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Area forecast discussion 
National Weather Service Medford or
1244 PM PDT Tuesday Jul 7 2015

Updated fire weather, aviation, and marine sections

Discussion...a close upper low off the California coast will be
a play maker for our weather for the next few days.
Today...Wednesday and possibly Thursday could be the most active
days in terms of thunderstorm coverage...frequency and severity.
The forecast challenge the next few days is detailing where storms
will be most frequent and the possibility that some storms could
be severe. Although there is a chance storms could be less
frequent Wednesday and Thursday due to ample cloud cover which
will limit the amount of daytime heating and thus instability.
This will be looked at in more detail this morning.

Right now...the models suggest thunderstorms could be most
frequent along the Siskiyous...western siksyou County and
Cascades late this afternoon and evening. The models show stronger
shortwaves moving into the area tonight, so there is a concern for
nocturnal storms. Made a few adjustments to the forecast today. increase the probability of precipitation along the Cascades and Siskiyous this
afternoon and evening and second adjust the sky cover to match
current visible image this morning. -Petrucelli


Fire weather...updated 1215 PM PDT, Tuesday 7 July upper
level low centered west of the central California coast will move
eastward through Friday and then northeastward Friday through
Saturday as a new trough of low pressure moves in from the Gulf of
Alaska. Red flag warnings look pretty well on track for today, with
storms already forming over the Cascades north of Crater Lake and
over the mountains of the east side. Model guidance indicates a middle-
upper level low pressure impulse moving in from the southeast late
this afternoon and evening and then another tomorrow afternoon. We
expect slow storm movement until later this afternoon, at which time
the impulse arriving should start to push storm activity westward
and west-northwestward off of the mountains at 10-20 miles per hour. There is
timing uncertainty associated with the incoming impulse, with it
arriving along the Cascade crest between 8pm PDT and 2am PDT. This
presents the potential that thunderstorms on the west side could
occur late this evening through the night. Thus, we'll be looking
into whether or not we need to extend warnings into the night. It
should be noted that portions of the east side to include eastern
624, 625, and 285 have wet enough fuels that warnings are not
currently in effect, though scattered thunderstorms are still
expected there.

Thunderstorms this afternoon and evening will produce rain in their
cores, which will be locally moderate to heavy, especially in
eastern Siskiyou and southern Klamath counties this evening. Due to
the very dry fuel conditions as well as the influence of the normal
northwest surface winds on the west side keeping the lower levels
dry, concerns for new fire starts are greatest west of the Cascades
with today's storms.

A continued threat of showers and thunderstorms through Friday with
an increasing chance of cooler weather and wetting rain with them is
likely to moisten fuels as we go forward in time. It's likely that
cloud cover will mute thunderstorm activity Thursday and Friday in
some areas, too. This weekend a new trough moving onto the West
Coast will result in some showers across the area, cooler
temperatures, and, likely, some gusty southwest afternoon and
evening winds. Btl


Aviation...for the 07/18z taf cycle...along the coast...MVFR
ceilings along the Coos coast will dissipate through noon. Along the
Curry coast expect areas of low clouds and fog to gradually
dissipate early this afternoon. Low ceilings with IFR/MVFR ceilings will
return to the coast this evening between 02-05z.

Away from the coast...generally VFR conditions are expected this
morning. Scattered thunderstorms are expected to develop early this
afternoon mostly over the mountains from the Cascades eastward. Late
this afternoon through late this evening these thunderstorms are
likely to get pushed off of the mountains and move into the valleys
across the east side and from the Cascades westward as a low
pressure impulse moves in from the southeast. Thunderstorms will be
possible mostly from the Cascades and Umpqua Divide south and
westward. Showers and isolated thunderstorms could persist into the
overnight period and Wednesday morning over Siskiyou County and from
the Umpqua Divide westward on the Oregon west side. These storms may
bring isolated MVFR ceilings as well as IFR visibilities in heavy
downpours, hail, and gusty variable winds. Btl


Marine...updated 900 am PDT, Tuesday, 7 July 2015... low pressure
centered off of the central California coast will drift eastward
this through Friday. This low will cause weak winds to prevail
through this week. There is a possibility of isolated
thunderstorms...mainly south of Cape Blanco today and
Wednesday...with the greatest chance on Wednesday. Low short period
northwest swell will continue through the week with several small
longer period south swells mixed in...resulting in combined seas
around 4 to 6 feet. A broader low pressure trough will push into the
area this weekend. Btl


Previous discussion... /issued 450 am PDT Tuesday Jul 7 2015/

Discussion...a closed low off the coast of California will
slowly push inland towards the San Francisco Bay area today,
finally crossing onshore Wednesday afternoon and evening. The
resulting flow pattern is pushing a substantial amount of moisture
into our region. Precipitable water values are forecast to reach
the top 1% to 10% of climatology later today and Wednesday over
much of the forecast area. With instability in place, and short
wave energy sliding overhead from the south, numerous showers and
thunderstorms are expected to develop this afternoon and evening.
With such high moisture values, these storms could drop heavy
rainfall, which would be a welcome sight in terms of the long term
dry conditions seen in southern Oregon and northern California.
Convective activity will once again be concentrated from the
Cascades east and Siskiyous south, but southeast steering flow
should be enough to bring some showers and storms west of the
Cascades, especially south of the Umpqua Divide Tuesday evening.
Flow will trend more easterly Wednesday, and although thunderstorm
chances will be concentrated slightly further south, very similar
storm behavior is expected, and cells could make their way across
the Cascades and into southwestern Oregon. Model solutions depict
instability remaining over the region through the overnight hours,
so nocturnal thunder is not out of the question tonight or
Wednesday night.

The low will begin to open and kick towards the northeast by
Thursday. However, moisture and instability will remain in the area,
and thunderstorms chances will continue Thursday afternoon and
evening. Friday afternoon will be the last hurrah of the departing
low, with afternoon and evening thunderstorms gradually ending from
south to north as the main belt of energy exits the region.

This will not be the last of the convection chances overall. The
second trough will begin to dig south just offshore through
Saturday, then slowly slide east across the forecast area Sunday
through Tuesday. Precipitation chances will continue through this
time, but as models have been inconsistent during this time frame,
confidence is low concerning any details in timing or placement.
Have therefore kept slight chance probability of precipitation in place through much of the
extended term.

The most noticeable change resulting form this pattern will be the
somewhat drastic drop in temperatures from the exceedingly hot
conditions seen over the past few weeks. High temperatures today are
expected to be around 5 to 10 degrees above seasonal normals, but by
this weekend and into early next week, we will likely see daytime
highs of 5 to 10 degrees below normal. With ample cloud cover
overhead, however, nighttime lows should remain at or near values
normal for this time of year. -Bpn


Mfr watches/warnings/advisories... flag warning from 2 PM this afternoon to 11 PM PDT this
evening for orz616-617-619>624. flag warning from 2 PM this afternoon to 11 PM PDT this
evening for caz280-281-284.

Pacific coastal waters...none.


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