Winter Storm Warning 14, NWS Medford, OR

Area Forecast Discussion
National Weather Service Medford OR
1046 AM PDT Sun Mar 26 2023

.Updated Aviation Discussion.

.DISCUSSION...Radar imagery shows that the region has cleared of
showers (except for a few small cells over the ocean), but plenty
of clouds persist. There will be a brief revival of rain/snow showers
this afternoon, mostly over the high terrain of
Coos/Curry/Douglas Counties and the south-central Cascades, but
these will be very scattered and light. Other than that, expect a
calm if cool day. The forecast is well on track and no changes
have been made this morning. The main focus for today will be the
upcoming storm system coming in later Monday; for more details on
this see the previous discussion below. -CSP

&&

.AVIATION...26/18Z TAFs...VFR ceilings are expected to be the
predominate condition through the TAF period. However, partial
terrain obscuration is expected through this evening, especially
along the Cascades, Siskiyous and higher terrain east of the
Cascades. If there's enough clearing tonight, low clouds could
develop towards 15z in portions of the Umpqua Basin, including
Roseburg, but confidence is not high enough to include it in the
TAF.

Even through it's outside of the TAF period, it's worth mentioning,
all of the ingredients are there for moderate to strong winds
starting Monday afternoon and lasting into Tuesday afternoon in the
Shasta Valley, Rogue Valley, including Medford, and Klamath Falls.
-Petrucelli

&&

.MARINE...Updated 900 AM PDT Sunday, March 26, 2023...Relatively
calm conditions will continue into Monday morning. There's good
agreement of a surface low deepening quickly Monday morning west of
the waters, then moving near 130 W Monday evening. The location of
the surface low at this time is expected to be west of Coos Bay near
130W. The low will then love south, but remain inside of 130 W
Monday night into Tuesday.

Strong gale force southeast winds and very steep wind-driven seas
are likely over all waters Monday afternoon through Tuesday evening.
Some of the guidance even suggest there could be a brief period of
storm force winds, mainly south of Cape Blanco Monday afternoon
into Monday evening, then again Tuesday morning as the surface low
moves closer to the waters.

Once the surface low moves south of the waters Wednesday morning,
we'll see conditions gradually improve with winds and wind driven
seas gradually diminishing. -Hermansen/Petrucelli

&&

.PREV DISCUSSION... /Issued 415 AM PDT Sun Mar 26 2023/

SYNOPSIS...A broad trough is over the region with northwest flow
aloft. Models support light isolated to scattered showers today
in portions of southwest Oregon and northern California.
Otherwise, expect unseasonably cool weather with colder than
normal temperatures today into Monday morning. Showers will taper
off late this evening and tonight, then a much stronger low
pressure system will approach the area on Monday. Then as this low
approaches the southern Oregon coast it will bring a strong front
into the area late Monday afternoon through Tuesday afternoon.
Models show a high chance for heavy snow accumulations in the
Mount Shasta region (including the I-5 corridor from Weed to
Dunsmuir) and higher terrain over western Siskiyou late Monday
through Tuesday. Additionally, moderate to strong gusty south
winds are likely over the mountains and into the Shasta and
southern Rogue valleys. The combination of snow and wind will
also create periods of near whiteout conditions in the Mount
Shasta region (Weed to Mount Shasta City area), especially Monday
night and Tuesday morning.

SHORT TERM...Light isolated showers are occurring this morning,
especially along the coast. Light showers are expected to develop
over inland areas during the day. Expect cool temperatures through
the day. Morning low temperatures will be in the 20s to lower 30s
across valleys west of the Cascades with lows in the mid to lower
30s at the coast. A frost advisory remains in effect for the Curry
coast  where temperatures have dropped to around 33 to 35 degrees
overnight/early this morning and skies are mostly to partially
clear. East of the Cascades, low temperatures this morning will be
in the teens to lower 20s. Afternoon temperatures today will be
slightly warmer than yesterday but still well below normal with
highs in the upper 40s to 50s in western valleys and in the mid
30s to near 40 in valleys east of the Cascades. Along the coast,
expect high temperatures in the mid to upper 40s.

Models indicate that showers will taper off late this evening and
tonight. However, a strong low pressure system will approach the
region. High clouds are expected spread over the area tonight.
Otherwise, expect dry weather with cold overnight temperatures
similar to this morning. Frost may be limited at the coast though
due to the increase in high clouds.

Models are showing good agreement on the track and timing of the
low pressure system moving towards the area tonight and Monday.
The GEFS, ECMWF ensemble and NAM all keep the surface low along
130W and near 43N during the day Monday, then track the low
closer to the coast near the southern Oregon and northern
California border Monday night into Tuesday. The surface low
pressure is forecast to deepen to around 977 to 985 as it moves
inside of 130W Monday night (when it is around 200 to 225 miles
off the coast of southern Oregon). Given the good agreement
between models and ensembles as well as between model runs on the
track, timing and strength of this low, there is high confidence
that this system will bring significant impacts into the area,
especially across southern and western Siskiyou County where
southerly flow will favor enhanced precipitation rates along with
gusty south winds.

