Area Forecast Discussion
Issued by NWS Medford, OR

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FXUS66 KMFR 231723
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
923 AM PST Tue Jan 23 2018

.SHORT TERM...A low moving into British Columbia will bring a cold
front into the coast late today into this evening. An upper
trough will drop down off the Pacific Northwest coast tonight into
Wednesday and the surface front will gradually move eastward
across the area.

Models are on track with this front bringing heavy rain to the
coast with inland areas seeing significant snow over the higher
mountains from the Cascades west, with areas of light to moderate
snow east of the Cascades. Heavy rain is expected to develop
along the coast by this evening and peak overnight into Wednesday
morning with amounts of 2 to 4 inches of rain expected for
coastal areas. Inland, moderate precipitation will spread across
the western portions of the area this evening and continue into
Wednesday. A mix of light to moderate precipitation will spread
east of the Cascades late tonight and Wednesday. Snow levels are
forecast around 5000 feet in the Cascades this evening as
precipitation increases, lowering to 4000 feet late tonight into
Wednesday. Precipitation along with lowering snow levels, will
bring significant snow to the higher mountain passes in the
Cascades, over the higher mountains in the Siskiyous and to
portions of western and central Siskiyou County. Also expect areas
of moderate to heavy snow to develop east of the Cascades in
northern and central Klamath county and over the higher terrain in
western Lake County on Wednesday. Winter storm warnings and
winter weather advisories are in effect. Please see the WSWMFR
for details.

Additionally, ahead of the front,  expect strong southerly winds
to develop along the coast this afternoon into this evening, and
inland today into tonight in the Shasta Valley and east of the
Cascades, including near Summer Lake. These inland areas will see
strong winds persist into Wednesday as the front gradually moves
across the area. High wind warnings are in effect for these
areas. Please see the NPWMFR for details.

The upper trough will move overhead Wednesday night and Thursday.
Of concern, models show a shortwave low moving around the trough
and into the region on Thursday. This will bring another round of
precipitation with relatively low snow levels. Snow levels down
to 1500 to 2000 feet are possible Wednesday night and Thursday
morning with light snow accumulations possible on lower mountains
in western areas, including the Sexton Summit area. Snow levels
may rise slightly during the day Thursday to 2500 feet. Given
model variability and the potential for impacts, will need to
monitor the track and timing of this shortwave with future model
runs.


&&

.AVIATION...23/12Z TAF CYCLE...Conditions are predominantly VFR
early this morning, but light rain has begun at the coast in advance
of a strong front. The front will bring slowly deteriorating
conditions and strengthening southerly winds today through tonight.
IFR will develop at the coast this evening with rain becoming
moderate to locally heavy then persisting through the night.
Conditions will remain mainly VFR farther inland but precipitation
and local MVFR with mountain obscuration will develop toward evening.
-DW
&&

.MARINE...Updated 800 AM PST Tuesday 23 Jan 2018...Seas are
steep and hazardous and will continue to build today into this
evening as a cold front approaches. The front will bring south gales
today into this evening then persist into early Wednesday morning.
Winds will diminish to advisory strength later Wednesday after the
front has moved inland, but seas will remain high and steep. The
next front will bring thunderstorms potential Wednesday night and
Thursday.

Seas will peak again at around 18 to 20 feet Thursday evening. Winds
will slightly decrease Friday then increase again ahead of the next
strong front Friday evening. A series of fronts are expected to move
through this weekend into early next week. -FB


&&

.PREV DISCUSSION... /Issued 229 AM PST Tue Jan 23 2018/

DISCUSSION...Have not made many changes to the going forecast
overnight, other than to fine tune winds in the areas where high
wind warnings are up for late Tuesday through Wednesday. This is
for portions of the coast, the Shasta Valley and the east side.
See details at NPWMFR. Also made some adjustments to the winter
weather headlines to account for changes in timing, and to add
Western Siskiyou County to the mix. More on that below, and at
WSWMFR.

