Area Forecast Discussion
Issued by NWS Medford, OR

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000
FXUS66 KMFR 100415
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
815 PM PST Sun Dec 9 2018

.DISCUSSION...The forecast is right on at the moment with
satellite imagery showing a frontal system shifting through the
forecast area. We`ve had 0.12 of an inch of rainfall here at the
airport through 8 pm, while coastal areas have had generally
0.50-1.00 inch, though some of the higher coastal RAWS have had up
to 1.25 inches. Snow levels are in the 4500 to 5000 foot range
with a covering of snow showing up in the most recent web cam
images from Lake of the Woods. There could be a little snow to
cover the roads too up at Siskiyou Summit, near Greensprings and
also Black Butte summit before precipitation winds down late
tonight/Monday morning. Models and satellite imagery do show a
weak surface wave developing along the front in Del Norte County
and this will move through the west side this evening, then out
east of the Cascades overnight. Latest radar trends don`t appear
overly impressive, but this wave may briefly enhance precipitation
rates in some areas from eastern Siskiyou to Modoc and southern
Klamath/Lake Counties in the 10 pm to 4 am time frame. Still looks
like about a half of an inch to 1 inch of snow at Klamath Falls
(rain changes to snow this evening) with perhaps 1-3 inches at
some of the higher passes of Highway 140 (Lake of the
Woods/Doak/Bly/Quartz), Highway 97 (Grass Lake) and in the Warner
Mountains overnight. Best snow will be in the Cascades above 5000
feet where 2-5 inches will accumulate. So, if traveling through
these areas, be prepared for a slick commute through Monday
morning.

An upper level trough will move through Monday, so rain/snow
showers in the morning will end for the most part by Monday
afternoon. After a brief break, the next disturbance will move
into the area Tuesday. For more details on that system, please
see previous discussion below. -Spilde

&&

.AVIATION...For the 10/00z TAFs...A cold front is moving inland and
areas of MVFR cigs/vis and local IFR are present  over the
area this evening. MVFR/IFR will become widespread tonight as the
front moves across the area, then continue into Monday morning.
Conditions will lift to VFR late Monday morning into early Monday
afternoon.

A couple other notes...freezing levels will behind the front
tonight to 3500 feet. Also, snow will impact KLMT tonight,
and an half inch to inch of snow possible. -CC

&&

.MARINE...Updated 800 PM PST Sunday 09 December 2018. Winds have
diminish this evening as a cold front is pushing inland. Seas will
remain steep and elevated across the waters through Wednesday night
at least. The next front is expected on Tuesday, and south winds
will become strong and gusty, strongest from Cape Blanco northward.
A period of gales and very steep seas is possible for the northern
waters Tuesday morning and Tuesday afternoon within 30 nm of shore.

Several heavy long-period swell trains are expected to impact the
coastal waters through the week. The first will arrive tonight,
following today`s front, with another arriving Wednesday into
Thursday, and a third possible Thursday night or Friday. These
swells are likely to produce very dangerous bar conditions, and may
also pose a risk to fishing apparatus and exposed infrastructure.

Of significance, models indicate that a very strong frontal system
is may move into the area over the weekend and very high and
dangerous seas are possible with this system. -CC

&&

.PREV DISCUSSION... /Issued 305 PM PST Sun Dec 9 2018/

SHORT TERM...The rest of this afternoon through Wed night...
The weather will become increasingly more active with time over
the next week, and the 7-14 day period is looking as if it will
be wetter than this current week, so expect frequent storm
activity and active weather over the next 2 weeks.

This afternoon south winds are still gusting 40-45mph in the
Shasta Valley at Weed Airport. Therefore, we`re keeping the wind
advisory going there through 4 PM PST this afternoon, which about
the time we expect rain to reach that area.

