626
FXUS66 KMFR 180537
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
1037 PM PDT Sat Aug 17 2024

...Updated AVIATION discussion...

.DISCUSSION...Pretty busy day on the radar today. There were a
handful of strong and severe storms in the region today, though
hail reports so far range from pea sized to around 1/2". One cell
passed through Medford area, dumping heavy rain in a short amount
of time and we recieved reports of downed trees and tree limbs in
the Phoenix area from strong winds. A rough count on lightning
strikes indicates about 1800 strikes recorded today, focused along
and west of the Cascades. Rainfall amounts range from 0.25"-0.50"
with some places recording 0.75" and even close to an inch. Even
the wildfires burning in Douglas County received some precipitation,
but amounts vary with the western fires (Homestead Complex)
receiving considerably more rain than the eastern fires (Diamond
Complex). Despite the lower amounts, fire activity should be
reduced, even if only temporarily, and this should bring a
reprieve from the persistent smoky conditions.

Thunderstorm activity has largely come to an end for the evening.
There are a few isolated showers on radar still, and satellite
indicated lightning detection does show a few cloud to cloud
strikes lingering in the region. Isolated showers are likely to
continue overnight, becoming more confined to areas west of the
I-5 corridor. Quieter conditions are expected for Sunday and into
next week. For more details, please see the previous discussion
below. /BR-y

&&

.AVIATION...18/06Z TAFS...Thunderstorm activity has calmed for the
moment across northern California and southern Oregon. A few
isolated showers indicate that moisture and energy remain in the
atmosphere, so stray thunderstorms and showers cannot be ruled out
overnight. Any overnight activity that does develop would most
likely be west of the Cascades.

Ceilings along the coast vary, with Brookings solidly at LIFR levels
and North Bend currently going between VFR and MVFR ceilings. Slight
shower chances (30-50%) will persist over coastal areas through
Sunday morning but should be cleared out by Sunday evening. Inland
areas are expected to remain at VFR through the TAP period. -TAD

&&

.MARINE...Updated 900 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.

&&

.PREV DISCUSSION... /Issued 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

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...None.

CA...None.

PACIFIC COASTAL WATERS...None.

&&

$$

MNF/TAD/MAS/CZS