Winter Storm Warning 14, NWS Medford, OR

Area Forecast Discussion
National Weather Service Medford OR
833 AM PDT Sat Mar 23 2019

.SHORT TERM...A shortwave trough is moving into the area this
morning and will remain in place this afternoon, then gradually
shift east in the evening. This will bring a showery and unstable
air mass to the CWA. A mix of rain and snow showers are occurring
this morning with snow levels around 3500 to 4000 feet west of the
Cascades and between 4000 to 4500 feet from the Cascades east.
Showers will increase during the day today. Snow levels will rise
slightly to 4000 to 5000 feet. Models continue to support a slight
risk for thunderstorms today, with the best chances from the
Cascades west during the afternoon as cold air aloft and
instability increase over the area. Storm motion will generally be
west to east between 5 to 15 kt, so periods of heavy rain are
possible with heavier showers or thunderstorms. Additionally small
hail is possible in any thunderstorms that develop today.
Scattered showers will continue into the evening with showers most
likely over the higher terrain and from I-5 east. Then expect
shower activity to diminish tonight.

&&

.AVIATION...For the 23/12Z TAFS...Most areas have vfr or mvfr
conditions. However, parts of the Umpqua Valley and Rogue Valley
have LIFR in fog around Roseburg and Grants Pass early this morning.

Unstable conditions in the post-frontal air mass should help to mix
and dissipate the fog this morning so expect LIFR vsby conditions in
these valleys to lift but low ceilings may linger into the morning.
Higher terrain obscurations will also occur.

Showers will become more common in the unstable airmass this
afternoon. Thunderstorms with small hail are also possible west of
the Cascades.

Sandler

&&

.MARINE...Updated 815 AM PDT Saturday, 23 March 2019...
High west swell will keep seas over 10 feet through Sunday morning,
continuing the small craft advisory for hazardous seas. This will
also result in hazardous bar conditions and higher than average surf
before diminishing Sunday morning.

After a brief break in the action Sunday, another robust frontal
system is expected to arrive Sunday night into Monday morning. This
system could warrant another gale warning but high end small craft
conditions are forecast at this time. Conditions should improve by
Tuesday, but long-term guidance suggests unsettled weather will
continue into next week. -Sandler/Sargeant

&&

.PREV DISCUSSION... /Issued 337 AM PDT Sat Mar 23 2019/

DISCUSSION...A frontal boundary is oriented south to north over
the middle of the forecast area, from Siskiyou County north to
northern Klamath County. Precipitation has been observed mainly in
Siskiyou County, and snow levels are currently around 5000 feet
and will lower to 4000 feet later this morning. A few inches of
snow is possible in areas east of the Cascades this morning as the
front pushes eastward, but currently there are no webcams showing
snow falling.

An unstable air mass and shortwave energy is moving into the coast
this morning. Infrared satellite imagery depicts a less unstable
atmosphere as open cell cumulus activity has decreased over the past
12 hours or so. Still, there is a slight chance of thunderstorms
over the coastal waters this morning due to -31C temperatures aloft.
This thunderstorm chance moves inland during the day, and latest
high resolution model output supports this. Also, upper level
deformation just east of the upper trough is indicated by the GFS20,
which would support enhanced convection. Brief periods of heavy rain
are possible with any of these showers or storms this afternoon.

Tonight, due to the cold air mass and some clearing skies, frost is
expected in valleys west of the Cascades. Freezing temperatures are
expected in the Illinois Valley and colder areas of the Rogue and
Applegate valleys. Those who have planted any frost-sensitive
vegetation will want to protect them tonight.

Sunday looks like a dry and partly to mostly sunny day with
temperatures above normal for this time of year. This could very
well be the warmest day for the next week. The brief period of
quiet weather changes Monday when a front approaches and brings
increasing south winds to most of the forecast area. The 700mb
south flow is expected to be anomalously strong for this time of
year, peaking at about 50kt early Monday morning. In addition to
moderate to strong winds in the Shasta Valley, portions of the
East Side, and over the mountains, these winds will support
periods of heavy precipitation over portions of Siskiyou County.
Given the cold air mass in place, we've issued a Winter Storm
Watch above 4000 feet. This means a period of heavy snow Monday
morning could impact Highway 89 at Snowmans Summit and Etna Summit
on Sawyers Bar Road. We have moderate confidence in snow levels,
and when looking at climatology, it's not rare for snow to impact
Mount Shasta City this time of year. However, for now we are
forecasting the main snow impacts to be higher than the Mount
Shasta City elevation. Monday's system will be wetter than recent
ones, and rain and mountain snow is likely for most areas on
Monday, with showers diminishing overnight. The long-term
discussion from Sunday's dayshift follows. Keene

Tuesday through Friday...An active pattern continues
with south to southwest flow aloft on Tuesday and a low pressure
system moving into the area late Tuesday through Wednesday. During
the day on Tuesday, expect showers to increase, especially across
southern and western portions of the area as a frontal system
approaches from the west. This front will spread light to moderate
precipitation across the CWA Tuesday evening through Wednesday
morning. Heaviest precipitation is expected across Curry, southern
Josephine and western and south central Siskiyou counties. Snow
levels are expected around 3500 to 4500 feet during this period.
Current models support light to moderate accumulating snow  possible
on higher passes Tuesday evening through Wednesday morning.

The closed low will push into the Oregon coast on Wednesday with
widespread showers expected over the area. Given the cold low
nearing the coast on Wednesday, may also see enough instability and
upper level dynamics for thunderstorms, especially over the marine
waters and into the coast. However the track of the low may be just
north of the CWA, so will continue to monitor this potential. Breezy
to gusty south winds are expected over the area on Wednesday, with
strong winds possible in the Shasta Valley and gusty winds east of
the Cascades. Then the low will move inland and open up into an
upper trough on Thursday. This will result in continued showers.
Snow levels are expected to be around 3500 to 4500 feet Wednesday
and Thursday but may lower to 3000 feet at times, Wednesday night
and Thursday morning. On Friday, the upper trough may begin to shift
eastward. However, depending on how quickly this trough shifts out
of the area, lingering showers are possible. -CC

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...
CA...Winter Storm Watch from late Sunday night through Monday morning
     for CAZ080-082-083.

Pacific Coastal Waters...Small Craft Advisory until 11 AM PDT Sunday for
     PZZ350-356-370-376.

CC