Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Medford or
333 PM PDT Tuesday Oct 13 2015
Discussion...the upper level Rex block pattern will persist
through Thursday. A high pressure ridge extends from the Desert
Southwest into the Pacific northwest while a closed low is offshore
from Baja California California. The ridge will persist while the low moves
northeast into Southern California. The result for our weather
will be another couple more days of near record to record high
temperatures inland. At the coast...light offshore winds will
persist through about noon on Wednesday. Then...the thermal
trough will shift inland with onshore winds developing in
Middle-level 700-500mb moisture over our area will begin to increase
on Thursday afternoon...but the upper low will still be far
enough away that the extent of convective development will likely
be a few low-topped cumulus clouds developing over the Cascades
Friday will be a day of transition as the California closed low
and a broad low far offshore both begin to kick the ridge
eastward. Upper level clouds will begin an increasing trend but
cooling will be very modest...just a few degrees lower than the
highs of Thursday.
Some pre-frontal showers may develop over the mountains from the
Cascades westward Friday night, and the models have trended a few
hours faster with the arrival of the main frontal rain band...
expected to arrive at the coast around 5 am PDT on Saturday.
The front looks to be a fast mover and bring a very noticeable
change to cooler weather. Inland highs on Saturday are expected
to be around 10-20 degrees lower than on Friday. The probability
of rain will be highest near the coast and from the Umpqua Divide
northward and there is at least a slight chance of rain across our
area. But there is still uncertainty regarding the extent of
rainfall on Saturday as the GFS has trended weaker and the European model (ecmwf)
has trended stronger with quantitative precipitation forecast amounts. The forecast trends toward
the ecmwfthe strength and speed of the front mean that rainfall
amounts are expected to be largely less than a quarter of an
inch...and less than a tenth of an inch from Jackson County
southward and eastward. Besides the possibility of rain...there is
a slight chance of east side thunderstorms with the frontal
passage expected to match up with peak heating in the afternoon
and early evening.
Weak...brief ridging is expected Sunday with increased confidence
that any lingering showers will come to an end around sunrise.
High temperatures look to be near normal and similar to Saturday
as the cooler air mass is offset by increased sunshine.
Model solutions are in better than usual agreement that a cold
front will brush past along the northern portion of our area
Monday into Monday night. The European model (ecmwf) is also a bit stronger than
the GFS with this system...but both models limit rainfall to the
coast and Umpqua basin northward. A high pressure ridge is
expected to briefly build northeast into the area on Tuesday.
The outlook beyond day 7 is for a series of weak systems to track
into the Pacific northwest.
Aviation...for the 13/18z taf cycle...VFR conditions will continue
for the next 24 hours with one exception. The exception will be
in the vicinity of North Bend where an area of IFR ceilings may
develop around 11z. But, with a weak offshore flow...confidence
is low that anything develops. -Petrucelli
Marine...updated 220 PM PDT Tuesday 13 Oct 2015...high pressure to
our north and a thermal trough along the coast will continue to
produce moderate north winds and heavy combined seas through
tonight, especially south of Cape Blanco. The thermal trough weakens
and moves out over the waters as high pressure to our north also
weakens Wednesday. This allows pressure gradients to relax and winds
to diminish and seas to subside. Relatively light winds and seas are
expected to continue Thursday and early Friday. Then a front
approaches Friday which may produce a period of moderate south winds
and seas. -Wright
Fire weather...updated 230 PM PDT Tuesday 13 Oct 2015...offshore
flow will continue through much of the week. This will result in
warm and dry conditions, with poor to moderate overnight recoveries.
Recoveries will be especially low over ridge lines, upper slopes,
and much of the east side. Improving conditions are not expected
until late this week and into the weekend, when an approaching
frontal system will push temperatures down and bring increasing
moisture to the region. While confidence is low on the exact
timing and amount, there is a chance for precipitation during the
weekend into early next week. -Fb/petrucelli
Pacific coastal waters...Small Craft Advisory until 11 PM PDT this
evening for pzz350-356-370-376.
Hazardous seas warning until 11 PM PDT this evening for pzz376.