385
FXUS66 KMFR 112129
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
229 PM PDT Sun Aug 11 2024

.DISCUSSION...We`ll be under the influence of an upper trough for
the next several days. This will bring increasing onshore flow
and a cooling trend. Today could be the last day of 90+F degrees
for a while here in Medford as the trough is expected to gradually
deepen near or just offshore of the PacNW coast through the week
and especially Friday into next weekend. Coastal areas are likely
to have nightly marine pushes. Inland locations will be dealing
with smoke from area wildfires that will be quite thick at times,
with some push east of the thickest smoke. But, weather- wise,
it`s pretty benign for the second week of August.

There is some weak instability and modest mid-level moisture near
the higher terrain in NorCal and northeastward into SE Klamath and
southern Lake counties this afternoon and evening, but really
nothing to trigger deep convection. Expecting some cumulus or even a
buildup or two into this evening, but there`s a less than 10% chance
of a thunderstorm. Tonight, the marine layer deepens and expect a
more significant push/formation of stratus into the Umpqua Basin,
which could bank up against the Rogue-Umpqua Divide and even cover
some of the eastern Douglas fires Monday morning. Some guidance is
even showing the clouds pushing over the Divide and into portions of
the Rogue/Illinois valleys, but probably not quite to here in
Medford. Some fog/drizzle are possible along the coast during the
early morning.

Meanwhile, an upper shortwave out over the ocean this evening will
swing through NorCal on Monday. With a bit more forcing, and enough
moisture/instability, we think there`s a little better shot at
isolated thunderstorms from NorCal northeastward across SE Klamath
and into Lake County. The remainder of the area will remain dry and
at least a few degrees cooler. There will be more westerly winds
Monday afternoon behind this system, so that should push the smoke
eastward, but a lot is dependent on fuel type, burn operations and
other non-weather factors. The pattern for the smoke should follow
a fairly typical ebb and flow with the thickest smoke settling into
the valleys at night/early morning and pushing eastward during the
midday/afternoon/evening for at least the next few days. This could
bring the thicker smoke into the Klamath Basin. Air quality products
are out for eastern Douglas, Jackson, Klamath and Lake counties and
these will be updated on Monday.

Thunder chances could linger into Monday night over the far east
side since it will take some time for the short wave to move out to
the east. Most areas should have a break Tuesday into Wednesday as
the upper trough weakens slightly and another disturbance approaches
from offshore. Temps moderate a bit by Wednesday, but remain
near to a bit below normal.

The next short wave moves in Wednesday evening into Thursday
evening and this could bring the next chance at
showers/thunderstorms. Since the flow aloft will remain generally
from the west-southwest, activity is most likely from the Cascades
eastward. Right now, PoPs stand at 20-30% Wednesday evening/night
and again Thursday afternoon/evening.

Models/ensembles then show the upper trough consolidating into a
closed low off the PacNW Friday into this weekend. There are still
many details to figure out here with just how deep the trough
will be and how far south it will get (and also if it`s closer to
the coast or even onshore). This will impact just how much, if any
precipitation we get this far south. Right now, since it is
still August, precip chances are on the low side. But, daytime
maximum temperatures should be about 5-10F below normal. -Spilde


&&

.AVIATION...11/18Z TAFS...IFR/LIFR conditions along/near the coast
this morning have cleared to VFR, but are expected to return this
evening and tonight, perhaps pushing well inland within the Umpqua
Basin and all areas west of the coastal ranges. Inland TAF sites
will be under mostly clear skies with breezy afternoon winds, with
widespread smoke impacts expected to produce MVFR/IFR visibilities,
especially in area valleys where smoke settles in overnight and
persists into the morning. -BPN


&&

.MARINE...Updated 230 PM Sunday, August 11, 2024...Wind speeds will
peak this afternoon combined with steep seas which will result in
conditions hazardous to small craft through this evening. The
probability for wind gusts of 34 knots or greater south of Cape
Blanco is roughly 40-60% where we have the current Small Craft
Advisory. These speeds will remain breezy/gusty until sunset before
slowly decreasing overnight. Thereafter, winds and seas will both
ease through at least the middle of the week. -Guerrero/Smith


&&

.FIRE WEATHER...Issued 200 PM Sunday August 11, 2024...After
another day of above normal temperatures today, the forecast period
will trend cooler through next week with increasing onshore flow,
improving humidities, and breezy afternoon winds as upper level
troughing settles in and remains over the Pacific Northwest.

We are expecting a relatively strong marine push tonight into
tomorrow, with extensive stratus and fog decks west of the Cascades
that may even push into the Rogue Valley by daybreak Monday. We also
expect increased winds through the day, especially east of the
Cascades. Despite these winds, rising humidities should keep us
below any critical thresholds, although it might briefly approach
those thresholds over eastern Lake and Modoc counties (FWZs 285 and
625).

With the weak upper trough overhead, some instability is expected to
develop this afternoon and again Monday afternoon. Instability is
marginal at best, with little or no trigger, and mid level moisture
is lacking, therefore the threat for thunderstorms are very low. The
most likely scenario will be cumulus build ups this afternoon over
the Cascades, higher terrain east of the Cascades, and in northern
California.

A shortwave will slide under the main trough and pass just to the
southeast of the area Monday. The potential for isolated
thunderstorms will therefore be higher Monday afternoon, mainly east
of the Cascades and across northern California, with increased mid
level moisture and the shortwave acting as the trigger. Given the
overall weak system, marginal instability, and the track of the
shortwave, only isolated storms are expected, and even then,
confidence is low.

With the trough remaining overhead through the week, we can not
completely rule out additional afternoon convection, particularly
Wednesday and Thursday afternoons as additional shortwaves pass
through the region. Again, we only expect isolated storms should any
develop, and confidence is pretty low across the board, especially
given that wildfire smoke has a tendency to suppress convective
development.

The main threat from any convection through this week will be
outflow winds, which could initialize not just from showers and
thunderstorms, but also any of the more robust cumulus fields.

Models are showing the potential for a much stronger, broader trough
arriving and setting up over or very near the area late this week
into this weekend. This will maintain our cooler temperatures and
higher humidities, and may also produce more widespread chances for
showers and thunderstorms Friday into Saturday. While wetting rains
are very unlikely, we will keep a close eye on how the system
develops, as cooler temperatures, higher humidities, and even a
small amount of rain will be very welcome to fire concerns. -BPN


&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...CA...None.

PACIFIC COASTAL WATERS...Small Craft Advisory until 2 AM PDT Monday
     for PZZ356-376.

&&

$$

MAS/BPN/CZS