Wildfire in California, July 31, 2021

Area Forecast Discussion
National Weather Service Medford OR
334 PM PDT Sat Jul 31 2021

.SHORT TERM...Tonight through Tuesday...We will remain in a
pattern with unusually high amounts of monsoonal moisture as the
most notable feature. The main item of interest in the short term
is continued isolated to scattered coverage of showers and
thunderstorms over portions of our area, with activity mainly
during the afternoon and evening hours...and mainly away from the
coast and Douglas County.

But, the first item of business was to extend the Heat Advisory
for valleys of Josephine, Jackson, and western and central
Siskiyou counties. It is now in effect through Monday evening,
with above normal high temperatures (by several degrees) trending
slightly higher...into the lower 100s, and also low temperatures
remaining much above normal (by about 15 degrees) in the mid 60s
to near 70. It will be a consistent story at the coast in the
short term with night and morning low clouds and fog accompanying
cool, near normal temperatures.

Showers today have been mainly east of the Cascades. Coverage and
intensity of showers will soon increase this afternoon into this
afternoon with scattered coverage expected over the higher terrain
of the Siskiyous and southern Oregon Cascades, extending across
south central Oregon. Coverage will be isolated over the rest of
the area, excluding the coastal counties and the Umpqua Valley.

In keeping with our unusually hot summer, this is a pattern with a
ridge that has built unusually far north along the Rockies with
substantial amounts of mid and high level moisture. The region of
highest moisture amounts is centered over Idaho, but its western
fringe is represented by the showers east of the Cascades. On
Sunday, the northward tracking stream of monsoonal moisture will
shift farther to the east. But, shortwaves will still be embedded
in a southerly flow aloft, the convective hindrance of high level
cloud cover will diminish, it will be a bit hotter, and 700-500mb
moisture amounts will still be supportive of isolated to scattered
showers and thunderstorms over roughly the same area as today.

The pattern will shift slightly on Monday with drier air moving
into California, but another late day episode of isolated showers
and thunderstorms from the southern Oregon Cascades eastward.

Model uncertainty increases for Tuesday. Heat will likely be at
similar levels as the previous few days, but the air mass will
start a drying trend. For now, we will follow the large majority
of model solutions (including the GFS and ECMWF) with a forecast
of a dry day. But, the NAM and SREF models are not quite as dry,
leaving open the door for the possibility of a few storms over
northern Klamath County and Lake County.

.LONG TERM...Wednesday through Saturday...In a real shift, a
series of Pacific troughs will become the dominant drivers of our
weather. Temperatures are expected to be slightly less hot
Wednesday then trend down to near normal for the remainder of the
week. This change to an onshore flow pattern will bring gusty
afternoon and evening westerly winds, strongest east of the
Cascades. It will also bring an influx of low level moisture to
the coast with a slight possibility of light showers as early as
Wednesday afternoon, and a higher probability with a frontal
passage in the range of Friday night into Saturday morning.
Lastly, the timing and track of the troughs will be crucial in
determining the location and extent of any risk of thunderstorms.
This type of pattern usually creates a concern focused over
northeast Oregon and eastern Washingtion, more so than in our
area.

&&

.AVIATION...For the 31/18Z TAF Cycle...Over the coastal waters,
along the coast, and in the coastal valleys...Widespread IFR cigs in
low clouds north of Cape Blanco. Model guidance suggests these low
cigs will not quite recede off the coastline so have kept KOTH in
IFR to barely MVFR through the afternoon.  Stark contrast to
locations Cape Blanco (K92S) southward to just north of KBOK where
it is VFR and will remain so the rest of daylight hours. Coastal
eddy will keep low clouds hugging the coastline from KBOK southward
into northern California.

