NWS Medford, OR wrote its last area forecast discussion before the first event at Aug 31, 7:51 PM PST, 4 hours 9 minutes earlier.
Area forecast discussion, Aug 31, 7:51 PM PST, 4 hours 9 minutes before
Area Forecast Discussion
National Weather Service Medford OR
851 PM PDT Thu Aug 31 2023
.DISCUSSION...The primary front has moved through. Did you notice
it? In some places it lead to a wetting rain of over a tenth of
an inch. Mainly in the Coast Range and across Douglas County. For
other places you might be right to be wondering where the rain
is. And as we watch a secondary band of showers dissipate across
Jackson and western Siskiyou County we will be left wondering a
while longer.
As it turns out, there was a lot of dry air out there! The
sounding from this afternoon revealed a dry layer 6kft deep
before a weak moist layer became present with another 3kft deep
dry layer stacked on top of it. So I'll say it anyway, as with
most early season fronts, this one just lacked the leading power
to get rain to many places. But now that things have moistened up
(a big boost to diminishing fire behavior and ultimately smoke
I'll point out), the showers that we anticipate for the coming 24
hours should have no problem getting rain to the surface. The
turbulence created by these fronts should also have a positive
effect on the smoke, continuing to disperse it a little more.
As the area of low pressure drops south through tonight the
showers it will generate Friday will be moving in from the
southeast. So areas of NorCal and the east side will experience
them first. Although rain amounts in Lake and Kalamth county may
be subdued at first because the drier surface air is likely going
to hang on a bit longer. The rain showers are forecast to spread
into S. Oregon late-morning on Friday (after roughly 10 am).
While doing so, some isolated thunderstorm activity is expected
to be embedded within it. The most likely area for these
thunderstorms is for both sides of the Cascades south of Crater
Lake, highest in the vicinity of Siskiyou Summit and into Western
Siskiyou County.
The more powerful the storm, the more rain it should produce. And
these thunderstorms could be big producers. If one moves over a
severely burned wildfire scar, debris flows will be possible. For
most areas though this will be a beneficial rainfall regardless
of amount as storms should be just mobile enough to prevent a
storm from producing flash flooding because it just sits over one
spot for its whole life cycle. So unless you are very near a
recent, severe burn scar, please don't be alarmed. Do take caution
to pay attention for hail, though. The environment looks primed
to support hail sizes ranging from pea size up to nickle sized,
which while the nickle size is not considered severe, it will
still hurt to get pelted in flurry of hail that size.
And of course there is the threat of lightning. Even with activity
anticipated to be isolated in nature there remains a low chance
that lightning counts could get into the hundreds. The SREF
guidance puts the chance for 100+ ground strikes at 2-4% in
western Siksiyou County, which the 4% isn't all that low for this
piece of guidance. The good news is that the wet storms should
help mitigate new fire starts due to the lightning. This high
density of lightning could pose a threat to anyone outdoors. Keep
an eye on the radar and have a plan or two of how to get to safety
if you are outdoors if a thunderstorm moves by.
-Miles
&&
.AVIATION...01/00Z TAFs...Low pressure moving south into the area
tonight will persist through Friday. This has resulted in a mix of
IFR and MVFR with mainly light showers and areas of terrain
obscuration for Coos, Curry, Douglas, and Josephine counties early
this evening, including North Bend and Roseburg. The lowest ceilings
are mainly near the coast. Meantime, conditions are mainly VFR
elsewhere.
The coverage of showers will increase across the area overnight,
with increasing areas of MVFR west of the Cascades, particularly in
southwest Oregon. A further increase in both the intensity and
coverage of showers will occur Friday afternoon into the evening,
with isolated thunderstorms expected to be embedded in moderate to
heavy showers. This includes Medford and Klamath Falls, with a
slightly lower probability of thunderstorms also for Roseburg.
