Area Forecast Discussion
Issued by NWS Medford, OR

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FXUS66 KMFR 260302

Area Forecast Discussion
National Weather Service Medford OR
802 PM PDT Sun Jun 25 2017

.UPDATE...The upper level wave responsible for kicking off
thunderstorms earlier today currently lies along a line roughly
from Cape Blanco to Lakeview. North of this line, showers and
thunderstorms continue, but due to the loss of daytime heating,
this convective activity has started to wane. To the south of the
line, a few isolated cells can be found, but very little lightning
activity has been associated with them for the past hour or so.
Another shortwave is expected to pass through later tonight, and
with elevated instability persisting through the overnight hours,
some nocturnal thunderstorms are possible. Have kept slight
chance to chance in the forecast, with the most likely areas to
see nighttime cells develop being the coastal ranges and the
Cascade crest. Have adjusted the forecast slightly to fit current
observations and to account for the latest model runs, but there
has been little overall change.

For more information on this and the remainder of the forecast,
see the previous discussion below. -BPN


.AVIATION...26/00Z TAF CYCLE...Along the coast, expect MVFR/IFR
ceilings to continue through the next 24 hours. VFR conditions
will continue everywhere inland from the coast through the TAF
period. Isolated to scattered thunderstorms are possible over
inland areas this evening and tonight, with the greatest threat
over the mountains. Expect gusty downdraft winds and lightning
with any storms. -Wright/BPN


.MARINE...Updated 800 PM Sunday, 25 June 2017...South winds will
persist over the waters today and Monday and bring continued fog and
low stratus. Seas will remain low through Monday. The thermal trough
returns Tuesday into Wednesday with gusty north winds and steep to
very steep choppy seas developing over the waters. The strongest
winds and seas are forecast from Cape Blanco south but periods of
small craft winds and seas are possible north of the Cape. These
conditions may persist through late in the week. -BPN/SBN


.PREV DISCUSSION... /Issued 232 PM PDT Sun Jun 25 2017/

DISCUSSION...The hot air mass is becoming unstable this afternoon, and
thunderstorms have developed over Northern California. 37 cloud
to ground lightning strikes have been observed so far, focused
over north-central Siskiyou County. More storms are expected to
develop as instability increases later this afternoon. Initially,
little rain is expected with the storms because of a very dry
subcloud layer, but rain is expected with storms later this
afternoon and evening. Lightning is expected to be focused over
the Siskiyous and Cascades where we have scattered thunderstorms
forecast. The broad shortwave that supports the afternoon storms
will move north and east out of the forecast area, but additional
forcing aloft will arrive overnight, and this could couple with
upper level instability to generated elevated thunderstorms.

Tomorrow the forcing becomes stronger as remnants of an upper low
moves towards Northern California, providing ample diffluence at
the left exit region of a 60kt jet. Additionally, the convective
inhibition that existed today won`t be there tomorrow, so that`s
more favorable for storm formation. Lastly, 0-6km bulk shear is
30kt+, and this will favor longer-lived, potentially severe
storms. Red Flag Warnings continue into tomorrow, and please see
the fire weather discussion below for more details.

Temperatures Monday drop well below Warning and Advisory criteria,
so we`ll let the Excessive Heat Warnings and Heat Advisories expire
this evening.

A cooler, breezy day is on tap Tuesday as onshore gradients increase
sharply. It will feel like a different climate on Tuesday compared
to today as high temperatures settle right around normal for this
time of year. Daily marine stratus intrusions are expected to bring
overnight and morning low clouds to at least the coast and Umpqua
Basin Monday through Wednesday. NAEFS output suggests relatively
zonal, west to east flow through early next week at least. What this
means is near to slightly above normal temperatures and little to
no thunderstorm activity are likely.


FIRE WEATHER...Updated 200 PM Sunday, 25 June 2017...Isolated
thunderstorms have already developed over western Siskiyou county as
a shortwave moves up from the south this afternoon. As conditions
continue to destabilize through the afternoon and evening,
thunderstorms should increase in coverage, especially over the
mountains. Thunderstorms will remain possible through the night as
an upper trough approaches, then they should again increase in
coverage and intensity tomorrow afternoon and evening as the main
trough arrives. However, the focus for lightning will shift to the
Cascades and east side tomorrow. It will also be cooler, but breezy
west winds and continued low humidities may fan any new fire starts.
Red flag warnings remain in effect primarily for the potential of
increased initial attack activity. The rest of the week should be
quieter as cooler weather dominates. -Wright


OR...     Red Flag Warning until 11 PM PDT Monday for ORZ621-623.
     Red Flag Warning until 11 PM PDT Monday for ORZ624.
     Red Flag Warning until 5 PM PDT Monday for ORZ620-622.
     Red Flag Warning from 11 AM to 11 PM PDT Monday for ORZ625.

CA...     Red Flag Warning from 11 AM to 8 PM PDT Monday for CAZ281.
     Red Flag Warning until 11 PM PDT Monday for CAZ280.

Pacific Coastal Waters...None.


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