Wildfire in Oregon, August 16 to 31, 2020

Area Forecast Discussion
National Weather Service Medford OR
854 PM PDT Sat Aug 15 2020

.DISCUSSION...There is some impactful weather to discuss this evening,
including heat, thunderstorms, and south winds that are
transporting smoke to the area. First, heat. Please see
weather.gov/medford for graphics that depict the record high
temperatures. In short, Medford reached 108 degrees, and this ties
a daily record for Aug 15, and Roseburg smashed the old record of
98 degrees set in 1933 with a high temperature today of 108
degrees! It...was...hot. Temperatures tomorrow will "cool" by a
few degrees, but it will still be hot. The Excessive Heat Warning
continues through Sunday to highlight the dangerous heat. We're
considering extending this into Monday because of the little
relief expected early next week. We'll see how the evening model
guidance trends.

There were about 120 cloud to ground (CG) lightning flashes this
afternoon and evening. A few more showers are ongoing. The lightning
was concentrated in Lake County with 103 CG flashes there.
Tomorrow's thunderstorm activity will be more significant and is
expected to stretch west of the Cascades. Significant triggers will
move up from the south to help initiate thunderstorms in our area
tomorrow, acting in concert with significant moisture (Precipitable
Water values 1in+) and instability. Evening with evening model data
coming in, there's still some uncertainty in how the situation
exactly plays out tomorrow, especially for areas west of the
Cascades.  Enough data, however, points to lightning west of the
Cascades that Red Flag Warnings continue for much of the forecast
area tomorrow, including portions of Jackson and Douglas Counties.
Lots of moisture and some strong winds aloft suggest these storms
will produce brief periods of heavy rain along with the potential
for strong winds.

Lastly, smoke moving up from Northern California into Southern
Oregon. The Red Salmon wildfire in Western Siskiyou County pumped
out a lot of smoke this afternoon, and it's moved into the Jackson
and Josephine Counties. Air Quality readings suggest we're in the
Unhealthy for Sensitive Groups range, and that could remain that way
through the evening per latest HRRR smoke model output.  Expect some
improvement tomorrow morning, but south flow will bring more smoke
to the area tomorrow evening. We've updated the forecast with the
latest HRRR smoke model output, which seems to be matching current
observations nicely.

Please see the previous discussions below for more details on the
thunderstorm potential and heat.

&&

.AVIATION...For the 16/00Z TAFs...VFR and clear skies are expected
to continue for most areas through the TAF period. One exception is
smoke from the Red Salmon Fire, which will reduce visibilities in
much of western Siskiyou County, with haze potentially spreading to
the north and northeast. Due to heat, density altitude could be a
concern today. Another concern is the possibility of isolated
thunderstorms this afternoon and evening, mainly over the high
terrain of the Cascades, Sikiyou, and Klamath mountains, and across
the East Side. Tonight, areas of IFR/LIFR conditions may affect the
coast and the coastal waters as a southerly surge brings low clouds
north. All other areas should remain VFR. -BPN/BR-y

&&

.MARINE...Updated 800 PM PDT Saturday 15 August 2020...South winds
will persist through Monday morning with low clouds and fog likely
returning sometime Sunday morning into the afternoon. Winds turn
northerly again by Monday afternoon but conditions will remain
relatively calm with light winds and seas through at least mid-week
when the thermal trough is expected to return. This event appears to
be trending to the weaker side, so while conditions may become
hazardous again by mid-week, it doesn't appear to become as
hazardous as the most recent event. Stay tuned as details become
more clear. /BR-y

&&

.FIRE WEATHER...Updated 200 PM PDT Saturday 15 August 2020...
We'll remain under the influence of a strong upper ridge, centered
over the Great Basin, the rest of this weekend into early next week.
This is bringing the hottest weather of the season thus far at
climatologically the driest time of the year. We're not
anticipating widespread lightning this afternoon, but a couple
of isolated cells could pop up near the Siskiyous and also the east
side mountains through this evening (and move off to the NNE
before dissipating later in the evening).

Meanwhile, a closed low is off the California coast. Deep southerly
flow will transport subtropical moisture northward, especially
in the mid and upper levels, through California tonight and
into our area on Sunday. Multiple disturbances will move northward
during this time and likely generate isolated to scattered
thunderstorms beginning as early as Sunday morning. With the
initial surge of moisture Sunday morning, convection is likely
to be elevated, and thus, dry. What we have seen a few times in the
past is that these events end up being a large canopy of mid and
high level cloud cover w/showers leading to very little in the way
of lightning. This is a concern, but given the extreme dryness
preceding this event and the potential for numerous new fire
ignitions with any lightning, we have decided to upgrade the Fire
Weather Watches to Red Flag Warnings with a start time of 15z
(8 am).