-DW
&&
.MARINE...Updated 800 PM....High pressure in the northeast Pacific
and low pressure extending from Oregon over the southern Oregon
coastal waters have brought steep seas and increasing north winds,
strongest over the outer waters. The north winds will continue to
increase into Friday evening with the steep seas persisting through
Friday night.
As the low moves farther offshore, southerly winds develop late
Friday night and continue through Sunday. This will result in a
transition to seas dominated by mixed northwest and fresh northwest
swells. As the low moves inland, north winds are expected to return
Sunday night and may increase to near Advisory strength again on
Monday into Tuesday. -DW
&&
.FIRE WEATHER...Updated 200 PM PDT Thursday, August 31, 2023...
The leading edge of a cold front is moving across the forecast area
at this time. Rainfall has been light associated with the front,
with early afternoon observations generally 0.20" or less along the
coast and over to the Umpqua Valley. At 2 pm, the incident
meteorologist on the Tyee Ridge complex north and west of Roseburg
reported 0.15" on the fires. Radar returns have been well maintained
along the front across Douglas County, but have been falling apart
as they come into Josephine County. So, not much being reported as
yet down around the Smith River complex. Still a reasonable chance
at a little measurable precipitation with the front this afternoon
into this evening, but probably not as much as models had previously
indicated. Farther east, it remains warm east of the Cascades and
into NE California, where some gusty westerly winds are expected to
develop this afternoon/evening. Humidity values should begin rising
a bit though as cloud cover increases and the front presses in from
the west.
The biggest changes will come tonight through Saturday as an
anomalous upper trough associated with the front closes off an upper
low that will settle southward and set up shop just to our southwest
in NW California (it could even briefly wobble offshore). This will
bring a much cooler, moist, showery weather pattern area wide. These
showers and also some embedded thunderstorms will likely produce
wetting rainfall in many areas, though amounts will vary widely due
to the showery nature of the precipitation. There will be a
noticeable temperature drop from today to tomorrow across NorCal and
over the East Side with most of the area running about 10-20F below
normal for early September. Along with that, there will be a notable
increase in humidity. Right now, two-day rainfall of 0.25-0.50"
seems to be the most likely amount across a large portion of the
area with 0.10-0.25" on the western and eastern fringes. The
highest likelihood of convectively enhanced rainfall is in the
NorCal mtns/foothills, up across the Cascades and over to western
Klamath County, where rain amounts could reach 0.50-1.00" in some
areas. A Flash Flood Watch remains in effect for Friday into Friday
evening for the potential for debris flows and flash flooding on the
McKinney and Head fire burn scars in western Siskiyou County.
Despite the lightning risk and the potential for new fire starts,
with the cooler air mass in place and plenty of moisture around,
fire weather concerns due to lightning are low.
An upstream trough should kick the upper low out to the east into
Nevada on Sunday, though recent models runs are a bit slower doing
this. As a result, while it will be a comparatively drier day for
most, there is still a chance of showers, especially in the Cascades
and Siskiyous.
The kicker disturbance is expected to zip through the PacNW
Sunday night into Monday with a chance of showers, mainly across N&W
sections (most of the precip will remain to our north). Behind that
system, the overall trend through next week is for a more
progressive pattern with temperatures rebounding closer to normal
and a couple of weaker fronts fizzling near the coast, but no
significant hot weather is in sight. -Spilde
&&
PREV DISCUSSION... /Issued 249 PM PDT Thu Aug 31 2023/
DISCUSSION...The front has been clearly defined by the incoming
cloud cover and stratiform rain today. North Bend picked up 0.08"
since this morning, and cameras this morning captured that with
wet roads. There were also reports of 0.10"-0.20" in northern
Curry and southern Coos counties. However, there have been lower amounts
from central Curry and Douglas counties south. This part of the
line has not been as steady as the northern edge, so for the
evening the rain chances will be less for counties to the south of
this area. With this track in mind, PoP chances have been
decreased for these areas.
