NWS Medford, OR wrote its last area forecast discussion before the first event at Sep 7, 10:05 AM PST, 2 hours 40 minutes earlier.
Area forecast discussion, Sep 7, 10:05 AM PST, 2 hours 40 minutes before
Area Forecast Discussion
National Weather Service Medford OR
1105 AM PDT Tue Sep 7 2021
.Updated Aiviation Discussion.
.UPDATE...No changes were necessary this morning, with all aspects
of the forecast seemingly on track. Smoke will continue for much
of the day, but when a shortwave trough arrives this afternoon and
evening, along with some showers at the coast, winds will
increase and provide some improvement to areas west of the
Cascades, with additional improvement possible tomorrow afternoon.
For more details on the forecast, see the previous discussion
below. -BPN
&&
.AVIATION...07/18Z TAFs...along the coast and just offshore. VFR
ceilings will continue into this evening, then marine stratus is
expected to return resulting in IFR ceilings. However a weak upper
level system is moving up from the southwest and will move through
later this evening which could be enough to disrupt the marine layer
enough where ceilings may not end up as low as what's in the North
Bend TAF.
Over the inland areas...Thick smoke from the fires in the region
will result in areas of MVFR/IFR visibilities through 0z west of the
Cascades, including Medford. Guidance suggest the lower level smoke
pushing out of Medford between 1-3z which should result in improving
visibility and have shown this trend in the TAF.
Meanwhile east of the Cascades, the smoke will continue to result in
MVFR visibility into this evening, but even Klamath Falls should see
visibility improve as the aforementioned upper level system moves
through tonight. However any improvement may only be brief because
there's some evidence smoke from the fires in northern California
could move back in resulting in MVFR conditions. -Petrucelli
&&
.MARINE...Updated 800 AM PDT Tuesday 07 September 2021...A thermal
trough near the coast will weaken today. Small Craft Advisory
conditions with gusty north winds and steep seas will diminish north
of Cape Blanco later this morning and south of Cape Blanco this
afternoon. After a day of moderated conditions Wednesday, a weak
front will move onshore Thursday into Friday. The thermal trough
will redevelop Friday night, and strong north winds and steep to
very steep seas will return to the area, lasting through the
weekend.
&&
.PREV DISCUSSION... /Issued 247 AM PDT Tue Sep 7 2021/
DISCUSSION...A ridge of high pressure is causing an upper-level
cut off low to pivot from offshore Southern California into the
general flow off the Northern California Coast. As the low gets
enveloped into the clockwise flow, sheared apart the whole way,
it will move onshore near the Oregon/Washington border Wednesday
morning. As a result, the synoptic pressure gradient will
increase today, enhancing gusty downslope flow along the east
slope of the coast range. Some of the momentum from this increase
in wind will further amplify the diurnal winds off the Cascades
promoting increased wildfire-spread potential. See PDXRFWMFR and
the FireWx discussion below for red flag details. However, an
effect of it being sheared apart as it advances north-
northeastward is that it will lose much energy.
For west side valleys today these breezy to gusty winds will
cause near critical fire weather conditions this afternoon. The
current forecast is finding a balance between the -relatively-
more moist global deterministic solutions, the very dry
statistical guidance, and the hi-res ensembles. Min values from
hi-res could see RHs drop to 10% or less. The Max MinRH values
are at 25%. Have confirmed a roughly 15% RH forecast for the
valleys, with a 30% chance it being less than 15% RH. It would
not be surprising to see one or two locations reach red flag
criteria for a brief period. The smoke may once again temper
things though, as it has been doing with the temperatures, as it
reduces the wind's momentum ever so slightly. Another place where
critical conditions are expected, but won't reach duration
criteria is in Western Siskiyou County.
Again, the upper low will shear apart, losing energy on its way
north. It will not have the energy or momentum to push the larger
ridge to the east. It is expected the ridge will remain
influential to high temperatures today. Smoke, much to this
forecasters fatigue over its presence, will be what keeps
temperatures 5 or so degrees lower than the guidance. However,
there is enough shortwave energy embedded within it that there
remains the chance for precipitation along the coast. The global
ensembles put the chance for 0.01" or greater along the coast
range at 30% to 50%. Perhaps a bit optimistic, have gone with a
peak of 40% in the southern coast range for Tuesday evening.
Accumulations will be light and the intensity will not be great.