As this low deepens and nears the area, models indicate that a
strong low level jet (50-70kt at 925 mb) will move over the
coastal waters and a strong mid level 700 mb jet (50-60kt) will
move inland. The stronger mid level winds combined with an
increasing north-south pressure gradient across the area, is
expected to result in moderate to strong gusty south-southeast
winds in the southern Rogue Valley and Shasta Valley. A high
winds watch (NPWMFR) for winds of 30 to 45 mph and gusting up to
60 mph remains in effect for the southern Rogue and Shasta
valleys. Strong gusty winds are also likely over the mountains and
possibly into the Scott Valley as well. Moderate gusty winds are
expected to affect the southern Oregon coast and valleys east of
the Cascades with breezy to gusty winds for most other areas.
Over the coastal waters, gales are likely. (Please see the marine
discussion below for details).

Light precipitation is expected to begin along the coast late
Monday morning into Monday afternoon, then spread inland during
the afternoon. Precipitation will then increase Monday evening
through Tuesday morning, with moderate to heavy precipitation
across western/southern Siskiyou, Curry, and into southwest
Josephine County. Elsewhere expect moderate to light
precipitation. Of note, locations in the southern Rogue and Shasta
Valley (north of Weed) may see only light precipitation late
Monday through Monday night, due to the effects of strong
downslope winds. This may be followed by a period of light to
moderate precipitation as the front moves through Tuesday morning.

There is a high chance for heavy snow across western and southern
Siskiyou County with light to moderate snow impacts elsewhere,
especially from the Cascades east and into higher terrain in Curry
County and into southwest Josephine Counties. Snow levels are
forecast to start at around 2500-3000 feet but then lower to
between 1500-2000 feet early Tuesday morning. The moderate to
heavy precipitation combined with low snow levels as well as
strong, moist southerly flow will bring a high chance for heavy
snow into the Mount Shasta region as well as for areas in western
Siskiyou County (mainly above 2000 feet elevation). The National
Blend of Models continues to indicate 24 hour snow totals of
around 12 to 24 inches in the Mount Shasta region (from Weed to
Dunsmuir) and in the McCloud area Monday evening through Tuesday
afternoon, with a potential for high snowfall rates of 1-2 inches
per hour Monday night and Tuesday morning. Additionally, with
heavy snow combined with strong gusty winds, especially from
north of Mount Shasta city to Weed and over exposed higher
terrain, expect near whiteout conditions at times Monday night
and Tuesday morning. Heavy snow is also likely in western
Siskiyou, especially above 2000 feet elevation (mainly for W.
Siskiyou Co west and south of the Scott Valley), with amounts of
10 to 20 inches possible. Moderate snow accumulations are possible
across portions of the Southern Oregon Cascades and northern
Klamath County (mainly north of Chiloquin) as well as the
mountains in eastern Klamath and western Lake Counties.

There is more variability in the model forecast for snow in
eastern Curry and southwest Josephine Counties. The National Blend
of Models indicates the heavy  snowfall amounts in the Siskiyous,
south and southeast of Cave Junction, including for the Grayback
Road area. Moderate to heavy snow is also possible in the
Kalmiopsis, mainly above 2500 feet. Of concern, as snow levels
lower early Tuesday morning with the back edge of the frontal
precipitation band, there is a chance a brief period of
accumulating snow down to valley floors in the Illinois Valley,
from Hayes Hill south, and in the Klamath River Valley near Happy
Camp.

We will continue to monitor this details of this storm and update
the forecast with further information.

PREV LONG TERM DISCUSSION... /Issued 259 PM PDT Sat Mar 25 2023/

Precipitation in the Mount Shasta area should gradually diminish
Tuesday afternoon into Tuesday night as the upper low moves south
towards the Bay Area. There could be some wrap around moisture in
portions of Northern California Tuesday night, including Mount
Shasta, but we're not expecting anything significant.

After Tuesday, we're not expecting any significant weather. Some
wrap around showers from the upper low that will be south of our
area could bring isolated to scattered showers mainly along and east
of the Cascades. A weak upper trough will drop down along the Oregon
coast Thursday which could trigger some showers Thursday afternoon,
but were not expecting anything significant.

Another front could move into the area Friday night with moderate
precipitation, followed by post frontal showers Saturday as another
cold air mass moves in from the northwest. The operational models
and the majority of ensemble members (ECMWF and GFS) show similar
solutions, although there's some variation on timing. -Petrucelli

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Frost Advisory until 9 AM PDT this morning for ORZ022.

     High Wind Watch from Monday afternoon through Tuesday evening
     for ORZ026.

CA...Winter Storm Watch from Monday afternoon through Tuesday morning
     for CAZ080>083.

     High Wind Watch from Monday afternoon through Tuesday evening
     for CAZ081.

PACIFIC COASTAL WATERS...Gale Watch from Monday afternoon through
     Tuesday evening for PZZ350-356-370-376.

&&