Winter Storm Warning 1, NWS Medford, OR

Area Forecast Discussion
National Weather Service Medford OR
841 PM PST Thu Jan 3 2019

.DISCUSSION...

No changes to the forecast tonight as a cold front approaches the
region tonight. Visibility has dropped yet again in some of the
valleys, yet high clouds should keep visibilities from dropping
below a mile tonight. With this cold front moving onshore, we'll
see rain north of the Umpqua divide and along the coast tonight
into the early morning hours of Friday.

We also cancelled the air stagnation advisory as low level winds
should get the air moving again Friday.

For more information beyond tonight, read the previous discussion
below.

-Smith

&&

.AVIATION...For the 04/00Z TAFs... A cold front will continue to
push toward the coast and into the western Umpqua Basin through
Friday morning. This is expected to bring low level wind shear at
and near KOTH through 14Z and at and near KRBG from 06Z-13Z. MVFR to
IFR is expected to develop with the front at those two airfields.
For KMFR and KLMT expect broken to overcast high clouds. Converging
light winds at the surface could result in MVFR and some fog in the
Medford area, but the airfield is not expected to be significantly
affected. VFR conditions are expected to areawide Friday afternoon.

-Lutz/Clarstrom

&&

.MARINE...Updated 820 PM PST Thursday, 3 January 2019...
Gales will continue at most areas north of Port Orford through
tonight as a frontal system moves in, weakens, and stalls. Southerly
wind wave dominated seas will gradually transition to west swell
dominated Friday. The front is then expected to lift back northward
as a warm front Friday through early Saturday. A strong area of low
pressure will then push toward the area from the southwest making
landfall near Brookings Saturday and bringing a rapid wind shift
with chaotic seas. A series of frontal systems is then expected
through early next week with periods of gales and high to very high
seas likely. Current guidance suggests southwesterly seas peaking in
the 20 foot range on Sunday afternoon and 20 to 25 feet Tuesday into
Wednesday.

-Lutz/Wright

&&

.PREV DISCUSSION... /Issued 300 PM PST Thu Jan 3 2019/

SHORT TERM...The upper ridge responsible for the stagnant
weather pattern over the area the past several days is flattening
in response to jet energy arriving from the Pacific. The main axis
of the jet continues to be directed well to our north this
afternoon, so most of the heavy precipitation associated with it
is in western Washington and southwest British Columbia. Down our
way, we've have some mid and high level cloud most of the day with
some breaks here and there. Nearest rainfall has been over the
coastal waters and along the coast near Newport, but hasn't made
it to our forecast area yet. West-southwest flow aloft will
strengthen tonight and a frontal system offshore will move onshore
Friday morning. This will bring rain to the coastal waters, the
immediate coast and areas north and west of the Umpqua Divide.
There will be a pretty distinct edge though to where the
precipitation gets as the flow becomes parallel to the front
causing the front to stall. Right now, that line appears to run
from about Roseburg to Brookings. South and east of that line,
precipitation will be hard to come by Friday. Even so, we should
have a bit better mixing, so this should bring an end to the air
stagnation advisory, which is in effect until 10 pm this evening.

An upper trough will then dig offshore, close off and spin up a
strong surface low (~985 mb) offshore. This will further back the
flow to the south and even southeast Friday night into Saturday.
The surface low is not forecast to deepen as it moves onshore, but
this is the kind of set up that could result in a windstorm for
many places. Guidance is indicating pressure gradients MFR-RDD
of around -7 to -10 mb with 700 mb winds in the 60-70 kt range.
With the flow aligned with the Rogue/Shasta Valleys, we are
likely to see pretty strong winds surface especially just ahead of
the low moving onshore. East side areas and the mountains are also
likely to have very strong winds. High wind watches are now up at
NPWMFR, so please refer to that product for the specifics. These
headlines may need to be revised/expanded, depending on the exact
track of the surface low. Coastal areas could have high winds too.
The upper trough/low will move onshore Saturday afternoon with an
expected wind shift from SE to SW, which could induce the
strongest gusts due to downward transport of high momentum air.

