NWS Medford, OR wrote its last area forecast discussion before the first event at Aug 31, 8:23 PM PST, 3 hours 37 minutes earlier.
Area forecast discussion, Aug 31, 8:23 PM PST, 3 hours 37 minutes before
Area Forecast Discussion
National Weather Service Medford OR
923 PM PDT Thu Aug 31 2017
.DISCUSSION...
No updates are needed this evening. Northeast flow across the area
will continue to spread dry continental air southwestward all the
way to the coast. This will limit coastal fog and stratus and
continue to keep area wildfires very active tonight through Friday.
Smoke from the wildfires will continue to be an air quality and
visibility concern through the weekend as hot, dry, and unstable
afternoons and mild nights continue. Please read below for further
details. BTL
&&
.AVIATION...01/00Z TAF CYCLE...Strong north winds at OTH with gust
to around 30 knots will continue until this evening but winds are
not expected to go completely calm overnight. As a result of the
offshore winds, conditions should remain VFR for most of the night
until winds decrease below 5 knots and low clouds reformed around 10-
12Z. Over the remainder of the area...IFR visibilities with the
thickest smoke will persist in southern Curry County and western
Siskiyou County. Elsewhere...areas of MVFR visibilities from
wildfire smoke will persist. There will be a gradual worsening trend
late this evening at LMT to MVFR visibility. /FB
&&
.MARINE...Updated 200 PM PDT Thursday 31 August 2017...The latest
observations at buoys 15 and 27 show breezy north winds above small
craft advisory. High pressure offshore and a thermal trough along
the coast will bring strong north winds and very steep short period
seas tonight into Saturday morning. Winds are expected to reach gale
force south of Port Orford beyond 5 nm from shore. Gales and very
steep wind driven seas are likely to peak this evening but linger
into Friday morning. The thermal trough is expected to drift
westward over the coastal waters Saturday and Sunday leading to
lighter winds and diminished seas. North Winds will briefly increase
again on Monday. Seas are expected to become swell dominated on
Wednesday for the first time in quite a while. /FB
&&
.FIRE WEATHER...Updated 345 PM PDT Thursday 31 August 2017...
There are a number of fire weather concerns: Gusty easterly winds
with low humidity this afternoon roughly in the area west of
Interstate 5, stronger northeast to easterly winds with poor
humidity recovery tonight, very hot temperatures Friday through
Monday, Haines 6 dry and unstable conditions moving from west to
east across the forecast area Friday into Saturday with the thermal
trough, and finally...the potential for a daily risk of
thunderstorms next week on Sunday through Thursday.
A dry air mass will gradually descend over the forecast area tonight
and remain through Sunday at least, with poor to moderate humidity
recoveries each night. Northeast winds will increase later this
afternoon and humidity will be very low, so a Red Flag Warning
remains in effect for the solid moderate-strength Chetco Effect
event continuing through early Friday morning. The Cascades,
Siskiyous, and Western Siskiyou County will also see gusty overnight
winds and poor humidity recoveries tonight, and they are included in
the Red Flag Warning. Northeast/east winds do diminish toward the
coast during the day on Friday, but we are not expecting a big push
of moist onshore winds to affect much of the Chetco Bar fire.
Instead, conditions on Friday become hot, dry, and unstable with
generally lighter winds for much of the forecast area as the thermal
trough begins to nudge inland. We have issued a Red Flag Warning for
these unstable conditions Friday for the Chetco Bar Fire, Horse
Prairie Fire, and Miller Complex of wildfires. As the thermal trough
moves farther inland Saturday, a Fire Weather Watch highlights these
dry and unstable conditions for the High Cascades Complex, the North
Umpqua Complex, the Pelican Butte, and the Naylox and Jade Creek
Wildfires...from the Cascades eastward. Models suggest this dry and
unstable pattern continuing Sunday, and we'll consider watches or
warnings in subsequent shifts.
Note that the air mass this weekend will be one of the hottest and
driest so far this summer, and certainly in the hottest 5% for this
time of year. Smoke will no doubt limit the amount of heating we get
at the surface, but clearer areas should be dangerously hot. Fire
crews will be impacted by heat even in the higher terrain areas.
