NWS Medford, OR wrote its last area forecast discussion before the first event at Aug 12, 1:37 PM PST, 53 minutes earlier.
Area forecast discussion, Aug 12, 1:37 PM PST, 53 minutes before
Area Forecast Discussion
National Weather Service Medford OR
237 PM PDT Thu Aug 12 2021
.DISCUSSION...Heat, wildfires, smoke--those are the three weather
and weather-related elements we're focused on in the near-term. A
record hot air mass is affecting the forecast area today (record
high temperature at 5000 ft for this morning's balloon sounding),
and the ridge of high pressure responsible for this really won't
budge until late Sunday. As is often the case, the heat is
lasting longer than models initially suggested days ago. We
maintain an Excessive Heat Warning for areas west of the Cascades,
and we have a Heat Advisory out for areas east of the Cascades.
We keep the Warning out through Saturday, and the Advisory
continues through Friday. This still looks reasonable per our
latest forecast. We continue to trim about 3 degrees off high
temperature forecasts for most of the area due to the smoke
preventing maximum solar insolation. Temperatures finally lower
some Sunday and moreso on Monday as upper troughing starts to
impact the region. How long this cool weather lasts is still a
question, and in relying on the National Blend of Models (NBM)
latest run, forecast high temperatures bump back above normal by
Wednesday after some below normal temperatures early in the week.
We looked closely at thunderstorm threat, with the idea in mind that
impacts from any lightning on the landscape could be quite
significant in terms of fire starts, given the ongoing extreme
drought conditions. Additionally, gusty winds from any storms could
cause problems for ongoing firefighting efforts. We expect
isolated to scattered storms tomorrow for most areas from the
Cascades and Siskiyous south and eastward. We've also added some
areas just west of the Cascades (Jackson County) in the scattered
thunderstorm coverage area, per the latest suite of Convection
Allowing Models (CAMs). There's usually one or two variables that
lower confidence in a thunderstorm forecast in this area, and the
main one for today/tomorrow is smoke. Smoke can lower surface
temperature and lessen instability, resulting in lower chances for
thunderstorms. The area we're most confident in seeing scattered
thunderstorms is east of the Cascades where afternoon smoke should
be thinner. Storm chances continue Saturday and Sunday, but
chances and coverage diminish compared to Friday. Winds increase
Sunday into Monday as we feel the cool front pushing through the
forecast area.
The long and the short of the smoke forecast is that we don't expect
widespread, significant improvement until Sunday, and more likely
Monday given the latest data. The atmosphere doesn't really
thoroughly mix until we get some increasing winds, and that won't
happen until perhaps Sunday afternoon, but significant improvement
may take until Monday. For now, continue to take precautions so you
avoid being impacted by this poor air quality. Stay indoors, use
quality air filters, and avoid strenuous activity if you venture
outdoors. Keene
&&
.AVIATION...13/18Z TAFs...Away from the coast...Smoke from local
wildfires continues to be the main impact to aviation for inland
areas. MVFR visibilities are occurring across much of the area with
local IFR conditions closer to the fires. Outside of smoke impacts,
VFR conditions prevail and are expected to do so through the TAF
period. There is a chance for thunderstorms this afternoon, mainly
from the Cascades and eastward and along the Siskiyous and
southward. Expect gusty and erratic winds in the vicinity of any
thunderstorms.
Along the coast...Marine stratus is impacting isolated areas along
the coast this morning, with a larger area of stratus lingering
about 10 nm offshore. Expect this stratus to push to the coast this
evening, bringing IFR/LIFR conditions to coastal locations. /BR-y
&&
.MARINE...Updated 200 PM PDT Thursday 12 August 2021...Sub-advisory
conditions will persist through the weekend with light winds and
northwest swell dominated seas that will gradually diminish to at or
below 6 ft by Friday afternoon. The thermal trough pattern will
return early next week, bringing the return of gusty north winds and
steep to very steep seas with advisory level conditions possible by
Monday afternoon. /BR-y
&&
.FIRE WEATHER...Updated 200 PM Thursday August 11 2021...The main
concern over the next two days ahead will be the potential for
thunderstorms this afternoon into this evening and Friday afternoons
and evening. More on this will follow later in the discussion
Excessively hot, very unstable and dry air across southern Oregon
and northern California will stick around through the weekend. The
models continue to show a haines 6 for the Skyline Complex, Jack
Fire, and Devils Knob Complex into this evening. Tomorrow the haines
is expected to be a 5, with some guidance suggesting a 4.
