NWS Medford, OR wrote its last area forecast discussion before the first event at Aug 1, 8:53 AM PST, 37 minutes earlier.
Area forecast discussion, Aug 1, 8:53 AM PST, 37 minutes before
Area Forecast Discussion
National Weather Service Medford OR
953 AM PDT Sun Aug 1 2021
.DISCUSSION...Plenty of thin smoke spread across the region today.
The newest smoke is from a number of fresh fires from Happy Camp
south and southeast. Some of these fires are showing up on
satellite RBG channels, showing increasing fire temperatures. Even
the fires not showing up on RBG channels, are showing up thanks to
rising smoke columns seen on visible channels. Offshore marine
stratus also makes up a large part of sky cover this morning.
There is a fairly expansive marine stratus shield over the coastal
valleys north of Cape Blanco. The cloud layer is hovering around
1000 feet and is very likely producing drizzle along the ridges.
We expect this stratus to recede back along the beaches by late
this afternoon before crawling back inland again tonight.
There are also thinning alto-CU over eastern Douglas County and
east. As these clouds thin, increased solar heating will begin to
trend us towards the possibility for a slight chance of
thunderstorms (Tstorms). However, the most favorable area for
thunderstorms continues to be along the border of Josephine and
Jackson County. While the thunderstorms along this area should not
be severe, they could be on the vigorous side. CAPE of 700-1000
J/kg, lapse rates of 8 c/km lend to an argument for strong
updrafts, resulting in strong outflow winds with 600-1100 J/kg of
DCAPE. Winds from cells in this area could push 40-50 mph outflow
winds and hail will be larger than the typical pea size and could
push nickle size provided the rain shaft can moisten the lower
level enough to mitigate melting, which current model solutions do
not show. Another mitigating factor is weak shear EBWD values
range from 20-32 kt, but since most of it is speed shear,
organization of cells will likely be limited. Instead, clustering
of a few simultaneously growing and decaying cores seems most
likely. Further, elevated CAPE values in the 0 to -20 degree C
range, are quite high at about 300 j/kg and is a sign that
frequent lightning is possible with these storms.
As the storms track north along the Jackson and Josephine county
line, they should begin to track to the NW, eventually decaying
in Central Douglas County. The favorable Tstorm environment
shifts east through the afternoon, so these cells should trend
downward as they head towards Douglas County. That said, the
environment does not decay quickly and lightning activity along
the umpqua divide is still a very real possibility.
The eastward shift in favorable Tstorm environment will trigger
another area of strong convection in the Prospect/Union Creek
area. Whether these storms track due north between Toketee Falls
and Diamond Lake or if they track NE towards Chemult will all come
down to timing. The earlier they trigger, the more likely they
retain a due north track, but a later ignition time would lead to
a more NE track. Probabilities for Tstorms taking the northerly
track towards Toketee Falls is between 8% and 20% depending on the
initializing model (GFS and NAM, NAM being the higher, which is
very common in our area). The probability of the NE track towards
Chemult are 10% and 45% (GFS/NAM). The mid-level environment is
more favorable from Crater Lake to Chemult with higher CAPE
values, but similar lapse rates and light shear again should keep
the severe threat low, but with 40-50 mph outflow winds and penny
or nickle size hail very much possible. The elevated instability
is higher along the N track rather than the NE track, so be on the
lookout for surprise lightning activity if the lower probability,
northerly path is where the storm activity takes.
Finally, there is very ample mid-level moisture with RHs at
700-500mb around 80% and PW values at or above 1", so these
thunderstorms should produce fairly strong rains directly under
there cores. Rain rates yesterday have been estimated between
1"/hr and 1.5"/hr and some minor debris flow issues, rapid rises
in small creeks, and ponding at poor drainage locations along
roads have been reported. It would not be surprising if isolated
cases of these issues are reported again. -Miles
&&
.AVIATION...For the 01/12Z TAF Cycle...Over the coastal waters,
along the coast, and in the coastal valleys...Widespread IFR
cigs/vsbys in low clouds and fog are expected this morning.
Clearing will occur inland but IFR will persist along the immediate
coast and offshore this afternoon. The lower conditions will move
back onshore late this afternoon and evening, then will persist
into Monday morning.
Over the remainder of the area...VFR conditions will persist into
Monday morning, but there will be isolated to scattered
thunderstorms this afternoon and evening with gusty winds. Smoke and
haze can also be expected from area wildfires with some visibility
limitations very near the fires, but probably remaining VFR
elsewhere. -Spilde
&&
.MARINE...Updated 230 AM PDT Sunday 1 August 2021...Offshore high
pressure and a thermal trough near the coast will both persist
through Tuesday. This will bring gusty north winds with steep,
choppy seas over the outer waters north of Cape Blanco, but also for
most other areas south of Cape Blanco. Areas of marine stratus and
fog will also persist. North winds will weaken Wednesday as a front
approaches the coast. That front will move onshore sometime Thursday
or Friday. The thermal trough and stronger north winds will return
though by next Sunday. -Spilde
&&
FIRE WEATHER...Updated 300 AM Sunday 1 August 2021...Deep
southerly flow will continue to bring monsoonal moisture into the
area today. Fire agencies have noted 14 new fire starts from
lightning across portions of southwest Oregon, with western Siskiyou
County also reporting numerous new fires. This occurred despite
wetting rains that occurred with many of these thunderstorms. So,
any thunderstorms are capable of creating new fires.
The main change to the forecast overnight was the addition of Fire
Weather Zones (FWZs) 620 and 622 to the Red Flag Warning for
abundant lightning today, expanding the area that included FWZ 623
and portions of FWZ 624, NOT including the Bootleg Fire. We did this
based on increasing trigger, moisture, and instability per the
latest models, and latest suite of high resolution models pointing
to storms forming near the CA/OR border this afternoon and pushing
north through Jackson and Josephine Counties. There is some chance
that these storms push north into Douglas County, affecting FWZs 616
and 617, but for now we think this coverage will remain isolated
there. See RFWMFR for details.
The focus of thunderstorm activity will move east Monday and a
warmer, drier, breezier, and more stable regime will develop in its
wake. A strong short-wave, as noted by ensemble means (this is
exceptional as ensembles tend to smooth out these features), will
approach northern California and southern Oregon Tuesday. This will
bring increasing winds to the forecast area, and dry and unstable
conditions (Haines 5-6) could promote fire growth in much of the
forecast area, but thunderstorms should be confined to mainly Lake
County.
Conditions will worsen on Wednesday as the short wave moves across
the area. Widespread Haines indices of 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. There will also be a slight chance of
showers and thunderstorms over northern Lake and Klamath counties.
Keene
&&
.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Red Flag Warning from 2 PM this afternoon to 10 PM PDT this
evening for ORZ620-622>624.
Heat Advisory until 11 PM PDT Monday for ORZ024-026.
CA...Heat Advisory until 11 PM PDT Monday for CAZ080-081.
Pacific Coastal Waters...Small Craft Advisory until 11 PM PDT Monday for PZZ356-370-376.