Excessive Heat in Oregon, August 27 to 28, 2019

Area Forecast Discussion
National Weather Service Medford OR
254 PM PDT Tue Aug 27 2019

.DISCUSSION...

Excessive heat will persist over the next 24 hours across southern
Oregon and northern California. We're expecting to break records
at a few of our climate sites today as highs push into the mid
100's in a few of the major cities. Overnight lows will be in the
upper 60's over the majority of the area and lower 70's at higher
elevations. Because of the warm overnight temperatures, relief
from the heat will be difficult for the elderly, young children and
those who are sick. We'll start a cooling trend Wednesday and
that cooling trend will continue into the end of the work week.

Eventually, the remnants of tropical storm Ivo will move in and
bring a chance for thunderstorms across much of the region. The
best chances seem to be east of the Cascades again, but we cannot
rule out thunderstorms west of the Cascades. We are not expecting
thunderstorms over and along the coast and into the Umpqua valley
because the airmass should be rather stable there. Because of the
uncertainty over certain portions west of the Cascades, we're
going to continue the fire weather watch. More can be read on the
fire weather side of things in the discussion below. We're not
expecting these thunderstorms to be strong. In addition, these
thunderstorms will probably be on the drier side of things instead
of the wet storms we saw on August 10th.

Another low will sweep across Oregon on Thursday. Models continue
to predict thunderstorms around northern Lake and northern Klamath
Counties for most of Thursday. We feel pretty confident this will
occur as most of the potential instability will be uninhibited.

As we move into Friday, another strong low over the Pacific will
attempt to move inland next Monday or Tuesday. It will likely lose
strength as it moves into the four corners high. As it stands
right now, there is a slight chance for some rain showers.
However, that will likely change as we move close to the start of
September as there is a fair amount of uncertainty towards next
week.

-Smith

&&

.AVIATION...For the 27/18Z TAFs...VFR conditions are expected for
inland locations throughout the TAF period. Along the coast, coastal
stratus, currently just south of Crescent City, will surge north.
It's difficult to be confident in timing of when this arrives at the
coast, but current models show this surge making it to Brookings
late this afternoon and then north of Cape Blanco early to mid
morning Wednesday.

-Keene

&&

.MARINE...Updated 230 PM PDT Tuesday, 27 August 2019...The thermal
trough pattern is weakening. But, steep fresh swell will maintain
conditions hazardous to small craft in the outer waters and near
Cape Blanco into this evening. Winds will turn southerly late today
through tonight, and areas of fog and low stratus will develop
across much of the waters. Sub-advisory southerly winds and light
seas will then continue through the week.

-Weygand

&&

.FIRE WEATHER...Updated 200 PM PDT Tuesday 27 August 2019...Hot and
dry conditions will continue through Wednesday, with the hottest and
driest day being today. Poor to moderate humidity recoveries over
the upper slopes and ridges are expected again tonight, and this
will continue the idea of fuels drying prior to thunderstorm threat
arriving Wednesday. There is still some uncertainty with the
thunderstorm forecast for Wednesday as this is a much different
pattern than the pattern that dealt us 1800 cloud to ground strikes
earlier in August. We expect less lightning with this event, but the
hottest/driest conditions are occurring ahead of the lightning, so
fuels will be more receptive for new starts.

The remnants of Tropical Storm Ivo have generated lightning through
the day over the offshore waters. Though this area of instability
isn't supposed to move directly over our forecast area (will weaken
and slip to west), moisture and energy associated with Ivo will
enter our forecast area from the south Wednesday morning.
Thunderstorms are expected to begin over Northern California late
morning and push northward as the day progresses. Moisture below the
clouds will be low, and we don't expect much rain with these storms.
Additionally, gusty winds and low humidity are expected for much of
the area Wednesday afternoon, so any new starts will could grow more
quickly in this environment.

We've upgraded the Fire Weather Watch to a Red Flag Warning for most
areas in Northern California and our Oregon zones along and east of
the Cascades. West of the Cascades we have cancelled the Fire
Weather Watch basically west of Grants Pass and for Central and
Western Douglas County. One area we're still uncertain about is
Eastern Fire Weather Zone 620, 622, and Western Fire Weather Zone
617. Latest data suggests just a slight chance of storms there, but
we're going to wait for another round of high resolution model data
to decide whether to upgrade to a Warning or not.

We also decided to extend the Red Flag Warning for northern Fire
Weather Zones 624 and 625 from Wednesday evening through Thursday
evening to account for expected continued thunderstorm activity
there.

Temperatures will cool further on Thursday, and afternoon humidity
will trend upwards by 5 to 10 percent.

-Keene

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Red Flag Warning from 11 AM Wednesday to 11 PM PDT Thursday for
     ORZ624-625.
     Heat Advisory until 8 PM PDT Wednesday for ORZ023.
     Red Flag Warning from 11 AM to 11 PM PDT Wednesday for
     ORZ617-621-623.
     Fire Weather Watch from Wednesday morning through Wednesday
     evening for ORZ617-620-622.
     Excessive Heat Warning until 8 PM PDT Wednesday for ORZ024-026.

CA...Excessive Heat Warning until 8 PM PDT Wednesday for
     CAZ080-081-083.
     Red Flag Warning from 11 AM to 8 PM PDT Wednesday for
     CAZ280>282-284-285.

Pacific Coastal Waters...Small Craft Advisory until 5 PM PDT this afternoon for
     PZZ350-356.
     Small Craft Advisory until 11 PM PDT this evening for
     PZZ370-376.