448
FXUS66 KMFR 171101
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
401 AM PDT Sat Aug 17 2024

.DISCUSSION...An impactful afternoon and evening of wet, windy
weather is imminent, with showers and scattered thunderstorms
from the coast, across the west side to Klamath and Siskiyou
counties, and also southwest wind gusts of 25 to 40 mph east of
the Cascades. Both conditions prompted the issuance of Red Flag
Warnings /PDXRFWMFR/. The forecast for today and tonight exhibits
very little change from yesterday`s expectations. As such, the
main points from the previous discussion are still fully valid.
This includes a low, but mentionable risk of flash flooding, and
a higher, 5-15% probability that thunderstorms could become
severe, reaching thresholds of 1 inch diameter hail or 58 mph
wind gusts. Convective available potential energy(CAPE) isn`t
all that high with values around 500 to 1000 J/Kg. However, there
is plenty of 0-6 Km shear forecast, along with steep lapse rates.

A broad, strong, slow-moving upper trough is offshore from Oregon.
It is expected to shift south-southeastward today, then
northeastward toward the Washington coast tonight through Sunday.
Rainfall amounts from the coast to the Cascades will be highly
variable, representing the nature of showers, but amounts of a
tenth of an inch to an inch should be common. Meantime, amounts in
far eastern Siskiyou and Klamath counties should largely be less
than a tenth of an inch.

Beyond this evening, late tonight into Sunday morning, lower
level moisture, closer to the center of the trough, will move
onshore. The main result is expected to be additional light rain
from the coast to the Coast Range, mainly for Coos, Curry, and
western and northern portions of Douglas counties.

Forecast confidence is high for Today through Wednesday, and more
generally is high for a continued relatively cool, trough
dominated pattern through next week.

The upper low is expected to lift sufficiently far to the north
during Sunday to allow the light, near coastal precipitation to
taper off before noon on Sunday. Sunday afternoon through
Wednesday morning looks to be a period of dry weather across our
area with continued below normal temperatures, variable cloud
cover, and typical late day breezes. A minority of ensemble
members, as well as the operational ECMWF do indicate a few light
showers on Monday and/or Tuesday, and we have continued with a
slight chance mention for near coastal showers. But, this
scenario doesn`t favor anything more than some virga.

Another trough and associated shortwave are expected to track
inland Wednesday afternoon with some slight potential for showers
and thunderstorms, highest from the Cascades east-northeastward into
south central and central Oregon.

Meantime, on Wednesday, the next strong, closed upper low is
likely to be situated over western Canada. From Thursday into
Saturday, there are important, signficant differences in the
strength, speed, and track of this next low. The extent of
similarity with today`s weather is highly uncertain. But,
compared with the past few days, the probability is trending
higher for another episode of wet, windy weather during the latter
portion of the week.

&&

.AVIATION...17/12Z TAFS...Marine stratus is rebuilding overnight,
bringing MVFR and IFR ceilings that are expected to remain
through the TAF period.

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

Active weather is expected west of the Cascades later this morning
and into the next TAF period. 25-50% thunderstorm chances will be
present over west side areas today, 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 as early as late morning, mainly in the
afternoon and evening, then tapering off tonight.

-TAD/Hermansen

&&

.MARINE...Updated 245 AM 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 will move into the waters today
and will likely bring showers with a slight chance of thunderstorms
into early Sunday morning. -Hermansen

&&

.FIRE WEATHER...(From Friday afternoon).

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 this afternoon to 9 PM PDT this
     evening for ORZ616-617-620>623.

     Red Flag Warning from 2 PM this afternoon to 8 PM PDT this
     evening for ORZ624-625.

CA...Red Flag Warning from 1 PM this afternoon to 9 PM PDT this
     evening for CAZ280-281.

     Red Flag Warning from 2 PM this afternoon to 8 PM PDT this
     evening for CAZ285.

PACIFIC COASTAL WATERS...None.

&&

$$