391
FXUS66 KMFR 141152
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
452 AM PDT Wed Aug 14 2024

.DISCUSSION...Satellite imagery this morning shows mostly clear
skies across the region. There is some marine stratus out over
the waters and along the coast south of Cape Blanco. There may be
some additional stratus development along the coast north of Cape
Blanco and into the Coquille Basin over the next few hours, but
this should be the extent of the marine layer for this morning.

We`ll be under the influence of an upper level trough through the
weekend and well into next week. This will continue the trend of
below normal temperatures with 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) remains
in effect for eastern Douglas, northern Jackson/Klamath and Lake
counties. Oregon DEQ has some air quality advisories out and we have
relayed those via AQAMFR.

A shortwave trough will move into the region late this evening and
overnight, and this will bring thunderstorm chances to the region
today and again Thursday. Models are showing modest instability and
best moisture from the Cascades over to the east side, but also
extending down into portions of NorCal. High resolution guidance
shows isolated thunderstorms developing over northern California
late this afternoon, then moving north-northeastward into Oregon
this evening and continuing into the overnight hours. While we
expect mostly isolated thunderstorm coverage (15% chance) today,
guidance does show the potential for more scattered coverage (25-40%
chance) as the trough moves overhead around sunset. There is some
uncertainty regarding how far west the shower/thunderstorm threat is
as some guidance shows some slight chances (15% chance) as far west
as the Rogue Valley/Medford and far eastern portions of Douglas
County. Have added a slight chance mention for these areas, but the
better chances are expected to be farther east. This system will
slow down as it moves through Oregon tonight into Thursday, so this
will keep the risk of showers and thunderstorms going. After a brief
lull in shower/thunderstorm activity early Thursday morning, we
expect another round of isolated to scattered thunderstorms in the
afternoon. The thunderstorm potential looks higher for Thursday with
the trough overhead during peak heating hours. The better dynamics,
however, will have pushed farther east and north, so thunderstorm
activity will shift slightly farther east and north as well.
Scattered coverage (25-40% chances) looks best over northern
Klamath/Lake counties centered on the Christmas Valley. The
atmosphere is more stable on Friday. Lingering moisture and weak
instability will be present, but thereisn`t a defined trigger coming
through the region, so thunderstorm chances drop out of the forecast
for Friday.

A deep, large scale trough will develop off the PacNW Coast late
Friday into Saturday and linger there well into next week. This will
continue the cooler than normal pattern, and also potentially bring
some precipitation chances to the region. This low will wobble
eastward and northward, and these "wobbles" could result in big
changes to the forecast depending the extent of the "wobble". This
is leading to low confidence in precipitation potential and
location. That said, there are a good amount of ensemble members
showing measureable precipitation for locations west of the Cascades
on Saturday afternoon. The other weather element to note with this
trough will be an increase in south to southwesterly winds. Guidance
is currently showing the potential for gusts of 30-40 mph east of
the Cascades and in the Shasta Valley on Saturday and Sunday
afternoons, so we`ll have to monitor this for fire weather concerns.
Despite the low confidence in the precipitation potential,
confidence is higher for these stronger winds as the location of the
low pressure will result in tightening pressure gradients, and
subtle "wobbles" likely won`t have big changes on this element.
Subtle changes in position/strength of this upper low can and will
play a role in if, when and where additional showers/thunderstorms
set up, so be on the lookout for daily forecast changes. /BR-y

&&

.AVIATION...14/12Z TAFS...Marine stratus has brought LIFR ceilings
over coastal areas south of Cape Blanco and is slowly approaching
areas north of Cape Blanco. This layer is expected to move over
those northern areas early this morning. There could be a few hours
of clearing this afternoon, with stratus likely to return this
evening.

Other areas should generally be at VFR through the TAF period. An
upper trough remains in place over the Pacific northwest, bringing
more westerly winds that help to limit smoke accumulation west of
the Cascades. However, that does also mean that smoke may impact
visibilities east of active fires. For this area, that includes
possible periods of moderate to heavy smoke over Klamath, Lake,
eastern Douglas, and western Siskiyou counties.

Upper level instability could bring some thunderstorm chances later
today. 15-30% thunderstorm chances are present over Lake, Klamath,
and eastern Siskiyou and Jackson counties this afternoon and
evening. These chances will decrease early Thursday morning, with 15-
20% thunderstorm chances lingering over Klamath and Lake counties
through the end of the TAF period. Lightning, locally heavy rain
showers, and gusty erratic winds are possible hazards around any
storms that do develop. -TAD

&&

.MARINE...Updated 200 AM Wednesday, August 14, 2024...Seas will
remain below advisory levels through the forecast period, with light
winds and low north-northwest swell. Showers over marine waters are
possible this weekend and going into next week. -TAD

&&

.FIRE WEATHER...Issued 400 AM Wednesday August 14, 2024...Upper
level troughing will remain the dominant weather pattern through
the forecast period. This will continue the trend of cooler than
normal temperatures, moderated humidities and breezy afternoon
winds. The two main fire weather concerns on the horizon are
thunderstorm potential today through Thursday and the development
of a deep upper low offshore over the weekend which could bring
some strong winds to the area.

First the thunderstorm potential...a shortwave trough will move into
the region later today and linger over the area through Thursday.
With ample moisture and modest instability, isolated to scattered
thunderstorms are expected beginning late this afternoon. Initially,
isolated thunderstorms are expected over portions of northern
California, then spreading north-northeastward into Oregon after
sunset and into the overnight hours. Isolated thunderstorm coverage
is the most likely scenario today mainly along and east of the
Cascades (possibly as far west as the Rogue Valley/Medford), but
scattered coverage is possible as storms move into Oregon across
Fire Weather Zones (FWZ) 624/625. We do expect a break in
thunderstorm activity early Thursday morning. The trough will linger
over the region Thursday afternoon, however, and this will provide a
better opportunity for scattered thunderstorms over FWZ 624/625 due
to daytime heating effects. Due to these factors, the Fire Weather
Watch has been upgraded to a Red Flag Warning for abundant lightning
on dry fuels...details can be found at RFWMFR. Storms are expected
to have some precipitation with them, but lightning strikes outside
of precipitation cores are possible and cause for concern.
Additionally, expect gusty and erratic winds near thunderstorms
with strong outflows that can travel a good distance from the
originating storm. The atmosphere will stabilize some on Friday.
There will be lingering moisture and instability, but without a
sufficient trigger/forcing mechanism, it`s likely there will be some
cumulus buildups east of the Cascades and over the higher terrain.

The next concern turns to a deep, large scale trough that develops
off the PacNW Coast late Friday into Saturday and lingers there well
into next week. This will bring some precipitation chances to the
region, though confidence is low on the details regarding where and
amounts. Ensembles suggest the best chances are along and west of
the Cascades, but there could thunderstorms to accompany these
precipitation chances. The other element of concern with this low
pressure (and where the higher confidence lies) is the potential for
strong south to southwest winds. Guidance suggests gusts of 30-40
mph for areas east of the Cascades and in the Shasta Valley on
Saturday and Sunday afternoons. These strong winds could combine
with low humidities (which ultimately depends on where precipitation
chances occur) and result in critical fire weather conditions. Stay
tuned for updates as subtle shifts in the position of this low could
have big changes on the forecast. /BR-y

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Red Flag Warning from 8 PM this evening to 11 PM PDT Thursday
     for ORZ624-625.

CA...None.

PACIFIC COASTAL WATERS...None.

&&

$$

MNF/TAD