Area Forecast Discussion
Issued by NWS Medford, OR

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FXUS66 KMFR 192236
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
236 PM PST Mon Feb 19 2018

.DISCUSSION...An upper ridge axis centered about 150W is
maintaining deep northerly flow over the forecast area
today...and will do so for much of the week. Today, afternoon
highs will likely register 15 to 20 degrees lower than normal.
Tonight`s temperature forecast, although seemingly straightforward
a few days ago, has become increasingly challenging. We are
basically forecasting a mostly clear sky scenario, so temperatures
are forecast to be lower than the suite of guidance shows. Our
reasoning is that we anticipate most low clouds to clear east of
the Cascades as a drier air mass aloft moves in, and we think most
of the high clouds should stay west of the Cascades. Additionally,
some snowpack will definitely nudge temperatures down quickly
tonight on the East Side. For the West Side areas, we`ve nudged
temperatures up a bit based on expected high clouds and some
lingering low clouds. Still, overnight temperatures are forecast
to reach near-daily record values. At the coast, we have upgraded
the Freeze Watch to a Warning tonight. Even though some high
clouds are expected there, the sheer coldness of the air mass
should allow for 32 degrees in many coastal locations.

Operational models have come into better overall agreement for a
surface low pressure to move down the coast Tuesday afternoon,
skirting the coast with precipitation. Given some warming and that
this precipitation will occur during the daytime, we don`t expect
coastal snow with this system. Then, a cold front pushes into the
forecast area from north to south Tuesday night into Wednesday
morning. Models have varied significantly just over the past 24
hours with the amount of precipitation with this system. There is
agreement that any snow impacts from this should occur mostly from
the Umpqua Divide northward, from about Sexton Pass north to Rice
Hill along I-5. A quick few inches of snow is possible, and since
it`s occurring at night, roads could become slippery. We are
awaiting a bit more consistency before issuing a Winter Weather
Advisory for this system. In addition to precipitation, there`s some
uncertainty how low snow levels will be Tuesday night. Right now we
are forecasting them to be about 500 ft, so just above Roseburg, but
Rice Hill and passes like Canyon Mtn Pass should be affected with
snow. Precipitation drops off significantly as the front pushes
south of the Umpqua Divide on Wednesday, and Medford could see a
half inch of snow. As was experienced on the previous storm, low
elevation snow impacts are not expected during the day, mainly due
to the late February sun angle and warming road surface
temperatures. Even though the bulk of precipitation diminishes
Wednesday morning, shortwave energy continues to ride north to south
on the eastern periphery of the upper ridge, and this brings
additional chances for light snow Wednesday into Thursday morning.

.LONG TERM...(Thursday through Monday)

A deep trough remains over our area on Thursday with cold northerly
flow aloft. Both the GFS and the ECMWF bring in some potent short
wave troughs and with low snow levels snow showers are possible
again across the west side valleys. Snow fall amounts will be light
in the valleys but upslope areas from northerly flow could see up to
an inch. Depending on how much clearing, Thursday night and Friday
morning could be quite cold with single digits to lower teens east
of the Cascades, and lower to mid 20s for the western valleys.

On Friday, the trough is expected to shift east into the Four
Corners with weak ridging. However flow aloft in our area continues
out of the NW with scattered upslope snow showers. Friday night
temperatures will moderate a bit with mid 20s to lower 30s for west
side valleys and mid to upper teens to the east. On Saturday both
the GFS and ECMWF drive a cold front through NW Oregon with
measurable precipitation for the Umpqua Basin and the coast, with
scattered showers elsewhere. Due to timing difference, confidence is
low to moderate on snowfall amount. If a system comes through
Saturday night, there will be a better chance for accumulating snow,
especially for low elevations.

Beyond Saturday night, models diverge wildly on timing of individual
front, but both the GFS and ECMWF suggest we will remain under
cloudy sky and showery pattern. -FB

&&

.AVIATION...19/18Z TAF CYCLE...Fog and some low level stratus are
impacting the smaller valleys in our region this morning near TAF
site RBG.  Those are burning off fairly quickly and we will see VFR
conditions prevail for today. Some terrain obscurations are expected
today with CIGS around 5000 feet. LIFR conditions are expected to
build back into the Umpqua valley tonight as the skies clear.

-Smith

&&

.MARINE...Updated 300 PM PST Monday 19 Feb 2018...Steep
seas will subside Monday evening as the low pressure system tracks
farther east.  A fresh swell will move into the area Monday night
through the next few days as the weather pattern remains active.
This may result in isolated areas of steep seas and seas that may be
hazardous to smaller crafts for the next few days.

Eventually, a stronger storm system is expected to build in the Gulf
of Alaska.  High steep swell will build and move into the waters
Saturday night into Sunday.

-Smith

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Freeze Warning from midnight tonight to 9 AM PST Tuesday for
     ORZ021-022.

CA...None.

Pacific Coastal Waters...Hazardous Seas Warning until 7 PM PST this evening for
     PZZ350-356-370-376.

$$

NSK/FJB/CZS


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