Area Forecast Discussion
Issued by NWS Medford, OR

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FXUS66 KMFR 201633

Area Forecast Discussion
National Weather Service Medford OR
933 AM PDT Tue Mar 20 2018

.UPDATE...A deformation zone remains oriented across the area,
supporting mid and upper level cloud development, primarily in the
southern portion of our CWA. We`re seeing some radar echoes in
Siskiyou County, but lower levels are quite dry, making
precipitation limited to virga. Left a chance/slight chance of
precipitation in for this afternoon, since more moisture should
move in, but don`t expect anything substantial. Heavy
precipitation will begin tomorrow afternoon as a trough approaches
from the southwest. Snow levels will start around 7000 feet, due
to southwest origins of the moisture. A cold front associated with
the trough will approach late Wednesday night. Expect gusty winds
over the east side, and snow levels to quickly drop low enough to
impact passes by Thursday evening. Temperatures aloft will
continue to drop Thursday night into Friday, as the main cold
pool moves overhead. Unstable conditions will support potential
for heavy mountain snow over the Coastal Range, Cascades, and
Siskiyous, and possible thunderstorms over the marine waters into
the Umpqua Basin Thursday night into Friday. The main focus today
will be on refining precipitation amounts and snow levels in the
near term. No substantial updates were made to the forecast this
morning... for more information see previous discussion. -MSC


.AVIATION...20/12Z TAF CYCLE...VFR conditions will persist into
Tuesday afternoon with areas of higher terrain becoming obscured.  A
warm front moving up from the south will bring widespread
precipitation to the area beginning Tuesday afternoon over northern
California and spreading north into Oregon Tuesday evening. From
Tuesday afternoon through Tuesday night from the Siskiyous south,
expect areas of MVFR cigs with widespread obscuration of higher
terrain. From Tuesday evening through Tuesday night north of the
Siskiyous and over the east side, isolated MVFR cigs will develop
with widespread obscuration of higher terrain.


.MARINE...Updated 200 AM PDT Tuesday 20 Mar 2018....A weak warm
front will move north through the waters Tuesday night, followed
by a cold front Wednesday night. South winds will increase ahead
of the cold front, but winds and seas will remain below small
craft advisory criteria.

A stronger front will move onshore Thursday night and this will
bring in stronger south winds and choppy seas.  Small craft
conditions are possible during this time, especially over the
northern waters with steep wind driven seas. Cold air aloft will
move over the waters Thursday night into Friday. Showers will be
widespread, and there may be some isolated thunderstorms as well.

Heavy west swell in the 11 to 13 foot range will develop Thursday
night into Friday. Seas will subside a bit Friday into Friday night,
but even heavier swell around 15 feet will move in Saturday. Seas
will subside Sunday. During this interval the weather pattern will
remain active, so there will be period of choppy wind waves on top
of the swell.


.PREV DISCUSSION... /Issued 330 AM PDT Tue Mar 20 2018/

SHORT TERM...A nearly stationary deformation zone is stretched
across the area extending between a weak departing trough in the
northerly stream and a broad upper low well offshore to the
southwest. This area has been the focus of weak radar echos
through the night with snow showers evident on the web cams across
the Cascades around Crater Lake and highway 97 north of Klamath
Falls. Little change expected today but additional forcing will
occur as some shortwave energy arrives from the south. Have
slightly more confidence in the GFS today as it continues to show
some light shower activity continuing while the NAM12 and higher-
resolution short term solutions indicate mainly dry weather -
apparently in response to building upper heights. Ample cloud
cover though and showers would be of minor impact nonetheless
with snow levels near 4500 ft rising above 5000 feet in the

Tonight through Wednesday the upper low well to the southwest
will fill and shift in our direction with increasing moisture. It
will be a wetter day across the board with increasing south winds
over the region. Snow levels will bounce up between 6500 and 7500

Later Wednesday through Wednesday night gets more interesting.
The arriving low phases with a northern stream digging trough
that will bring a strong cold front to the coast by around
midnight, and then through the CWA through early afternoon
Thursday. This should be a fairly vigorous frontal passage with
ample moisture for robust rain and snow rates, gusty south winds
in advance, and unstable conditions with thunderstorms possible at
least across the coast and Umpqua. Given the strong baroclinicity
indicated with 700-500 millibar lapse rates nearing 7 deg C/km,
some isolated thunder is not out of the question elsewhere inland
with this front. We will be considering a wind advisory for
sometime Wednesday night through early Thursday across the east
side and Shasta Valley.

Thursday night through the weekend we`ve not shifted the existing
forecast and the previous forecast discussion still is quite
valid after viewing successive model runs. Will repeat that well-
written piece below. -MTS

LONG TERM...Thursday night through Monday...Indeed, by this time
frame, we will have shifted into astronomical spring. However,
winter will keep a firm grip on the weather as multiple disturbances
swing through the area beneath a deep, anomalous upper trough.

The first front will shift east of the area Thursday evening. The
cold pool aloft offshore will arrive overnight at the coast, and
then Friday morning west of the Cascades. One disturbance will move
through the area Friday, then another Friday night, followed by one
more on Saturday. The core of the cold air mass associated with this
trough (-35 to -38C) is colder than the one that moved through the
area late last week. So once again, we`ll be looking at very low
snow levels with the potential for snow on the valley floors Friday
through Saturday (especially if the timing of precipitation is

This time of the year, it`s pretty hard for snow to accumulate much
at the lower elevations given the warming of the ground and the
higher March sun angle. This is especially true during the daylight
hours, but if precipitation arrives at night or very early in the
morning, there could be snow accumulation down as low as 500-1500
feet in some areas. This is possible both early Friday and Saturday
mornings. While each valley west of the Cascades seems to have its
own microclimate, perhaps the most vulnerable to this type of
situation is the Illinois Valley, where locally heavier
precipitation can really drag down the snow level at times. Right
now, it`s too early to say for certain how much snow will fall, but
this event will continue the late winter trend of building the
mountain snow pack. This is good news with respect to water storage,
since most areas are still running significant precipitation
deficits for the water year. But, it`s not so great news for
travelers, as there`ll likely be snow impacts, perhaps at low
elevations, but especially over the higher passes.

The expected cold weather also presents an issue for local
agriculture, especially this weekend west of the Cascades as the
upper trough lifts out. Cloud cover/showers will be the limiting
factor for freezing temperatures, but if there`s enough clearing
Saturday night or Sunday night, temperatures could drop into the 20s
in the typically colder spots.

Model guidance shows another disturbance arriving in the NW flow by
early next week, with perhaps another bout of precipitation for the
forecast area Monday/Tuesday. -Spilde




Pacific Coastal Waters...None.


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