523
FXUS66 KMFR 232342
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
442 PM PDT Mon Sep 23 2024

.UPDATED AVIATION Discussion...

&&

.AVIATION...24/00Z TAFS...VFR conditions will prevail across the
region through the TAF period with patchy high level clouds
streaming through the area. LIFR/IFR cigs/vis are likely to return
to the coast north of Cape Blanco between 03z-06z, then around
Brookings for a brief period around 12-15z. /BR-y

&&

.PREV DISCUSSION... /Issued 221 PM PDT Mon Sep 23 2024/

DISCUSSION (Today through Sunday)...

Noteworthy items for this forecast include the well above normal
temperatures (10+ degrees) expected tomorrow with some areas
approaching 100 degrees. Additionally, we have a chance for rain
showers with this incoming system starting Wednesday morning, but
this should be confined to the westside with the best chance for
accumulation along/near the coast. Also with this system, strong
wind speeds (areas of 25+ mph sustained) are forecast on the
eastside, and this will be coupled with low RH values, so we are
looking at the threat for fire weather conditions Wednesday
afternoon. Please see the latest fire weather discussion below for
further information. Thereafter, we have seasonable temperatures
and dry conditions forecast through the weekend.

Further Details:

A ridge of high pressure aloft will start to expand over the
PacNW today and tomorrow. This will lead to well above normal
temperatures in the afternoon hours. Brookings is currently at 90
degrees as of writing this (~2pm) thanks to an ongoing Chetco
Effect. Here in the Rogue Valley, we are forecast to reach 90...or
close to it. Medford will likely see temperatures passing 90
degrees tomorrow as we are currently forecasting 97 degrees.
Temperatures in the 80s expected on the eastside for tomorrow. The
probability for Medford to reach 100 degrees tomorrow is about
10%, so we may not reach triple digits tomorrow. The record for
Medford on September 24th is 99 degrees, so we will be within
record territory.

The 500mb pattern will undergo a transition today into tomorrow
as a Rex Block pattern develops tonight, and then transitions to
an Omega Block pattern across the CONUS late Tuesday afternoon.
Blocking patterns are tricky sometimes, but this Rex Block seems
to be progressive in the sense its not really "blocking" the flow
too much (pseudo blocking pattern?). However, we should probably
keep an eye on this transition as the block could disrupt rainfall
timing and chances. A closed low on westside of the Omega Block
is expected to ride the western periphery of high pressure center
over the Four Corners Region. This trough is progged to go
north/northeast through the PacNW on Wednesday. This southwesterly
flow will allow for rain showers to develop and spread west to
east through the day on Wednesday. Showers are likely not going
too far into the evening, and the current thinking is showers are
unlikely after 9pm. Not expecting any thunderstorms to develop
with instability forecast to be very low (<100 J/kg). Broadly
speaking, this is one of those "high PoP, low QPF" scenarios for
Wednesday. Rainfall might make it into eastern Douglas County and
parts of Josephine County, but chances farther east really dwindle
for rainfall. Along and near the coast has the best chance for
rainfall accumulation, with the probability for greater than 0.10"
over 12 hours around 10%-50%. That said, the chance for rain
itself along/near the coast is about 35%-80%. In other words,
expecting light rainfall amounts for areas west and north of a
line from roughly Cave Junction to Roseburg. Areas south and east
of this line will be lucky to get a couple hundreths (0.01") of
rainfall.

-Guerrero

MARINE...Updated 200 PM Monday, September 23, 2024...A thermal
trough remains in place over the Oregon coast. The trough will
continue bringing gusty northerly winds through early Tuesday
morning. A Small Craft Advisory remains in place for wind-built
steep seas of water south of Bandon through Tuesday at 11 AM.
Periods of very steep and hazardous seas are possible south of
Pistol River late Monday night into early Tuesday morning.

After the current Advisory ends, northwest swell will keep seas
in the 7 to 8 foot range with longer periods keeping seas below
Advisory levels in the middle of the week. Northerly winds will
increase towards the end of the week and combine with the steep
northwest swells.

-TAD/Smith

FIRE WEATHER...Issued 200 PM Monday, September 23, 2024...
Offshore flow of recent days will ease today through tonight as
the thermal trough that is present along the coast weakens. The
thermal trough will shift northward and move inland on Tuesday.
Where it is hot today (Brookings), it will be much cooler on
Tuesday as marine air takes hold. However, inland areas will have
the hottest day of the next 7 days. West side valleys will rise
well into the 90s with a few spots flirting with the 100-degree
mark. Winds on Tuesday will be fairly typical with afternoon gusts
of around 15-20 mph and minimum RH values in the 15-25% range.

The main fire weather risk comes on Wednesday as an upper trough
swings through the PacNW. While the associated marine push will
send some clouds and even a little rainfall onto the coast/Umpqua
Basin (a few hundredths to a tenth of an inch for the Bottom Creek
fire), gusty W-SW winds are expected in advance of the trough
from the Cascades eastward. At least a few hours of windy
conditions in portions of eastern Siskiyou, Modoc and into
Klamath/Lake counties. Expect most of those areas to have 15-20
mph sustained winds during the afternoon with frequent gusts of 35
mph. Local RAWS guidance for some of our most exposed RAWS sites
(Coffee Pot, Summit, Summer Lake, Timber Mtn) is showing peak wind
gusts in the 40-45 mph range. The limiting factor though is that
minRH values are shown to be in the 15-20% range, slightly too
high for a Fire Weather Watch. We`ll continue to headline this
risk though in the Fire Weather Planning Forecast.

After that trough, there`ll be a break Thursday, then another
weak trough/marine push is expected Friday afternoon/night. This
one doesn`t look as strong as the Wednesday trough.

The next stronger trough is shown to arrive over the weekend.
This may increase W-SW winds again over the East Side/Modoc County
Saturday afternoon/evening (winds may be similar to Wednesday)
and RHs may be down a bit as well. This could then induce another
thermal trough with a period of offshore NE winds Sunday into
early next week.

-Spilde

&&

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

PACIFIC COASTAL WATERS...Small Craft Advisory until 11 AM PDT Tuesday
     for PZZ356-370-376.

&&

$$