Area Forecast Discussion
Issued by NWS Medford, OR

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FXUS66 KMFR 261053
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
353 AM PDT Tue Sep 26 2017

.DISCUSSION...An upper ridge axis centered just off the coast will
lean in over the PacNW and gradually become positively tilted
over the next 48 hours. A surface thermal trough will also remain
along the coast during this period. This will lead to warm weather
with offshore breezes along much of the coast through tomorrow.
Over inland areas, temperatures will warm about 5 degrees today
compared to yesterday, and this will put us a few degrees above
normal. As the thermal trough moves inland later tomorrow into
Thursday, temperatures will warm even more and peak at around 10
degrees above normal. At the coast, it will start to cool off
tomorrow evening as the flow turns onshore in the wake of the
thermal trough, and then Thursday will be much cooler with marine
stratus approaching the coast if not moving onshore. All of this
was well covered in the going forecast, and few changes were made
to the forecast through Thursday.

Significant changes were made to the forecast starting Friday and
continuing on through the rest of the period. Models are in
agreement in showing a frontal system moving into the waters early
Friday and then onshore during the day. There are differences in
timing and amounts of rain with the front, and while a good part
of this front`s precipitation will likely be sacrificed in
moistening the airmass, higher pops certainly seemed warranted,
especially along the coast and across the north.

The front then moves east on Saturday, and we are left with moist
westerly flow over our area. This pattern lends itself to at least
some chance for rain, focused along the coast and northern areas
into Sunday. Later Sunday into early next week model consistency
goes out the window to a degree. The ECMWF and Canadian show a
potentially very wet system impacting our area as early as Sunday
evening (Canadian) or Monday (ECMWF). By contrast, the GFS, which
had a similar solution in earlier runs, almost entirely dropped
this system on the 00Z run and showed a weakening system along the
coast and an upper system moving down inland through the Great
Basin. Given the poor run-to-run consistency with the GFS as well
as its very large ensemble spread during the period, we leaned
much more on the ECMWF/Canadian solutions. Many adjustments
(mostly wetter and cooler) were made to the forecast from Saturday
into early next week. -Wright

&&

.AVIATION...26/12Z TAF CYCLE...At the coast north of Cape Blanco
and in the lower Umpqua Valley northwest of Roseburg...areas of
IFR/LIFR cigs/vis are expected this morning through around 16z,
then should clear to VFR around or after 17z. Patchy fog may
develop near RBG for a few hours this morning. Elsewhere, VFR is
expected through the TAF period. -Spilde

&&

.MARINE...Updated 345 AM PDT Tuesday, 26 September 2017...High
pressure offshore and a thermal trough along the coast will
maintain gusty north winds and steep wind seas through tonight.
Winds and seas will increase today with small craft conditions
south of Cape Blanco expanding north of Cape Blanco this evening.
Occasional gusts to gale force are possible this evening mainly
from Gold Beach south beyond 5 nm from shore. Very steep wind-
driven seas are expected to develop by this evening from Brookings
south between 10 and 45 nm from shore. North winds and steep seas
will gradually diminish by Wednesday afternoon. Light southerly
winds and light seas are expected Wednesday night into Thursday. A
cold front will push through on Friday with southerly winds
becoming northwesterly. Seas will increase a bit this weekend as a
northwest swell moves into the waters. -Spilde

&&

.FIRE WEATHER...Updated 345 AM PDT Tuesday, 26 September 2017....
Warming and drying will continue over the area today through
Thursday. Breezy northeast to east winds are expected this morning
and again tonight into Wednesday morning on the mid slopes and
ridges of Western Siskiyou and Curry Counties bringing dry air all
the way to the coast. The higher peaks along the Cascades will
have breezy winds tonight into Wednesday morning and then maybe
again Wednesday night into Thursday morning. Overall, it will be
breeziest tonight, then it will be driest Wednesday night into
Thursday morning as offshore flow weakens and the thermal trough
pushes inland. Marine air will have more influence along the coast
by Thursday, so it will be cooler there with higher humidity.

Inland, temperatures are expected to peak with lowest
humidity values both Wednesday and Thursday afternoons with the
thermal trough advancing inland. It should be noted up front
that we are not expecting critical fire weather conditions (that
would result in Fire Weather Watches/Red Flag Warnings) at
any point this week. But, with dry, unstable conditions expected
Wed/Thu, if there was a period of time where fire activity picks
up, that would be it.

By the end of this week, model guidance is in agreement showing a
cold front moving through the area on Friday. Westerly winds will
become gusty for much of the forecast area Friday afternoon and
evening and the front could bring some light rain along
the coast, north of the Umpqua Divide and over to the Cascades.
Rain amounts should be light, however.  Most NE California
and East Side locations should stay dry, but isolated showers
could make it east of the Cascades. It will turn much cooler
over the weekend with most areas running 10-15 degrees below
average. Some showers will persist north and west of the Umpqua
Divide.

Models diverge early next week, so no significant changes were
made to the forecast Sunday night onward. -Spilde

&&

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

CA...None.

Pacific Coastal Waters...Small Craft Advisory until 11 AM PDT
     Wednesday for PZZ356-376.
     Small Craft Advisory from 5 PM this afternoon to 11 AM PDT
     Wednesday for PZZ350-370.
     Hazardous Seas Warning from 5 PM this afternoon to 11 PM PDT
     this evening for PZZ376.

$$



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