bringing some areas of light rain to northwest
portions of the area.

Satellite imagery shows clouds moving into the area from the
north as a weak front approaches. Based on observations and radar
data, some spotty areas of light rain are occurring just north of
the CWA in the Eugene and surrounding areas. Models continue to
show areas of light rain spreading into Coos and northern Douglas
Counties and into portions of the Southern Oregon Cascades today.
However, models have trended lower on precipitation amounts. So
light rain of a few hundreths of an inch is expected and rain may
be spotty in coverage. From the Umpqua Divide south, east of the
Cascades and across Northern California, dry conditions are
expected today.

Of note, this morning, fog remains in the Illinois Valley and in
the lower Rogue Valley from around Rogue River/Grants Pass to
Agness. Fog has been locally dense in some areas early this
morning, such as near Merlin. However, expect fog to gradually
improve during the mid morning, clearing in the late morning or
around noon.

Precipitation chances will increase tonight into Monday as strong
upper level jet moves over the area and an upper level trough
sinks south across the region. The trough will be centered inland
just to the east on Monday and we will be in a moist northwest
flow pattern. Expect light rain/snow to spread across southwest
Oregon tonight into Monday morning with the best chance for
measurable precipitation from the Cascades west and into the
Chemult area. However, a chance to slight chance for light
precipitation will extend into northern California and across areas
east of the Cascades as well. Snow levels are forecast to be
around 3000 to 3500 feet elevation on Monday and light
precipitation will continue through the day Monday and into Monday
night. Snowfall totals of 2 to 6 inches are expected in the
Southern Oregon Cascades Monday through Monday night. So light
snow accumulation are expected to affect the higher passes in the
Southern Oregon Cascades during this period.

For Tuesday, a strong low pressure system takes aim for the
southern Oregon coast. This will low will approach the area
Tuesday morning and afternoon then move inland Tuesday night. With
this low expect significant impacts, including to holiday travel.
Heavy snow is possible above 2500 feet and winter storm watches
are in place. In addition high winds are possible at the coast and
over the coastal waters. Please see the WSWMFR and NPWMFR for
details as well as the discussion below.

&&

.AVIATION...For the 24/12Z TAFs...VFR conditions will continue
through this evening, with the exception of IFR fog in the Umpqua
Basin near Roseburg and Rogue Valley near Grants Pass. These IFR
conditions will continue through the morning and improve some to
MVFR and then possibly VFR as a cold front approaches from the west
this afternoon. This front will bring mountain obscuration to areas
along and west of the Cascades late this afternoon. Another front
will bring some valley rain and mountain snow to the area by early
Monday morning. Ceilings will lower to MVFR for areas from the
Cascades west initially and then spread east of the Cascades Monday
afternoon. Keene

&&

.MARINE...Updated 845 AM PST Sunday 24 November 2019...A
front is moving through the waters this morning and is briefly
disrupting the northerly winds, but north winds will redevelop this
afternoon and strengthen some. Strongest winds will be south of Cape
Blanco this evening. Another front Monday will bring moderate north
winds to most areas. Moderate west swells will continue to produce
conditions hazardous to small craft through Monday.

The majority of model guidance now points to a rapidly deepening low
pressure approaching the waters Tuesday. Confidence is increasing
that the coastal waters will be impacted by a period of at least
gale force winds. We have issued a Storm Watch to account for the
growing possibility of storm force winds for at least a portion of
the coastal waters. Right now the best chance for storm force winds
is from Cape Blanco southward, but there still is a risk for storm
force winds north of Cape Blanco, so that area has been included in
the Storm Watch. All mariners should plan now for the potential of
dangerous conditions on Tuesday. Please keep a close eye on updated
forecasts as we get closer to Tuesday. We'll likely be issuing
either a Gale or Storm Warning in the next 24 hours depending on
what future data suggests.

A period of strong north winds follow behind the low early
Wednesday, and then Small Craft Advisory conditions are likely to
return later on Wednesday.

