Area Forecast Discussion
Issued by NWS Medford, OR

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000
FXUS66 KMFR 210409
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
909 PM PDT Wed Sep 20 2017

.DISCUSSION...The main impacts from a broad, cold upper level
trough are showers, snow showers, and temperatures at 10 to 20
degrees below normal. The cold temperatures will continue through
Friday. The majority of lingering showers will continue through
around midnight then activity will diminish further into early
Thursday morning. The snow level is around 5000 to 5500 feet. On
Thursday, after a morning lull, the heating of the day and a
couple of shortwaves in a moderately strong north flow aloft will
generate a slight chance to chance of afternoon and early evening
showers.

The upper level trough axis now over the area will track to
eastern Oregon on Thursday then shift into Idaho on Friday. This
will be sufficient to allow a more stable air mass to develop over
the area with a slight chance of afternoon showers only over the
Warner Mountains in eastern Lake and Modoc Counties. A warming and
drying trend will develop this weekend and continue into mid-week.


&&

.AVIATION...21/00Z TAF CYCLE...From the coastal waters to the
Cascades, expect mostly VFR ceilings with frequent showers and
isolated thunderstorms through this evening with terrain obscured.
Areas of MVFR ceilings are possible in heavier showers/thunderstorms.
Expect widespread turbulence due to gusty winds aloft and at the
surface. There were even a few pilot reports of low-level wind
shear at the Medford Airport earlier. East of the Cascades, VFR
will prevail through this evening with some shower, but heavier
showers could produce local MVFR.

Later tonight into Thursday morning, freezing levels will fall to
near 5000 feet overnight into Thursday morning, perhaps a bit lower
near the Cascades. Expect a mix of MVFR/VFR with terrain obscuration
continuing, but winds will subside. -Spilde/Smith

&&

.MARINE...Updated 830 PM PDT Wednesday, 20 September 2017...
Steep northwest swell will continue through Thursday afternoon with
seas hazardous to small craft. There remains a slight chance for
thunderstorms over the waters through tonight. Weak low pressure
moving through the area Thursday will be followed by increasing
northerlies and wind driven seas Friday through the weekend as a
thermal trough builds along the coast. -Schaaf/DW

&&

.PREV DISCUSSION... /Issued 245 PM PDT Wed Sep 20 2017/

DISCUSSION...The remains of the cold front are just about out of
the area now. Cold air pool will continue to move over the
forecast area this evening. Scattered showers following the front.
A few are popping up to 45 dBZ or so then quickly back down. There
have been a few lightning strikes over the coastal waters through
the day. Over land there have only been cloud-to-cloud strokes.

The long wave upper trough is over the Pacific Northwest.
The strongest instability associated with the cold pool has stayed
over the waters and the Umpqua Basin. A significant embedded
short wave will rotate around the bottom of the trough tonight.
The short wave will stay far enough off shore to keep the coldest
air and instability over the water later. So, the threat of
thunderstorms will decrease over land but remain for the waters.

Drier, more stable air will move over the area late Thursday night
and into Friday. The last chance of showers will exit Lake and
Modoc Counties Friday afternoon.

Areas east of the Cascades will again see cold overnight low
temperatures tonight and Thursday night. Good confidence for the
cold temperatures Friday morning so went ahead with the advisory
and warning. See PDXNPWMFR for details on the frost advisories
and freeze warnings.

The long wave trough becomes stationary over the Rocky Mountains
Friday which puts the Pacific Northwest under a dry northerly
flow. The ridge will strengthen raising temperatures to normal or
slightly above for next week with no precipitation. JBL

FIRE WEATHER...Updated 240 PM PDT Wed, 20 September 2017....
An upper level disturbance will continue to move through the
area into this evening with gusty winds, showers, and isolated
thunderstorms. The strongest cells have been able to produce
moderate to heavy rainfall at times, with up to 0.25 of an
inch/hour rainfall rates. This could still cause debris flows
and/or wash out rural forest roads into early this evening near the
Umpqua North Complex (FFAMFR), but we have allowed the watch to
expire for areas near the Chetco Bar fire.

Snow levels will fall to around 5000 feet (perhaps a bit lower near
the Cascades) tonight, but snow impacts will mostly be above 6000
ft, where a few inches could still accumulate. Any weakened
trees from wildfire will be susceptible to falling due to
the gusty winds in the highest terrain and especially where
some snow may weigh them down. That threat will persist
through this evening.

Overall, temperatures will be around 20 degrees below normal
through Thursday. Showers will still be around Thursday, but
won`t be as numerous as today.

A long duration warming and drying trend is then expected Friday
through next weekend, with temperatures likely above normal again
early next week. We are expecting broad northeast to east flow over
the weekend, and afternoon humidity will likely lower into the 20s
and 30s across the forecast area. Model trends are suggesting a dry
and warm period that lasts through next weekend. With the recent
rainfall, models may be somewhat overdoing the amount of drying at
least initially this weekend, so in the MaxRH/MinRH grids, we
haven`t gone quite as dry as the guidance.

The Climate Prediction Center`s 8-14 day outlook is calling for
enhanced chances for above normal temperatures and below normal
precipitation. So, the current wet and cool period will likely be
followed by a steady and long-duration warming and drying trend.
-Spilde

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Frost Advisory from 2 AM to 9 AM PDT Friday for ORZ029>031.
     Freeze Warning from 2 AM to 8 AM PDT Thursday for ORZ029>031.

CA...Frost Advisory from 2 AM to 9 AM PDT Friday for CAZ084-085.
     Freeze Warning from 2 AM to 8 AM PDT Thursday for CAZ084-085.
     Frost Advisory from 2 AM to 8 AM PDT Thursday for CAZ084-085.

Pacific Coastal Waters...Small Craft Advisory for hazardous seas
     until 11 AM PDT Thursday for PZZ350-356.
     Small Craft Advisory for hazardous seas until 5 PM PDT Thursday
     for PZZ370-376.

$$

DW/JBL/BMS



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