Area Forecast Discussion
Issued by NWS Medford, OR

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247
FXUS66 KMFR 051231
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
430 AM PST Mon Dec 5 2016

.Updated aviation discussion.

.Short term...The focus of this shift has been on a cold system
dropping south along the PacNW coast this morning which will bring
rain and snow to our area later today through tomorrow morning.
Precipitation has ended this morning, but this break will be short
as all models show precipitation breaking out over the mountains
first late this morning, then over the rest of the area through
the afternoon. Snow levels are already down to around 2000 feet,
and while they will tick up a bit this afternoon, they will come
right back down to many valley floors tonight just as
precipitation intensifies. Aided by strong upper jet support and
low level orographics, snowfall along the west slopes of the
mountains will be heavy, particularly above 2000 feet. The 4km WRF
shows a peak of up to 18 inches in the Cascades between Lake of
the Woods and Crater Lake, and our current forecast is in line
with this guidance.

Snow will also fall over lower elevations, probably down to as
low as 1000 feet, later tonight and Tuesday morning. While
snowflakes will probably be seen in the inland west side cities
tonight and Tuesday morning, accumulations are expected to be
isolated and minor due to warm surfaces. Snowfall and associated
impacts will be more significant just outside the cities and over
higher elevations. A variety of winter storm warnings and
advisories have been issued for this afternoon through Tuesday
morning. See PDXWSWMFR for details.

This clears out later Tuesday, and this will be followed by very
cold weather area wide. Low temperatures Tuesday night will be in
the single digits on the east side to teens and low 20s on the
west side. Much of the coast may also see a freeze.

Models then show a warm front pushing across our area later
Wednesday and Wednesday night. With cold air in place this could
be an interesting scenario, particularly for the east side. This
looks like a potential significant overrunning snowstorm for the
east side with some freezing rain or sleet mixed in at the end as
warm air takes control. This will be looked at by future shifts,
but be aware of the potential for significant winter weather on
the east side mid week. -Wright

.Long Term...Thursday through Saturday Night...Despite some timing
differences of about 12 hours or so among them, the GFS and ECWMF,
as well as their respective ensemble members, are are still showing
a significant system arriving by Thursday morning. The 12Z
deterministic GFS has trended farther south with the best moisture
and dynamics, but this appears to be the outlier among its ensemble
members. Have therefore leaned heavily on the ECMWF, which was more
in line with the GEFS and its own ensembles. There has also been a
slight increase in the strength of the warm front, which has
resulted in higher snow levels at the start of the system, but has
also given the area a slightly better scenario for light freezing
rain to fall along the western foothills of the Cascades, the
valleys of Siskiyou county, and the valleys of the East Side.
Confidence in this scenario is low, and much will depend on timing
and temperatures swings of only a degree or two, so as of now there
is only a wide ranging period of slight chances as the warm front
passes overhead Wednesday night into Thursday morning. Despite the
uncertainty and low-end chances, those traveling along roads in the
areas of concern should keep an eye on the forecast for the latest
updates, as only a very light amount of ice can make for very
hazardous conditions.

Once the warm front passes, the next set of major concerns arrives
with the cold front Thursday afternoon into Friday. Winds do not
appear to be overly strong, but will likely approach Warning and
Advisory criteria for many areas Thursday. Meanwhile, moisture
transport with this system has maintained a much higher than normal
strength over the past several days, so heavy precipitation is also
likely. Snow levels during the height of the precipitation should be
between 4000 and 5000 feet, and the result could be heavy snow along
many area passes, as well as significant mountain snow. Due to local
effects, snow levels in the Mount Shasta City area may be even
lower. Again, confidence is low but improving in the timing of the
system, but confidence is much higher than normal for this far out
in regards to the likelihood of heavy rain and snow, given the
consistent signature in nearly all model suites.

Snow levels will fall quickly behind the system to around 3000 feet,
with post-frontal showers continuing into Saturday. Beyond this,
moist, nearly zonal onshore flow is expected to continue through the
remainder of the forecast term. A few shortwaves within this flow
could easily traverse the area anytime while the pattern persists,
but confidence in the timing and strength of any single system is
very low, so have maintained at least a slight chance of
precipitation through the weekend. -BPN

&&

.AVIATION...05/12Z TAF cycle...Valleys west of the
Cascades...MVFR cigs with areas of IFR to LIFR lower clouds and fog
continue through the early morning and then should lift to VFR later
morning or early afternoon.  MVFR conditions will return as a cold
front brings precipitation to the area late this afternoon and
evening. This front will bring widespread mountain obscuration with
it.  At the coast periods of MVFR conditions are expected before the
cold front and precipitation brings overall lower ceilings late this
morning into early afternoon. East Side areas should remain mainly
VFR, with the potential for morning patchy freezing fog at locations
like Klamath Falls. The front may bring lower ceilings and mountain
obscuration overnight and Tuesday morning.

Very low freezing levels means there is a chance for snow all the
way to valley floors late tonight and Tuesday morning, lowering
visibilities and making for slick runways in heavier showers. SK

&&

.MARINE...Updated 330 AM PST Monday 5 December 2016...Seas will
remain steep through Tuesday evening. A weak front will move
onshore today...and unsettled showery weather will persist this
afternoon into Tuesday. There may be a brief period of small craft
north winds beyond 30 NM off the coast Tuesday. We have elected to
keep the advisory for seas only for clarity while mentioning the
winds in the MWW text. High pressure will briefly build
Wednesday...then a stronger frontal system will move through
Wednesday or Wednesday night into Thursday. Gale force winds are
possible with this front, and an active weather pattern is likely
to continue Friday into Saturday, although confidence is low on
wind/seas details at this time due to model differences. SK

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory from 4 PM this afternoon to 10 AM PST
     Tuesday for ORZ029-030.
     Winter Weather Advisory from 4 PM this afternoon to 10 AM PST
     Tuesday for ORZ021-023-024.
     Winter Storm Warning from 4 PM this afternoon to 10 AM PST
     Tuesday for ORZ021-023-024.
     Winter Storm Warning from 10 AM this morning to 10 AM PST
     Tuesday for ORZ027-028.
     Winter Weather Advisory from 10 AM this morning to 10 AM PST
     Tuesday for ORZ025.
     Winter Storm Warning from 10 AM this morning to 10 AM PST
     Tuesday for ORZ025.
     Winter Weather Advisory from 10 AM this morning to 10 AM PST
     Tuesday for ORZ027-028.
     Winter Weather Advisory from 10 PM this evening to 10 AM PST
     Tuesday for ORZ026.
     Winter Storm Warning from 10 PM this evening to 10 AM PST
     Tuesday for ORZ026.

CA...Winter Weather Advisory from 10 PM this evening to 10 AM PST
     Tuesday for CAZ080.

Pacific Coastal Waters...Small Craft Advisory for hazardous seas
     until 10 PM PST Tuesday for PZZ350-356-370-376.

$$



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