Winter Storm Watch 2, NWS Medford, OR

Area Forecast Discussion
National Weather Service Medford OR
309 AM PST Fri Jan 6 2017

.DISCUSSION...Very cold, arctic air remains entrenched over our
area this morning. It's downright cold in Medford with a current
temperature of 10, and today's low temperature record of 9 (from
1937) will probably be reached. I'd call it outrageously cold
over parts of the east side. Currently, an Agrimet station in
Lorella, OR is showing 27 below zero! As for official records,
Klamath Falls is sitting at -19F, and that is way below their
record for today, which was -9 from 1977.

Aside from some freezing fog and stratus in the valleys, today
will be mostly sunny and cold with high temperatures at or below
freezing over much of the area save for the coast and Umpqua
basin. High clouds will be increasing this evening, but with snow
cover over much of the area, we expect to radiate well this
evening. That will set the stage for a wintry mix of precipitation
as a warm front comes in later tonight and Saturday morning.

Low pressure sits out between 140 and 150W, and a moisture plume
originating near the Hawaiian Islands is being drawn north ahead
of the system toward us. This moisture will overrun the very cold
air in place. Model soundings are more or less unanimous in
keeping the profile below freezing for all areas inland from the
coast through mid morning Saturday. Models are also faster in
bringing precipitation in, with all showing a tenth to a quarter
of an inch of precipitation across the west side by dawn, which
will likely start as snow tonight and then transition to freezing
rain and rain Saturday. While snow amounts are not expected to be
excessive, the combination of new snow, freezing rain, and rain on
already very cold ground will make for treacherous travel
conditions late tonight through Saturday morning. Ensemble
temperature plume diagrams and metro road forecasts lend
confidence to this idea, as both show surface conditions getting
well below freezing tonight and remaining there through early
Saturday.

Winds will eventually scour the cold air out of the valleys, and
it will be all rain from then on, but until then it's going to be
messy. Typically, the narrower valleys such as the Illinois and
Evans and Grants Pass areas hold onto cold air the longest, and
this is where the greatest potential for freezing rain exists. The
Rogue valley often scours quickly, although as mentioned,
soundings do hold on to cold air here well into Saturday morning.
While this looks messy, it should be stressed that it WILL change
to rain Saturday afternoon, and this is NOT going to be a repeat
of Tuesday's massive snowstorm.

We do expect to see a lot of snow in the Mt. Shasta area before it
turns to rain Saturday night. The east side will also see a good
shot of snow Saturday followed by the potential for freezing rain
Saturday night.

All of this will be covered by a winter storm watch. We've also
issued a winter weather advisory for the Cascades where a steady
but not unusually heavy snow is expected from this system. There
may also be a bit of freezing rain in the mountains, but we don't
expect too much trapped cold air as snow levels rise rapidly
tonight. See PDXWSWMFR for details.

The focus this morning was on the wintry weather through Saturday,
and the rest of the forecast hasn't been changed. The applicable
portion of the previous discussion follows for reference. -Wright

.PREV DISCUSSION... /Issued 242 PM PST Thu Jan 5 2017/

Snow levels will be coming up Saturday night with heavy
precipitation moving into southwest Oregon and Northern California
late Saturday night through Sunday night. It will be at this time
where snow levels will jump up to around 7500 to 8000 feet.
Therefore, the combination of heavy rain and snowmelt, especially
in the lower elevations could bring a higher chance for flooding.
See ESFMFR for more details. -Petrucelli

Monday through Thursday...Snow levels lower and precipitation
becomes showery on Monday as cold air aloft moves in.  This does not
mean heavy rain (or mountain heavy snow) is still not possible, but
it will likely not be a steady, heavy rain. Snow levels on Monday
will lower to around 3500 feet, so expect snow impacts along I-5
from Siskiyou Summit southward, over the Cascades passes, and east
of the Cascades.

Attention then shifts to another atmospheric river event for
Tuesday. GEFS and GFS Integrated Vapor Transport (IVT) tools show a
broad swath of deep moisture pointed at northern California and
Southern Oregon. The European model shows a very similar solution,
so this increases our confidence in heavy precipitation and snow
levels bumping back up to around 6000 feet. With soils expected to
be saturated from previous rain and melted snow, Tuesday's heavy
rain will bring sharp river rises and the potential for urban and
small stream flooding.

Lowering snow levels occur on Wednesday behind the front before yet
another low pressure system ushers in moderate to heavy
precipitation on Thursday.

Even though we can't point to any specific main stem rivers that we
expect to flood, precautions should be taken now to protect areas
susceptible to urban or small stream flooding. It's unknown exactly
how much snow melt will exacerbate any potential for flooding, but
it definitely will result in saturated soils, leading to more runoff
and a higher potential for flooding. FJB

&&

.AVIATION...06/06Z TAF CYCLE...Widespread VFR will prevail
overnight through Friday evening. Patchy valley fog and low clouds
will result in IFR/LIFR in some of the valleys west of the
Cascades/Siskiyou County and also in the Klamath Basin. This
will likely impact the terminal at KRBG and perhaps also at
KMFR and KRBG. -Spilde

&&

.MARINE...Updated 230 PM PST Thursday 5 January 2017...Weak high
pressure will build into the area this evening with both winds and
seas remaining light. A front will move across the waters Saturday
afternoon and evening. This system will bring at least small craft
advisory level south winds to the area and gales are possible.
Several models solutions indicate a brief period of gales are
possible over the waters Saturday afternoon and evening. Behind the
front...additional strong fronts and building steep to very steep
seas are possible Sunday through Tuesday. By Tuesday, very dangerous
seas are possible. The combination of a strong front bringing strong
south winds on Tuesday and building mixed seas from the southwest
and west may bring combined seas to 15 to 18 feet with a dominant
period of around 10 seconds. Confidence is low to moderate on the
details due to model variability. Will need to monitor this forecast
with future model runs. -CC

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Storm Watch from this evening through late Saturday night
     for ORZ029>031.
     Winter Weather Advisory from 10 PM this evening to 10 PM PST
     Saturday for ORZ025-027-028.
     Freeze Warning until 10 AM PST this morning for ORZ021-022.
     Winter Storm Watch from this evening through Saturday morning
     for ORZ024-026.
     Winter Storm Watch from this evening through Saturday morning
     for ORZ023.

CA...Winter Storm Watch from this evening through late Saturday night
     for CAZ084-085.
     Winter Storm Watch from this evening through Saturday afternoon
     for CAZ080.
     Winter Storm Watch from this evening through Saturday evening
     for CAZ082-083.
     Winter Storm Watch from this evening through Saturday afternoon
     for CAZ081.

Pacific Coastal Waters...Small Craft Advisory from 7 PM this evening to
     4 PM PST Sunday for PZZ350-356-370-376.
     Hazardous Seas Watch from late tonight through Sunday afternoon
     for PZZ370-376.