Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Medford or
910 am PDT Friday Jul 25 2014
Discussion...with offshore flow for much of the night the only
lingering clouds are in the Umpqua basin inland...and that should
urn off quickly this morning. Current forecast looks on track and
will not update this morning. Sven
Previous discussion...the warming trend will continue today with
skies remaining clear to mostly clear and inland high temperatures
returning to around normal. The main concern for today relates to
fire weather as gusty north to northeast winds in southwest Oregon
this afternoon will combine with low relative humidities. Winds
will be strongest in the Illinois and Applegate valleys with gusts
of 15 to 25 miles per hour expected. From Saturday into next week, it will be
even warmer and still very dry but northerly winds will not be
quite as strong.
Our area is between an upper trough in the Gulf of Alaska and a
ridge that extends from the Desert Southwest to The Rockies while
a strong thermal trough is in place over northern California. The
warming trend is still on track to continue into early next week
with inland high temperatures forecast to remain above normal all
week as the ridge retrogrades. Another couple of consistent
aspects of the forecast are strong northerly coastal winds that
will be strongest during the afternoon and evening hours, and then
from Sunday through the week...afternoon and evening instability
with a slight chance of thunderstorms. Confidence is high that the
large scale pattern will persist next week. There is uncertainty
in the detail of which days and which areas will have the greatest
instability/highest probability of thunderstorms.
Middle-level moisture will still be limited on Sunday afternoon, but
the 00z GFS and European model (ecmwf) both depict a shortwave offshore from Eureka
early on Sunday that will track northeastward and could initiate
isolated afternoon and evening thunderstorms from Siskiyou County
across the southern Oregon Siskiyous and Cascades.
On Monday, middle-level monsoonal moisture is expected to track
farther north along The Spine of the Sierra Nevada with a slight
chance of thunderstorms that will be highest east of the Cascades.
Moisture and instability are forecast to increase Tuesday
afternoon and there is another shortwave expected to be embedded
in the north to northeastward flow aloft. There is a likelihood
that thunderstorms will produce higher rainfall amounts but also
the possibility of increased occurrence compared to the preceding
couple of days.
Behind the shortwave, the focus for convection may shift north
and east of our area on Wednesday. But, the GFS still indicates
instability and moderate middle-level moisture amounts over our area
that would support at least a slight chance of thunderstorms.
Confidence is not high in the timing of the smaller scale features
that far into the future.
For the end of next week, the question becomes the timing at which
the ridge will move east and the Pacific trough brings an onshore
push with cooler coastal temperatures and slightly cooler inland
temperatures. Middle-level winds are likely to remain southerly
Thursday into Friday with sufficient moisture and heat to continue
a slight chance of afternoon and evening thunderstorms...and the
possibility of night-time convection depending upon the presence
of any shortwaves.
Aviation...based on the 25/12z tafs...
Areas of MVFR/IFR stratus/fog are forming over the coastal valleys
of northern Curry, Coos and Douglas counties this morning as well as
in the Umpqua basin. These ceilings may briefly impact koth, but stand a
better chance at sticking around for a few hours at krbg. These
should burn off by middle-late morning. The remainder of the area will
remain VFR for the next 24 hours with a breezy afternoon over the
Coast Ranges and the west side valleys. A mix of IFR/MVFR stratus
likely returns to the north coast and Umpqua valley overnight
Marine...updated 500 am PDT 25 July 2014...
A thermal trough will persist over the southwest Oregon coast today
through at least early next week resulting in high confidence in the
marine forecast during this time. This will result in moderate to
strong north winds and steep to very steep wind driven seas. The
strongest winds and highest seas will occur in the outer waters
south of Cape Blanco, but may nudge into the southern inner waters
at times in the afternoons and evenings. The thermal trough
pattern will likely persist through next week. Spilde
Fire weather...with a warming trend expected today, afternoon
minimum relative humidity will be lower than it was
yesterday...down to around 15 percent. A thermal trough in
northern California/southwestern Oregon will lead to gusty north-
northeast winds through the Illinois and Applegate valleys this
afternoon/evening. The pressure gradient between high pressure
offshore and the thermal trough is not quite as strong today as it
was yesterday. But, still think we could see some gusts in excess
of 20 miles per hour. These conditions meet critical fire weather thresholds
and as such we have issued a red flag warning at rfwmfr. It will
also be very dry across the rest of northern California and the
east side with minimum relative humidity values below 10 percent in many cases.
But in those areas, winds this afternoon are expected to remain
below 20 miles per hour. Spilde
or...red flag warning from 1 PM this afternoon to 8 PM PDT this
evening for orz620.
Pacific coastal waters...Small Craft Advisory for winds until 11 PM
Sunday for pzz350-356-370-376.
Small Craft Advisory for hazardous seas until 4 PM PDT Monday
Hazardous seas warning until 5 am PDT Monday for pzz356-376.
Gale Warning until 5 am PDT Monday for pzz376.