814
FXUS66 KMFR 162349
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
449 PM PDT Fri Aug 16 2024

...Updated AVIATION discussion...

.SYNOPSIS...

Saturday will be an active weather day with strong winds east of
the Cascades and a slight risk of severe storms along and slightly
west of the Cascades. Cooler than normal conditions persist
through the short and long term forecasts.

&&

.SHORT TERM...Now into Tuesday

Plenty of cumulus clouds are on satellite this afternoon with
some marine stratus holding on along the coast and portions of
Douglas County. Smoke is still billowing out from active fires in
the area and some of that smoke should be heading southwards this
late afternoon. However, with a low off the Oregon coast, low
level winds should become more southerly overnight through
Saturday. This should result in more smoke from the Boise fire
moving into sections of our forecast area. There is still an air
quality alert out for Lake and Klamath Counties as smoke from the
many complexes in the Central Cascades continue to flow into those
two counties.

By Saturday, an upper level low off the coast of Oregon will bring
plenty of rain and thunderstorms to the region. There is a
slight(15-25%) chance that these thunderstorms are severe.
Convective available potential energy(CAPE) isn`t all that high
with values around 500 to 1000. However, there is plenty of shear
around 0-6 km, which is what we usually lack for severe storms in
this section of the US. Lapse rates look good along the spine and
east of the Cascades, which suggests some stronger and sustained
updrafts with the right shear. Right now, hail seems to be the
main concern, although strong gusty winds up to 60 mph are not out
of the question. The high resolution ensemble forecast(HREF)
system is picking up on some supercell structures in our CWA as
well, which increases the risk of severe storms in the Oregon side
of our warning area.

As for flash flooding and debris flows, there is some risk of that
happening given how wet the storms are. However, these storms will
be moving quickly, which should decrease the amount of
precipitation they can put down over a small area over a short
period of time (15 minutes or so). These short intense bursts of
rain is usually what triggers these events. Therefore, we decided
to not issue a flash flood watch for Saturday`s thunderstorm
event given the low probability of something occuring. However,
it`s worth stating again that there is a low risk.

It will also be very windy east of the Cascades on Saturday, which
is more of a fire weather risk during this time of year. However,
some model output statistical guidance suggested wind gusts
around 45 to 50 mph over higher exposed terrain east of the
Cascades. This would usually suggest a wind advisory, although we
felt the fire weather products pretty much covered this threat.

Heading into Sunday, the upper level low wanders a bit and moves
a bit farther north. Southerly flow should still remain dominant,
which will result in smoke from the Boise fire still moving into
our area if it still burning with enough intensity.

-Smith

.LONG TERM...Tuesday into next Friday

This rather progressive pattern will persist into the extended
forecast with a deep wave swinging into Oregon with plenty of
cooler moist onshore flow. This will keep temperatures below
normal for Tuesday onwards. Our normal high in Medford is usually
around 90 this time of year, and we`re forecasting highs the 80`s
for all of next week with low temperatures in the mid to lower
50`s in many of the valleys.

Models are currently showing another upper level low settle off
the Oregon coastline sometime around Thursday afternoon and
Thursday night. This will result in a low threat of thunderstorms
across our forecast area. Right now, the probability is right
around 10 percent, although that could trend higher as we move
through the start of next week.

-Smith

&&

.AVIATION...17/00Z TAFS...A few scraps of marine stratus are
clinging to the Oregon coast to start the TAF period but are quickly
clearing out. The cloud layer is expected to rebuild later this
evening, bringing MVFR and IFR ceilings that are expected to remain
through the TAF period. Periods of MVFR visibilities are possible
overnight into Saturday morning as well, but there is low confidence
in the timing.

Wildfire smoke is not expected to be a widespread problem as
southerly winds will help to prevent accumulation. Visibility near
the fires in eastern Lane and western Siskiyou counties may still be
locally limited.

Active weather is expected west of the Cascades late Saturday
morning and into the next TAF period. 25-50% thunderstorm chances
will be present over west side areas on Sunday, with the highest
chances over Jackson, Josephine, and eastern Douglas counties.
Abundant lighting, erratic gusty winds, and localized heavy showers
are possible. Hail development will also be a concern. Thunderstorms
are expected to move quickly to the north. Approximate timing for
activity to start has been included in the Medford and Roseburg
TAFs, with thunderstorm chances lingering into early Sunday morning.
-TAD

&&

.MARINE...Updated 200 PM Friday, August 16, 2024...Relatively
calm conditions are expected for the next several days with light to
moderate winds and seas remaining 5 feet or less. A rather strong
upper low for this time of the year will move into the waters
Saturday and will likely bring showers with a slight chance of
thunderstorms into early Sunday morning. -DW/Smith

&&

.FIRE WEATHER...The main fire weather concerns over the next 24-36
hours will be two-fold. One, for some strong, gusty winds in
combination with low relative humidity over portions of the East
Side and in NE California Saturday afternoon and evening. And two,
for thunderstorms expected near and west of the Cascades during the
same time period.

