Area Forecast Discussion
Issued by NWS Medford, OR

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334
FXUS66 KMFR 230414
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
914 PM PDT Tue Aug 22 2017

.DISCUSSION...This evening, there is good short term model
agreement with the NAM, SREF, HRRR, and RAP models all verifying
quite well with the positioning of scattered thunderstorms. The
intensity of thunderstorms has decreased with the end of day-time
heating. But, these storms have produced substantial rainfall and
even more substantial amounts of lightning. Amounts in Modoc
County have been generally around a half inch with over 2 inches
of rain at Blue Mountain in the northwest part of the county.
Over 1300 lightning strikes have been recorded this afternoon and
evening over Klamath, Lake, Siskiyou, and Modoc Counties with
nearly two-thirds of those recorded in Modoc County.

A line of very slow moving thunderstorms has been in place east
of the Cascades focused from Modoc into eastern Klamath County. It
is expected to expand westward to the Cascades over the next few
hours and continue overnight with most of the activity through
around 2 or 3 AM. The forecast looks on track with a slight chance
of a thunderstorm overnight as far west as Fort Jones in Siskiyou
County and the Rogue Valley in Jackson County...but the majority
of continued activity from the Cascades to Klamath County and
western Modoc County. Activity is forecast to diminish toward
sunrise then increase again during Wednesday with thunderstorms
mainly from eastern Klamath County into Lake County through the
afternoon then shifting northeast out of our area during Wednesday
evening.

A focus for the night shift will be on the extent of warming and
drying with a strong and lenghty period of ridging aloft likely
late in the week into early next week.

&&

.AVIATION...23/00Z TAF CYCLE...Along the coast and over the coastal
waters...areas of IFR cigs/vsbys over the coastal waters will move
onshore this evening...then burn back to the coast by Wednesday
afternoon. The lower conditions will move back onshore Wednesday
evening.  Over the remainder of the area...Areas of MVFR vsbys in
smoke...possibly IFR near fires...will persist through Wednesday
evening.  Isolated to scattered thunderstorms south of the Siskiyous
and east of the Cascades will diminish but not end this
evening...and will become scattered east of the Cascades Wednesday
afternoon.

&&

.MARINE...Updated 130 PM PDT Tuesday 22 August 2017...A weak
thermal trough near the coast will strengthen Thursday...bringing
increasing north winds and steep seas to the area into the
weekend.

&&



.PREV DISCUSSION... /Issued 326 PM PDT Tue Aug 22 2017/

DISCUSSION...Over the next 7 days the weather conditions will evolve
from warm and unstable to cool and stable, to hot and dry...with
continual periods of smoke affecting the forecast area.

The next two days will feature thunderstorm chances, focused today
over northern California, extending into Southern Oregon later this
afternoon and evening. There is potential for nocturnal storms as
disturbances aloft make their way south to north into Oregon.
Tomorrow a cold front will approach the forecast area and
gradually introduce a westerly component to the upper level winds,
drying out the midlevels and stabilizing the atmosphere. This
will push the threat from thunderstorms eastward tomorrow,
primarily along and east of the Cascades. We have Red Flag
Warnings out at RFWMFR for the expected abundant lightning on dry
fuels today and tomorrow. Some of the storms will produce heavy
rain, and we have a Flash Flood Watch at FFAMFR to highlight the
threat for flash flooding on some of the recent wildfire burn
scars in Siskiyou and Modoc Counties. Note that we do expect
plenty of lightning outside of storm cores, so new fire starts are
still expected despite some wetting rainfall.

Smoke is another big concern, impacting a majority of the forecast
area. In general, air quality has trended similar or worsened
compared to yesterday. The HRRR smoke models suggest some
improvement tomorrow but with similar wind flow, we have maintained
an Air Quality Alert out through tomorrow afternoon for much of the
West Side, Cascades, and portions of the East Side to around Highway
97. Brookings and the Curry County coast should continue to see
periods of poor air quality through tomorrow at least, based on weak
winds, overnight drainage northeast winds down the Chetco drainage,
and the abundant smoke from the nearby large fire.

A cooler air mass, not surprisingly, will accompany the cold front
on Thursday, but this trend towards lower temperatures will be very
brief. 850mb temperatures, a good indicator for trends of surface
temperatures, rise starting Friday and peak Monday or Tuesday. 850mb
temperatures reach the top 5 percent for this time of year on
Monday per GEFS output, and temperatures could reach 100 degrees
in the Rogue Valley. The surface temperatures will partially
depend on the amount of smoke in the area, so that will have to be
continually evaluated. Needless to say, the fire environment will
become hotter and drier late this week into the weekend. We have
increased northeasterly overnight winds near the coast as the
thermal trough strengthens into the weekend. Looking at cross-
sections of the atmosphere on Monday, relative humidity values
throughout the column are all 10 to 25 percent...indicating an
extremely dry air mass.

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Red Flag Warning until 11 PM PDT this evening for
     ORZ621-623>625.
     Red Flag Warning from 1 PM to 8 PM PDT Wednesday for ORZ624-625.
     Flash Flood Watch until 11 PM PDT this evening for ORZ028.

CA...Red Flag Warning until 11 PM PDT this evening for
     CAZ280>282-284-285.
     Red Flag Warning from 1 PM to 8 PM PDT Wednesday for CAZ285.
     Flash Flood Watch until 11 PM PDT this evening for
     CAZ080-084-085.

Pacific Coastal Waters...Small Craft Advisory until 5 AM PDT Thursday
     for PZZ356-376.

$$

DW/NSK/JRS



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