Hazardous Seas Warning 85, NWS Medford, OR

Area Forecast Discussion
National Weather Service Medford OR
1011 PM PST Thu Dec 29 2016

Updated AVIATION Section

.DISCUSSION...Made a few minor adjustments to sky cover/fog in
the Rogue/Illinois/Umpqua Valleys this evening. A cold front
stretches from western Washington to just north of our coastal
waters this evening. This front will continue to drop southward,
passing through the CWA overnight through Friday. Some very light
precipitation will accompany this system as it swings through the
area. Overall, expect less than a tenth of an inch of liquid
precipitation most areas (many won't see any at all)...with less
than an inch of snow where it does snow, so just slight or
negligible impacts. The upper trough associated with the front
will eventually develop into a closed low as it drops southward
into California Friday night.

All eyes are still focused on the west side valley floor snow
potential Saturday night through Sunday night followed by the
Arctic blast expected next week. 00z NAM and GFS are coming in,
and, at first glance, we do not see any major changes to what the
day shift presented below. One thing that should be noted is that
the impact from Sunday's system could be very large based on the
timing of snow. If roads on the west side are wet initially (due
to unfrozen ground/melting snow), with the cold air arriving, a
layer of ice could form beneath any new-fallen snow by Sunday
evening, creating very hazardous driving conditions. Those
planning any travel through SW Oregon and northern California
Sunday into Monday are urged to keep this in mind and check the
forecasts for the latest updates.

Any snow that does accumulate will stick around for a while too.
Next week will be the coldest air we have felt since the early
December 2013 cold snap, which brought low temperatures in the
single digits in Medford and 20-25 degrees below zero over the
east side. Make preparations now if you or your home/property is
susceptible to extreme cold. The frigid weather will also present
a danger to the homeless, pets and livestock. -Spilde

&&

.AVIATION...30/06Z TAF CYCLE...A cold front will allow MVFR to
develop along the coast overnight, including at KOTH overnight.
Conditions may lower to IFR Friday morning with light rain/drizzle.
Conditions improve back to MVFR after 15z, then VFR after 20z.

VFR at KRBG right now should lower to LIFR overnight (after 08z),
but the cold front will bring a little rain or drizzle around 13-
16z and conditions should become MVFR. Expect VFR to return
Friday afternoon as the front pushes south of the area and a north
breeze kicks in. MVFR/IFR may return there again late Friday evening
or overnight. IFR/LIFR will prevail in the Rogue/Illinois Valleys,
including at KMFR through Friday morning. The cold front will push
through Friday afternoon and conditions should improve following the
front into the evening with northerly breezes developing.

VFR will prevail over the east side, though conditions may lower
briefly to MVFR in any light snow that develops along the front
Friday afternoon and evening. -Spilde

&&

.MARINE...Updated 200 PM PST Thursday 29 December 2016...High west
swell will keep seas elevated and dangerous to small craft for the
remainder of the week. Confidence continues to be moderate to low on
the upcoming high to very high swell event Sunday and Monday with
models continuing to vary run to run, with a peak now around 16 foot
range on Sunday evening. A very cold air mass is still expected to
move into the region Sunday into Monday, with snow possible down to
the beach Monday morning. /FB

&&

.PREV DISCUSSION... /Issued 341 PM PST Thu Dec 29 2016/

SHORT TERM... Through Sunday night...
High pressure has created strong inversion conditions across the
forecast area. Temperatures above the valley floors this morning
were 20 degrees cooler than they were two thousand feet higher.
This has low clouds and fog in the Illinois, Upper Rogue, and
Scott Valleys, while most other areas have cleared out resulting
in a mild afternoon. The inversion will persist tonight into early
Friday before dissipating with the arrival of a fairly dry cold
front Friday afternoon.