First, the winds. A strong mid-level jet will move across NE Cal and
south-central Oregon in advance of low pressure spinning offshore
Saturday afternoon and evening. This will bring increasing S-SW
winds east of the Cascades with models indicating a high probability
(60-80% chance) of wind gusts exceeding 40 mph. In-house fire
weather RAWS guidance is showing peak wind gusts near 50 mph for
some of our more exposed RAWS locations. In addition, afternoon
relative humidity will bottom out in the 10-15% range. We have
upgraded the Fire Weather Watch for FWZ 285 (Modoc) in California
and FWZs 624/625 in Oregon,  to a Red Flag Warning (2-8 pm PDT).
mainly south and east of Summer Lake. The main area of concern
includes the Warner Peak fire in eastern Lake County. Winds will
also be gusty in the Shasta Valley up into portions of the Klamath
Basin, but humidities there should be slightly higher.

Next, the thunderstorms. The low offshore will send a very potent
shortwave northward across NW California and SW Oregon Saturday,
providing plenty of forcing/lift. With good mid-level moisture
available and enough (modest) instability, this will induce showers
and thunderstorms across a large portion of the west side (from the
Cascades west) during the afternoon/evening. Models are showing some
showers moving onshore as early as Saturday morning, but it should
take until midday for the thunderstorms to really get going. This is
where the forecast gets tricky. This isn`t your typical mid-August
thunderstorm event since we`ve had a cool-down in the last week or
so and subsequent moistening of the low-levels and fuels to some
degree. Usually, these events are preceded by hot, dry weather. In
that case, this would be a slam dunk fire weather regime. But, these
storms are also likely to be decent rain producers with PWs in the
0.75-1.00 inch range (even up to 1.25 farther north). Recent model
runs of the SPC HREF convection allowing models (CAMs) are showing a
high probability (60-80% chance) of lightning from the Cascades
westward to central Douglas/portions of Jackson, Josephine and
western Siskiyou counties. It is in these areas that SPC SREF also
shows a fairly high probability of >100 cloud to ground lightning
strikes. These storms will be moving north at a pretty good clip (40-
50 mph) and they may be more cellular (as opposed to clusters) at
least initially. This could allow for strikes outside rain cores.
Local fuels agencies are still indicating high fire danger despite
the recent cool down and there is potential for numerous new fire
starts, due to the expected amount of lightning. For these reasons,
we have decided to issue a Red Flag Warning for FWZs 616, 617, 620,
621, 622, and 623 in Oregon and FWZs 280 and 281 in California. It
should also be noted that the Storm Prediction Center has much of
the aforementioned area in a marginal to slight risk for severe
storms, which means there`s a 5-15% chance of wind gusts exceeding
58 mph and large hail in excess of 1 inch in diameter. It is
important for aircraft and ground resources/personnel to be aware of
this risk for safety concerns. Individual storm cells will have a
tendency to become clustered/more organized as they move north
(toward Lane County), where sustained heavy rainfall rates
(>0.50"/hr) are more likely. The shortwave disturbance should lift
north of the area by around 9 pm Saturday evening and the lightning
risk will end.

By Sunday, the low will wobble a bit more offshore and lift
northward, so while there is still a slight chance of showers near
and west of the Cascades, most areas will be dry. It will be breezy
again over portions of the East Side and humidity will continue to
be down around 15%, but wind gusts should be at least 10-15 mph
lower than on Saturday. We`ll headline this in the Fire Weather
Planning forecast.

Beyond that, things should calm down a bit with the low moving
northward toward Vancouver Island Monday, then opening up into a
trough Tuesday and eventually onshore Tuesday night. This should put
the area in a mostly dry SW flow aloft. However, models are showing
a shortwave moving through NorCal Tuesday night, so we`ll have to
watch for some shenanigans with that. If you can believe it, yet
another upper low is shown to drop south along the British Columbia
Coast mid next week, then offshore the PacNW Thu/Fri. Will we do
this all over again? We`ll see. -Spilde

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Red Flag Warning from 1 PM to 9 PM PDT Saturday for ORZ616-617-
     620>623.

     Red Flag Warning from 2 PM to 8 PM PDT Saturday for ORZ624-625.

CA...Red Flag Warning from 1 PM to 9 PM PDT Saturday for CAZ280-281.

     Red Flag Warning from 2 PM to 8 PM PDT Saturday for CAZ285.

PACIFIC COASTAL WATERS...None.

&&

$$