Over the remainder of the area...VFR conditions will persist into
Sunday morning, but there will be isolated to scattered
thunderstorms this afternoon/evening with gusty winds. Some
shower activity may persist through the night. There will also be
smoke from area wildfires, but probably not enough to drop
visibilities below VFR. -Sargeant/Spilde

&&

.MARINE...Updated 230 PM PDT Saturday 31 July 2021...Offshore high
pressure and a thermal trough near the coast will both persist
through Tuesday, bringing gusty north winds to the area. Steep
seas are expected south of Cape Blanco through Saturday night,
then will extend north into the outer waters from Florence to
Cape Blanco Sunday into at least Monday. Winds and seas will
remain highest south of Cape Blanco. Areas of marine stratus and fog
will also persist mainly from Cape Blanco northward and from
Brookings southward. North winds will weaken Wednesday as a front
approaches the coast. -Sargeant/Spilde

&&

.FIRE WEATHER...Updated 300 PM Saturday 31 July 2021...Deep
southerly flow will continue to bring monsoonal moisture into the
area through the weekend. It will not be as hot over the inland
areas due to more cloud cover, but afternoon temperatures will still
be above normal, with much warmer than normal overnight low
temperatures.

For today, the SPC SREF shows a broad area of 30 percent chance of
thunderstorms for portions of Oregon and northern California, mainly
in Siskiyou County north and east into the Cascades. Although the
probability for more than 100 strikes in an area has diminished
somewhat, there could still be abundant lightning this evening.
Satellite shows that the bands of showers east of the Cascades are
starting to put out some in-cloud lightning in some places, while
the clearer skies west of the Cascades are starting to show cumulus
buildups. Overall, expect thunderstorm activity to develop, and then
as steering flow shifts from southeast to southwest, expect the
storms to start moving northward and and westward this evening, over
many of the same areas that saw thunderstorms yesterday (and
including the east side this time).

Fire agencies have noted 14 new fire starts from lightning across
portions of southwest Oregon, with western Siskiyou County also
reporting numerous new fires. It is worth noting that despite
wetting rains that occurred with many of these thunderstorms, new
starts did ignite. So even though wetting rains are more likely in
today's storms, lightning may still create new fires and holdovers.
Therefore, will continue the Red Flag Warning for this reason.

The moisture will continue moving into our area on Sunday, bringing
cloud cover, cooler temperatures, and widespread showers.
Instability remains relatively high, especially along the Cascades
and portions of fire zones 624, but NOT including the Bootleg or
Jack Fires. Have upgrade the Fire Weather Watch to a red flag
warning for Sunday. Did not include 625 since it was a very small
area of Klamath and Lake Counties, and storms there will likely be
wet and more isolated in nature. Please see RFWMFR for details on
the Red Flag Warnings.

Thunderstorm chances begin pushing eastward on Monday as conditions
abruptly change to become warmer, drier, and breezier--typical for
summer time. One thing of note is that a strong short-wave, as noted
by ensemble means (this is exceptional as ensembles tend to smooth
out these features), approaches northern California and southern
Oregon on Tuesday. This could begin bringing more hot, dry, windy,
and unstable conditions. This gets even worse on Wednesday as the
disturbance moves across the area. Widespread Haines 5 and 6 is
expected over much of the area and any existing fires could see some
rapid spread, especially east of the Cascades. Winds along the
Winter Rim will also be particularly breezy, which was added in this
forecast package. The situation will need to be monitored as next
week could become the next problem in the fire weather world.
Stability increases on Monday west of the Cascades, so we're not
expecting thunderstorms there, but the threat continues east of the
Cascades. Then on Tuesday, the air mass becomes stable and dry
enough to keep thunderstorms out of the forecast. -Schaaf

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...     Red Flag Warning from 2 PM to 10 PM PDT Sunday for ORZ623-624.
     Heat Advisory until 11 PM PDT Monday for ORZ024-026.
     Red Flag Warning until 10 PM PDT this evening for ORZ621>625.

CA...Red Flag Warning until 10 PM PDT this evening for CAZ280.
     Heat Advisory until 11 PM PDT Monday for CAZ080-081.

Pacific Coastal Waters...Small Craft Advisory until 11 PM PDT Monday
     for PZZ356-376.
     Small Craft Advisory from 5 AM Sunday to 11 PM PDT Monday for
     PZZ370.

DW/Schaaf/Sargeant