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Area forecast discussion 
National Weather Service Medford or
259 PM PDT Friday Aug 28 2015

Discussion...looking at the forecast, it would appear we have
jumped ahead a couple of months. In reality, what we are going to
experience is a late fall to winter type wind and rain storm,
during what would normally be the thick of fire season. Across the
board, model solutions are depicting a system that defies anything
seen in late August since at least 1979, if not longer. In short,
expect wetting rains over the Cascades and all points west, and
gusty winds along the coast, the higher terrain, and across the
east side.

The low responsible for this event is currently offshore, showing a
rather impressive baroclinic Leaf, or rather, a very unseasonably
impressive cloud feature. The associated upper level trough is
digging south and will continue to rotate to a negative tilt. This
feature is known to produce rapidly deepening surface lows, and this
one is no exception. With pressures at the center currently sitting
at around 1007 millibars, models are suggesting the pressure could
drop to as low as 992 millibars by 5 am tomorrow morning. This will
be more than enough to produce storm force winds offshore and along
the immediate coast, particularly at The Headlands and convective available potential energy. Flows
across the mountains and east side will also bring strong winds down
to the surface there. Have issued wind warnings/advisories for the
coast and east side, as well as the Shasta Valley, and also a high
surf advisory for the coast north of Humbug Mountain. Some areas
within the warnings/advisories may not quite reach criteria, but
given the high level of recreation this time of year, including
fishing, tourism, and camping, as well as the beginning of some
hunting seasons this weekend, have issued for these areas anyway due
to the potential for numerous and extensive impacts. Also of note,
with drought stressed trees and leaves remaining on vegetation,
winds will have extra potential to down or damage trees. Travel,
utilities, and campers should be on extra alert.

In terms of rainfall, this storm system would not normally cause US
to bat an eye, assuming it was late in the fall or in the winter.
Instead, we are expecting wetting rains for all areas along and west
of the Cascades, and here it is late August. The first round of rain
will accompany the frontal passage late tonight and tomorrow,
followed by showers associated with the upper level trough late
tomorrow through Sunday. All told, up to an inch of rain would not
be out of the question along the coast, while in the coastal ranges
in far southwest Oregon, an inch and a half or more is possible.
Inland, the west side valleys can expect between a tenth and a
quarter inch, while the mountains could reach up to three quarters.
Overall, a significant rain event for fire season, and one that
could also quench area wildfires and clean the area of our
persistent smoke. While it is much to early to say this is a fire
season ending event, pending a possible return to heat and dryness
in September, it may very well be the beginning of the end.

Beyond this weekend, the Pacific northwest will remain in a cooler
and wetter regime. Models have placed a series of troughs overhead
for the next week at least, and each one will have the potential to
bring rain and wind, although at this time, none look anywhere near
as significant as the first. Generally speaking, expect high
temperatures to remain around 10 degrees below normal through next
week, with an intermittent chance of rain and showers. With the
cooler temperatures and precipitation in the forecast, do not be
surprised if the peak of Montana Shasta displays a hint of new white
coverage sometime this next week.



&&

Aviation...for the 28/18z taf cycle...smoke continues to produce
MVFR visible in spots this afternoon, otherwise, conditions will remain
VFR into the evening. A strong low pressure system will move north
along the coast overnight into Saturday morning. The main aviation
impact from this storm will be strong winds along the Coast,
Mountains, and east side late tonight and Saturday morning with
breezy winds elsewhere. Prior to the onset of winds, low level wind
shear is expected, especially along the coast and into the Umpqua
basin. The front will also bring rain and MVFR visible/ceilings to west side
locations as it passes Saturday morning. -Wright



&&

Marine...updated 200 PM PDT Friday 28 August 2015...light winds
and seas are expected today. Then a surface low will near the
waters tonight and track north along the Oregon coast Saturday,
bringing increased winds and seas tonight and Saturday.
Confidence is high regarding the details of this low pressure
system as it nears the coast tonight and early Saturday. A period
of moderate to strong southerly winds is expected to begin tonight
with the strongest winds developing early Saturday morning through
around noon Saturday. This will create small craft winds and seas
across all of our waters with winds increasing to gales to storm
force winds and very steep wind driven seas by early Saturday,
mainly for the waters north of Gold Beach. Winds are expected to
lower some Saturday afternoon but areas of gales and very steep
hazardous may continue into the afternoon, especially north of
Gold Beach. This low pressure system will also bring moderate rain
with occasionally heavy rain and low visibilities possible. As the
surface low approaches the waters tonight into early Saturday
morning, there is also a slight chance of thunderstorms,
especially north of Cape Blanco. Multiple long period swells are
also expected this weekend with increased sneaker wave activity
possible. Conditions will improve some on Monday, but moderate
long period northwest swell will develop over the waters and
continue through Wednesday.

&&

Fire weather...updated 215 PM PDT Friday 28 August 2015...gusty
southwest winds and dry conditions will continue over the Shasta
Valley and east side into this evening, but conditions are expected
to stay below warning criteria due to slightly increased humidities.
Humidities are already much higher west of the Cascades and will
continue to increase as a frontal system approaches.

Later tonight, strong surface low pressure will track north along
the front as it approaches the coast and bring strong winds to the
Coast, Mountains, and east side, with breezy conditions elsewhere.
Rain will move onshore Saturday morning and bring a wetting rain to
much of the west side. Amounts will be much lighter on the east
side Saturday.

While we will get rain over many of our ongoing fires, there are
specific concerns about this storm with respect to those fires and
the personnel Manning them. There is the potential for moderate to
heavy rain over all fire areas from the Cascades west. This is
expected to be a gentle soaking rain rather than convective, high
intensity rain, so burn scar flooding and debris flows are not a
huge concern. But it is something to think about. Also, snags,
burned trees, and other material that have been weakened over the
Summer have the potential to come down from wind, the added weight
of rainfall, or a combination of both. Firefighters should remain
very aware of their surroundings this weekend, especially on or
around existing fires and burn scars.

A secondary front may bring a little additional rain to west side
locations on Sunday, otherwise, conditions will improve. General
troughiness will remain over the pacnw through the upcoming week, so
cooler and more humid than normal weather is expected. Rain will be
possible again middle week as another front moves through. -Wright



&&

Mfr watches/warnings/advisories...
or...High Wind Warning from 2 am to 2 PM PDT Saturday for orz021-022.
High surf advisory from 5 am to 5 PM PDT Saturday for
orz021-022.
High Wind Warning from 2 am to 5 PM PDT Saturday for orz030-031.
Wind Advisory from 2 am to 5 PM PDT Saturday for orz029.

California...High Wind Warning from 2 am to 5 PM PDT Saturday for caz085.
Wind Advisory until 11 PM PDT this evening for caz081.

Pacific coastal waters...gale watch from late tonight through Saturday afternoon for
pzz350-356-370-376.
Small Craft Advisory for winds from 2 am to 5 am PDT Saturday
for pzz350-370.
Hazardous seas watch from late tonight through Saturday
afternoon for pzz350-356-370-376.
Small Craft Advisory for hazardous seas from 2 am to 5 am PDT
Saturday for pzz350-370.
Small Craft Advisory for winds from 2 am to 5 PM PDT Saturday
for pzz356-376.
Small Craft Advisory for hazardous seas from 2 am to 5 PM PDT
Saturday for pzz356-376.

$$

Bpn/thunderstorm

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