&&

.PREV DISCUSSION... /Issued 437 AM PST Sun Nov 24 2019/

DISCUSSION...Areas of valley fog have developed once again this
morning, but significant upper level cloudiness has developed as
well, a sign of the approaching weak cold front that will pass
through the region later today. Light rain is expected with the
front today, mainly over the coastal mountains, Umpqua Basin, and
Southern Oregon Cascades. The rest of the forecast area should
remain dry. Showers should decrease in coverage this evening, but
will continue to linger through tonight.

Tonight into Monday, a strong upper level jet will push into the
area as an upper trough deepens inland. With this pattern, models
show light precipitation spreading into much of southwest Oregon
and portions of northern California Sunday night through Monday.
Snow levels are expected to be around 2500 to 3500 feet during
this period, so expect light rain at the coast and across western
valleys with light snow or a rain/snow mix for areas in the
mountains and east of the Cascades. The best chance for
accumulating snowfall will be in the Cascades north of Highway
140, where 2 to 6 inches total snowfall is expected Monday
through Monday night.

The big "Welcome to Winter" event arrives Tuesday. A very deep and
cold upper-level trough, originating from Alaska, will dig south
along the coast, and after joining forces with a rapidly
deepening shortwave approaching form the eastern Pacific, take
station over southern Oregon and northern California by Tuesday
evening. There has been considerable attention paid to the various
model suites and ensemble members over the past several days, and
in the latest runs, the wide range of solutions has narrowed
significantly in regards to the track and strength of the surface
low that will accompany the shortwave into the coastline Tuesday
afternoon and evening. The resulting scenario is one of a rapidly
deepening low, a "bombing cyclone," tracking into the coast
somewhere between Cape Blanco and Cape Mendocino Tuesday evening.
The latest deterministic models are in good agreement, pushing
the low inland right along the Oregon California State line. This
event is therefore very likely to produce significant and wide-
ranging impacts during one of the busiest travel periods of the
year.

The first, but not least, concern will be winds, especially along
the Curry County coast, the higher terrain, and the Shasta and
Rogue Valleys. With a rapidly deepening low along with significant
pressure gradients (on the order of 10 to 15 mb between Redding
and Medford, and well above 15 mb between Eureka and Gold Beach),
damaging winds are possible, and tree and power line damage are
significant concern, especially along the Curry County coast
where wind gusts could top 60 mph. Inland, the winds just above
the surface align very well with the Shasta and Rogue Valleys, and
as a result, areas from Weed to Ashland could see strong gusts.
Meanwhile the mountains and East Side, where snow is likely to be
falling, could see significant drifting and visibility
reductions.

This leads to the primary concern: heavy snow during a period of
much higher than normal travel. A number of factors could make
this a significant snow event. First, pre- existing cold air will
be in place, with a deepening compact surface low tracking just to
the south. Also, the warm front associated with the low is
expected to drape itself just to the south as well, allowing for
significant overrunning over the region. Then we have upslope
flow, a result of the aforementioned gradients and upper level
winds. These pieces of the puzzle should all combine to produce
heavy snow accumulations for a wide area, including Interstate 5
from Dunsmuir to Ashland, many local passes, especially across the
Cascades and Siskiyous. For specific on forecast snow amounts, see
the Winter Storm Watch at PDXWSWMFR.

Now is the time to prepare for the increasing chance for winter
weather. If you plan to travel Tuesday afternoon into Tuesday
night, it may be a good idea to reconsider your plans. At the very
least, stock your vehicle's emergency travel kits, check the
vehicle's tires/chains, and know road conditions before you go.

Snow levels of 2500-3000 feet are expected as precipitation
begins in earnest Tuesday, but could drop down to the southern
Oregon valley floors once the surface low passes and the cold air
pours into the region. The bulk of the precipitation should end
before this occurs, but showers will be ongoing, and we can not
rule out the chance for low elevation snow Wednesday morning.
Precipitation should be light enough to prevent snow from
accumulating, but this is not locked in stone. So stay tuned for
the latest updates.

