Fire Weather Watch 38, NWS Medford, OR

Area Forecast Discussion
National Weather Service Medford OR
901 AM PDT Sun Sep 12 2021

.DISCUSSION...Observations along the coast show that the marine
stratus last night was able to produce drizzle at a couple of the
RAWS sites (Flynn Prairie and Seven Mile Creek) as well as at the
airport in Brookings. We've massaged the sky/wx grids to better
reflect these conditions this morning.

An interesting phenomena is showing up on the radar this morning
(at 1518 UTC) that might lead the untrained eye to think there is
a lone thunderstorm off the coast of Pt. St. George. However, the
atmosphere sometimes is just so that the radar beam is bent
downward, called "ducting", and due to the curvature of the Earth,
the beam is able to hit the surface. In this instance, the beam
is likely picking up the higher surface (ocean) waves developing
due to the increasing north winds caused by a strengthening
thermal trough. This can occur when there is a sharp inversion
with high humidity/moisture beneath the inversion (ie - marine
layer). We have those exact conditions this morning. Conditions
recently, however, at 1556 UTC have changed already (the sun is
coming up and the atmosphere is warming) and the phenomena, while
still there, is less evident.

Otherwise, the rest of the forecast is on track. Please see the
previous discussion below for the details on conditions inland
today, including high temperatures, breezy afternoon winds and
smoke/haze impacts. -Spilde

&&

.AVIATION...12/12Z TAFs...Marine stratus along the coast is
resulting in LIFR conditions, but is expected to burn off by this
afternoon to VFR. VFR conditions prevail inland today with another
day of improved visibility over interior SW Oregon as smoke blows
eastward over the Cascades. Isolated areas near and downwind of area
fires may experience local LIFR/IFR conditions in smoke. /BR-y

&&

.MARINE...Updated 825 AM PDT Sunday 12 September 2021...No
changes. Moderate north winds will continue south of Cape Blanco
this morning as the thermal trough begins to reestablish itself.
However, the thermal trough gets briefly interrupted this morning
as a weak, dry front moves through the region. A larger northwest
swell near 9 feet at 13 seconds will combine with choppy wind seas
and maintain advisory level conditions south of Cape Blanco
through this morning.

The thermal trough really takes hold this afternoon and strengthens
into early this week. North winds increase and seas become very
steep and hazardous from Gold Beach south with steep seas spreading
north of Cape Blanco today into Monday. Gales are possible Monday
afternoon in the southern waters from Gold Beach southward and a
Gale Watch has been issued for this. This pattern is expected to
continue through mid-week, with the strongest winds and steepest
seas expected during the afternoons and evenings. /BR-y/Spilde

&&

.PREV DISCUSSION... /Issued 442 AM PDT Sun Sep 12 2021/

DISCUSSION...A dry and warm air mass is expected over the
forecast area through the middle of the week as a thermal trough
dominates our local weather. A dry cool front will push through
from the west today, resulting in afternoon temperatures trending
a few degrees lower than yesterday. Additionally, afternoon
breezes in most places will trend higher than yesterday. As fires
likely become more active in a breezier and drier environment,
smoke will increase across much of the forecast area. Yesterday
featured relatively good air quality for some areas, with Medford
likely seeing its first "moderate" air quality day in 3 weeks.
Still, other areas struggled to see improved air quality, such as
Klamath, Lake, and Siskiyou Counties.

The thermal trough (generally moderate in strength) will peak in its
influence Monday and Tuesday, affecting firefighting efforts with
breezy north to northeast winds, especially for fires west of the
Cascades. Additionally, the air mass will become drier, resulting in
long burning periods. More can be found in the fire weather
discussion below. In these offshore flow/east wind patterns, the
Brookings area can get quite warm, and this will be no different. We
have forecast Brookings temperatures into the 80s both Monday and
Tuesday.

After Tuesday's inland push of the thermal trough, a cooling trend
is expected Wednesday through the weekend as a train of disturbances
moves toward the area from the Northeast Pacific waters. These will
initially be dry and bring increasing westerly to northwesterly
winds. Perhaps the first classic wet cold-season-type front
approaches on Friday, and winds will ramp up as we get 10,000-foot
winds (30kt or so) that we haven't seen since springtime.

For the upcoming frontal system, models have generally trended
wetter over the past 24-48 hours, and confidence is increasing for
widespread rainfall for at least Southern Oregon Cascades
westward. This trend is a good sign as the beneficial nature of
any additional wetting rains cannot be overstated. The beneficial
impacts include decreasing threat from area wildfires, long-
lasting air quality improvement (improving mental/physical
health), aids to firefighting efforts, a minor dent to ongoing
drought, benefits to area vegetation, etc. Latest data suggests
moderate/50% chance for wetting rainfall (using 0.25 inches or
greater in 24 hours for this discussion) in the Saturday
timeframe for Douglas, Coos, and Curry Counties and Cascades in
general. Lower chances exist for valleys in Jackson and Josephine
County as well as Siskiyou County. Still lower potential for
wetting rains exists east of the Cascades. It should be noted that
latest data has trended wetter even south of the OR/CA border,
but again, would like to see more model runs that support wetting
rains here.

