NWS Medford, OR wrote its last area forecast discussion before the first event at Sep 1, 1:32 PM PST, 28 minutes earlier.
Area forecast discussion, Sep 1, 1:32 PM PST, 28 minutes before
Area Forecast Discussion
National Weather Service Medford OR
232 PM PDT Sun Sep 1 2024
.SHORT TERM...
Like yesterday, there is some cumulus clouds building east of the
Cascades this afternoon, mostly in northern Lake County. Unlike
yesterday, the dynamics are a little better today. The upper level
low is closer to the coast and divergence aloft should be better
as well. Therefore, we feel confident there will be some
thunderstorm activity in northern Lake County, the question is
where and how much. Right now, the probabilities suggest 15%
chance of a cloud to ground flash within 20 miles in northern Lake
County. This seems pretty reasonable based on the marginal
instability data and the latest HREF model outputs.
The threat of thunderstorms will carry into the evening hours and
even into tonight. The locations that stood out the most were
along the coast and eastern Lake County. GFS BUFR soundings near
buoy 15 showed 300 CAPE with limited convective inhibition and
that is north of the low. In addition, the HREF is picking up on
spotty showers or weak thunderstorms in eastern Lake County
overnight.
Aside from thunderstorms, winds in the Shasta Valley will be a bit
stronger today. The upper level 700 mb flow aligns well with the
Shasta Valley. Local MOS guidance suggests gusts up to 40 mph near
Weed and sustained winds around 20 to 25 mph near Montague.
By Labor Day, this upper level low will start to track farther
east while bringing plenty of mid level cloud cover. In addition,
isolated thunderstorms are expected in the high central Cascades
and northern Lake and Klamath Counties. The most likely time for
storms will be in the afternoon although some cloud to ground
lightning could occur in morning as the mid level moisture seems
to be driving the storm development.
It will also be windy a wind day in eastern Lake County. This is
more of a fire weather concern, although wind gusts could reach up
to 40 mph over the higher exposed terrain over there.
Once the low moves onshore on Labor Day, cooler air will follow
in it's wake with temperatures trending about 10 to 15 degrees
cooler on Monday afternoon. This surge of cooler air will push us
closer to our normal highs for this time of year, if not a few
degrees cooler.
.LONG TERM... Monday night into next Sunday.
As the low departs the region, there are some hints of a thermal
trough starting to build along the southern Oregon coastline with
high pressure building in the Willamette Valley. Northeast flow
is notable in the Kalmiopsis wilderness. This thermal trough
builds northwards through Tuesday with high temperatures pushing
higher along the coast and west of the Cascades. Brookings will
likely be in the lower 80's by Tuesday afternoon and should repeat
that on Wednesday afternoon.
After Wednesday, high pressure will likely remain in control
through Friday before an upper level low likely parks off the
northern California coast again. Another upper level low will
mean more thunderstorms in the forecast, although it will really
depend on how much moisture is arriving to the region. Some models
are placing the low farther to the south, near San Francisco
versus as far north as Eureka. In any case, it will be something
to watch as more data comes in.
It's also worth noting the NBM is thinking we'll have another
warm up with highs in the lower 100's around Thursday and Friday.
Heatrisk will likely remain in the moderate level as the low
temperatures are able to fall below the critical 65 degree
threshold. Nights indeed are becoming longer, so the main threat
here are people over exerting themselves without adequate
hydration during the warmest time of day.
-Smith
&&
.AVIATION (18Z TAFs)...Marine stratus remains solidly along the
Oregon coast, with IFR ceilings and visibilities at Brookings and
low MVFR ceilings over North Bend. The stratus layer will remain
through the TAF period, although some improvement, including periods
of clearing, is possible this afternoon, then again on Monday
afternoon.
