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Area forecast discussion 
National Weather Service Medford or
927 am PDT Wednesday Sep 17 2014

Discussion...made some minor adjustments to the forecast for this
morning. The current observation at Weed is showing south winds
blowing at 26 miles per hour with gust to 37 miles per hour. The models suggest south
winds will continue there through this evening before gradually
tapering off overnight tonight. Therefore have updated the winds
there. Radar is showing some returns along the coast, but so far
little or nothing has reached the ground. As the upper low
continues to push east towards the coast, we'll see a higher
chance of precipitation along the coast. The models still hint at
some weak instability from about Crater Lake north and we'll keep
a slight chance mention there. Additionally the NAM also shows
some weak elevated instability at 850 mb moving over the marine
waters and possibly along the coast this afternoon. At the same
time there is cooling aloft at 500mb and a weak trigger, so we'll
add in a slight chance for the above mentioned areas this

Moderate to strong winds along and east of the Cascades will
continue to be an issue and they are already starting to pick up
over the middle slopes and ridges. Gusty southwest winds are expected
to persist into this evening, then gradually decrease tonight as
the upper trough moves inland. -Petrucelli


Aviation...based on the 17/12z taf cycle...VFR conditions will
persist through mid-morning..except for local MVFR visibilities in smoke
near wildfires. Areas of MVFR ceilings in showers will develop at the
coast late this morning...spreading inland over the Coast Range and
Umpqua basin this afternoon and evening with higher terrain becoming
obscured. It will be windy this afternoon and evening east of the
Cascades and in the Shasta Valley. -Jrs


Marine...updated 300 am PDT Wednesday 17 Sep 2014...low pressure now
approaching the coastal waters from the southwest will move to
just off the extreme northern California coast Thursday...then
weaken. This will bring some light to moderate southerly winds to
the area...but below small craft criteria. Offshore high pressure
and a thermal trough along the coast will both build Friday...then
weaken Saturday. Small craft northerly winds and steep seas will
develop Friday...then diminish Saturday. Light to moderate northwest
swell will then dominate the seas Sunday into Monday as winds will
be mostly light.


Previous discussion... /issued 316 am PDT Wednesday Sep 17 2014/

Discussion...the first wet system of the season will arrive
today. Much welcome rain is expected to fall over much of southern
Oregon and northern California during the next two days, however,
forecasted rainfall amounts are not that impressive. Winds will be
very breezy over the east side and in the Shasta Valley today as
well. Unfortunately, after a brief period of wet and cooler
weather, warmer temperatures return for the weekend. Beyond that,
there is a great deal of uncertainty, as the synoptic models begin
to diverge significantly despite the very good agreement in the
short term.

First off, the system expected today and tomorrow. Current water
vapor satellite imagery shows the trough offshore, poised to make
its move inland over the next 48 hours. The surface front will
likely push through the area this evening, bringing very breezy winds
to the east side and the Shasta Valley, while the upper level trough
passes overhead tomorrow afternoon. Models have trended a bit wetter
with this system over the past several cycles, and shifted the focus
of the rain further south. These trends place the bulk of the
moisture moving onshore along the southern Oregon coast and far
northern California, where rainfall is very much appreciated given
the current fire weather concerns. However, this is the first such
system of the season, and models may not be handling the system that
well. There is some concern that the solutions are overdone, given
the time of year, and that the forecasted precipitation pattern and
moisture flow do not line up well with what we would normally expect
with such a system. Despite all this, feel very confident that many
areas will get a wetting rainfall, especially along the coast and
much of the west side, including western Siskiyou County, and
although the forecasted precipitation amounts are not spectacular,
they have been bumped up slightly to come into line with the more
southerly moisture flow apparent in the latest model solutions. Have
also kept a slight chance of thunderstorms in the forecast over the
northern Cascades this afternoon, extending over the entire east
side Thursday afternoon, as the trough axis swings overhead.
Instability is not fantastic for convective development, but a
strong trough this time of year will most likely steepen lapse rates
enough to fire off a cell or two.

The main trough exits to the east by Saturday, but the southern half
of the trough is forecast to cut off and take up residence over
Southern California. Meanwhile, ridging will build in over our area,
and the thermal trough is expected to return. Therefore, what had
appeared in many of the previous forecast cycles as an extended
period of near normal temperatures. Now lasts for just for a few
days before above normal temperatures make their return through the
weekend. Offshore flow will return as well, and despite the rain,
the area will quickly dry out again.

Heading into next week, the cut off low over Southern California
will slowly work its way east, and although it will pull a good
amount of moisture north, its location will likely keep all of that
moisture to our south. The synoptic models do produce a very large
broad trough over the Pacific next week, but while the European model (ecmwf) brings
that trough, and plenty of rain, inland by midweek, the GFS builds a
stronger ridge overhead, thereby keeping the forecast area dry. This
GFS solution is quite different than just 24 hours ago, while the ec
has been rather consistent. Have therefore leaned towards the ec
solution, and kept a slight chance of precipitation in the forecast
for midweek, with above normal temperatures gradually dropping to
more seasonal values. Uncertainty is very high, and chances are
equally high that the extended forecast will change significantly
over the next several days. -Bpn


Mfr watches/warnings/advisories... flag warning until 9 PM PDT this evening for orz624-625. flag warning until 9 PM PDT this evening for caz284-285.
Red flag warning until 9 PM PDT this evening for caz281.

Pacific coastal waters...none.


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