Scientific Forecaster Discussion

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Area forecast discussion 
National Weather Service Medford or
321 am PDT Tuesday Jul 29 2014

Discussion...a strong upper ridge remains parked over The Rockies
and Great Basin and will remain parked there through the weekend
at least. This will result in continued hot weather over southwest
Oregon and northern California. Water vapor imagery clearly shows
substantial monsoonal moisture flowing around the southern and
western periphery of this ridge. Moisture has yet to increase
substantially over US, but it will become deeper and more
widespread over our region over the next few days. The resulting
increase in instability will produce rather volatile conditions
and greatly increased chances for thunderstorms.

Two of the main ingredients for thunderstorms around here
(instability and moisture) will be present through the remainder
of the week, so thunderstorms are a good bet in the usual
locations from the Siskiyou Mountains and Cascades eastward. All
the guidance, including the sref and Storm Prediction Center lightning and
thunderstorm guidance, hit these areas all week. A third
ingredient (trigger) usually results in much more widespread
activity and can often get storms over onto the west side as well.
The main challenge will thus be to identify and then time any
shortwaves in the soupy southerly flow. Given the very unstable
conditions, any shortwave activity will have the potential to
produce a lot of thunderstorm activity, so these will be very
important, especially as we attempt to ascertain thunderstorm
activity on the west side.

A number of shortwaves can be seen on water vapor this morning.
A very weak one is likely moving through right now and may be
responsible for some weak elevated convection across northeast
California this morning. While these cells aren't doing anything
at the moment, middle level lapse rates are still fairly steep, so it
wouldn't take much to get them going. The models then show a
diffuse area of vorticity drifting through today. I don't think
this will be much of a trigger, and there is a bit of cap expected
west of the Cascades. But at least scattered storms are expected
in the mountains and east side.

There is a much stronger shortwave off the Santa Barbara coast
this morning, which already has elevated convection with it
(including one lightning strike). I'm not sure this wave is being
handled well by any of the models, but pure extrapolation puts it
here tonight. This may be why the GFS holds onto storms through
the night on the east side. I did expand the probability of precipitation over there
tonight for that reason.

I can't really identify any particular trigger on Wednesday, but
we ought to see an uptick in convection simply because of the
increase in moisture and instability. There is, however, a very
impressive shortwave off the central Baja California California peninsula
(currently around 25n 120w) that may arrive here Thursday.
Unfortunately, the models all handle this differently. The European model (ecmwf)
brings it north to our area Thursday and breaks out a considerable
amount of convection as a result, even on the west side. The GFS
takes it in south and east of US, which means thunderstorms would
likely still be confined mostly to the east side. Not sure which
to believe at this point, but will have to watch closely.

I made no changes to the extended, but hot unsettled weather may
continue right on into next week. -Wright


Aviation...based on the 29/00z tafs...

Widespread LIFR ceilings and IFR visible in low marine stratus and fog will
persist along the entire coast and the Coquille Valley as well
through middle morning. Low clouds and fog are expected to clear
back from the coast around 19z. Inland...expect VFR conditions
through early afternoon. Isolated to scattered thunderstorms are
expected to develop though in the afternoon Tuesday over northern
California and from the Cascade foothills eastward in Oregon.
These storms will bring gusty winds and local MVFR ceilings are


Marine...updated 300 am PDT 29 July 2014...

Models remain in agreement that the current pattern...with some
slight day-to-day variation...will persist this week. High pressure
is centered offshore and a thermal trough is over northern
California...which will maintain steep wind wave dominated seas and
areas of Small Craft Advisory winds. Through middle-week, the strongest
winds will occur in the outer waters south of Cape Blanco and
during the afternoon and evening hours. Models indicate that winds
will increase slightly on Wednesday and then moreso on Thursday and
Friday. Conditions may increase to gales with very steep seas late
in the week for areas south of Cape Blanco, with moderate north
winds and steep seas over the coastal waters from Cape Blanco north.


Fire weather...current water vapor image shows midlevel moisture
being ushered into the forecast area from two areas. First...a big
swath of moisture is sourced from a circulation over southern
Utah...and the second is farther away...moving from south to north
up from the subtropical Pacific. Energy embedded within these
moisture feeds will maintain the chance for thunderstorms through
the week at least.

Relatively high precipitable water values tomorrow...ranging from
0.75 inches east of the Cascades to 1 inch west of the
Cascades...indicate rain will accompany these storms and moreso as
one heads west. Of course lightning is expected outside of storm
cores and this has the potential to start fires. Storm motion will
generally be from southwest to northeast and this will keep the main
thunderstorm threat generally east of Josephine County...and really
mostly from the Cascades eastward. A red flag warning covers this
threat of lightning.

Wednesday looks similar in terms of instability...and thus the Fire
Weather Watch has been issued. Moisture values do creep up some on
Wednesday so rainfall should be more significant with the storms.

Thursday may feature more in the way of upper level energy...and
storm motion will be more south to thunderstorms could
cover a broader swath of the forecast area.


Mfr watches/warnings/advisories... flag warning from 2 PM this afternoon to 11 PM PDT this
evening for orz617-621>625.
Fire Weather Watch from Wednesday afternoon through Wednesday
evening for orz617-621>625. flag warning from 2 PM this afternoon to 11 PM PDT this
evening for caz280>282-284-285.
Fire Weather Watch from Wednesday afternoon through Wednesday
evening for caz280>282-284-285.

Pacific coastal waters...Small Craft Advisory for winds until 11 PM PDT
Thursday for pzz350-356-370-376.
Small Craft Advisory for hazardous seas until 11 PM PDT Thursday
for pzz350-356-370-376.


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