Area Forecast Discussion
Issued by NWS Medford, OR

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FXUS66 KMFR 211243
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
443 AM PST Wed Feb 21 2018

.SHORT TERM...The forecast for this morning is generally on
track. A few tweaks were made, but no major changes for today. A
band of precipitation associated with an upper level disturbance
is moving through the Umpqua Basin right now. Roseburg has been
right on the edge of snow for several hours now, and it has
changed to snow at times, but they`re just a couple of degrees too
warm for any significant accumulations in the city. The
surrounding hills have no doubt been at least dusted with snow.
The heaviest part of the band is now moving into the Umpqua
Divide, so rain/snow will gradually taper off in Roseburg the rest
of the morning.

It has been snowing hard at Canyon Mountain Pass, and although
it`s hard to see the road surface, we`ve seen plowing going on up
there on the cameras, so it is definitely accumulating. This band
continues to slide south, so passes further south will start to be
affected, if they`re not already. A winter weather advisory
remains in effect for all these locations, and that looks good.
Although snow should be diminishing in the north, we`ve still got
a while until sunrise, so further cooling will probably allow some
places that got mostly rain to ice up. We`ll keep the advisory
out just to keep awareness up.

It remains to be seen what happens to this band when it gets over
the divide and down into the Rogue and Illinois Valleys. Since
yesterday, models have been showing this band weakening as it gets
past the divide, and they continue to do so. As we`ve been saying,
we`ll likely see snow falling around Grants Pass and Medford by
sunrise, but accumulations should be light. That still seems like
the most likely scenario, but we`re watching closely. It has also
been snowing hard in the Cascades and will probably snow in the
Siskiyous as well. But little of this snow is expected to survive
beyond the mountains, so the east side and California shouldn`t
see much. This will all gradually diminish and clear out this
afternoon.

One lingering effect from today`s system will be to bring a
slightly cooler airmass into our region, and this sets up another
snowfall potential tonight into Thursday. Models generally show
the next system to be wetter than the current one, and given that
it will be slightly cooler as well, snow impacts may be more
widespread. To avoid confusion with the current advisory, we
won`t be issuing any headlines for tonight just yet. But if we
did, it would likely be much of the same area, but would probably
also include the Rogue and Illinois Valleys as well. Pops and QPF
have been raised for late tonight and Thursday morning on the
west side and in the mountains. -Wright

LONG TERM...The long-term will likely feature below normal
temperatures and multiple rounds of precipitation. For Friday
and Saturday, deep northerly flow should keep moisture limited
with snow levels generally between 1000 and 2000 feet. To start
off, shortwave ridging will bring mostly dry conditions to the
area on Friday. Then, weak shortwave energy moves through Saturday
morning and could bring around an inch of snow to West Side
valleys, then a few more inches to the mountains later Saturday as
snow levels rise.

Deep flow shifts westerly on Sunday ahead of a frontal system that
should bring significant precipitation to the forecast area,
likely lasting from Sunday through Tuesday. Snow levels late
Sunday, just ahead of the front, will be around 2500 feet, and the
bulk of precipitation is expected Sunday night through Monday.
However, additional energy will move southward along the Pacific
Northwest coast and could bring additional precipitation with
lowering snow levels.

In short, the extended features below normal snow levels and light
to moderate precipitation (with the exception of Sunday
night/Monday morning that could feature some heavy
precipitation)...resulting in winter weather impacts to at least
mountain passes and possibly valley floors west of the Cascades.

-Keene

&&

.AVIATION...21/12Z TAF CYCLE...Areas of MVFR cigs
will persist along the coast, and periodically over the west side in
showers, through midmorning. The Umpqua basin will see a general
break from precipitation, though a shower or two is possible.
The Rogue Valley probably won`t get more than a dusting of snow, but
expect terrain obscuration and lower conditions at times. KLMT will
likely be shadowed by the Cascades, and the main impact of any snow
there will be areas of lower cigs and terrain obscuration in the
Cascades. Most of the area will clear to VFR by this afternoon,
but lower conditions may persist over the Cascades and Siskiyous
through the day Wednesday.

&&

MARINE...Updated 400 AM PST Tuesday 20 Feb 2018...Low pressure is
moving southward along the coast. Seas will be steep at times
due to wind waves and fresh swell. Another low will move onshore
from the north/northwest late tonight. North winds will increase
over the waters behind the low Thursday. Winds and seas will be
relatively light Thursday into Friday. A weak front will move
onshore Friday. A stronger front will move onshore Sunday, and
there may be heavy northwest swell as well.

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory until 10 AM PST this morning for
     ORZ023-024-026.

CA...None.

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

$$

TRW/MSM


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