Area Forecast Discussion
Issued by NWS Medford, OR

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FXUS66 KMFR 182221
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
221 PM PST Sun Feb 18 2018

.DISCUSSION...Today has been a mixed bag of weather across the
forecast area, from partly to mostly sunny skies in Medford and a
high temperature of 42...to frequent snow showers and a high of
33 degrees at Camas Mountain (1450 feet) on Highway 42 towards the
coast. Most of the Cascades has seen snowfall all day. It`s tough
to gauge the snowfall amounts in the mountains thus far, as
SNOTEL machines typically have a hard time reporting accurate
reports during heavy snowfall, so we`ll have to wait until things
calm down a bit before gauging new snowfall.

The battle between mid February sun angle and cold air
advection will be won by the cold air this evening as the sun sets,
and all areas along and west of the Cascades will have a shot at
getting at least a coating of snow...all the way to the coast.
Models suggest scattered snow showers through about 10pm when
showers start diminishing from north to south across the forecast
area. So the Roseburg and Camas Mountain areas should see most snow
showers end by about midnight. The Rogue Valley and places southward
should see precipitation diminish early Monday morning. We`ve left
the Winter Storm Warning out for areas mostly above 2000 feet west
of the Cascades...and some lower elevation areas like Camas Mountain
on Highway 42 where we expect more accumulation. Winter Weather
Advisories continue for other areas along and West of the Cascades
through 10am PDT. Though the most significant snow should end by
4am PDT or earlier (again, ending from north to south through the
night), we`ve decided to keep the Advisories going through 10am
because folks will likely wake up to some snowy/icy roads in the
early morning, and this should only improve once some morning
warming occurs.

Any roadways that see some melting snow on them today, or slush,
will quickly freeze tonight as temperatures lower to the teens East
Side and 20s West Side. High temperatures tomorrow will struggle to
get out of the 30s west of the Cascades and 20s East Side, and then
tomorrow night, the coldest temperatures of the year are expected.
Teens are expected west of the Cascades and near-zero to below-zero
overnight temperatures are forecast. These represent daily record
low temperatures for places like Klamath Falls and Alturas, and we
are forecasting 18 degrees in Medford, which will tie the previous
record.

Tuesday afternoon through Sunday night...Models agree on low
pressure sliding south along the coast, and keeping the bulk of
the precipitation over the waters through Tuesday afternoon.
However, there are differences between the models in how the
offshore low interacts with the shortwave trough digging southward
east of the Cascades. The EC does interact with the shortwave and
thus, generates showers over the area, north of the California
border, early Wednesday morning. Snow levels will continue to be
low during this time frame and if the EC does verify, low
elevation snow will be possible. Both the GFS and NAM don`t
interact with the other shortwave and keep conditions dry through
Wednesday afternoon. It`s tempting to buy into those models given
the agreement between them, but the EC has been consistent with
it`s solution for multiple runs now so it`s difficult to discredit
that. Have left part of the EC model in the current forecast
resulting in chance to slight chance PoPs along the coast and for
most locations north of Lake of the Woods. With this system moving
by offshore, cloud cover will be increased, and this will keep
Wednesday morning temperatures relatively warmer. So instead of
teens for western valleys, temperatures will be in the mid
twenties. And instead of single digits to below zero values east
of the Cascades, temperatures will be in the low teens.

Timing and position differences continue with the next system expected
Thursday afternoon. The GFS is faster and further east with the next
low and would have precipitation in the area by late Wednesday
night. The EC on the other hand digs the energy offshore, bringing
it into central California with the bulk of the precipitation
arriving Thursday morning and afternoon. Although the finer details
are unclear at this time, snow levels will still be low enough to
for low elevation snow concerns.

The parade continues this upcoming weekend with another system
digging south along the coast Saturday afternoon. Again the GFS is
faster with the precipitation with this system, but overall there is
better agreement with the position of this system. Like the Thursday
system, snow levels look to be similar bringing another round of low
elevation snow concerns.

Timing and position differences in the long term make for low
confidence in details of each system, but it does look like there
will be multiple chances for low elevation snow, or at the very
least lower elevation pass impacts due to wintry weather. The simple
difference of the timing of each system will have big impacts on the
resultant weather. If a system comes through at night, there will be
a better chance for accumulating snow, especially for low
elevations. On the other hand, if the bulk of the precipitation
moves through during the day, the relatively higher February sun
angle will limit accumulations due to day time heating and warmer
road surfaces. Stay tuned as details become clear. /BR-y

&&

.AVIATION...18/18Z TAF CYCLE...An arctic front is bringing strong
northwest winds which will continue through today, then become
northerly and diminish tonight. LLWS will be a concern at KRBG and
KMFR until the strong winds surface. West of the Cascades...Cigs
will initially be VFR with areas of MVFR cigs/vsbys in showers with
higher terrain obscured. However, as snow levels drop, areas of IFR
cigs/vsbys in snow showers will develop. The lower conditions will
then persist through Sunday night. Over the Cascades and
Siskiyous...IFR cigs/vsbys in snow showers will prevail through
Sunday night with terrain obscured.  East of the Cascades...IFR
cigs/vsbys near snow showers, but VFR conditions will prevail away
from the showers.

&&

.MARINE...Updated 950 AM PST Sunday 18 Feb 2018...Strong low
pressure has moved onshore north of the area, causing northwest
winds to increase to near gale force and seas to become very steep
and hazardous. Total wave heights will peak at 20 to 25 feet late
Sunday night into Monday morning.  These seas will be a combination
of fresh swell and wind wave with a period of 10 seconds increasing
to 14 seconds Sunday night, resulting in very steep seas. A High
Surf Advisory has been issued for the conditions. Winds will
actually reach gale force over small portions of the the outer
waters through this evening, but they will not be widespread enough
for a gale warning, and the seas will constitute the larger impact
anyway. Seas will gradually subside Monday afternoon and night,
followed by relatively calm conditions Tuesday through Wednesday.
Another area of low pressure will bring another round of small craft
advisory winds and seas Wednesday night into Thursday.

The incoming system will bring very cold air to the waters Sunday
night through Tuesday. Snow is possible all the way to sea level
Sunday and Sunday night. Sub-freezing temperatures are possible
over the waters Sunday night and Monday night, especially near the
coast.

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory until 10 PM PST this evening for
     ORZ029-030.
     Winter Weather Advisory until 4 AM PST Monday for ORZ027-028.
     Freeze Watch from late Monday night through Tuesday morning for
     ORZ021-022.
     Freeze Warning from 1 AM to 9 AM PST Monday for ORZ021-022.
     Winter Weather Advisory until 7 AM PST Monday for ORZ021-022.
     High Surf Advisory until 4 AM PST Monday for ORZ021-022.
     Winter Storm Warning until 4 AM PST Monday for ORZ023>026.
     Winter Weather Advisory until 4 AM PST Monday for
     ORZ023-024-026.

CA...Winter Weather Advisory until 4 AM PST Monday for CAZ080.
     Winter Weather Advisory until 4 AM PST Monday for
     CAZ081-083>085.

Pacific Coastal Waters...Hazardous Seas Warning until 4 PM PST Monday for
     PZZ350-356-370-376.

$$

NSK/MNF/SBN



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