Otherwise, the frontal system moving into the area has pushed an
initial precipitation band out ahead of it that has remained
fairly healthy on radar. Rain is falling across much of the Rogue
Valley now, and flakes are flying at Mount Ashland. Over the next
hour we expect snow to reach the Oregon Cascades and Siskiyous
with snow levels between 4500 and 5kft. Total new snow amounts
through Monday morning are expected to be generally in the 2 to 4
inch range above 5kft, though locally higher amounts up to about
6 inches are possible. Also, for the elevations between 4kft and
5kft lighter accumulations of 2 inches or less are expected, with
about a half inch expected at Klamath Falls and on Siskiyou
Summit. These lighter amounts at these lower elevations will be,
in part, due to above freezing high temperatures today resulting
in a warmer ground, thus limiting accumulations. It should be
noted, however, that the 18Z NAM12 does show some higher amounts
east of the Cascades tonight, so the current weak wave that
appears to be developing along the coast near Crescent City needs
to be watched this evening to ensure it behaves as we`re
expecting. Elsewhere, we`ll see some beneficial rainfall,
generally a tenth of an inch or more, from the Cascades westward,
and over an inch in portions of Curry County.

The next frontal system arriving on Tuesday and continuing into
Wednesday will come in on a more westerly to northwesterly flow
and compared to today`s, which is coming in on a more southwesterly
flow. We`ll probably need some wind advisories east of the
Cascades mainly for the Summer Lake area. On the front end of the
Tuesday frontal system cold air could become trapped, yielding
lowish snow levels initially. Model guidance is suggesting a
stronger westerly flow in the mid-levels/at 700MB with this
frontal system, so we expect a bit more precipitation with this
one in the west side valleys. Amounts should be a quarter inch or
more from the Cascades westward in Oregon, and a half inch to inch
of water is expected in the mountains, Umpqua Basin, and coastal
counties. With 700MB winds perpendicular to the Cascades at about
50 knots, the highest snow amounts will be in Cascades versus the
Siskiyous. We expect 6 to 12 inches in the mountains north of
Highway 140, with lesser amounts south of there.

We`ll then catch a relative break Wednesday afternoon through
Thursday morning. BTL

LONG TERM...Thursday through Sunday...Weak ridging over the area
Thursday morning will quickly shift east with a southwest flow
aloft. The models are in pretty good agreement keeping most, if
not all precipitation north of our area on Thursday as a warm
front stays to our north. Have kept in a slight chance to chance
over the northwest part of the forecast area, but it could easily
end up flat out dry. Elsewhere, confidence is moderate to high
we`ll stay dry on Thursday. In fact we could have some clearing
along with a favorable wind flow for mild temperatures in the
Rogue Valley.

Confidence is higher we`ll have a fairly active weather pattern in
the works from Thursday night until the end of the forecast period.
A series of fronts will move through the forecast area bringing
valley rain and mountain snow. The main question will be the timing
of each system which is not completely clear at this time. The one
thing that is common among the models is the pattern will be
progressive. This means storms will quickly move in and out, thus
limiting the duration of moderate to heavy precipitation. This will
be good news in terms of keeping flooding concerns for the burn
scars in check. In other words, flooding concern from the burn scars
should remain low.

The first front will move into the area either later Thursday night
or Friday morning. One ofthe operational models shows the front
coming in sooner while other show a later arrival. Snow levels are
expected to be around 4500 feet Thursday night, but could lower to
4000 feet in heavier precipitation, so it looks like the higher
passes like highway 140, Diamond Lake and Crater Lake will get
accumulating snow. Right now, Siskiyou summit should be mainly rain,
but wet snow could mix in at times if precipitation becomes heavy.

The other concern will be wind east of the Cascades as the 700 mb
winds are showing sustained wind speeds around 55 to 60 knots which
means we could be looking at high winds east of the Cascades near
the Summer Lake area and over higher terrain.

Post frontal showers follow Friday with snow levels between 3500 and
4000 feet, depending on the intensity of precipitation. Weak ridging
builds in Friday night giving us a relative break in the action.

Another strong storm is expected Saturday evening or night. The
timing on this one is in question because the models are showing
varying solutions. Thus confidence in the forecast for Saturday is
low. Because of timing issues, it`s hard to really pin down how high
and how low snow levels will be.  Sunday remains active with a upper
trough over our area with a cooler air mass aloft. -Petrucelli

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...None.

CA...None.

Pacific Coastal Waters...Small Craft Advisory until 10 AM PST Thursday
     for PZZ350-356-370-376.

$$

MAS/BTL/MAP/NSK



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