Severe Thunderstorm Warning 19, NWS Medford, OR

Area Forecast Discussion
National Weather Service Medford OR
244 PM PDT Wed Aug 5 2020

.DISCUSSION...A 18z sounding from the IMET on the July fire
complex showed similar instability and lapse rates to the model
data, but the shear was 5kt higher for all elements. This
increases confidence that any thunderstorms in Modoc, Lake, and
eastern Klamath Counties will be capable of producing hail nearing
one inch. These storms could also have strong outflow winds with
them as well. Siskiyou County is still not showing an environment
as capable of the one inch hail, but pea sized hail should be
anticipated with any thunderstorm.

Thursday morning, increased QPF along the coast by a few
hundredths, so areas from Coos Bay north could see up to one tenth
of accumulation, with areas from Port Orford north receiving
around 0.05" of accumulation. The front looks to hold together
more than has been shown the past few days, so northern Douglas
County could also observe a few hundredths of rain. However, it is
more likely this will result from accumulating drizzle and be
contained more to higher elevations in the mid-morning. The front
will also bring strong west winds along and east of the Cascade
crest.

Following this, the weather mellows out onshore, but a strong
thermal trough takes up over the marine waters and is likely to
cause gale force winds. Offshore flow looks modest with easterly
850 to 825 mb winds around 20 to 25 knots. Temperatures will warm
into the new week for the whole area. Highs are expected to hit 99
on Monday in Medford with 106 expected in Happy Camp and Somes
Bar. No heat related advisories are anticipated at this time, but
if overnight lows continue to warm as the afternoon highs have, a
short duration advisory may be considered.

There are indications of a trough returning early next week. Most
solutions maintain an open wave pattern and the GFS deterministic
has backed off on showing a cutoff low. We will monitor for
trends towards a cutoff low pattern as it is favorable for
thunderstorms, but for now have excluded them from the forecast.
-Miles

&&

.AVIATION...For the 05/18Z TAF Cycle...IFR/MVFR ceilings at the
coast and in the Umpqua Basin will clear by early afternoon to VFR.
We expect IFR conditions (perhaps low MVFR) to return to the
coast/coastal valleys around 02Z and then reach the Umpqua Valley
and KRBG by around 09z, but the KRBG ceilings could be a low VFR
ceiling around 3500 feet. An approaching cold front will bring a
chance for showers to the coast Thursday morning.

Elsewhere, inland areas will have breezy conditions this afternoon,
and VFR conditions are expected 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.
~Keene/BTL

&&

.MARINE...Updated 230 PM PDT Wednesday, 5 August 2020...
A borderline Small Craft Advisory event continues south of Port
Orford and beyond about 3 NM from shore, with north winds 20 to 25
kt and seas around 6 ft. Winds and seas will weaken tonight 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 by Saturday. 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. Keene/BTL

&&

.FIRE WEATHER...Updated 230 PM PDT Wednesday 5 August 2020...The
main concern in the forecast period will be scattered thunderstorms
late afternoon into the evening. Scattered coverage is expected in
portions of the east side in Oregon, and western Siskiyou county.

The upper trough is moving inland in northern California.
Instability, mid level moisture and trigger will be higher east of
the Cascades and Western Siskiyou county, and a Red Flag warning is
in place for these areas.

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.

The operational run of the GFS is showing an upper low deepening of
the coast of Central California early next week, with instability in
northern California Monday and Tuesday. However, most models are not
showing this, and have not added this at this time. This will be
something to focus on as we approach this time period. -Sven

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Red Flag Warning until 11 PM PDT this evening for ORZ624-625.

CA...Red Flag Warning until 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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