Area Forecast Discussion
Issued by NWS Medford, OR

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FXUS66 KMFR 051550
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
850 AM PDT Wed Aug 5 2020

.UPDATE...Changes to PoP and Weather grids for this afternoon.


&&

.DISCUSSION...Satellite showed some early morning convective
clouds moving through Modoc and Lake county already. The IMET on
the July Complex fire reported these clouds producing virga. The
clouds were not tightly packed and there is plenty of clearing
behind them, so this should not inhibit convection this afternoon.
If anything, it shows how primed that area is for thunderstorms
this afternoon.

Reduced PoPs in the Klamath Falls area from 2-5PM PDT, removing
the slight chance. There is still a slight chance in that area
from 5-8PM.

In lake county, shear has trended up and lapse rates have
increased to more than 9 C/km, so have added hail to the gusty
thunderstorms already in the forecast. The latest runs of the HRRR have
initiated multiple strong thunderstorms off the Warner mts in
Modoc county and have them moving north into the favorable
environment over Lake County. It is certainly not impossible the
strongest storm or two of the day in this area reaches severe
criteria.

The previous shift noted that shear and CAPE, combined
with minimal convective inhibition, over the Trinity alps in
Siskiyou County. Have not added small hail or gusty winds to the
forecast yet, but may be making that addition last minute if the
00z Hi-res runs continue to show a favorable environment. Also,
steering winds remain weak, under 15kt, so any storms should stay
latched to the terrain and not move too far from its initiation
area.

Expanded discussion on the thunderstorms and weather following
today can be found in the previous discussion below.    -Miles

&&

.AVIATION...For the 05/12Z TAF Cycle...LIFR/IFR/MVFR ceilings
with some visibility restrictions and higher terrain obscurations
will continue across Coos County into the Umpqua Valley this
morning. Low clouds will also spread inland across Curry County, but
are likely to be blocked by the higher Coastal Range Mountains.
Humbug Mountain to Gold Beach could remain mostly clear, however.

Elsewhere, inland areas will continue to have VFR conditions, except
over and near the Scott Valley and areas east of the Cascades this
afternoon into this evening. In those areas isolated to scattered
thunderstorms- some of which could be strong or even severe-  are
expected. These thunderstorms will be capable of gusty outflow winds
and, possibly, micro-bursts. Thunderstorms are possible at KLMT this
afternoon, but those should move east of there by evening.

Over southwest Siskiyou County near the Red Salmon Fire there`s
likely to be some local MVFR visibility restrictions due to smoke.
~BTL/Sandler

&&

.MARINE...Updated 835 AM PDT Wednesday, 5 August 2020...
The thermal trough remains weak this morning, as shown by relatively
light conditions at the buoys and stratus clouds remaining over much
of the marine coastal waters area. Therefore, the Small Craft
Advisory that had been in effect has been cancelled until this
afternoon, when northerly winds and steep seas are expected to build
to advisory levels south and west of Humbug Mountain. Winds and seas
will weaken Wednesday night as a cold front moves through.

The thermal trough will then re-strengthen Thursday afternoon
bringing the return of advisory level conditions south of Cape
Blanco. Friday through the weekend winds and seas are likely to
strengthen further with advisory level winds and seas expected to
expand to areas north of Cape Blanco. Gales are likely for a portion
of the area south of Cape Blanco with advisory levels winds and seas
expected for all areas south of the Cape. It should be noted that
Friday through early next week could be a prolonged upwelling event
for the coastal waters with multiple consecutive days of hazardous
conditions. ~BTL/Sven

&&

.PREV DISCUSSION... /Issued 349 AM PDT Wed Aug 5 2020/

DISCUSSION...

There are some interesting observations to be made on satellite
this morning. First, a fair amount of stratus is pushing into the
Umpqua valley as of 245 am this morning. In addition, one could
look at the water vapor imagery on satellite and see the upper
level low rotating near Humboldt and Trinity Counties. Looking at
the water vapor, there appears to be a fair amount of low level
and mid level water vapor moving into the region.

