Area Forecast Discussion
Issued by NWS Medford, OR

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FXUS66 KMFR 212124

Area Forecast Discussion
National Weather Service Medford OR
224 PM PDT Thu Sep 21 2017

.DISCUSSION...The upper trough axis is east of area which has
allowed things to quiet down considerably compared to this time
yesterday. However, there is a weak disturbance that is moving down
along the coast. Most, if not all the precipitation should stay
offshore, but could not rule out isolated through this evening. The
HRRR  high res model does not much of anything through this evening.
Therefore we`ll keep isolated showers in the forecast with very
light QPF amounts. Plenty of cloud cover remains and that will be
the case through this evening. If we don`t get any breaks in the
cloud cover, there`s a chance we could set a new record for the all
time record low max temperature which is 61 set back in 1945. Right
now were at 58 degrees.

Most locations and hours will be dry tonight. Cloud cover should
dissipate some, but not a lot. Despite the cloud cover, were still
expecting temperatures to drop near or below freezing tonight
east of the Cascades and a freeze warning and frost advisory remains
in effect. Also could not rule out patchy fog in some westside
valley locations late tonight if there`s enough clearing.

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. Another cold night is expected Friday night east of the
Cascades and there will be less cloud cover and wind. So a freeze
watch and frost advisory will be issued, but this time it will only
be for southwest Klamath County, northeast Siskiyou and northwest
Modoc counties. Please see NPWMFR for details on the locations of
warnings, advisories and watches for tonight and again Friday night.

Dry and warmer weather returns this 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.

Confidence is high we`ll remain dry for all of next week with above
normal temperatures. The upper ridge is expected to gradually move
over our area Tuesday through Wednesday with continued dry weather
and max temperatures averaging close to 10 degrees above normal.

The models are in general agreement showing the ridge axis shifting
east late next week with some cooling as a weak front approaches the
coast. -Petrucelli


.AVIATION...21/18Z TAF CYCLE...VFR will prevail the rest of today.
However, partial terrain obscurations will persist and local MVFR is
possible in heavier showers. Showers will be more hit-or-miss though
compared to the last few days. IFR/MVFR ceilings and visibilities
will redevelop in most west side valleys overnight into Friday
morning as well as at the coast, mainly from Cape Blanco northward.
Other than patchy fog, VFR will prevail elsewhere tonight into
Friday morning. -Spilde


.MARINE...Updated 200 PM PDT Thursday, 21 September 2017... Steep
seas will return this evening for areas well offshore. Winds will be
light today as weak low pressure drifts southward through the
coastal waters bringing with it more showers. Friday afternoon and
evening, north winds and seas will begin to build with small craft
advisory level conditions expected for much of the area south of
Cape Blanco by Friday evening. This will begin a prolonged period of
northerlies that will maximize in strength each afternoon and
evening as thermal troughing returns and persists. /FB


.FIRE WEATHER...Updated 200 PM PDT Thursday, 21 September 2017....
The cold upper level trough that brought the wet, unsettled
weather over the area earlier this week will gradually move into
the Great Basin and the Intermountain West over the next 24-
48 hours. Mostly light to moderate northerly flow will persist on
the back side of this trough, and while this is generally a dry
flow for the area, isolated to scattered showers will persist.
Showers will be most numerous near the coast, inland west of the
Cascades, and in southern Lake and Modoc Counties through this
evening, then tend to diminish after midnight. Isolated
showers are still possible Friday and Friday evening, but any
amounts will be very light.

Meanwhile, high pressure offshore will begin to build into the area.
This will result in a thermal trough developing along and near the
coast. Light northeast winds associated with the development of the
surface thermal trough may begin as early as late tonight/early
Friday morning over the SW Oregon coast range and western Siskiyou
County mountains, but humidity recovery should again be very good.
The thermal trough will strengthen this weekend and result in east
to northeast winds each night and morning through at least Tuesday.
Nighttime humidity recoveries will gradually worsen and afternoon
humidity will likely lower to 20-30% across most of the forecast
area. Temperatures are expected to be 5 to 10 degrees above normal
and warmth will likely reach all the way to the coast (mainly south
of Cape Blanco) each afternoon.

With recent rainfall, models may be somewhat overdoing the amount
of drying, at least initially. So, for this weekend we`ve leaned
toward the consensus guidance rather than seeking the drier
solutions (as we were doing prior to the significant rainfall). We
have lowered minimum RH values slightly across the area early
next week.

At midweek, the thermal trough is expected to weaken and move
north/inland. As such, the northeast winds will abate and we`ll
expect more of a marine influence along the coast Wednesday onward.
It will remain dry and warm inland. Extended model guidance is
hinting at a potential (mostly dry) frontal passage late next week,
which could result in stronger afternoon winds, especially over the
east side. We`ll continue to monitor this for any trends.

CPC`s 8-14 day outlook as well as weeks 2-4 on the CFSv2 continue to
indicate enhanced probabilities of above normal temperatures and
below normal precipitation. So, a steady and long-duration warm and
dry period is expected, albeit tempered by the declining sun angle
associated with the changing of the seasons. Spilde/BTL


OR...Frost Advisory from 2 AM to 9 AM PDT Friday for ORZ029>031.
     Freeze Warning from 2 AM to 9 AM PDT Friday for ORZ029>031.
     Frost Advisory from 2 AM to 9 AM PDT Saturday for ORZ029.
     Freeze Watch from late Friday night through Saturday morning for

CA...Frost Advisory from 2 AM to 9 AM PDT Friday for CAZ084-085.
     Freeze Warning from 2 AM to 9 AM PDT Friday for CAZ084-085.
     Frost Advisory from 2 AM to 9 AM PDT Saturday for CAZ084.
     Freeze Watch from late Friday night through Saturday morning for

Pacific Coastal Waters...Small Craft Advisory for hazardous seas until 11 AM PDT Friday
     for PZZ370-376.

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