NWS Medford, OR wrote its last area forecast discussion before the first event at Sep 14, 1:37 PM PST, 1 hour 53 minutes earlier.
Area forecast discussion, Sep 14, 1:37 PM PST, 1 hour 53 minutes before
Area Forecast Discussion
National Weather Service Medford OR
237 PM PDT Mon Sep 14 2020
.DISCUSSION...Smoke is mainly confined to the interior valleys,
except for the Shasta Valley where gusty south winds have allowed
the smoke to disperse some. Meanwhile smoke will remain pretty much
status quo for the other valleys through at least Tuesday morning.
This is because an inversion will continue in these areas which
means the mechanism is not there for the smoke to become more
dispersed. Therefore the dense smoke advisory has been extended
through Tuesday morning. Please see NPWMFR for more details.
Because of the smoke, little of any improvement in air quality is
expected for at least the next few days and this has also been
extended until noon Tuesday. Please see AQAMFR for more details.
East of the Cascades, smoke is not as think and should remain this
way through this evening this is because winds have mixed down near
the surface due to increasing instability as temperatures heat up.
Winds and low relative humidity will result in critical fire
conditions. Details on this will be in the fire weather discussion.
Further offshore is a weakening cold front and this will continue to
dissipate as it moves closer to the coast. With that said, could not
rule out intermittent light rain at the coast from around Cape
Blanco north later this evening and tonight with a trace to maybe a
0.02 of an inch at best.
Tuesday the best chance for precipitation will remain offshore, but
could not rule out off/on light rain at the coast. However most
hours will be dry as the upper low remain offshore and makes little
if any progress eastward.
There's good agreement the upper low will remain offshore until it
finally arrives at the coast Thursday. Until then, we can expect
much of the same with off/on light rain over the marine waters and
near and at the coast with precipitation chances dropping off fairly
quickly further inland.
Thursday, the upper low will finally move closer to the coast and
this will be the time where we'll see increasing chance of
precipitation along the coast and west of the Cascades with rain
chances Thursday morning highest near and at the coast. Instability
is marginal, but does exist along the Cascades, portions of Siskiyou
county and northwest Klamath County and a slight chance of
thunderstorms remain in the forecast.
The upper low finally moves on shore Thursday night and sets up over
the area Friday. It's during this time where most locations will
have a good chance of getting measurable precipitation. Marginal
instability exist Friday afternoon, but not enough to include a
slight chance of thunder at this time. Later shifts will want to
take another look at this.
The upper low will progress through Friday night with dry weather
returning Saturday. There's some evidence supporting a strong
thermal trough setting up along the coast with increasing offshore
flow in southwest Oregon and flat out dry weather Saturday through
Sunday. This is a ways out and some of the details could change, so
stay tuned for the latest updates.
Looking beyond Sunday, there's increasing evidence we could be
heading back into another prolonged period of dry weather as a zonal
flow sets up Saturday through the early part of next week, followed
by an upper trough digging offshore with ridging building over the
area. It's a ways out there, so it's possible the details on this
could change. -Petrucelli
&&
.AVIATION...For the 14/18Z TAFs...Wildfire smoke will be the main
impact during the TAF period. This will limit visibilities to
IFR/LIFR across much of the area. Partial improvement can be
expected this afternoon, especially along the coast and east of the
Cascades once gusty winds increase this afternoon. That leads us to
the second concern, gusty winds. Expect these to develop again this
afternoon and evening. The strongest wind gusts of 20-30kt will be
from the Shasta Valley eastward in northern California to areas east
of the Cascades in Oregon. Right now, guidance indicates a chance of
rain along the coast toward the end of the TAF period (late
tonight). Confidence wasn't high enough to include as a prevailing
weather group, so have maintained the PROB30 -RA to KOTH after 08Z.
-Spilde/BR-y
&&
.MARINE...Updated 130 PM PDT Monday 14 September 2020....Expect
periods of reduced visibilities due to marine stratus, fog or
wildfire smoke today. South winds have begun increasing ahead of a
cold front moving toward the coast. These gusty south winds will
result in steep, choppy seas this evening into Tuesday morning,
especially beyond 5 nm from shore along with rain at times.
Conditions will improve later Tuesday, but additional fronts will
keep rain chances going throughout the week. -Spilde/BR-y
&&
.FIRE WEATHER...Updated 200 PM PDT Monday 14 September 2020... Smoke
continues to be the deciding factor for much of the forecast, with
wind strength and temperature/humidity highly dependent on smoke
conditions through Thursday. Winds have at least partially cleared
areas across the East Side and over the ridgelines, allowing for
rising temperatures and lowering humidities there, but much of the
West Side and northern California remain socked in, especially in
the valleys. Soundings and satellite imagery confirm the inversion
is keeping those valleys capped under roughly 2500 to 3000 feet.
This will keep temperatures down and humidities somewhat higher in
these areas, and will also limit mixing and therefore keeping winds
calmer as well. Conditions are not expected to change much Tuesday
or Wednesday, as the offshore upper low slowly but surely tracks
towards the coast. South to southwest winds aloft will remain in
place, producing gusty winds east of the Cascades and across the
Modoc. Humidities in these areas remains low, thus Daily Red Flag
Warnings continue for each afternoon, likely through Wednesday. For
more details on expected conditions and locations of the Red Flag,
see the PDXRFWMFR.
Due to the wind direction aloft, the Shasta Valley could see periods
of channeled south winds during the afternoons. Some of this wind is
the result of mixing, and some is not, so while winds could pick up
from Weed to Montague, we do not expect Red Flag criteria to be met
for more than a brief time.
There is light at the end of this tunnel. Overnight recoveries
continue to be moderate to good across the area, although daytime
humidities remain low. As the upper low makes its way ever closer to
the coast, those daytime humidities will slowly but surely improve,
and by Thursday, they should rise high enough to prevent any
additional Fire Weather headlines (although breezy to gusty winds
are likely to continue). With the trough will also come shower
chances, first along the immediate coast and west of the coastal
ranges Tuesday into Wednesday, then for locations farther inland
Wedensday into Thursday. Models have backed off on overall
precipitation amounts, and as a result wetting rains may not be very
common outside of the coast, the Umpqua Basin, and the northern
Cascades, but overall most portions of the area have a decent chance
to receive measurable rainfall. There is a chance for a thunderstorm
or two to develop Thursday and perhaps Friday, but this will once
again hinge on the smoke coverage. Moisture may be limited as well,
but with the low moving directly overhead, can not rule out some
convection as it passes.
Any relief from the smoke and dry conditions may only be short
lived. Long term models show ridging returning next weekend, with a
return of the thermal trough pattern. It will be interesting to see
if the smoke cover remains thick enough to weaken the thermal
trough, but we will have to wait to find out. Otherwise, warm and
dry conditions are expected to return by next week, should model
trends hold. -BPN
&&
.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Red Flag Warning until 9 PM PDT this evening for ORZ623>625.
Red Flag Warning from noon to 8 PM PDT Tuesday for ORZ624-625.
Fire Weather Watch from Wednesday afternoon through Wednesday
evening for ORZ624-625.
Dense Smoke Advisory until noon PDT Tuesday for ORZ023-024-026.
CA...Red Flag Warning from noon to 8 PM PDT Tuesday for CAZ284-285.
Fire Weather Watch from Wednesday afternoon through Wednesday
evening for CAZ284-285.
Dense Smoke Advisory until noon PDT Tuesday for CAZ080-081.
Red Flag Warning until 9 PM PDT this evening for CAZ284-285.
Pacific Coastal Waters...Small Craft Advisory until 8 AM PDT Tuesday for
PZZ350-356-370-376.