Area Forecast Discussion
Issued by NWS Medford, OR

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FXUS66 KMFR 150315 CCA
AFDMFR

Area Forecast Discussion...CORRECTED
National Weather Service Medford OR
815 PM PDT Fri Aug 14 2020

.DISCUSSION...Some high clouds are moving overhead from the south,
signaling a shift towards deep southerly flow and a much warmer
air mass. While it cooled off significantly last night,
temperatures will trend significantly higher tonight and moreso
Saturday night. Dangerously hot afternoon temperatures are
expected tomorrow. What once seemed like a heat event that could
break a few daily record high temperatures Saturday, models
continue to trend warmer, and now it looks like many daily
records will be broken or tied west of the Cascades, with the
potential of a few more records on Sunday. To provide context for
the significance of the expected heat, we are forecasting 106
degree temperatures for Roseburg, and this is 20 degrees above
normal in what is already the hottest time of the year.
Additionally, we`re forecasting the hottest temperatures in
Medford since 2017.

For tonight`s update, based upon the collective guidance and
analog forecast temperatures, we`ve increased overnight low
temperatures by a few degrees Saturday night west of the Cascades.
Here are some tips to help keep yourself and your loved ones safe
during this heat. Please check our website at weather.gov/medford
for more heat safety tips. A host of Excessive Heat Warnings and
Heat Advisories continue through Sunday evening, highlighting the
hottest period of this heatwave.

* Monitor older adults, young children and those who are sick.

* Take plenty of rest breaks in shade or air conditioning.

* Drink water before, during and after activities.

* Rivers are dangerously cold and have strong currents. Wear a life
  jacket if swimming.

Thunderstorm chances gradually increase this weekend, entering the
forecast tomorrow and expanding in coverage on Sunday. We have A
Fire Weather Watch out for Sunday for parts of Northern California
and areas east of the Cascades for gusty winds and low humidity,
and we also expect some lightning in those areas. There`s more on
this in the Fire Weather discussion below. Please see the
previous discussions below, which detail the upcoming heat and
thunderstorm potential.

&&

.AVIATION...For the 15/00Z TAFs... Clear skies are expected to
continue for most areas through the TAF period. One exception is
smoke from the Red Salmon Fire, which will reduce visibilities in
much of western siskiyou County, with haze potentially spreading to
the north and northeast. Breezy to gusty north winds at the coast
and into the Umpqua Valley, including at KRBG will diminish after
sunset. There may be some patchy IFR/LIFR conditions that develop in
the Coquille Basin late tonight/early Saturday morning and could
briefly affect KOTH. If anything does develop, it will be short
lived with conditions improving to VFR by 16-17z Saturday. /BR-y

&&

.MARINE...Updated 730 PM PDT Friday 14 August 2020...A thermal
trough along the coast is bringing gusty north winds and elevated
seas. Steep seas will be present across most of the waters with very
steep and hazardous seas occuring roughly south of Gold Beach and
beyond 10 nm from shore. Winds and seas will gradually subside
tonight into Saturday once the thermal trough moves inland, then a
wind reversal to southerlies is expected late Saturday into Sunday.
Light wind and calmer seas, fog, low clouds, and a possibility of
drizzle will likely result. Relatively calm conditions will continue
through Tuesday, then gusty north winds and steep seas may redevelop
Wednesday, especially for areas south of Cape Blanco. -BPN/CC

&&

.FIRE WEATHER...Updated 200 PM PDT Friday 14 August 2020...
A strong upper ridge will build into the area this weekend and
persist into next week bringing the hottest stretch of weather this
summer thus far. It will be very hot across inland areas with high
temperatures challenging or exceeding record levels. Humidities will
remain rather low as well, but the upper level trough off the West
Coast will allow deep southerly flow to bring mid-level moisture
northward into the area Saturday. This will bring an isolated threat
of lightning to portions of the area during the afternoon and
evening. Moisture, forcing and instability aren`t overly robust, but
models support isolated thunderstorm development. Mid-level flow is
shown to be around 15 kt or so and this motion should allow any
storm that is able to develop to move northward up over the
Siskiyous into Jackson and eastern Josephine Counties over to the
Cascades. There could also be a couple of isolated storms from Modoc
County up into Lake and northern Klamath County. It should be noted
that while we think coverage of storms won`t be sufficient for a Red
Flag Warning, any lightning at all will bring with it the threat of
new fire starts. We`ll continue to headline the Fire Weather
Planning Forecast.

By Sunday, we`ll be on the western periphery of an upper high over
the Great Basin. A closed low out around 130W will open up and eject
northeastward offshore. Mid-level flow ahead of this system is
expected to increase to around 35-45 kt around 600 mb. This should
lead to some gusty winds due to strong diurnal mixing and downward
transport of momentum during the afternoon/evening. We have issued a
Fire Weather Watch for the increased winds along with low humidity
for the Shasta Valley to NW Modoc and some areas east of the
Cascades. See RFWMFR for the specific details. In addition, PWs
increase to 1.00 inch or greater as subtropical moisture works its
way northward. Moisture, instability and forcing will be greatest
along and east of the Cascades, so that is the area where isolated
thunderstorms are most likely, but a slight chance of storms exists
just about everywhere from I-5 eastward in Oregon and east of the
Red Salmon complex in NorCal.