Aside from those observations, thunderstorms are still the main
concern this afternoon and evening. Models continue to target
eastern Klamath County and western Lake County as the hot spot of
thunderstorm activity. The trinity mountains in south western
Siskiyou County should also see storm activity. Convective
available potential energy varies from 600 - 1000 j/kg. That is
nothing to scoff at for our region and is on the higher side than
what we usually see in the summer. Convective inhibition appears
to be very limited during the afternoon and evening hours, so
storms should have no problem forming under weakly forced
conditions. 0 - 6 km bulk shear is around 25 to 30 knots in some
areas. This is stronger than what we usually see in Oregon, so
some of the storms will sustain a little longer and might produce
some gusty winds.

Because of everything mentioned above, an isolated severe storm
is possible in south central Oregon, especially south of Bend near
Chemult and Crescent. Overall, we feel confident with the
scattered thunderstorm wording over south central Oregon and south
western Siskiyou County as thunderstorm probabilities are around
50% for those areas.

As for Wednesday night into Thursday morning, a low will swing
into the state of Washington and bring a cold front into most of
the coastline around 7am Thursday. There is currently a 15 to 25
percent chance for light rain along the coast north of Cape
Blanco. The rain accumulation should vary between 0.01 to 0.05
inches along the coast. This cold front will bring cooler
temperatures to the region and will likely be the coolest day
observed for some time. Although the forecast does contain highs
in the upper 80`s for Medford today, this short stretch of
weather, today and Thursday, will be the first time we`ve seen 80
degrees in Medford since July 12th.

After the upper level low traverses east, we`ll see a coastal
thermal trough build during Friday. Offshore flow looks modest
with easterly 850 to 825 mb winds around 20 to 25 knots. This
offshore flow will likely push Brookings into the upper 70`s and
perhaps up to 83 on Sunday if the MEXBOK bulletin is to be
believed. This warming trend will be observed across the whole
county warning area from Thursday into Monday with each day
slightly warmer than the last. Highs are expected to hit 98 on
Monday in Medford with 105 expected in Happy Camp and Somes Bar.

The last thing to mention is what some of the models are depicting
next Tuesday. Ensemble members suggest some troughing off the
coast of California yet again. The operational GFS does show a
cutoff low meandering around the Pacific. Right now, we`ll just
keep note of it and wait until we`re confident enough to throw
thunderstorms in the forecast. Cutoff low forecasts can change
significantly run to run, especially almost a week away.

-Smith

FIRE WEATHER...Updated 300 AM PDT Wednesday 5 August 2020...The
main concern in the forecast period will be scattered
thunderstorms Wednesday. This scattered coverage is expected in
south central Oregon and south western Siskiyou county.

For today, the upper trough will move inland in northern
California. Instability, mid level moisture and trigger will be
higher east of the Cascades and northern California. Therefore,
confidence is higher we`ll have scattered thunderstorms east of
the Cascades in Oregon and western Siskiyou county, with more
frequent thunderstorm activity late Wednesday afternoon and
evening. Because of the scattered thunderstorms, we have kept the
red flag warning up for Wednesday in FWZ 624, 625 and 280.

Thursday, the upper trough shifts east and any threat of storms will
be east of the forecast area. Temperatures will be cooler and
relative humidity higher, however gusty winds are expected in the
afternoon due to a combination of a stronger winds aloft and
tightening pressure gradient.

Friday through next weekend should remain dry with a thermal trough
developing along the coast with increasing east to northeast winds
near and at the ridges in fire zones 618, 619 and western 280.

-Sven/Smith

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Red Flag Warning from 2 PM this afternoon to 11 PM PDT this
     evening for ORZ624-625.

CA...Red Flag Warning from 2 PM this afternoon to 11 PM PDT this
     evening for CAZ280.

Pacific Coastal Waters...Gale Watch from Friday afternoon through Sunday evening for
     PZZ356-376.
     Small Craft Advisory until 11 PM PDT this evening for
     PZZ356-376.
     Small Craft Advisory from 2 PM Thursday to 2 PM PDT Friday for
     PZZ356-376.

$$



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