Wildfire in Oregon, August 1 to 17, 2019

Area Forecast Discussion
National Weather Service Medford OR
655 PM PDT Wed Jul 31 2019

The Aviation section has been updated...

.UPDATE...

We're seeing some radar returns in Siskiyou county this evening
and we've already observed some cloud to ground strikes about 10
miles west of Mt Shasta City. As a result, we added a slight
chance for thunderstorms near Mt Shasta and areas to the northwest
near Macdoel California. Lightning should be few and far between
as the potential instability is still quite limited around the
region this evening.

Read the discussion for more information beyond this evening.

-Smith

&&

.AVIATION...For the 01/00Z TAFs...
Some cumulus build-ups west of the towns of Weed and Mount Shasta
have developed into an isolated thunderstorms, putting down a couple
of bolts of lightning, thus far, this evening. Through 8pm PDT and
northeast of Tennant erratic winds and lightning will be possible
from the Trinity Alps northeast to Weed and in the higher terrain
northeast of Tennant and southwest of Tule Lake.

Otherwise, IFR to LIFR patchy fog and low clouds are expected to
form again overnight along and near the coast south of Gold Beach
and north of Cape Blanco. The timing and extent of this is
uncertain, but most of the guidance is indicating this to form late
and burn off early, generally in the 09Z to 17Z time frame. We're
expecting KRBG to remain cloud free, so formation of low clouds in
Douglas County will probably only make it to Elkton and Camas Valley.

Elsewhere, VFR will generally prevail. However, expect some 5 to 10
mile visibility smoke south and southeast of the Milepost 97 Fire
into the Rogue and Shasta Valleys this evening gradually dissipating
overnight into the middle of the day Thursday. Another round of
smoke will push south and southeast late Thursday afternoon and
evening. BTL

&&

.MARINE...Updated 145 PM PDT Wednesday 31 July 2019...A weak thermal
trough is along the coast resulting in relatively light north winds
and seas. A front will approach the coastal waters Thursday, and
then move onshore early Friday. Winds will turn south to southwest
on Thursday but are expected to remain fairly light, gradually
increasing late in the day. Winds will increase some Thursday night
into Friday morning, nearing small craft advisory levels, as the
front moves on shore. This may result in a period of steep seas
Thursday night and Friday morning, mainly across the outer waters
beyond 10 nm from shore from Cape Blanco north. Saturday and Sunday,
expect a return of gusty north winds as the thermal trough
redevelops. Steep seas and small craft winds are possible mainly
from Cape Blanco south this weekend. The thermal trough will linger
into early next week but may gradually weaken.

-CC

&&

.FIRE WEATHER...Updated 200 PM PDT Wednesday, 31 July
2019...Conditions east of the Cascades will be at least moderately
dry and unstable with Haines Index values of 5 and 6 in Lake and
Modoc Counties. These dry and unstable conditions will affect the
Tucker Fire. These conditions will last in the afternoons into the
evening through Friday.

On Thursday an approaching front will help ramp up winds some in
most areas, and with very dry air over Modoc County, we've issued a
Fire Weather Watch for single digit humidity and gusts into the 20s
(mph). Additionally, ahead of the front there should be just enough
moisture, instability, and trigger to support thunderstorms from
Siskiyou County and the Siskiyous in Southwest Oregon northward into
the Cascades and over Norther Klamath and Lake Counties.

A weak front move through Friday morning with light showers expected
near the coast and possibly inland into Douglas county, but amounts
will be light and confidence is low that they will bring any rain to
the Milepost 97 fire.

Thereafter, a brief warming and drying trend is expected Saturday
and Sunday before yet another upper low approaches from the
northwest and likely brings a cooldown with generally stable
westerly to southwesterly flow aloft. There is one potential day for
lightning per the latest model data, and that's Monday. However, mid
level moisture is marginal at best, and do not have enough
confidence to add at this time. Some energy from the southwest
squeezes under the approaching upper low and reaches the forecast
area Monday morning. We'll keep an eye on this feature to see if
models come into better agreement with what we can expect.

Sven

&&

.PREV DISCUSSION... /Issued 241 PM PDT Wed Jul 31 2019/

Discussion...A look at the larger picture shows an upper low
centered in the southern part of the Alaska panhandle and a strong
ridge centered just east of the four corners. In between is a stable
southwest flow which includes our area. In our area skies are clear.
The smoke from the Milepost 97 fire is mainly confined to around and
just northeast of the fire perimeter, but the smoke is showing signs
of migrating southeast towards the Rogue Valley and we should expect
to see an increase in smoke later this afternoon into this evening.
Even then guidance is suggesting the concentration of the smoke
should not be as bad as it has been the last few days.

Dry weather will continue through at least tomorrow morning. An
upper trough approaching our area could bring the ingredients
necessary to warrant a slight chance of thunderstorms mainly for
the northern Cascades and portions of northern California. Two
things are working against thunderstorm development late tomorrow
afternoon and evening are moisture and weak trigger. Thus would not
be surprised if nothing happens, but at the same time could not rule
it out, so we'll keep a slight chance in the forecast, but did scale
back the coverage area.

A weak front will approach the area Friday and bring light
precipitation mainly to the coast, north of Cape Blanco, Coos and
northwest Douglas County. Even then precipitation amounts will be a
few hundreths of an inch at best. However the Milepost 99 fire
probably won't see any rain at all.

After Friday, we'll remain dry through at least the middle of next
week. The one day we'll have to keep a watch on will be next Monday.
The GFS continues to show an upper trough moving from the southwest
Monday morning while the ECMWF has the low further offshore. Even if
we take the GFS at face value, the timing of the upper trough will
be moving in during the morning with the greatest instability
centered north of our forecast area. For now we'll follow the
previous shift lead and keep it dry, but this could be subject to
change.

Temperatures will be above normal through the forecast period with
slight cooling Friday as the weak upper trough moves through.
Temperatures will warm up Saturday through at least next week.
However were not expecting any significant warming with the mean
ridge expected to be centered near the four corners.

-Petrucelli

&&

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

CA...Red Flag Warning from 2 PM to 8 PM PDT Thursday for CAZ285.

Pacific Coastal Waters...None.