Area Forecast Discussion
Issued by NWS Medford, OR

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000
FXUS66 KMFR 181615
AFDMFR

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

.SHORT TERM...A strong incoming front will impact the area
beginning late tonight through Thursday followed by a colder
pattern with showers and mountain snow on Friday.

The impacts expected from this storm include:

*Rain Impacts: Ponding on roadways, debris flows or mud slides
 possible near recent burn scars near the coast and in western
 Siskiyou County on Thursday.
*Slippery, snow-covered roads over the higher passes in the
 Cascades near Crater Lake and Diamond Lake on Friday. Light snow
 on highway 140 near Lake of the Woods.
*Travel may become difficult for high profile vehicles on
 Thursday 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.
*Dangerous conditions for beach goers with high surf conditions
 along the coast late Thursday afternoon through Friday.
*Dangerous seas developing in the coastal waters tonight through
 Thursday and continuing on Friday. Strong gale force winds will
 accompany these seas tonight and Thursday morning.

Today is expected to be the last dry day for the week. Models are
on track showing a strong front moving towards the area tonight
and Thursday morning then moving inland late Thursday through
Thursday night. Rainfall is expected to begin along the coast
tonight into Thursday morning, becoming heavy Thursday afternoon
and evening. Rain will spread inland across areas from the
Cascades west during the day Thursday then across areas east of
the Cascades Thursday evening and night. Strong winds are expected
ahead of and with the frontal passage. South winds will pick up
over the mountains and along the coast tonight. South winds will
peak along the coast Thursday morning with gusts of 35 to 45 mph
likely for coastal locations with higher gusts of up to 60 mph
near Cape Blanco. Inland areas will see south winds peak Thursday
afternoon into Thursday evening. South winds are expected to gust
to around 40 to 50 mph in many valleys east of the Cascades and in
the southern Shasta Valley on Thursday with higher gusts of 60
mph possible in portions of Lake County and over the Warner
Mountains in eastern Modoc county. Of note, these strong winds
over eastern areas will develop in advance of the precipitation
Thursday afternoon with rain moving into the Shasta Valley and
areas east of the Cascades late Thursday afternoon into Thursday
evening.These winds will lower Thursday night behind the frontal
passage. Behind this front widespread to numerous showers are
expected Thursday night into Friday along with snow levels
lowering to 4000 to 5000 feet. This will will bring light to
moderate snow to the passes in the Cascades Thursday night through
Friday afternoon.

Expect only a break in the active pattern Friday evening then a
strong and moist warm will spread precipitation into the area
Friday night and Saturday. This next system will bring moderate
to locally heavy rain from the Cascades west with light to
moderate precipitation east of the Cascades and in Northern
California. Snow levels are expected to rise to above pass
levels on Saturday. Another period of strong winds is possible
with this warm front on Saturday. The strong winds will be mainly
over the mountains although gusty winds are likely over much of
the area on Saturday.



&&

.AVIATION...18/12Z TAF Cycle...In the coastal waters MVFR conditions
are widespread right up to the coast down to Port Cape Blanco.
Orford. Expect little change in coverage throughout the day. From
Port Orford south MVFR conditions are farther offshore beyond 10nm
from shore. Inland, low clouds and MVFR cigs extends into NW Douglas
County. IFR conditions at RBG appear to be isolated event and do not
show up on satellite imagery. RBG should improve to VFR by 18Z at
the latest. Elsewhere, VFR is expected.

The next front will bring more clouds to the area. Aside from the
coast, conditions inland are expected to remain VFR until late
tonight with local MVFR inland. /FB

&&

.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 places.

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


&&

.PREV DISCUSSION... /Issued 411 AM PDT Wed Oct 18 2017/

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

*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 tree falls 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.
-Schaaf


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

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
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
     ORZ021-022.

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 until 11 PM PDT this evening for PZZ350.
     Gale Warning from 11 PM this evening to 11 AM PDT Thursday for
     PZZ350-370.
     Hazardous Seas Warning from 5 PM this afternoon to 11 PM PDT
     Friday for PZZ350-370.
     Small Craft Advisory until 11 AM PDT Thursday for PZZ370.

$$

CC/CC/CC



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