Regarding thunderstorms this afternoon and evening, the idea that
they will be isolated and far and few still looks on track and if
any storms were to develop, they are likely to be isolated dry. This
is being supported by various model soundings which are showing an
inverted "V" sounding which typically indicate that if any storms
develop, they will produce little or no rain because of the extreme
dry layer in the lower levels which will likely result in any
precipitation evaporating before it reaches the ground. The latest
SPC SREF thunderstorm guidance does not show anything that
impressive. Meanwhile the latest HRRR models indicate the areas that
have the best chance for isolated thunderstorms is in Fire Weather
Zone 285 and even then it's not until early this evening. One
variable that could put a cap on how much thunderstorms develop is
all the smoke in the sky. The latest visible image shows the smoke
mainly concentrated west of the Cascades and Siskiyou County in
northern California and thinner in Modoc and Lake County which
coincides where the HRRR is showing convective feedback. Despite the
smoke, the HRRR model suggest storm development is there because of
the moist unstable air aloft in the atmosphere.
Tonight, there is some indication of elevated instability east of
the Cascades and portions of the west side. Odds are there won't be
any storms lingering in the overnight period, but at the same time
cannot rule it out given the amount of elevated instability and weak
trigger. Therefore the lean on the side of caution, have opted to
keep a slight chance of storms in for tonight. Keep in mind, the
most likely scenario is nothing will happen, but at the same time
the threat is not zero.
Thunderstorms chances are higher on Friday. Mid level moisture will
be higher, instability will be greater and a slightly stronger
trigger. There is a low potential for storms starting late Friday
morning, but those chances will increase in the afternoon. The SPC
SREF has since backed off some on the probability of thunderstorms
(20%), but also the probability of greater than 100 cloud to ground
strikes which still remains mainly along the Cascades and across
portions of western Siskiyou County. At the same time several of the
CAM (Convective Allowing Models) suggest a more extensive area of
scattered thunderstorm coverage east along and east of the Cascades,
Northern California and portions of the westside to include Fire
Weather Zones, 622 and southeast Fire Weather Zone 620.
Keep in mind, this scattered coverage is dependent on how much
instability is able to build under the smoke from the fires.
Although there is more moisture in the atmosphere for storms on
Friday, the subcloud layer remains very dry with elevated bases
which could limit the amount of rainfall with some of these storms.
However steering winds will be light which means storms that do form
will move slowly, thus increasing the opportunity for wetting rains.
However confidence on this is not high.
Also gusty winds are likely with any thunderstorms on Friday. Given
the above mention reasoning and increasing confidence, the Fire
Weather Watch has been upgraded to a Red Flag Warning for the same
areas that were in the watch, except we will add fire weather zone
622 and east to southeast 620, except for fire zone 282. Please see
RFWMFR for details.
On Saturday the upper flow is expected to transition from the south
to southwest. Instability is still there, but not as much as Friday
and mainly east of the Cascades and portions of northern California.
However there are some indications that storms could be more
frequent along the Cascades and in portions of Fire Weather Zone 280.
For now, we'll keep a slight chance of storms in these areas, but
this is subject to change.
Sunday the upper level trough will approach the area from the west
and the trough axis is expected to be west of the area Sunday
afternoon, this will result in greater instability east of the
Cascades, and thus a higher chance for thunderstorms east of the
Cascades and portions of Northern California. Also winds ahead of
the upper trough will be higher (700mb winds between 15-25 kts)and a
stronger pressure gradient at the surface, therefore it's likely we
could be dealing with gusty winds Sunday afternoon and evening.
After Sunday, we'll have a cooling trend as an upper trough begins
to settle into the area. This will be cooler temperatures, but then
out concern will shift from thunderstorms to stronger winds and low
relative humidities. -Petrucelli
&&
.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Red Flag Warning from 2 PM to 11 PM PDT Friday for
ORZ616-617-620>625.
Heat Advisory until 9 PM PDT Friday for ORZ029>031.
Excessive Heat Warning until 11 PM PDT Saturday for ORZ023>028.
CA...Red Flag Warning from 2 PM to 11 PM PDT Friday for
CAZ280-281-284-285.
Heat Advisory until 9 PM PDT Friday for CAZ083>085.
Excessive Heat Warning until 11 PM PDT Saturday for CAZ080>082.
Pacific Coastal Waters...None.