The cold front on Friday does not have a lot of moisture with it,
but does have a substantial cold air with it. While precipitation
amounts with this front will be a tenth of an inch of water or
less with this front and less than an inch of snow where it does
snow, snow levels will lower to be near 2000 feet by the time
precipitation ends Friday into Friday night. Models are well
agreed with sweeping this front through and faster by sunrise
Saturday morning, so temperatures have been adjusted downward
accordingly. This will sweep below freezing temperatures to all
areas except the immediate coast for Saturday morning.

A lull in the weather will occur on Saturday, and then an at first
shallow, but increasingly deeper moisture layer will arrive with a
cold front arriving from the northwest. This front will encounter
cold air already in place across much of the area Saturday night
into Sunday, with snow levels beginning near 1500 feet. Model
thicknesses 850mb temperatures and 1000-850mb thicknesses are
unanimous in indicating that precipitation will very likely begin
as snow even at the Medford Airport Sunday morning. West side snow
levels will lower to 1000 feet by Sunday afternoon as very cold
air, to the tune of -16C or colder at 700mb settles in, and then
to at or very near the beaches Sunday night through Monday
morning. The biggest question is how much the snow will accumulate
at elevations where the ground is not frozen and surface
temperatures go above freezing on Sunday. Forecast snowfall
amounts have been adjusted to account for this as much as
possible. That said, we expect to issue winter weather advisories
for most of the West Side and mountains for Sunday. A winter storm
watch has been issued for Eastern Douglas County where snow
amounts will reach warning levels. For the Rogue Valley current
estimates are for 1 to 4 inches for the town centers Sunday into
Monday, with more above 2kft in some areas. Please see point-and-
click forecasts on the website for further information.

The very cold, possibly daily record breaking temperatures due in
behind the snowfall next week mean that the snow related slippery
impacts are likely to linger for most of us through next week.
BTL

LONG TERM...Monday Through Thursday Night...
The Rex block expected to be in place over the Eastern Pacific
will be the primary cause if uncertainty during the extended term,
mainly as a result of the models' struggles predicting the
behavior of this type of feature. Despite this, the GFS and ECMWF
are in fairly good agreement, both depicting a very cold pattern
for our area.

For much of the long term, southern Oregon and Northern California
will remain under north flow aloft, but with high pressure and east
winds at the surface. With snow likely to be on the ground over a
significant area, and arctic air moving in, the coldest temperatures
of the season are forecast through at least midweek. Models are
producing midlevel temperatures for the first part of the week that
are similar to those recorded during the cold outbreak of early
December 2013. As a result, it appears very likely that maximum
temperatures will barely make it to freezing anywhere but the
immediate coast by Wednesday, and low temperatures are expected to
drop into the single digits and teens on the West Side and well
below zero on the East Side. These are close to or below daily
records, just like in 2013, and those who are, or have property,
susceptible to extreme cold should plan and prepare now.

Also during this time, a thermal trough is expected to develop to
our south. Normally a major feature during the summer, the large
temperature difference between here and central California is
expected to produce a similar wind pattern, that of easterly, or
offshore, flow. Normally, this would produce some downslope warming
to the east of the Cascades, but with the cold air in place, the
winds will most likely just flow over the top of the valley
inversions, and little downsloping will result. Whether or not the
valleys will fog in remains to be seen, but given the lower
humidities and the very cold temperatures, frost may occur other
than fog, keeping the skies clear and producing even lower
temperatures overnight.

Models are showing another system approaching the area late
Thursday, but there are some timing differences, mainly a result of
the aforementioned upstream block. With the cold air in place, and
another front passing there, there is the likelihood of low elevation
snow by the end of the week. Given the current pattern, the models
are likely to push back the arrival of this front over the next
several days, so have kept mention of precipitation out of the
forecast for the current cycle. -BPN

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Air Stagnation Advisory until 3 PM PST Friday for
     ORZ024-026-029-030.
     Winter Storm Watch from Saturday evening through Sunday evening
     for ORZ025.

CA...None.

Pacific Coastal Waters...Small Craft Advisory for hazardous seas
until
     4 PM PST Saturday for PZZ350-356-370-376.

MAS/BTL/BPN/FJB