Rain accumulations are possible from the Illinois Valley west and
north of the Umpqua divide mainly along high terrain. Even with
the moisture we do not expect many mid-slopes or ridges to
observe full recoveries except around the Coquille Valley. Marine
stratus will intrude inland some overnight and patchy fog should
be present over the ocean.
After the cut off low does much to squash the ridge, a more robust
trough will compress it and move its center east closer to the Texas
Panhandle. This will allow for some cooling temperatures as it moves
onshore Wednesday into Thursday. 25-40 kts of mid-level flow will
pass over the southern Cascades and will be felt along the higher
terrain. MinRHs will generally be on the rise above 15% but isolated
critical conditions will still be possible especially for fire
weather zone 624. This will also bring improved smoke conditions
for the westside valleys. Smoke moving northeast from the fires
in Northern California will be concentrated east of the Cascades.
Smoke in the Klamath Basin and other east side valleys could
become fairly dense.
As the back side of the trough swings through the state on Friday
precipitation chances increase east of the Cascades. Hope should
not rise too much because most showers will produce virga instead
of rain. The global ensembles put the peak chance for 0.01" at
70% but even this close in time there remains too many
differences in the character of the trough to believe the
environment will be that favorable -did raise them to a hopeful
35%- . This is because some solutions have an environment
favorable for thunderstorms while other do not. For now have left
thunderstorms out of the forecast, but the probabilities are
nearing our mentionable threshold of 15% in eastern Lake County.
Through the weekend we should observe more seasonal temperatures,
along with an increase in humidities and cloud cover, as well as
improved ventilation, which should keep the smoke from
accumulating so much within the local area. There is then loose
agreement that a cresting wave of high pressure will pass over the
PNW sometime during the middle to end of next week.
-Miles
&&
FIRE WEATHER...Updated 130 PM PDT Monday, 06 September 2021...
This afternoon, winds will be generally lighter (compared to
yesterday) across inland areas. Still, expected to see some
afternoon/early evening breezes and dry humidities in Modoc and
southern Lake County. Winds lower tonight and will see another night
of moderate to poor recoveries across the ridges in Northern
California and east of the Cascades. Additionally, widespread and
thick smoke will continue to affect western Siskiyou, Jackson,
eastern Douglas and portions of Klamath counties today and tomorrow.
Other inland areas will see thinner smoke. Of note, where the smoke
is thickest, temperatures will be around 5 to 10 degrees lower than
guidance today and Tuesday.
On Tuesday, an upper level trough, currently located off the
California coast, will move north-northeastward towards the area.
Breezy to gusty afternoon winds are expected across most inland
areas Tuesday with critical fire weather conditions expected to
develop for areas east of the Cascades in FWZs 624, 285 and eastern
284. A red flag warning is in effect for these areas Tuesday
afternoon and evening. Other areas will also see breezy to gusty
winds and low humidities Tuesday afternoon/early evening, including
Josephine, Jackson, Siskiyou Counties and into the Cascades. Local
Fire Weather Guidance and SPC HREF indicate we'll near RFW criteria
in these areas. However, it appears that the duration of the dry
humidities combined with gusty winds is short and will likely not
reach criteria in those areas. Therefore, chose to keep a headline
for these conditions on Tuesday. As moisture increases from the west
Tuesday evening into Wednesday morning expect a slight chance for
light showers west of the Cascades, with the best chance over the
coast mountains. Also expect overnight recoveries to trend higher.
On Wednesday, the upper trough will move inland to the north
resulting in afternoon temperatures trending slightly lower, RHs
trending moister and winds trending stronger. Gusty winds are
expected during the afternoon and early evening, strongest east of
the Cascades (with gusts of 25 to 35 mph expected). However,
humidities will trend moister with RHs in the mid to upper teens for
these areas, keeping conditions just under critical thresholds. We
will continue to monitor these forecast conditions on Wednesday,
especially in FWZ 625 where RHs are likely to be driest and RHs
lowest.
Thursday and Friday, another upper trough moves into the region.
This may bring another period of gusty winds with relatively dry
humidities to areas east of the Cascades Thursday afternoon/evening.
Additionally, may see a slight chance for showers or thunderstorms
over far eastern zones, mainly on Friday afternoon/evening. -CC
&&
.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Red Flag Warning from 1 PM this afternoon to 8 PM PDT this
evening for ORZ624.
CA...Red Flag Warning from 1 PM this afternoon to 8 PM PDT this
evening for CAZ284-285.
Pacific Coastal Waters...Small Craft Advisory until 2 PM PDT this afternoon for
PZZ356-376.