Inland areas are expected to remain VFR. Gusty winds are possible
around and east of the Cascades this afternoon, with winds
decreasing later in the evening. Slight chances of thunderstorms
remain in the forecast east of the Cascades this afternoon and
evening, with the highest chances (10%) in northern Klamath and Lake
counties. -DW
&&
.MARINE...Updated 200 PM Sunday, September 1, 2024...Low clouds and
fog will persist into Monday along with light swell dominated seas
and light southerly winds. Increasing north winds accompany the
development of a thermal trough on Tuesday with steep to very steep
seas possible. This thermal trough will be short lived and winds
will weaken on Wednesday with improved conditions expected into
Thursday. North winds are likely to increase again Thursday evening
into early Saturday. -DW
&&
.FIRE WEATHER...Issued 200 PM Sunday September 1, 2024...The pattern
is transitioning today as low pressure continues to approach the
region. The good news is that recoveries will improve for Monday and
Tuesday mornings, then trend lower again for the latter half of the
week. Temperatures will trend slightly cooler today, and daytime
humidities will trend ever so slightly higher in response. However,
it will still be very dry across the interior with low teens common
across the region away from the coast and south of the Rogue-Umpqua
Divide. As low pressure offshore moves closer to the area this
evening, winds will trend a bit stronger. However, with the slight
increase in minimum humidities, wind/RH concerns will be near
critical and not quite reaching Red Flag threshold for portions of
the Shasta Valley, Oregon Cascades and the west side of the Cascade
foothills (Eastern Fire zone 622, and fire zone 623). This low will
create gusty winds in the Shasta Valley where critical humidities
also exist, but conditions are not expected for a 6 hour duration.
Therefore, we will continue to headline the near or briefly critical
conditions for these areas. In addition, conditions are generally
unstable and new and existing fires could spread somewhat quickly
and even become plume-dominated.
This upper level low pressure will move inland through the day on
Monday, and this will bring the strongest winds to the region.
Humidities will again trend higher for most of the area, but will
still remain in the low teens across portions of the Fremont-Winema
Forest and across the Modoc. The strong winds combined with
critically low RHs will result in critical fire weather conditions
Monday afternoon, and a Red Flag Warning was issued this across
portions of Fire Weather Zones 624/625/285. Details can be found at
RFWMFR.
Lastly, the thunderstorm potential...as has been talked about for
days now, this low pressure moving through region will bring at
least a slight chance for thunderstorms mainly along and east of the
Cascades. The track of the low pressure has shifted slightly
northward over recent model runs, and this will focus thunderstorm
potential across northern areas mainly from Crater Lake/Summer Lake
and northward. Cumulus buildups are already occurring across the
higher terrain both today and Monday, but mid-level moisture is
marginal for thunderstorm development west of the Cascades and south
of the OR/CA border. That's not to say a very isolated shower or
thunderstorm in those areas isn't possible in this pattern, but the
chance is very low (5%-10%). It is worth noting that portions of the
Oregon Coast Range and Coast have already seen lightning strikes
(north of the Douglas/Lane County border), so this is why we're
carrying that 5-10% probability.
Regarding tonight's nocturnal potential with low pressure pushing
inland early Monday morning, guidance continues to show minimal
moisture available at the mid-levels for thunderstorm initiation as
a dry slot moves into the area overnight Sunday. The exception will
be over far eastern areas where mid-level moisture will be
sufficient for at least "showers" and maybe an isolated thunderstorm
over far eastern Lake and Modoc Counties between midnight and
sunrise Monday. Convective allowing guidance has shown some cumulus
build ups and showers firing over the Warner Mountains and eastward
during the night, so this lends additional credence to the
possibilities here -- with how dry conditions are at the surface,
storms would almost certainly produce only little rain that actually
reaches the ground before it evaporates. The aforementioned showers
would likely result in virga and not actual rainfall.
Otherwise, as low pressure moves inland Monday and daytime heating
increases instability, isolated thunderstorms are expected across
those northern areas previously mentioned. Given the marginal
moisture associated with this low, confidence is moderate for
isolated coverage at best both today and Monday.
After a cooler Monday, the thermal trough and high pressure aloft
returns on Tuesday. This will bring the return of a pattern quite
similar to what occurred last week, with RHs and recoveries trending
lower and temperatures trending higher. -Schaaf/BR-y
&&
.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Fire Weather Watch from Monday morning through Monday evening
for ORZ624-625.
CA...Fire Weather Watch from Monday morning through Monday evening
for CAZ285.
PACIFIC COASTAL WATERS...None.
&&