775
FXUS66 KMFR 151743 AAA
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
1043 AM PDT Thu Aug 15 2024

.DISCUSSION...The upper low that brought abundant thunderstorms to
mainly the east side continued to exit northeast out of the area
overnight and early this morning. A few rain showers remain over
northern Lake and Klamath counties this morning...but will move
out within the next hour. Otherwise...the focus of the forecast
today will be thunderstorm potential on the east side. The short
term models run after run continue to shift the initiation
east...with confidence diminishing as far as the scattered nature
of thunderstorms. We will be evaluating and make any necessary
edits within the next few hours.

Once we get through today...we will be taking a much closer look
at the low forecast to impact the region over the weekend.
Confidence continues to increase that areas along and west of the
Cascades will see wetting rain.

More to come...stay tuned!

-Riley

.AVIATION...15/18Z TAFS...Ceilings at the coast and across coos and
coastal Douglas Counties will be VFR for much of the day before
returning to to MVFR late evening and overnight. Guidance is
suggesting around a 30 percent chance for IFR stratus, but left it
out of the TAF due to the unsettled nature of the pattern.

Thunderstorm chances this afternoon and evening are generally
limited to Lake and Klamath counties, with 20% chances at the
southern end and 40% chances along the northern edge of these
counties. Extra caution is advised around any storms that do
develop, as thunderstorms can generate lightning, gusty erratic
winds, and locally lower ceilings and visibilities. Chances for
activity drop to single digits late tonight.

Wildfire smoke also continues to complicate conditions. Winds will
generally come from the west and brief periodic moderate to heavy
smoke over western Siskiyou, northern Lake, northern Klamath, and
eastern Douglas counties with MVFR conditions in their vicinity and
downwind. Light to moderate smoke is possible over most inland
counties through the TAF period. -Schaaf/TAD


&&

.PREV DISCUSSION... /Issued 459 AM PDT Thu Aug 15 2024/

DISCUSSION...It was an active evening on radar with isolated to
scattered thunderstorms that continued until around midnight.
During this time, around 1200 lightning strikes have been
recorded...most numerous across Klamath and Lake Counties, but
also in portions of central and eastern Siskiyou County. Rainfall
amounts with these recent storms have been rather impressive.
0.25" is a common report at observation sites across these areas,
with even reports of 0.50" occurring at several locations.
Thunderstorm activity has significantly dwindled as of this
writing (~430am), but there are some lingering showers across far
northern Klamath County in the Diamond Lake/Chemult area and
northward. Dry conditions persist for the remainder of the area
with satellite imagery showing some mid-level cloud cover over
much of the West Side.

This shower/thunderstorm activity was brought to you by a shortwave
trough passing through the region yesterday evening, which was the
trigger needed to tap into some elevated instability and moisture.
This trough will linger over Oregon today and, after a break in
activity through this morning, it will reinvigorate
shower/thunderstorm activity this afternoon and evening. The focus
of storms today will be shifted east and northward, with the best
chances (25-40%) expected over Klamath and Lake Counties north of
Highway 140 where high resolution guidance continues show another
round of isolated to scatter thunderstorm activity. The one "fly in
the ointment" leading to some uncertainty in the extent of
thunderstorm development will be the amount of smoke expected east
of the Cascades today. Given that thunderstorm development today
will largely be surface based, smoke could be a limiting factor as
it can act as a cap on surface based instability. In the event that
surface heating is able to overcome the limiting effects of smoke,
have continued the Red Flag Warning for portions of Fire Weather
Zones (FWZs) 624 and 625 for abundant lightning on dry fuels for
today, and details can be found at RFWMFR. The atmosphere is more
stable on Friday. Lingering moisture and weak instability will be
present, but there isn`t a defined trigger coming through the
region, so thunderstorm chances largely drop out of the forecast for
Friday. It`s likely there will be some cumulus buildups east of the
Cascades and over the higher terrain and there`s a small chance (5-
10%) a few of those buildups could result in a brief shower east of
the Cascades.

For the remainder of the area, dry conditions will continue along
with below normal temperatures (afternoon highs in the mid to upper
80s for West Side Valleys and upper 70s to low 80s for areas east of
the Cascades). Despite the reprieve from the summer heat, smoke will
be a continued nuisance through the forecast period, and probably
longer. An Air Quality Alert (issued by partnering agencies) is in
effect for Klamath and Lake counties, and we have relayed those via
AQAMFR.

The cooler than normal pattern will continue as a deep, large scale
trough develops off the PacNW Coast late Friday into Saturday and
lingers there well into next week. The center of the low will be at
it`s southern most location on Saturday before it drifts northward
Sunday into early next week. As such, Saturday will be the day with
the highest precipitation potential through at least mid-week.
Confidence is increasing on the potential for measurable
precipitation (even possibly wetting rains) for much of the area
along and west of the Cascades as ensembles are in remarkable
agreement. When looking at the chance of total precipitation amounts
ending at 5 pm Saturday (for areas along and west of the Cascades),
there is a 80-90% chance of at least 0.01" (measurable
precipitation), and about a 40-60% chance of at least 0.10". For
wetting rains along and west of the Cascades, there is about a 25-
30% chance of 0.25". Even locations in northern California from the
Shasta Valley westward have similar chances, albeit lower chances
for 0.10" (30%) and 0.25" (10%). Models indicate ample instability
to go with this moisture so there is moderate confidence in
thunderstorms occurring west of the Cascades on Saturday. The
uncertainty lies in how much lightning there will be...will it be a
convective event with abundant lightning or will it be a broad area
of showers with embedded thunderstorms. Once this timeframe is
resolved by high resolution models, we`ll evaluate the need for any
fire weather products.

Uncertainty increases for areas east of the Cascades regarding
precipitation chances. There will likely be a sharp gradient between
where precipitation chances drop off and the concern for strong
winds combined with low humidities results in critical fire weather
conditions. Current guidance suggests this gradient will be along
and near the Warners, extending northward to Winter Rim/Summer Lake
area. West of that area, the chance for 0.01" of precipitation is
about 50%, while the chance for 0.10" drops to 15% and only a 10%
chance for 0.25". Farther east towards Lakeview and Alturas, the
chance for 0.01" of precipitation is only 10% with no solutions
indicating a chance for higher amounts. The bigger concern east of
the Cascades (and where the higher confidence lies) will be strong
south to southwest winds. Guidance is currently showing the
potential for gusts of 30-40 mph east of the Cascades and in the
Shasta Valley. There will likely be areas where these strong winds
combine with low humidities and result in critical fire weather
conditions. Exactly where and how low humidities fall is where the
uncertainty lies at this point in time, and will ultimately depend
on where precipitation chances occur. Right now, it looks like the
best chances for critical fire weather conditions exist over much of
FWZ 625 and portions of the Modoc in FWZ 285. Stay tuned as we
refine the details over the coming shifts.

From Sunday onward, low pressure will drift northward and
precipitation chances will come to an end. We`ll remain under a
cooler than normal pattern with gusty afternoon breezes. Shortwaves
will rotate around this low pressure and into the region through the
forecast period, though the timing of these are difficult to
pinpoint at this time range. Stay tuned and monitor for updates as
these shortwaves could result in either enhanced afternoon winds
(possible fire weather concerns) or thunderstorm chances at some
point next week. /BR-y


&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Red Flag Warning until 11 PM PDT this evening for ORZ624-625.

CA...None.

PACIFIC COASTAL WATERS...None.

&&

$$