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Area forecast discussion 
National Weather Service Medford or
257 PM PDT Wednesday Jul 9 2014

Discussion...isolated thunderstorms have developed with abundant
moisture and weak instability. The 12z Medford sounding showed
1.38 inches of precipitable water which is greater than the 99th
percentile for this time of year. The latest Storm Prediction
Center mesoscale analysis shows convective available potential energy around 500 j/kg. This
analysis also shows convective inhibition north and west of Mount
Ashland which is where a shower dissipated. The greater
instabilities are over southern Klamath...Siskiyou...and Modoc
counties which match well with the strongest thunderstorms. We
received a report of one-half inch hail and nearly one-half inch
of rain from a storm that passed through Tennant early this afternoon.

A challenging forecast through the weekend. Once again there will
be higher than normal moisture along with weak instability which could
produce isolated thunderstorms Thursday afternoon from the
Cascades east and Siskiyous south. Thursday night looks interesting
as a wave lifts across central California. The models show a 250
mb jet of 60 knots with northern California in the left-front
quadrant of the jet in difluent flow. Could see middle level
instability and the possibility of thunderstorms developing
overnight and moving into northern California...and possibly
southern Oregon.

Friday afternoon and evening have the best potential for
thunderstorms but with caveats. The 250 mb jet is in a good
location to our southeast with much of our area in difluent flow
aloft. The upper trough approaches close to peak heating.
However...the amount of instability is in question. If there is a
lot of cloud cover from morning convection that does not clear
then the instability will be less but if it clears in the
afternoon then there would be more thunderstorm development and
possibly a few severe storms.

A southerly flow aloft persists into the weekend with another low
offshore of California approaching on Sunday. Isolated
thunderstorms remain in the forecast in this pattern. A ridge
aloft begins to build early next week which means an enhancement
of the Summer heat and less thunderstorm activity.


Aviation...isolated thunderstorms east of the Cascades and south
of the Siskiyous will dissipate near and after sunset. Otherwise
VFR conditions inland. At the coast...gusty northerly winds will
weaken this evening. IFR conditions will persist around Brooking
into Thursday. IFR conditions will return elsewhere at the coast
mainly north of Cape Blanco as far inland as the Coquille Valley
late tonight.


Marine...offshore high pressure continues with a thermal trough of low
pressure along the southern Oregon coast. Moderate to strong north
winds will continue into Thursday with steep to very steep seas
over the waters. Highest seas and strongest winds will be south of
Cape Blanco. Both winds and seas are expected to decrease late in
the week into Saturday. The thermal trough pattern is expected to
redevelop on Sunday and strengthen Monday.


Fire weather...current visible and radar images show isolated
thunderstorms mainly confined to northern cal and along and east of
the Cascades. Models show storm motion will be from west to
southwest to east to northeast. Storms are expected to be slow
movers, so a few of the heavier cells could produce locally heavy
rain and small hail given that precipitable waters remain high for this time of
year...around 1.00 inch some clouds are developing and building west
of the Cascades near the Siskiyou/Josephine County line, but the cap
is stronger, so don't expect much to develop from this. Forcing for
thunderstorms remains weak Thursday, but there's a slight chance
again in northern California and over the east side. Thursday night
through Friday will be a key period to watch closely and much will
depend on the timing. Currently, the models are in similar agreement
showing strong negatively tilted trough moving up from northern cal
into southwest Oregon with plenty of monsoonal moisture being
advected northward. The models suggest much of the instability could
be elevated and jet energy at 250 mb will move into the area which
could result in more frequent storms. Steering winds are expected to
be from the south which will favor increased thunderstorm coverage
into the valleys west of the Cascades. -Petrucelli

mfr watches/warnings/advisories...

Pacific coastal waters...Small Craft Advisory for winds until 8 PM PDT Thursday for
Small Craft Advisory for hazardous seas until 5 am PDT Friday
for pzz350-356.
Hazardous seas warning until 11 PM PDT this evening for
Small Craft Advisory for hazardous seas until 11 am PDT Friday
for pzz370-376.
Gale Warning until 2 am PDT Thursday for pzz376.
Hazardous seas warning until 11 am PDT Thursday for pzz376.



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