We`ll remain in S-SW flow aloft through at least Tuesday, so the hot
weather looks to continue, albeit not quite as hot as over the
weekend. Isolated thunder chances continue in north Cascades and
over far eastern sections both Monday and Tuesday afternoons.
-Spilde

&&

.PREV DISCUSSION... /Issued 258 PM PDT Fri Aug 14 2020/

DISCUSSION...
Temperatures this afternoon were running 3 to 6 degrees warmer
than they were this time yesterday at most inland locations as of
2 PM PDT. Meanwhile, along the Curry County coast, a delay in the
expected wind reversal there has brought temperatures into the
90s at Brookings Airport at the elevation of 459 feet making it
23 degrees warmer than it was there 24 hours ago.

Meanwhile, high clouds are edging northward in southerly flow
across northern California and are just now beginning to move over
the southernmost portion of the forecast area. These clouds mark
the outer fringe of an area of low pressure that will affect our
weather this weekend into early next week.

Tonight through Saturday morning temperatures are expected to be
about 10 degrees warmer than they were last night due to the
combination of a warmer air mass pushing in from California and
some high level cloudiness moving through.

Heat, moisture, and instability are expected to increase Saturday,
and Excessive Heat Warnings and Heat Advisories remain in effect
for most of the forecast area through most of the weekend.
Hydration will be key to anyone spending any appreciable time
outdoors remaining cool. Please see our heat product statements
for other tips and reminders. Of note, temperatures will still
cool off appreciably overnight into the morning hours for those
seeking relief from the heat.

While moisture and instability increase Saturday, the lifted
condensation level for this air mass is at about the freezing
level. This means that cloud bases will be high with any
convection that forms Saturday afternoon and evening, at about
600mb, or nearly 14,000 feet above MSL. It also means that
convective clouds that do form will have a difficult time creating
enough charge differential for cloud to ground lightning. That
said, if a thunderstorms does form Saturday afternoon and evening,
it`s unlikely to produce rainfall, so this yields concerns for
starting fires from lightning and causing gusty winds. Our
official forecast has a slight chance of thunderstorms for mainly
Saturday evening across the Siskiyou County west of the Shasta
Valley northward into Jackson and Josephine counties to the
Umpqua Divide and then into the Oregon Cascades, and then arcing
across northern Klamath and Lake counties into the Warner
Mountains. Numerical model guidance suggests that conditions are
marginal for thunderstorms to form, but we`re likely to see at
least a few cloud to ground flashes, but suspect the total for the
day will remain under 100.

Southerly flow increases across the area Sunday with 30 to 40
knots in the 700-500mb layer coming across the Siskiyous by late
morning. It`s likely to be a breezy day for many areas, in
addition to being hot. On top of that, there will continue to be a
threat of isolated thunderstorms from about the Marble Mountains,
Jackson County, and eastern Douglas County eastward. Models
suggest the greatest potential for thunderstorms Sunday to be in
the Cascades from about Crater Lake NP northward. PWATs are
expected to be 1 to 1.3 inches Sunday with storm motions of 30
knots. Thus, in addition to concerns for lightning started fires
and gusty winds that day, we`ll also need to be on the lookout for
locally heavy rainfall, especially with any training thunderstorm
activity.

As we move into Monday model consistency and agreement
deteriorates. The majority of the model guidance pushes the
threat of lightning to at least areas east of the Cascades to our
eastern fringes, whereas the NAM12 continues a west side threat
with PWATS of 1 to a whopping 1.5 inches. Have generally
discounted the NAM12, at this point, as the other models seem to
be trending away from it and it`s out on both the fringe of
climatology and the edge of it`s time range.

Thereafter, it`s expected to cool down to temperatures 5-10
degrees above normal. ~BTL

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Fire Weather Watch from Sunday afternoon through Sunday evening
     for ORZ624.
     Heat Advisory from 8 AM Saturday to 11 PM PDT Sunday for
     ORZ027>029.
     Excessive Heat Warning until 11 PM PDT Sunday for ORZ024-026.
     Heat Advisory until 11 PM PDT Saturday for ORZ021-022.
     Excessive Heat Warning from 8 AM Saturday to 11 PM PDT Sunday
     for ORZ023-025.

CA...Heat Advisory from 8 AM Saturday to 11 PM PDT Sunday for
     CAZ082>084.
     Excessive Heat Warning until 11 PM PDT Sunday for CAZ080-081.
     Fire Weather Watch from Sunday afternoon through Sunday evening
     for CAZ281.
     Fire Weather Watch from Sunday afternoon through Sunday evening
     for CAZ284-285.

Pacific Coastal Waters...Small Craft Advisory until 5 PM PDT Saturday for
     PZZ350-356-370-376.
     Hazardous Seas Warning until 5 AM PDT Saturday for PZZ376.

$$


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