Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Medford or
116 am PDT Wednesday Sep 2 2015
Discussion...02/00z NAM/GFS in.
Southwest flow aloft is increasing as a digging long wave trough
approaches the coast. This trough will move into the Pacific
northwest Tuesday night into Wednesday...forcing the storm track
farther south. The initial surface front will move onshore later
this morning. With little upper level support it will be weak. The
north coast will get most of the precipitation...and it is already
raining there. Inland amounts from the Cascades west and
Siskiyous north will be very light. Far northern Klamath County
may get some measurable rain too. Inland high temperatures will
cool down to 5 to 10 degrees below normal...except over lake and
Modoc counties...where the highs will be 3 t0 5 degrees below
A stronger front will move in this afternoon and evening...and
this will reinforce the first front. Precipitation will still be
light...but it will spread to the east side tonight. As typically
happens with incoming fronts...winds will be gusty east of the
Cascades this afternoon and evening.
Once the long wave trough sets up over the Pacific northwest...it
will hang around a while as short waves diving down the backside
of the trough will keep it digging into Friday night with a slow
eastward progression. Thursday and Friday look to be cool with
inland highs 10 to 15 degrees below normal. There is the potential
that frost/freeze headlines may be needed for the agricultural
interests east of the Cascades and in northern California.
Cool air aloft would normally support shower activity...and the
model solutions have finally hit on this. The NAM...ec and GFS are
all much wetter than previous runs...especially on Friday. At
this point it looks like it won't be unstable enough for
thunderstorms Thursday...so just went with showers then east of
the Cascades. Friday looks to be more unstable with more
widespread activity...so going with areas of scattered showers
from the Cascades east and Siskiyous south...as well as the Rogue
Valley. However...with lifted indices topping out at around
-2...thunderstorm coverage will likely remain isolated.
By Saturday morning...the trough axis will be over central and
eastern Oregon and heading east. This will shift the instability
to the east and northwest flow aloft will bring in more stable
conditions for the west side and Cascades. By Saturday evening the
showers will be limited to the far east side. It will be warmer
too with inland highs 3 to 5 degrees warmer than the Friday highs.
Sunday and Monday look to be dry with a warming trend as
northwest flow aloft becomes zonal. Inland highs will warm to near
normal values Sunday...then go above normal Monday.
The GFS keeps Tuesday dry...but the ec suggests another front will
move onshore around that time.
Aviation...for the 02/06z taf cycle...a front gradually moving
inland will bring low clouds and showers to the coast, the Umpqua
valley and into the southern Oregon Cascades. MVFR ceilings with
isolated IFR ceilings are expected along the coast through early this
morning...with conditions gradually lifting to VFR around 17z-18z.
Inland, expect MVFR ceilings in the Umpqua valley. Local MVFR ceilings may
also spread into the Grants Pass area. Clouds and areas of showers
will also bring widespread mountain obscurations from the Cascades
west in southern Oregon through the morning...with partial
obscurations continuing into the afternoon. Elsewhere, expect VFR
conditions through the taf period. /Cc
Marine...updated 830 PM PDT Tuesday 1 September 2015...a cold front
will move over the waters tonight into Wednesday morning bringing
some showers to the area...then a trough will move through late
Wednesday through Friday with some additional showers possible.
Winds will remain generally light with moderate seas dominated by
west swell into Wednesday morning. Seas will increase further
Wednesday evening and night as west to northwest swell increases
over the waters. Swell will reach a peak Wednesday night...then
gradually lower. The coastal waters will become dominated by
moderate to strong north winds Thursday through the weekend. Winds
will be strongest in the outer waters south of Cape Blanco with the
potential for Small Craft Advisory level winds to develop in that
area as early as Thursday evening. Winds will diminish during the
overnight hours then increase again Friday afternoon and evening.
More widespread coverage of advisory level winds is possible on
Saturday, then winds are forecast to reach a peak on Sunday
afternoon with gales possible in the southern outer waters. Strong
northerly winds are likely to persist through Labor Day. -Wright/cirrocumulus
Pacific coastal waters...none.