Area Forecast Discussion
Issued by NWS Medford, OR

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000
FXUS66 KMFR 201142
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
442 AM PDT Wed Sep 20 2017

.SHORT TERM...A cold upper trough will remain over the region
today into Thursday. A frontal system will move inland today as a
shortwave moves around the upper trough and into the area. This
pattern will bring rain with lowering snow levels and a mix of
rain/snow developing for locations above 5000 feet elevation.
Much cooler than normal temperatures, gusty winds in the
mountains, and isolated thunderstorms are also expected today.
Then the upper trough will deepen to the south but remain over
the region tonight and Thursday. Another disturbance is forecast
to move down the backside of the trough into the area from the
north-northwest Thursday. This next disturbance is weaker though
and expect shower activity to be mainly concentrated near the
coast and coastal mountains and into the Umpqua valley and
Southern Oregon Cascades with mainly isolated to scattered
showers elsewhere. The upper trough will gradually shift eastward
Thursday night into Friday with decreasing showers over the area.

Today, expect shower activity to be greatest along the coast,
into the Umpqua and over the Southern Oregon Cascades. Moderate
rain is expected at times along the coastal mountains and into
the Cascade foothills. Also expect isolated thunderstorms to
affect Coos and Curry county with a slight chance for
thunderstorms inland as well across northern portions of the
area, including northwest Josephine, Douglas, northern Klamath
and northern Lake counties. Thunderstorms will bring a chance for
moderate to locally heavy rain. The main concern with any heavy
rain will be the risk for flash floods or landslides over area
burn scars, including the Chetco and Umpqua North complex near
Highway 148. A flash flood watch remains in effect for these two
wildfire burn areas. Please see the FFAMFR for details.

Snow levels are currently high, around 7000 feet or higher, and
will lower to around 5000 to 6000 feet late this morning into this
afternoon across southwest Oregon. Note, across northern
California and for areas in Lake county, expect snow levels to
remain above 6000 feet into this evening, lowering to around 5000
feet tonight as showers diminish. The lowering snow levels during
the day today combined with numerous showers over western areas,
will bring snow or a rain/snow mix to the higher elevations in the
Southern Oregon Cascades and Siskiyous today. With strong moist
onshore flow, expect areas of moderate precipitation in the
Cascades. Snowfall amounts of around 1 to 4 inches are expected
above 5000 feet in the Southern Oregon Cascades through tonight
with higher amounts of 3 to 6 inches above 6000 feet elevation,
especially in the Crater Lake area. Shower activity will gradually
decrease late this evening into tonight but expect some continued
light showers over the Southern Oregon Cascades through the
night.

Gusty winds will continue to impact the higher terrain as a
strong mid level jet moves across the region. West winds with
gusts of around 30 to 50 mph are possible this morning into this
afternoon especially over the Cascades and mountains east of the
Cascades. A wind advisory has been issued for eastern mountains
in the Lake county and eastern Modoc county where gusts of 45 to
55 mph are expected today. These winds will peak this morning into
early this afternoon then gradually lower late in the afternoon.
Please see the NPWMFR for details.

Behind this frontal system, expect winds to lower and showers to
decrease in coverage and intensity tonight with continued light
shower activity becoming focused on the Southern Oregon Cascades.
A very cold air mass aloft will allow freezing or near freezing
temperatures to develop across areas east of the Cascades in
Klamath, Lake, eastern Siskiyou and Modoc counties tonight and
early Thursday morning. Freeze warnings and frost advisories are
in effect for these areas. Please see the NPWMFR for details.

Then on Thursday, models continue to show a weak low moving down
the coast and bringing showers to the area, with activity mainly
focused over the coastal waters, along the coast and into the
Umpqua and Southern Oregon Cascades. Elsewhere expect a slight
chance to chance for showers on Thursday. Confidence on the amount
and extent of showers is only moderate since models have shown
some variability on the strength and track of this system.


&&

.AVIATION...20/06Z TAF CYCLE...From the coast to the Cascades, a mix
of MVFR/VFR ceilings with frequent showers are expected through
Wednesday evening with terrain obscured. A slight chance of
thunderstorms will develop around 20Z over the coastal waters with
this instability spreading across the coast to areas north of the
Umpqua Divide for the afternoon and evening. Snow levels around 8000
ft will fall to around 5500 ft during the morning. East of the
Cascades, VFR will dominate but local MVFR and numerous showers are
expected. Low level winds will be gusty and strong during the
morning and afternoon. -Smith/DW

&&

.MARINE...Updated 830 PM PDT Tuesday 19 September 2017... A cold
front is expected to push into the marine waters later tonight and
south winds will increase as a result.  However, these winds are
expected to stay within Small Craft Advisory thresholds.  Steep
northwest swell will also continue to impact the area tonight as the
low pressure system in the Gulf of Alaska slides down into our
region.  We can expect these steep northwest seas to continue into
Wednesday before the swell heights start to decrease late Wednesday
into early Thursday.

Eventually, high pressure will develop and a thermal trough will
position itself along the coast and south of Cape Blanco.  We are
getting into that time of year when the temperature difference
between land and water is not going to be that great,  so winds
associated with this thermal trough will be small craft advisory at
best. -Smith/DW


&&

.PREV LONG TERM DISCUSSION... /Issued 529 PM PDT Tue Sep 19 2017/

Friday will be a transition day as the upper trough shifts east
with a dry northerly flow. Still could not rule out a few showers
along the northern Cascades and eastside, but they should be the
exception. Dry and warmer weather returns next weekend as an upper
ridge builds over the eastern Pacific. At the surface, a thermal
trough will develop with increasing offshore flow in southeast
Oregon and Western Siksyou County. There is increasing confidence we
could remain in a prolonged period dry and warmer weather for most
of next week and beyond, but this is not a slam dunk. -Petrucelli

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Freeze Warning from 2 AM to 8 AM PDT Thursday for ORZ029>031.
     Wind Advisory until 5 PM PDT this afternoon for ORZ030-031.
     Flash Flood Watch until 11 AM PDT this morning for ORZ022-024.
     Flash Flood Watch through this afternoon for ORZ025.

CA...Wind Advisory until 5 PM PDT this afternoon for CAZ085.
     Frost Advisory from 2 AM to 8 AM PDT Thursday for CAZ084-085.
     Freeze Warning from 2 AM to 8 AM PDT Thursday for CAZ084-085.

Pacific Coastal Waters...Small Craft Advisory for hazardous seas until 5 AM PDT Thursday
     for PZZ350-356-370.
     Small Craft Advisory until 11 PM PDT Sunday for PZZ376.

$$

CC



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