A significant frontal system will arrive Tuesday bringing a wide
variety of weather including strong, gusty winds, heavy rain, and
higher elevation snow to the forecast area through Wednesday.
Rain will arrive at the coast during the mid-late morning, then
push inland during the afternoon. Strong southerly winds in
advance of this system will lead to some downslope flow in the
west side valleys, particularly the Shasta and Rogue Valleys,
where it may not rain much at all until Tuesday night into
Wednesday. Much like with the last couple of fronts, we`ll likely
get breezy for a while here in the Rogue Valley, especially at the
south end near Ashland, but probably not quite to levels that
would warrant an advisory. Rainfall through Wednesday evening
will be quite heavy along the coast with amounts 1.50-3.00 inches
north of Cape Blanco and 3.00-5.00 inches south of there, focused
in the Coast Range. Amounts of 1.50-3.00 inches will also occur in
western Siskiyou County up into southern portions of the Illinois
Valley. The remainder of the west side will have mostly 0.75-1.50
inches north of the Umpqua Divide, but 0.25-0.75 inches south.
Snow levels will be a bit higher with this system, but still near
Mount Shasta City. Rather than rehash snow amounts here, I`ll
refer you to the winter storm warnings, which can be viewed at
WSWMFR. Colder air will arrive Wednesday night into Thursday with
plenty of rain/snow showers.

Models have trended generally wetter for Friday night through
Sunday, especially along and near the coast. There is clear trend
toward less high pressure ridging in from the south Sunday into
Monday in the latest model runs. Along with this, models have
also trended further south, faster, and cooler for the frontal
system early next week.

The long term period begins with an anomalously cold trough of low
pressure extending from Alaska southward to southern California
along the West Coast. Model guidance has collectively trended
toward a slightly stronger onshore flow for Thursday and Friday
which equates to more shower activity and precipitation. Snow
levels continue to be on track to generally be between 1500 and
2500 feet, +/- 500 feet. Thus, we`ll be on the lookout for
snowfall on the lower passes such as Jacksonville Hill and the
Sexton Summits. Snow showers will also be possible on or near the
valley floors of the Rogue and Illinois valleys. Of note, the
ECMWF keeps the bulk of the precipitation in the Thursday to
Friday time frame along and near the Coast Range and across
northern California, while the GFS pushes it further inland. Both
models, however, have trended further inland with shower activity
for Friday.

As mentioned above, the forecast for this weekend has trended
wetter/cloudier. The ECMWF still indicates something of a break in
the precipitation Saturday afternoon into Sunday, but only about 6
hours long in the Reedsport area. The GFS, on the other hand, shows
the break during the Sunday afternoon to Monday morning time frame,
with heavy precipitation along the coast Saturday and Sunday.
Coastal interests, in particular, should keep an eye on the details
of the forecast for this weekend because the higher end
precipitation amount solution could push some waterways, especially
smaller ones, to near bankfull. Most likely, amounts will be lesser,
but the realm of possibility does lead us to some concern.

The frontal system early next week arrives at the coast on Sunday on
the ECMWF and on Monday on the GFS. Snow levels, once again, appear
as if they will roller coaster, beginning near 6kft and falling to
near 1500 feet in showers Tuesday into Wednesday.

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...High Wind Warning from 4 AM to 10 PM PST Wednesday for
     ORZ030-031.
     Winter Weather Advisory from 4 AM to 10 PM PST Wednesday for
     ORZ029-030.
     Winter Storm Warning from 4 AM to 10 PM PST Wednesday for
     ORZ029-030.
     High Wind Warning from 4 PM this afternoon to 4 AM PST Wednesday
     for ORZ021-022.
     Winter Storm Warning from 4 AM to 10 PM PST Wednesday above 5000
     feet in the for ORZ027-028.

CA...Winter Weather Advisory from 4 AM to 10 PM PST Wednesday below
     3500 feet in the for CAZ082.
     Winter Storm Warning from 4 AM to 10 PM PST Wednesday above 3500
     feet in the for CAZ082-083.
     Winter Storm Warning from 4 AM to 10 PM PST Wednesday for
     CAZ080.
     High Wind Warning from 4 PM this afternoon to 10 AM PST
     Wednesday for CAZ081.

Pacific Coastal Waters...Gale Warning until 4 AM PST Wednesday for PZZ350-356-370-376.
     Hazardous Seas Warning until 4 PM PST Friday for
     PZZ350-356-370-376.

$$

CC/CC/CC


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