Area Forecast Discussion
Issued by NWS Medford, OR

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885
FXUS66 KMFR 201709
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
909 AM PST Tue Feb 20 2018

.DISCUSSION...

A pretty nice cloud deck is currently moving over our region and
there appears to be very few breaks between the clouds. Decided to
cool off daytime highs down to 40 in the west side valleys even
with most of the guidance showing temperatures in the mid 40`s.

We`ll focus on winter weather impacts Wednesday and Thursday for
the upcoming forecast cycle. Have pretty high confidence in the
winter weather advisory for Wednesday. See previous discussion
below for more information beyond tonight.

-Smith

&&

.AVIATION...20/06Z TAF CYCLE...VFR conditions will continue for
south of the Umpqua Divide and east of the Cascades, though mid
level clouds will bring partial terrain obscuration. A low
developing along the coast will bring a chance for more
precipitation along the coast and in the Umpqua Basin, bringing
periods of MVFR or even IFR cigs this afternoon. Cold conditions in
the Umpqua Basin will bring another opportunity for light snow.

-Cohen

&&

.MARINE...Updated 500 AM PST Tuesday 20 Feb 2018...Seas are
gradually diminishing and will continue lowering over the next few
hours. Low pressure will move south along the outer waters this
afternoon and evening. Seas are expected to be steep at times as
fresh swell from this low moves in from the west. Offshore high
pressure and a trough near the coast will wet up on Wednesday.
Another low will move south Wednesday night into Thursday...causing
winds to ramp up in the outer waters Thursday.

-Cohen

&&

.PREV DISCUSSION... /Issued 512 AM PST Tue Feb 20 2018/

SHORT TERM...The focus this morning has been on the potential for
snow tonight into Wednesday morning as a low pressure system
slides down the coast. More on that in a moment.

We have seen some light snow at times along and north of the
Umpqua Divide as moist, northwest flow is squeezed over the
mountains. Radar is overshooting all of this, but satellite shows
the associated cloud features well. There is currently in a clear
area on satellite over the Umpqua, but there are more clouds
upstream, so we will probably see more flurries around. These
won`t amount to much, but motorists will see the snow skittering
around on the roadways at times. We`ve even seen a few snowflakes
around Medford, but these will amount to even less.

For the remainder of today, a low pressure system will move south
off the coast and bring rain to the waters and probably the
immediate coastline. There will be an increasing chance for a mix
of rain and snow in the west side valleys this afternoon, but
most if not all the precipitation will stay offshore.

A trailing shortwave aloft will move in behind the low pressure
late this evening and tonight. Models consistently show a
deformation band of precipitation dropping south with the
shortwave. Snow levels will be fairly low again tonight - around
500 feet - and model soundings show an all snow profile over most
inland west side locations. It`s coming at night, so it looks like
it will be mostly snow inland from the coast. The main question
is snowfall amount. Deterministic, MOS, and ensemble guidance all
indicate at least an inch of snow at Roseburg with higher amounts
in the surrounding mountains, including the I-5 passes between
there and Grants Pass. Confidence is high enough that we`ll see
some accumulating snow in these areas tonight to issue a winter
weather advisory, and one will be sent soon.

Further south, we`ll probably also see at least light snow
develop by tomorrow morning in Jackson and Joesphine counties.
While all the guidance also shows an inch or so of snow in Medford
and GP, the deformation band is shown to weaken as it moves south
of the Umpqua Divide. I don`t have as much confidence in snow
amounts further south, so they`ve been left out of the advisory
for now. Snow is still in the forecast, but later shifts will re-
evaluate the need for headlines further south. There will be a
break with just light flurries or showers around later Wednesday
as we reload and wait for the next disturbance.

-Wright

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.

-Bunnag

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory from 10 PM this evening to 10 AM PST
     Wednesday for ORZ023-024-026.

CA...None.

Pacific Coastal Waters...None.

$$

Smith/Wright/Bunnag/Cohen



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