Area Forecast Discussion
Issued by NWS Medford, OR

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FXUS66 KMFR 211020
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
320 AM PDT Fri Jul 21 2017

.DISCUSSION...Dry weather will prevail today and Saturday across
much of the CWA as an upper level ridge, albeit weak, builds over
the area. This will bring offshore east or northeast winds to the
mid slopes and ridges during the night and early morning hours
resulting in fairly typical, but significant drying. Winds will
then shift to more northerly or even northwesterly in the
afternoons/early evenings. The marine layer this morning won`t be
quite as deep as it was yesterday, but areas of stratus/patchy
fog will impact areas within about 25-35 miles of the immediate
coast. Inland areas will warm up by about 3-5 degrees compared to
yesterday, with temperatures topping out in the low 90s in the
Rogue Valley and in the mid 80s in the Umpqua Basin/over the East
Side. A couple of wildfires burning in the SW portion of the CWA
will cause some low-level smoke in the lower Klamath River/Chetco
Valleys today. Temperatures will get a boost again on Saturday as
the air aloft warms. With model 850 temps in the lower 20s
Celsius, expect high temperatures here in Medford up near 100
degrees F (99F, officially at the airport). It should also get
warm due to the offshore northeast winds at the coast in Brookings
with highs in the low 80s. A bit deeper marine push is expected
Saturday night. Expect stratus to fill in north of Cape Blanco
along with patchy drizzle, but these should erode Sunday morning.

Attention shifts to an upper air disturbance that will approach
the northern California coast Sunday, then slowly move
northeastward across the area Monday and Tuesday. Models continue
to waver on the timing and strength details regarding this system,
but the flow will turn southerly in advance of this system
allowing moisture to be drawn northward into northern California
and SW Oregon Sunday afternoon/evening. With strong surface
heating expected, there should be enough instability to allow at
least isolated thunderstorms to develop during the late afternoon
and evening. It should be noted that right now, we are not
predicting temperatures to be quite high enough for heat
advisories, but high temperatures will be close to 100 degrees
here in the Rogue Valley for 3 days (Sat-Mon). If conditions
change and temperatures are a few degrees higher, advisories may
be necessary.

The low will remain SW of the area Sunday night, and while we`ll
lose some of the instability, showers/isolated thunderstorms will
linger. With the slow movement of this system, we`ll maintain
chances for showers and isolated thunderstorms both Monday and
Tuesday. A kicker trough is expected to move through the PacNW
ejecting the low east-northeastward and out of the area at
midweek. This should effectively end the lightning threat and
temperatures will drop back closer to normal. -Spilde

&&

.AVIATION...21/06Z TAF CYCLE...Along the coast and over the coastal
waters...Areas of MVFR/IFR cigs will develop tonight and persist
into Friday morning...clearing to VFR by Friday afternoon.  VFR
conditions will then persist into Friday evening...possibly into
Saturday morning. Over the remainder of the area...VFR conditions
will prevail through Saturday morning.

&&

.MARINE...Updated 200 AM PDT Friday 21 July 2017...Winds and seas
will remain relatively calm through this morning as high pressure
builds to the west. The thermal trough will strengthen late this
afternoon through the weekend, producing gusty north winds and steep
seas, likely reaching gale force as early as Saturday afternoon
south of Cape Blanco. A period of hazardous seas warning conditions
is also likely for portions of the area between Florence and Cape
Blanco Sunday morning through Monday morning. The thermal trough
will persist into early next week, though it will weaken Tuesday and
Wednesday. /FB

&&

.FIRE WEATHER...Updated 230 AM Friday 21 July 2017...The two
main concerns are: easterly winds with moderate ridge top humidity
recoveries tonight and Saturday night, and a slight chance to chance
of thunderstorms Sunday afternoon through Tuesday evening. First, it
will remain hot and dry but with a slight improvement in humidities
and decrease in wind speeds to closer to normal levels.

Humidity recoveries will be poorest tonight 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
shortwave disturbance. 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. The highest
probability for thunderstorms Sunday afternoon and evening lies to
the east of the coastal ranges up to and including the Cascades. By
Monday afternoon, the area shifts to the East Side, but still
includes the Cascades. The latest ECMWF solution has trended towards
the GFS, which is concerning given the wider area of thunderstorm
chances and the more robust instability, and given the model
agreement, confidence is increasing in the likelihood of a multi-day
thunderstorm event. /FB

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...None.

CA...None.

Pacific Coastal Waters...Gale Watch from Saturday afternoon through
     Monday evening for PZZ356-376.
     Hazardous Seas Watch from late Saturday night through late
     Sunday night for PZZ350-370.

$$

MAS/FJB/JRS



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