The focus of the best instability is shown on the models to be on
the NW periphery of this potential convective area Sunday
afternoon/evening. This is from northern and eastern Jackson
County up across the Umpqua Divide and eastward to the Cascades with
another area across northern Klamath/Lake Counties. While all
locations within the Red Flag Warning could see new fire starts, it
is these areas that stand the highest chance of lightning-caused
ignitions. Even though PWs increase to 1.00 or greater and rainfall
will accompany storm cores, rainfall may be brief due to fast mid-
level flow and storm motions of 25-35 kt. And, it should be noted
that since the mid-level flow is strong, any of the stronger cells
will have the ability to produce gusty outflows of up to 55 mph.
Guidance shows most of the activity ending by around 03z (8pm), but
have maintained the end time of 06z (11 pm) as a buffer. Activity
should wane or end overnight.

Another shortwave will rotate north-northeastward Monday. There
should be another round of scattered thunderstorms across portions
of the east side, so have hoisted a Fire Weather Watch for Fire
Zones 624 and 625 in Oregon and 285 in California.

We'll remain in S-SW flow aloft through at least Tuesday, so the hot
weather looks to continue, albeit not quite as hot as over the
weekend. Isolated thunder chances continue in north Cascades and
over far eastern sections Tuesday afternoon. Temperatures should
ease back but remain above normal Wed-Fri next week with little or
no chance of thunderstorms. -Spilde

&&

.PREV DISCUSSION... /Issued 300 PM PDT Sat Aug 15 2020/

DISCUSSION...Temperatures are within a couple of degrees of daily
record highs as of 2 PM PDT this afternoon. Therefore, confidence
is high that we're going to see a number of records fall today.
Roseburg has already tied their record of 98 and North Bend is
within a degree of a 101 year old record of 84. Please keep in
mind that you need to make extra effort to remain hydrated today
due to the very hot and dry air mass in place.

Cumulus fields are developing in the areas that we've been
expecting them. We've also already seen a few significant cumulus
build-ups across the Modoc that are showing up on radar in the
last hour. However, there have been no CG flashes yet, nor has
there been any in-cloud lightning detected. Models and our going
forecast continue to indicate a slight chance of thunderstorms
developing this afternoon and evening from the Marble Mountains
to the western edge of the Shasta Valley northward into the
Siskiyous and Jackson County to north and northeast of Crater Lake
NP and across northern Klamath County, Lake, and Modoc counties.
If a storm does form, it will produce little or no rainfall.
Current convective cloud development has been coincident with
where the coolest air is at 500MB. While that coolest air is
currently east of the Cascades, models suggest it will shift to
areas along and near the Cascades through this evening.

Red Flag Warnings have been issued along with the pre-existing
Excessive Heat Warnings and Heat Advisories for Sunday. The Red
Flag Warnings are for expected lightning on very dry vegetation
along with gusty outflow winds. It will be generally breezy Sunday
and thunderstorms across the area will be capable of producing
gusty outflow winds of 40 to 60 mph. While increasing precipitable
water values of 1 to 1.5 inches by Sunday afternoon means that
these thunderstorms will be capable of producing moderate to heavy
rain rates, storm motions of 20 to 40 mph means that storms are
unlikely to be over any particular area for very long before
moving off. Therefore, if you will be outside Sunday between 8am
and 11 pm PDT where we're forecasting thunderstorms, be ready to
take shelter should it become necessary to do so. While we have
moderate confidence in our going forecast for thunderstorm
coverage, there continues to be a high amount of spread in
possible solutions from the models due to the highly convective
and, therefore, chaotic nature of this weather event. Some of the
Convective Allowing Models (CAMs) suggest cloud cover and rain
with embedded thunder moving through Sunday morning with isolated
thunderstorms in the afternoon while others suggest thunderstorms
with little rain in the morning followed by severe thunderstorms
in the RFW warning area (to include on the west side in Jackson
and Douglas counties) in the afternoon to early evening.

While the heat and thunderstorm threat goes down with time, the
forecast has generally trended to keep thunderstorms in the
forecast a bit longer further west for Monday and Tuesday. The
NAM12 continues to be farthest west with the lingering threat,
while the GFS20 is further east. Collectively, the models have
generally trended toward the thunderstorm threat being from the
Cascades eastward for Monday and Tuesday. High PWATs of 1 to 1.4
inches Monday and Tuesday mean that thunderstorms could be quite
wet where they do occur. ~BTL

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Red Flag Warning from 8 AM to 11 PM PDT Sunday for
     ORZ617-621>625.
     Fire Weather Watch from Monday afternoon through Monday evening
     for ORZ624-625.
     Blowing Dust Advisory until 9 PM PDT this evening for ORZ031.
     Heat Advisory until 11 PM PDT Sunday for ORZ027>029.
     Excessive Heat Warning until 11 PM PDT Sunday for ORZ024-026.
     Heat Advisory until 11 PM PDT this evening for ORZ021-022.
     Excessive Heat Warning until 11 PM PDT Sunday for ORZ023-025.

CA...Red Flag Warning from 8 AM to 11 PM PDT Sunday for
     CAZ280>282-284-285.
     Fire Weather Watch from Monday afternoon through Monday evening
     for CAZ285.
     Heat Advisory until 11 PM PDT Sunday for CAZ082>084.
     Excessive Heat Warning until 11 PM PDT Sunday for CAZ080-081.

Pacific Coastal Waters...None.