Area Forecast Discussion
Issued by NWS Medford, OR

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000
FXUS66 KMFR 010431
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
931 PM PDT FRI SEP 30 2016

.DISCUSSION...An evening update to the forecast is not necessary.
A significant change in the weather is underway. The initial
effects on our area from a broad trough over the eastern Pacific
were cooler temperatures, increased cloud cover, and breezy to
windy westerly winds...including east side gusts of 25 to 40 MPH.
These winds have begun to diminish. The trough is offshore and
will swing into the coastal waters tonight into Saturday morning with
showers and isolated thunderstorms. This instability will be
mainly over the waters but thunderstorms may reach coastal
communities also.

Showers on Saturday still look to be isolated to scattered and
mainly in western Oregon roughly from Interstate 5 in Douglas and
Josephine Counties westward. The main event will be a cold front
and post-frontal showers late Saturday night into Sunday evening.
Snow levels look to be around 5500 Feet with up to 3 inches of
snow expected.

There is still notable model disagreement beyond Sunday. The 00Z
GFS has sped up the next front and now has it encroaching upon the
coast toward the end of Monday afternoon with inland showers
during Monday night. Shortwaves in a northwest flow aloft are
expected to continue periods of showers Tuesday through Wednesday.
Another strong trough may impact the Pacific Northwest on Thursday
or Friday.

&&

.AVIATION...01/00Z TAF CYCLE...Over the coastal waters...along the
coast...and in the Umpqua Basin. CIGS will be mainly VFR with
isolated MVFR cigs in rain showers. The lower conditions will spread
to the lower Umpqua Basin tonight with higher terrain becoming
obscured. These areas will clear to VFR by Saturday afternoon. Over
the remainder of the area...VFR conditions will prevail through
Saturday...but some higher terrain will become obscured.

Note: The ceilometer at FAA site KOTH is out of service, so there
will be no ceiling observations available between 04Z and 14Z. /FB

&&

.MARINE...Updated 830 PM PDT Friday 30 September 2016...Low
pressure will sweep across the waters tonight into Saturday
morning with isolated thunderstorms. A cold front will bring
strong south winds Saturday into Saturday night. Low pressure
lingers over the waters Sunday then another front will arrive late
Monday. Steep swell will likely follow Monday night into
Wednesday. /DW

&&

.PREV DISCUSSION... /Issued 158 PM PDT FRI SEP 30 2016/

SHORT TERM...An upper level trough just off the Pacific Northwest
coast will bring a strong shortwave into the area tonight and
Saturday, resulting in a cool showery pattern. A stronger low
will then move around the trough and the low and upper trough will
shift over the area Saturday night and Sunday. This will bring
widespread precipitation and cool temperatures to the area on
Sunday.

Ahead of the trough, strong gusty winds have developed east of the
Cascades with gusts of 30 to 40 mph. These winds will continue
into the evening. Strongest winds are expected in Southeast
Klamath, Eastern Siskiyou, Lake and Modoc counties. Also where
humidities will be low this afternoon, mainly over southeastern
Klamath, southern Lake and portions of Modoc County, expect
critical fire weather conditions. A red flag warning is in effect
for increased fire danger in these area. See the RFWMFR for
details.

Tonight into Saturday, the upper trough will nudge inland as a
strong shortwave rotates around the trough and into the region.
Cooler conditions are in store on Saturday with a chance of
showers from the Cascades west and showers likely along the coast.
The shortwave moving around the base of the trough is associated
with a -25 deg C cold pool at 500 mb and weak instability which
will contribute to the potential for isolated thunderstorms over
the waters and coast. Current satellite and observations show very
cold cloud tops and isolated lighting associated with the
shortwave offshore. This cool pool and shortwave are forecast to
move into the coastal waters by late this evening and into the
coast tonight. Expect a slight chance for thunderstorms for the
coastal waters and along the coast this evening and tonight.

On Sunday, a low moving around the trough will shift inland and
the main upper trough will settle over the area. Model forecasts
continue to show this system bringing moderate to brief heavy rain
to the coast with widespread light to moderate precipitation
across inland areas on Sunday. Snow levels are expected to be
lower, down to 5500 to 6000 ft, Saturday night into Sunday. So
brief snow accumulations are possible, especially in the higher
portions of the Cascades and Siskiyous such as near Crater Lake or
Mount Ashland. Ground temperatures are still warm though so any
accumulations will likely be brief. This low will also bring very
cold air aloft, which combined with weak instability will bring a
slight chance of thunderstorms to the coastal waters and along the
coast, mainly south of Cape Blanco. Models also show weak
instability across inland areas Sunday afternoon. This may result
in some stronger showers and even the chance for small graupel or
ice pellets, especially in Northern California and from the
Cascades east. Overall, cloudy, showery and cold conditions are
expected across the area Sunday. High temperatures are expected
to only reach the 50s in the valleys with much cooler temperatures
in the 30s and 40s in the mountains.

There is good agreement in the model forecasts that a shortwave
ridge will build into the region on Monday. This will result in a
break or decrease in showers over the area and a slight increase
in temperatures. However, a warm front moving into the ridge will
bring a chance for precipitation  Monday evening into Tuesday.
Model solutions show considerable variability on the intensity of
this front and amount of precipitation though. So confidence is
low in where and how much precipitation will fall. Will need to
monitor future model runs.

LONG TERM...Wednesday though Friday, models continue to show
increased variability in the weather pattern. Overall, expect the
potential for a continued progressive pattern with additional weak
fronts affecting the area Wednesday into early Thursday then a
stronger front and upper trough possible late Thursday and Friday.


&&

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

CA...None.

Pacific Coastal Waters...Small Craft Advisory from 2 PM to 8 PM PDT
     Saturday for PZZ350-356-370-376.

$$

DW/CC/FJB



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