749
FXUS66 KMFR 172334
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
434 PM PDT Sat Aug 17 2024

...Updated AVIATION discussion...

.DISCUSSION...

Lots of fast moving thunderstorms on radar this afternoon as an
upper level low off the coast and embedded short wave kick off
thunderstorms across the region. They haven`t looked very
impressive over the last few hours, perhaps because a lack of
convective available potential energy. However, there is plenty of
directional shear for these thunderstorms and models are still
predicting some stronger storms later this afternoon and evening
mostly in the Cascades. In general, we`ve seen wind gusts mostly
around 45 mph with one gust up to 58 mph over higher terrain with
mostly pea size hail. However, that could change in the coming
hours as models are still initiating convection through 3Z.
Because of this, a severe thunderstorm watch has been issued until
9 pm for Jackson, Douglas and Siskiyou Counties.

Storms will move north through the evening and decrease in
coverage overnight. A few models are still holding onto storm
activity along the Cascades. In addition, the latest NBM does
have some low(15%) chance of thunder probabilities along the
coast overnight.

By Sunday, the low bringing all the weather is starting to move
north and the weather should be more fall like with only a few
showers lingering around Douglas County Sunday morning. Conditions
will continue to dry into Sunday evening with some cooler than
normal temperatures persisting.

The cooler westerly winds will remain in place for Monday and
Tuesday, which will result in highs in the lower 80`s and some
cooler lows overnight. The low bringing all this weather will
eventually shoot eastwards and high pressure will build in it`s
wake east of Oregon on Wednesday. Temperatures won`t warm up too
much, but the ridge will block the progression of the next upper
level low.

Deterministic models and likely some ensemble members are
anticipating another low to sit off our coastline around Thursday.
This type of pattern during the summer usually means a few
thunderstorms. The NBM probability of precipitation increased
significantly compared to 24 hours ago and we added a slight
chance of thunderstorms in there as well.

Ensembles are hanging onto the 20% chance of showers or
thunderstorms by Friday into the weekend. These upper level lows
like to wobble and things will probably change between now and
early next week. -Smith

&&

.AVIATION...18/00Z TAFS...Thunderstorms continue to move over
northern California and southern Oregon from the south. Gusty winds
and hail are the main threats from these storms and these could
impact areas west of the Cascades (including the Roseburg and
Medford terminals) through the evening. While the chances of storms
meeting severe criteria are slight, hazardous conditions have been
observed through the afternoon. Hail, heavy showers, and gusty
erratic winds remain possible through the evening and into Sunday
morning

Once storm activity calms, conditions should be fairly normal.
Ceilings along the coast are expected to fall to IFR overnight
before lifting to MVFR late Sunday morning. Scattered clouds may
remain over inland areas, but ceilings will be VFR to end the TAF
period. -TAD


&&

.MARINE...Updated 145 PM Saturday, August 17, 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 is bringing showers and storms
today. While storms will dissipate this evening, showers may
continue into early Sunday morning. -Schaaf/Hermansen

&&

.FIRE WEATHER...Offshore low pressure will continue to send
shortwaves northward across NW California and SW Oregon into this
evening, providing plenty of forcing/lift for thunderstorms.
Satellite and radar are showing numerous showers and thunderstorms
developing early this afternoon as the shortwave encounters good
moisture and instability with steep mid-level lapse rates near and
west of the Cascades. These storms will move to the north (or even
NNW) at a pretty good clip. A recent timed cell had a storm motion
at 35-40 mph. Initially, individual fast-moving cells are likely to
produce lightning, possibly outside of rain cores which could result
in numerous new fire starts. As such, a Red Flag Warning remains in
effect for abundant lightning on dry fuels for FWZs 616, 617, 620,
621, 622, and 623 in Oregon and FWZs 280 and 281 in California.
In addition to frequent cloud to ground lightning, storms today will
also have the ability to produce strong (40-57 mph) to perhaps
severe (>58 mph) wind gusts and large hail in excess of 1 inch in
diameter and the Storm Prediction Center has much of the
aforementioned area in a marginal (5-14%) to slight risk (15-29%)
for severe storms. These storms are also likely to become
significant rain producers with PWs in the 0.75-1.00 inch
range (even up to 1.25 farther north). Eventually, individual cells
could organize into clusters or even train for a while over similar
areas. This could result in heavy rain rates (>0.25"/hr at times)
with peak rates of 0.50-0.75"/hr. By the time the storms end, most
of the area from the Cascades west will have had a wetting rainfall
of >0.25" (60-90% chance). Areas that get under training cells may
receive more than 1.00 inch of rain today. While we aren`t expecting
widespread flash flooding, there is a small possibility (5% chance)
of a flash flood/debris flow (best chance is near the more
significant fires in eastern Douglas County). All of these risks
are safety concerns, and important details for aircraft and ground
resources/personnel to be aware of. Please be safe out there this
afternoon/evening. The Red Flag warning ends at 9 pm PDT this
evening, but activity could end a little sooner than that in NorCal
(6-8 pm) as it moves off toward the north.

Farther east, a strong mid-level jet well in advance of the
offshore low will bring strong, gusty S-SW winds into this
evening for NE Cal and south-central Oregon. Guidance still shows
sustained wind speeds of 15-30 mph with gusts in the 35-50 mph
range. Model probabilities are showing 60-80% chances of
wind gusts exceeding 40 mph and in-house fire weather RAWS guidance
is showing peak wind gusts 40-50 mph for some of our more exposed
RAWS locations. A Red Flag warning remains in effect until 8 pm
PDT for FWZ 285 (Modoc) in California and FWZs 624/625 in Oregon.
This also 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 into this evening, but humidities there should be
slightly higher.

Shower activity should wane overnight west of the Cascades. By
Sunday, the offshore low will wobble a bit more and lift northward,
so while there is still a 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%. Even so, wind gusts should be a good 10-15 mph lower
than today. We`ll maintain the headline for 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 with maybe some cumulus
buildups Tuesday afternoon. Models do show another shortwave moving
through NorCal Tuesday night into Wednesday, so we`ll have to watch
for some shenanigans with that. Right now, it appears to be moving
through in the morning, which isn`t favorable for precipitation, but
some slower/stronger solutions generate a slight thunder risk east
of the Cascades Wednesday afternoon/evening.

After that, yet another upper low is shown to drop south along the
British Columbia Coast mid next week, then offshore the PacNW coast
Thu/Fri. There are still some position/strength detail
uncertainties to be worked out with this system, so we`ll be
watching that as we head into next week. -Spilde


&&
.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Red Flag Warning until 9 PM PDT this evening for ORZ616-617-
     620>623.

     Red Flag Warning until 8 PM PDT this evening for ORZ624-625.

CA...Red Flag Warning until 9 PM PDT this evening for CAZ280-281.

     Red Flag Warning until 8 PM PDT this evening for CAZ285.

PACIFIC COASTAL WATERS...None.

&&

$$