Area Forecast Discussion
Issued by NWS Medford, OR

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FXUS66 KMFR 201702

Area Forecast Discussion
National Weather Service Medford OR
1000 AM PDT Thu Jul 20 2017

.UPDATE...Current forecast looks on track and will not update this


.AVIATION...20/12Z TAF CYCLE...The southern end of a weak cold
front has brought IFR with patchy drizzle to the coast early this
morning. It also has sent a deep marine layer with MVFR stratus into
the Umpqua Basin. The lower conditions will burn back offshore by
this afternoon. A shallower marine layer will return IFR to the
coast after 03Z this evening. Over the remainder of the area...VFR
conditions will prevail through tonight. -DW


.MARINE...Updated 815 AM PDT Thursday 20 July 2017...A weak cold
front is producing some light rain and drizzle north of Cape Blanco
this morning. Winds and seas will remain relatively calm through
Friday morning as surface high pressure builds to the west. The
thermal trough will strengthen Friday through the weekend, producing
gusty north winds and steep seas, possibly reaching gale force as
early as Saturday afternoon south of Cape Blanco. The thermal trough
will continue into early next week. Light to moderate winds and seas
are likely by mid-week. -DW/BPN


.FIRE WEATHER...Updated 600 AM Thursday 20 July 2017...The two main
concerns are: easterly winds with moderate ridge top humidity
recoveries Friday night and Saturday night, and a slight chance to
chance of thunderstorms Sunday afternoon through Tuesday evening.
First, it will remain hot and dry through Friday but with a slight
improvement in humidities and decrease in wind speeds to
closer to normal levels.

Humidity recoveries will be poorest Friday night but still worse
than normal into early next week, and there will be a warming trend
into Sunday with easterly low level winds. This will be due to a
ridge building into the area with a deepening thermal trough near
the coast.

Of even greater concern is a low offshore of California that will
move slowly northeast toward Oregon. There are still model
differences in the track and timing of the low and its associated
shortwaves. But, there is general agreement that it will pull
monsoonal moisture northward and increase instability Sunday
afternoon then continue to affect our area at least through Monday
evening. Shortwaves rotating around the low could continue
thunderstorm activity during the overnight hours. It is also
possible that weaker instability will follow after Monday evening
and linger through the middle of the week. The highest probability
for thunderstorms is over the mountains from the Cascades eastward
on Sunday afternoon and evening. There is no reason to favor either
model at this early point, but the GFS solution is more concerning
than the ECMWF. It indicates a more widespread and longer lasting
event. -DW


.PREV DISCUSSION... /Issued 338 AM PDT Thu Jul 20 2017/

DISCUSSION...The synoptic pattern will transition to a flat
ridge today...which will build north along the west coast through
Saturday. This will make for continued dry weather at least
through Saturday. Skies will remain mostly for the
marine layer. Satellite imagery indicates low clouds in along the
coast north of Cape Blanco inland to Myrtle Point and Powers.
Stratus should make it into the Umpqua Valley by sunrise...and
burn off later this morning. Temperatures across the CWA today
will be similar to yesterday.

Friday morning will feature a return of marine fog and stratus to
the coast, but the marine layer won`t be nearly as deep as today,
so do not expect it to creep into the Umpqua Valley. Friday will
generally be 3 to 5 degrees warmer than Thursday.

Upper level pressure over the Great Basin will build Friday into
Saturday...bringing very warm temperatures Saturday and Sunday.
Afternoon highs should be within a few degrees of 100 in west side
valleys Saturday afternoon... and temperatures will peak Sunday
afternoon... with highs of 95 to 105 expected in west side valleys
and mid 90s in east side valleys.

The latest runs of the GFS and ECMWF depict a closed low
developing off the coast of northern California...progressing
northeastward and approaching the coast Sunday. Several shortwaves
will rotate around the low, possibly triggering thunderstorms.
The setup of these waves has varied widely with different model
runs. The EC is showing considerably less instability, so there is
high uncertainty. For now, since the GFS has shown variations in
location of vort maxes, have covered a large area from western
Siskiyou County northward and northeastward... and much of
Klamath...Lake...and Modoc Counties with a slight chance of
thunderstorms. Any thunderstorms that develop will have high cloud
bases, and expect gusty winds and low rainfall in areas under
tstorms. The last several runs of the GFS have depicted a few
strong shortwaves developing over the east side Sunday
night...which could trigger nocturnal convection. The parent upper
low will move into the forecast area Monday... possibly
triggering tstorms Monday afternoon through Tuesday night, mainly
over the east side and on ridges in Siskiyou County. -MSC




Pacific Coastal Waters...None.


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