By Wednesday afternoon, most of the energy will be south of the
area, but the upper level trough will remain in place, so showers
are still possible with snow levels expected to be between
1500 and 2000 feet. Given the convective nature of the showers,
and there could be sun breaks, roads should be mainly wet.
However, if a heavier shower moves overhead, it could result in
brief road snow accumulations.

Wednesday night things begin to dry out, but it will be cold. So
the concern turns from road wet or snow covered roads to ice. This
will especially be the case where roads are already wet and in
areas that are sun sheltered.

Looking at Thursday and beyond, current models and ensembles show
a break in the active weather with only isolated showers but cold
conditions Thursday into Friday. This is followed by a robust low
pressure system and front approaching the area Saturday. This
next frontal system may bring precipitation into the area late
Saturday with increased winds possible over the coastal waters and
in the mountains. Snow levels are expected to rise some ahead of
the front on Saturday and be around 2500 to 4000 feet elevation.
-BPN

AVIATION...For the 24/12Z TAFs...VFR conditions will continue
through this evening, with the exception of IFR fog in the Umpqua
Basin near Roseburg and Rogue Valley near Grants Pass. These IFR
conditions will continue through the morning and improve some to
MVFR and then possibly VFR as a cold front approaches from the west
this afternoon. This front will bring mountain obscuration to areas
along and west of the Cascades late this afternoon. Another front
will bring some valley rain and mountain snow to the area by early
Monday morning. Ceilings will lower to MVFR for areas from the
Cascades west initially and then spread east of the Cascades Monday
afternoon. Keene

MARINE...Updated 315 AM PST Sunday 24 November 2019...A
front is moving through the waters this morning and is briefly
disrupting the northerly winds, but north winds will redevelop this
afternoon and strengthen some. Strongest winds will be south of Cape
Blanco this evening. Another front Monday will bring moderate north
winds to most areas. Moderate west swells will continue to produce
conditions hazardous to small craft through Monday.

The majority of model guidance now points to a rapidly deepening low
pressure approaching the waters Tuesday. Confidence is increasing
that the coastal waters will be impacted by a period of at least
gale force winds. We have issued a Storm Watch to account for the
growing possibility of storm force winds for at least a portion of
the coastal waters. Right now the best chance for storm force winds
is from Cape Blanco southward, but there still is a risk for storm
force winds north of Cape Blanco, so that area has been included in
the Storm Watch. All mariners should plan now for the potential of
dangerous conditions on Tuesday. Please keep a close eye on updated
forecasts as we get closer to Tuesday. We'll likely be issuing
either a Gale or Storm Warning in the next 24 hours depending on
what future data suggests.

A period of strong north winds follow behind the low early
Wednesday, and then Small Craft Advisory conditions are likely to
return later on Wednesday. Keene/Wright

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Storm Watch from Tuesday morning through Wednesday
     morning for ORZ029-030.
     High Wind Watch from Tuesday morning through late Tuesday night
     for ORZ021-022.
     Winter Storm Watch from Tuesday morning through Wednesday
     morning above 2500 feet in the for ORZ023>026.
     Winter Storm Watch from Tuesday morning through Wednesday
     morning for ORZ027-028.

CA...Winter Storm Watch from Tuesday morning through Wednesday
     morning for CAZ081>083.
     Winter Storm Watch from Tuesday morning through Wednesday
     morning for CAZ080.
     Winter Storm Watch from Tuesday morning through Wednesday
     morning for CAZ084-085.

Pacific Coastal Waters...Storm Watch from Tuesday morning through late Tuesday night for
     PZZ350-356-370-376.
     Small Craft Advisory until 10 AM PST Tuesday for
     PZZ350-356-370-376.

CC/CC/CC