The models are showing increasing monsoonal moisture moving up on
the backside of the ridge into our area Sunday along with a weak to
moderate trigger. This brings the possibility of thunderstorms back
into the picture. Next Monday and Tuesday now look more favorable
than Sunday for thunderstorms, and an unstable air mass will likely
persist through Thursday. -SK/DW
&&
.PREV DISCUSSION... /Issued 350 PM PDT Thu Aug 31 2017/
Updated fire weather discussion.
DISCUSSION...However brief, the break in the smoke over our area
has come to an end. East winds, developing behind the departing
trough and under the building ridge, has started to push smoke
back into the same areas that were plagued by smoke earlier in the
week. There appears little chance for any relief from this smoke
in the near future, with the upper level ridge remaining in
control through at least the middle of next week. An Air Quality
Alert, in coordination with Oregon Health Authority and local
governments, will continue until further notice for a large
portion of southwestern Oregon.
The upper level ridge will be the source of another problem as
well. Working in conjunction with a strengthening thermal trough,
the ridge will allow temperatures to values approaching 20 to 25
degrees above normal for the first part of September. The thermal
trough, currently located along the coast, will gradually shift
east into Saturday, producing the warmest inland temperatures
Saturday afternoon. The feature will then slowly progress further
east, but heat will persist through at least Monday or Tuesday.
One limiting factor for the heat will be the smoke, as a thick
layer of smoke can block out enough incoming sun to keep highs 7
or 8 degrees below what they would otherwise be, while a thin
layer may only drop highs by a degree or two. Considering smoke is
likely to be rather thick over much of the area, have dropped
temperatures to around 5 degrees below most model guidance, but
this still results in very warm conditions, and some local daily
records may still be under threat. Have upgraded the Excessive
Heat Watch to an Excessive Heat Warning for most of the Inland
West Side and Western Siskiyou county, and to a Heat Advisory for
parts of central Siskiyou county, where overnight lows should be
cooler and impacts will not be as severe.
Some relief form the heat may arrive early next week in the form
of convection and thunderstorms. As the upper level ridge
gradually builds to our east, a broad weak area of troughing is
expected to develop offshore of California. This will produce a
pattern that will feed tropical moisture, some of which some may
be remnant parts of Tropical Storm Lidia, up through California
and into southern Oregon. The Thunderstorm threat begins late
sunday in Sikiyou and Modoc counties, then spreads north into
nearly all of our Oregon zones by Monday. Some model solutions
then keep a broad area of precipitation and thunderstorms over
the entire forecast area through Wednesday, even persisting
convection overnight. The GFS has been a stalwart proponent of
this scenario, but the ECMWF has kept the region mostly dry,
despite having a similar overall pattern. Have kept or expanded
the areas of thunderstorm chances from early to midweek, but kept
them at slight chance due to the uncertainties of location and
timing, and the lack of agreement across model suites. What storms
do develop should be wet, and perhaps this may offer some relief
to area fires, at the unfortunate risk of starting a few more.
-BPN
&&
.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Fire Weather Watch from Saturday afternoon through Saturday
evening for ORZ617-623-624.
Excessive Heat Warning from 11 AM Friday to 11 PM PDT Monday for
ORZ021>028.
Red Flag Warning until 9 AM PDT Friday for ORZ618>620.
Red Flag Warning from 1 PM to 8 PM PDT Friday for
ORZ616-618>621.
Red Flag Warning until 9 AM PDT Friday for ORZ617-621-623.
CA...
- Excessive Heat Warning from 11 AM Friday to 11 PM PDT Monday
for CAZ080.
- Heat Advisory from 11 AM Friday to 11 PM PDT Monday for
CAZ081-082.
- Red Flag Warning until 9 AM PDT Friday for CAZ280.
Pacific Coastal Waters...Small Craft Advisory until 11 PM PDT Friday for
PZZ350-356-370-376.
Gale Warning until 8 AM PDT Friday for PZZ356-376.
Small Craft Advisory for hazardous seas from 8 AM to 11 PM PDT
Friday for PZZ350-356-370-376.
Hazardous Seas Warning from 8 AM to 11 PM PDT Friday for PZZ376.
BTL/BPN/NSK/FJB