All in all, we're optimistic about upcoming significant rainfall
chances, but with a dose of experience-driven caution given that in
this extreme drought we've seen signs of significant rainfall before
that have failed to produce. Keene

AVIATION...12/06Z TAFs...Coastal stratus later tonight returns with
LIFR condition expected at the coast, then burning off by Sunday
afternoon to VFR. VFR conditions prevail inland Sunday with another
day of improved visibility over interior SW Oregon as smoke blows
eastward over the Cascades. Isolated areas near and downwind of area
fires may experience local IFR conditions in smoke, with LIFR
visibility in smoke possible over many areas in northern California
and east of the Cascades in south-central Oregon. Stavish

MARINE...Updated 215 AM PDT Sunday 12 September 2021...Moderate
north winds will continue south of Cape Blanco this morning as the
thermal trough begins to reestablish itself. However, the thermal
trough gets briefly interrupted this morning as a weak, dry front
moves through the region. A larger northwest swell near 9 feet at 13
seconds will combine with choppy wind seas and maintain advisory
level conditions south of Cape Blanco through this morning.

The thermal trough really takes hold this afternoon and strengthens
into early this week. North winds increase and seas become very
steep and hazardous from Gold Beach south with steep seas spreading
north of Cape Blanco today into Monday. Gales are possible Monday
afternoon in the southern waters from Gold Beach southward and a
Gale Watch has been issued for this. This pattern is expected to
continue through mid-week, with the strongest winds and steepest
seas expected during the afternoons and evenings. /BR-y

FIRE WEATHER...Updated 400 AM PDT Sunday, 12 September
2021...A thermal trough will tend to dominate the fire weather
environment through the middle of the week. Today a dry front will
bring a relative maximum in west to northwest general winds. Tonight
into Monday north to northeast winds will trend higher, reaching
moderate speeds (strongest near the coast) Monday afternoon into
Monday evening. Gusts to 30 mph are expected over the higher terrain
of Western Siskiyou County and into the Southwestern Oregon
Kalmiopsis and Coast Ranges. Overnight and afternoon humidity will
bottom out Monday night into Tuesday as the thermal trough begins to
push inland some.

Winds will then trend higher again on Wednesday afternoon, shifting
to a more westerly to northwesterly direction as upper troughs to
the north begin to influence the region and disrupt the thermal
trough some. Several days of gusty afternoon westerly to
northwesterly winds can be expected Wednesday into Friday, impacting
areas east of the Cascades the most.

I'll copy/paste what was written in the main section of the forecast
discussion here, adding a bit more detail: For the upcoming
frontal system, models have generally trended wetter over the past
24-48 hours. This is a good sign as the beneficial nature of any
additional wetting rains cannot be overstated. The beneficial
impacts include decreasing threat from area wildfires, long-
lasting air quality improvement (improving mental/physical
health), aids to firefighting efforts, minor dent to ongoing
drought, benefits to area vegetation, etc. Latest data suggests
moderate+ confidence for wetting rainfall (0.25 inches or greater)
in the Saturday timeframe for Douglas, Coos, and Curry Counties
and Cascades in general. Lower confidence exists for valleys in
Jackson and Josephine County as well as Siskiyou County. Lower
potential for wetting rains exists east of the Cascades. It should
be noted that latest data has trended wetter even south of the
OR/CA border, but again, would like to see more model runs that
support wetting rains here.

More specifically, for some area wildfires, chances of wetting
rains (using 0.25 inches for this discussion) for eastern Douglas
County have generally trended up from 30% to 50% from a day ago,
from 15% to 30% over far Western Siskiyou County, but hang around
10 to 15% for Eastern Siskiyou County.

All in all, we're optimistic about upcoming significant rainfall
chances, but with a dose of experience-driven caution given that in
this extreme drought we've seen signs of significant rainfall before
that have failed to produce. Keene

&&

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

Pacific Coastal Waters...Gale Watch from late tonight through late
     Monday night for PZZ356-376.
     Small Craft Advisory until 5 AM PDT Tuesday for PZZ356-376.
     Hazardous Seas Warning from 2 PM this afternoon to 5 AM PDT
     Monday for PZZ356-376.
     Small Craft Advisory from 2 PM this afternoon to 5 AM PDT
     Tuesday for PZZ350-370.

MAS/NSK/MNF