Another aspect to this storm is the potential for heavy
snow in the Mount Shasta region. Right now, it appears that
precipitation will begin very late Friday night/early Saturday
morning in Siskiyou County, then a period of moderate/heavy
precipitation is expected late Saturday morning through Saturday
evening as upslope flow is maximized by the mid-level 60-70kt
winds. We have issued a winter storm watch for elevations above
3000 feet for 6-10 inches of heavy, wet snow for western and
southern Siskiyou County. Some of the higher elevations above 4000
feet could see more than a foot of new snow. Confidence in snow
impacting Interstate 5 near Mount Shasta City on Saturday is high,
but confidence in the snow level is a little less certain. This
will be driven mainly by precipitation rates. If the precipitation
is heavy enough, it could drag the snow level all the way down to
Dunsmuir at times. Model guidance, however, is unanimously
keeping the snow level 3000 feet and above. Please refer to the
WSWMFR product for more details. Additional advisories may be
necessary, so stay tuned for updates. We are also expecting snow
over the Cascades and higher passes, including Siskiyou Summit,
from Saturday afternoon through Saturday night. Precipitation will
become showery, however, Saturday night as the upper trough
swings through and the upper flow becomes more west-southwesterly.

It won't end there. Another strong storm will develop over the
Pacific and we'll do it all over again as it moves onshore late
Sunday into Monday. This particular storm could be even stronger,
so expect more wind/rain/snow headlines during the coming shifts.

-Spilde

LONG TERM...Monday through Thursday...An active pattern is expected
through mid week. Models continue to show a strong and moist warm
front moving into the area early Monday, lifting northward Monday
evening. A surface low and cold front will approach the Pacific
Northwest coast Tuesday. Models show variability on the timing of
when this front moves inland. Overall, the ECMWF and GEFS support
the low nearing the southern/central Oregon coast Tuesday evening
and night then shifting north along the Oregon coast Wedensday. This
system is expected to bring the potential for strong winds and
additional rain and higher mountain snow into the area Tuesday night
into Wednesday.

On Monday morning, as a warm front moves into the area, expect snow
levels to gradually rise as moderate to locally heavy precipitation
spreads across the area. Snow levels are expected to initially start
out around 4000 feet Monday morning, rising to around 5000 feet
Monday afternoon then to 6000 feet Monday evening. These snow levels
will remain elevated Monday night and Tuesday. Moderate to heavy
precipitation is expected to decrease Monday evening when the warm
front lifts north but showers will continue over the area into
Tuesday. Rain and snow impacts are expected with this warm front on
Monday with moderate to heavy rain likely at the coast and in the
coastal mountains. Also snow accumulations are expected to continue
over the higher mountain passes Monday morning.  Some strong gusty
winds are possible ahead of the warm front Sunday night into early
Monday, mainly in the Shasta Valley, over the mountains, in the
Summer Lake area as well as at the coast.

Behind the warm front, expect showers Monday night into Tuesday
followed by a strong surface low gradually bringing a cold front
into the area. Models have slowed the timing of the cold front and
now forecast it to gradually move inland Tuesday night into
Wednesday as the surface low approaches the coast then moves
northward along the Oregon coast. Moderate to heavy precipitation is
possible, especially across Curry county and into western and
southern Siskiyou county.  There remains variability on the track of
the surface low as well as the timing of the frontal system. How
close the low approaches the southern/central Oregon coast will be a
factor in how strong winds will be with this system late Tuesday
into Wednesday. The current GFS and ECMWF show a surface low moving
inside of 130W and approaching the southern or central Oregon coast
Tuesday night. With this low, models show an increasing surface
pressure gradient as well as strong winds aloft. Models indicate 700
mb winds increasing to between 50 to 80 kts over the area Tuesday
evening into Wednesday and 850 mb winds increasing to 50 to 80 kts
near the coast. This will likely bring strong winds into the area
ahead of the low. The areas with the highest potential for strong
winds are along the coast, over the mountains, in the Shasta Valley
and possibly east of the Cascades in the Summer Lake area. Other
locations, such as the Rogue Valley may also see strong gusty winds
ahead of this system. Will continue to monitor future model runs for
the warm front on Monday and surface low late Tuesday/Wednesday to
refine the forecast.

Showers may continue Wednesday night into Thursday as an upper
trough moves over the area. However, confidence is low in the
details for this time period as there is significant variability in
model forecasts.

-Clarstrom

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...High Wind Watch from late Friday night through late
     Saturday night for ORZ030-031.

     High Wind Watch from Saturday morning through
     Saturday evening for ORZ028.

     High Wind Watch from Saturday morning through Saturday
     evening for ORZ026.

CA...Winter Storm Watch from late Friday night through Saturday
     evening above 3000 feet in the for CAZ080.

     Winter Storm Watch from late Friday night through Saturday
     evening above 3000 feet in the for CAZ082-083.

     High Wind Watch from late Friday night through Saturday evening
     for CAZ081.

Pacific Coastal Waters...Small Craft Advisory until 10 AM PST Sunday for PZZ350-356-376.
     Gale Warning until 4 AM PST Friday for PZZ350-356-370-376.
     Hazardous Seas Warning from 4 AM to 4 PM PST Friday for
     PZZ350-356-370-376.