Friday is still favored for the best rain chances with this system
as the upper trough evolves to an upper low as it moves south. The
heaviest rain will be found east of the Cascades and in northern
California. These areas will also have the chance for isolated
thunderstorms, although the afternoon update includes pushing the
thunder chances west into the rest of Siskiyou County and through
Jackson county and eastern Douglas County. Flash flooding is a
concern for areas with recent burns, so a Flash Flood Watch has been
issued for tonight through Friday for the McKinney and Head burn
scars. There are also chances for scattered thunderstorms across
western Siskiyou County tomorrow afternoon. Amounts reaching
0.25"-0.50" west side are possible through Friday and Saturday.
Temperatures are still forecast to be 5 to 15 degrees below normal
in the afternoons with minimal changes to morning temperatures.
After lingering showers Saturday, activity will wind down through
the rest of the weekend as the low moves south. Into early next week
there are models showing a general trough pattern off the coast
which are favoring dry conditions.
-Hermansen
FIRE WEATHER...Updated 200 PM PDT Thursday, August 31, 2023...
The leading edge of a cold front is moving across the forecast area
at this time. Rainfall has been light associated with the front,
with early afternoon observations generally 0.20" or less along the
coast and over to the Umpqua Valley. At 2 pm, the incident
meteorologist on the Tyee Ridge complex north and west of Roseburg
reported 0.15" on the fires. Radar returns have been well maintained
along the front across Douglas County, but have been falling apart
as they come into Josephine County. So, not much being reported as
yet down around the Smith River complex. Still a reasonable chance
at a little measurable precipitation with the front this afternoon
into this evening, but probably not as much as models had previously
indicated. Farther east, it remains warm east of the Cascades and
into NE California, where some gusty westerly winds are expected to
develop this afternoon/evening. Humidity values should begin rising
a bit though as cloud cover increases and the front presses in from
the west.
The biggest changes will come tonight through Saturday as an
anomalous upper trough associated with the front closes off an upper
low that will settle southward and set up shop just to our southwest
in NW California (it could even briefly wobble offshore). This will
bring a much cooler, moist, showery weather pattern area wide. These
showers and also some embedded thunderstorms will likely produce
wetting rainfall in many areas, though amounts will vary widely due
to the showery nature of the precipitation. There will be a
noticeable temperature drop from today to tomorrow across NorCal and
over the East Side with most of the area running about 10-20F below
normal for early September. Along with that, there will be a notable
increase in humidity. Right now, two-day rainfall of 0.25-0.50"
seems to be the most likely amount across a large portion of the
area with 0.10-0.25" on the western and eastern fringes. The
highest likelihood of convectively enhanced rainfall is in the
NorCal mtns/foothills, up across the Cascades and over to western
Klamath County, where rain amounts could reach 0.50-1.00" in some
areas. A Flash Flood Watch remains in effect for Friday into Friday
evening for the potential for debris flows and flash flooding on the
McKinney and Head fire burn scars in western Siskiyou County.
Despite the lightning risk and the potential for new fire starts,
with the cooler air mass in place and plenty of moisture around,
fire weather concerns due to lightning are low.
An upstream trough should kick the upper low out to the east into
Nevada on Sunday, though recent models runs are a bit slower doing
this. As a result, while it will be a comparatively drier day for
most, there is still a chance of showers, especially in the Cascades
and Siskiyous.
The kicker disturbance is expected to zip through the PacNW
Sunday night into Monday with a chance of showers, mainly across N&W
sections (most of the precip will remain to our north). Behind that
system, the overall trend through next week is for a more
progressive pattern with temperatures rebounding closer to normal
and a couple of weaker fronts fizzling near the coast, but no
significant hot weather is in sight. -Spilde
&&
.MFR WATCHES/WARNINGS/ADVISORIES...
OR...CA...Flash Flood Watch from 8 AM PDT Friday through Friday evening
for CAZ080-081.
PACIFIC COASTAL WATERS...Small Craft Advisory until 11 PM PDT Friday
for PZZ350-356-370-376.
&&