Frost Advisory 25, NWS Medford, OR

Area Forecast Discussion
National Weather Service Medford OR
816 AM PST Sat Nov 18 2017

.DISCUSSION...No major updates planned this morning. There's a
Dense Fog Advisory this morning for the Rogue and Illinois
Valleys. A look at area web cams, as well as the drive in this
morning, shows thick fog and low visibilities around the area for
these valleys. Guidance is still indicating that the fog will burn
off today but have extended the timing of some patchy fog into
the early afternoon for the Rogue, Illinois, and Umpqua Valleys as
it will likely take a little longer to dissipate fully. There's
the chance that it never goes away for some locations like Grants
Pass, but still think it may clear, at least briefly, for most
areas.

Temperatures were quite chilly last night. Mid to upper teens were
reported east of the Cascades, low 20s in the valleys of northern
California, and upper 20s in the Rogue Valley. A quick glance at
temperatures in the Coquille and Umpqua Basins, low to mid 30s,
show that the frost advisory and freeze warnings were on
track.Those have been allowed to expire with temperatures
continuing to warm up as the morning progresses.

The focus for today will be on the chance for any additional
frost/freeze products for tonight into Sunday morning as well as
fine tuning some details on the system due in the area by Sunday
afternoon. For more details from the previous shift's thinking,
see the discussion below. /BR-y

&&

.AVIATION...18/12Z TAF CYCLE...Expect MVFR/IFR ceilings/visibility
in stratus and fog in the valleys along the coast and north of the
Umpqua Divide this morning, especially around sunrise. These will
expand over the west side valleys and also in the valleys of western
Siskiyou County. VFR will prevail east of the Cascades, but patchy
fog may develop in some of the deeper valleys, like the Klamath
Basin. VFR will return to most areas this afternoon, but low
ceilings may last through mid-afternoon in some of the deeper west
side valleys. /FB

&&

.MARINE...Updated 200 AM PST Saturday 18 November 2017...High
pressure will remain over the coastal waters with mostly light winds
and low seas today. South winds will increase again ahead of the
next front Sunday with gales and very steep seas likely Sunday
afternoon. Confidence is high that seas will build to around 15 feet
Sunday night from Gold Beach north, with seas possibly building to
17 feet north of Coos Bay. Winds could decrease some Monday night
but are expected to increase to near gale again on Tuesday. As a
result we have extended the Gale Watch through Wednesday. Unsettled
weather will then continue through much of next week. /FB

&&

.PREV DISCUSSION... /Issued 410 AM PST Sat Nov 18 2017/

DISCUSSION...High pressure is building into the area, and showers
have ended. Cold temperatures and fog are expected tonight. Frost
advisories and freeze warnings remain in effect for much of the
Umpqua Basin and the coastal zones inland from the coast for
through mid-morning. It will be even colder elsewhere, but we are
beyond the growing season in other locations. Fog has developed in
many west side valleys and along the coast, and this is only
going to become thicker and more widespread as the night goes on.
Pretty much all our valley locations are expected to have fog by
sunrise, including the east side valleys. With warm air building
in aloft, there is also some question as to how soon (or if) fog
clears out. Guidance still shows fog decreasing by late morning,
and we'll stick with that for now. However, there is a chance that
some valleys, the Umpqua, for example, clear very late or not at
all. This will make for a challenging temperature forecast... if
fog lingers all day, afternoon temperatures will be lower than
expected.

Otherwise, today should be a quiet weather day. Tonight into
Sunday morning, we'll see another round of fog and cold
temperatures... though temperatures will be slightly warmer than
this morning. Frost will again be possible. Fog won't stick around
as long Sunday, as the ridge axis will have shifted eastward...
allowing some mixing. An upper trough front will start
approaching, and on the front side of the trough, a stream of
moisture with tropical origins will bringing heavy precipitation
as early as Sunday evening. This system will be a wet one, but not
absurd for this time of year. From Sunday evening through Monday
evening, expect around 2 inches of rain near Brookings... 2 to 3
inches in the Coastal Range... 1 to 1.5 inches in the Illinois
Valley... up to an inch in Roseburg... and up to a half inch in
the Rogue Valley. Snow levels will start relatively low on the
east side... around 5000 feet due to cold air in place, but the
moist, warm airmass should bump them up to above 8000 Monday
morning.

For the 00z run, model agreement is surprisingly good... showing a
continued wet pattern at least through Thursday, with high snow
levels. Projections start to diverge Friday... but we'll probably
a ridge build in by next weekend, bringing drier conditions.
-MSC/TRW

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Dense Fog Advisory until 9 AM PST this morning for ORZ024-026.

CA...None.

Pacific Coastal Waters...Gale Watch from Sunday afternoon through Wednesday afternoon for
     PZZ350-356-370-376.

MNF/MSC/FJB