Area Forecast Discussion
Issued by NWS Medford, OR

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FXUS66 KMFR 181111

Area Forecast Discussion
National Weather Service Medford OR
411 AM PDT Wed Oct 18 2017

.DISCUSSION...Timing for impacts will be Wednesday night through

*Impacts: Ponding on roadways, debris flows near burn scars near
 the coast and western Siskiyou County.
*Slippery, snow-covered roads over the higher passes in the
 Cascades near Crater Lake and Diamond Lake. Light snow on highway
 140 near Lake of the Woods.
*Travel may become difficult for high profile vehicles due to
 strong southerly winds along the coastal headlands, Shasta
 Valley, and east of the Cascades.
*Isolated trees may fall near burn scars due to high winds.

Wednesday will be the last relatively dry day across southern
Oregon and northern California with temperatures slightly above
normal before the first strong front of the season arrives across
the Pacific Northwest Wednesday night.

This frontal system is expected to be stronger than what is
typical for this time of year, and model guidance has come into
better agreement regarding the details of this system. The biggest
concerns will be the strong winds and moderate to heavy rain
across the area.

Precipitation totals will be significant, especially at the
coast. Rain rates will be less than a quarter of an inch an hour,
but some coastal areas could see 1.5 inches in 6 hours and some
inland areas will get 1 inch in 6 hours. Rivers are still low and
soils are dry from the summer, but a few impacts could still be
felt, particularly for coastal rivers. This will be mainly in the
form of debris flows and flash flooding for the Chetco Bar
and Eclipse Complex burn scar areas. Have issued a Flash Flood
Watch for those two areas (details at FFAMFR). This does NOT
include any nearby cities.

Additionally, there have been some recent issues with rock falls
along highway 138 between mile post 45 and and 53 in Douglas
County associated with the Umpqua North Wildfire Complex. Rain
rates in that area will be about fourth tenths of an inch in 6
hours, which is well below our traditional guidance for issuing
any flash flood products. That being said, the aforementioned
issues could create travel concerns. Thus, will allow the day
shift to coordinate with ODOT to see if a flash flood watch would
be beneficial there.

This will be one of the first wind events in our forecast area
for the season. The strongest winds will occur at the coast
Wednesday night into Thursday with Shasta Valley and East Side
winds ramping up Thursday into Friday morning. This could create
some travel difficulties, especially for high profile vehicles.
There may be some treefalls due to the winds, particularly near
recent burn scars. The evening shift has issued a high wind watch,
and wind advisory (NPWMFR) for areas east of the Cascades.

Snow levels will start off above 8000 feet on Thursday and will
drop to 4000 to 4500 feet by Friday evening as the precipitation
comes to an end. Some of the higher passes could see some moderate
snow, particularly near Crater Lake and Diamond Lake; but lower
passes like highway 140 near Lake of the Woods will only see light
snow. Siskiyou Summit may see a few flurries on Friday, but it is
more likely that the precipitation will end before it changes
over to snow. Confidence is not high enough quite yet for snow
amounts, so will allow the day shift to determine if any winter
products are needed.

The long wave upper trough will move onshore Friday, and short
waves will continue to move through the trough. This will support
widespread shower activity into Friday. The offshore upper level
ridge will amplify as it approaches the coast, and this will cause
the front to lift back north as a strong and quite wet warm front
Saturday, followed by a weaker trailing cold front Saturday night
into Sunday.

The offshore ridge will be quite strong by this time just off the
coast. Low level flow will turn easterly Sunday night into Monday,
so the area will dry out quickly at that time. Easterly flow will
weaken Monday into Tuesday, but it will remain dry with no
incoming systems initially. A weak front may bring some
precipitation to the north coast and Umpqua Basin Wednesday.


.AVIATION...MVFR conditions are expected offshore and along the
Coos County Coast, including at KOTH for the remainder of this
morning. KOTH could even see periods of IFR, but expect those
periods to be limited in nature. Elsewhere, VFR is expected for
most of the day before the front arrives from west to east
bringing reduced visibilities and ceilings due to rain. Once the
rain arrives, terrain obscurations will be expected. -Schaaf


.MARINE...Updated 300 AM PDT Wednesday 18 October 2017...South
winds will increase today across the waters ahead of a strong cold
front. Confidence has increased to high for gales from Cape
Blanco northward tonight with warning level seas most other

Long period and large northwest swells (around 20 feet at 17
seconds) build into the waters late Thursday into Friday behind the
front, creating dangerous surf and bar conditions. The tail end of
the cold front is expected to move back northward as a warm front,
and winds are expected to increase again Friday night and Saturday
with guidance showing Gale Force Winds again. SK/FB


.HYDROLOGY...Updated 400 PM PDT Tuesday 17 October 2017... There
will be some hydrologic concerns with the Wednesday night into
Friday system. Currently, soils are dry and rivers are low, but
there will be enough rain to create possible debris flows near
recent burn scars. The river levels will not be an issue itself,
but the debris may obstruct river flows at times, creating
elevated river levels upstream of the block at first, then
downstream of the block if and when it breaks free. If you come
across a flooded road, find an alternate route. Remember to turn
around, don`t drown.

Debris floating down the river combined with high surf may create
dangerous bar conditions near the Port of Brookings. -Schaaf


OR...Wind Advisory from 11 AM to 11 PM PDT Thursday for ORZ029>031.
     High Wind Watch from Thursday morning through Thursday evening
     for ORZ030-031.
     Flash Flood Watch from Thursday morning through Thursday evening
     for ORZ022-024.
     High Surf Advisory from 5 PM Thursday to 11 PM PDT Friday for

CA...Wind Advisory from 11 AM to 11 PM PDT Thursday for CAZ084-085.
     High Wind Watch from Thursday morning through Thursday evening
     for CAZ085.
     Flash Flood Watch from Thursday morning through Thursday evening
     for CAZ080.
     Wind Advisory from 8 AM to 8 PM PDT Thursday for CAZ081.

Pacific Coastal Waters...Small Craft Advisory from 5 PM this afternoon to 11 AM PDT
     Thursday for PZZ356-376.
     Hazardous Seas Warning from 11 PM this evening to 11 PM PDT
     Friday for PZZ356-376.
     Small Craft Advisory from 11 AM this morning to 11 PM PDT this
     evening for PZZ350.
     Gale Warning from 11 PM this evening to 11 AM PDT Thursday for
     Hazardous Seas Warning from 5 PM this afternoon to 11 PM PDT
     Friday for PZZ350-370.
     Small Craft Advisory from 11 AM this morning to 11 AM PDT
     Thursday for PZZ370.


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