Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Medford or
253 am PST Friday Nov 27 2015
Discussion...a quiet, but cold, period is in store for the
forecast area through this weekend. Changes are expected early
next week, and if long term model solutions are to be believed,
that will be the beginning of an extended period of wet and windy
weather for our region. However, confidence remains too low for
certainty in any details of the long term forecast.
Currently, a blocking ridge remains over the Pacific northwest,
and this is preventing any of the storms and system in the Pacific
from reaching our area. This ridge, along with high pressure at
the surface, has kept cold air locked in over US, and thus resulted
in another night of frigid temperatures. Have continued the frost
and hard freeze headlines for this morning, as current
observations are well in line with the forecast. For example,
temperatures at Rogue Valley Airport, arguably the warmest spot in
the valley, have dropped to 25 degrees so far, with several hours
of cooling remaining. Elsewhere, such as in the Umpqua valley, low
clouds and fog have prevented radiative cooling, and temperatures
there remain relatively warm.
As the air mass moderates over the next several days, expect
temperatures to warm slightly, approaching more seasonable
values. East of the Cascades, with snow on the ground,
temperatures will remain colder than normal, with little change
until after the weekend.
The change in the pattern will depend on when the blocking ridge
is finally kicked off to the east, finally allowing the preferred
storm track to return to our region. Models are notorious for
breaking down blocks too fast, and usually, there would be little
reason to believe that this scenario would be much different.
However, all model suites, including the GFS and European model (ecmwf) and their
respective ensembles, depict the ridge breaking down Monday, under
pressure from a frontal system that weakens considerably as it
moves onshore. Suspect that precipitation from this front would
not make it east of the Cascades, as it will be hitting the walls
of both the Cascades and the blocking ridge at the same time.
Models do show a second, much stronger, system coming in around
Thursday, and this one would be clear of any inhibitions from the,
by then, non-existent block.
While confidence is lower than average with this scenario, it is
interesting that all of the models are depicting a very similar
chain of events. With this in mind, have depended on a general
blend of the ec and GFS solutions, with at least a slight chance
of precipitation in the forecast for all areas beginning early to
middle week. That being said, would not be surprised to see things
slow down over the next several forecast cycles, with The Block
eventually remaining in place for a longer period than currently
Aviation...for the 27/12z taf cycle...VFR conditions will prevail
through Friday night...except for areas of late night through middle-
morning IFR ceilings/visibilities in freezing fog over some of the west side
valleys. The lower conditions will affect krbg...but the remaining
taf locations will likely remain VFR. -Jrs
Marine...updated 200 am PST Friday 27 Nov 2015...winds will be
light with slight swell through Friday...then westerly swell will
gradually build...peaking at around 10 feet Monday night. A weak
cold front will move onshore Monday...and this will bring Small
Craft Advisory winds to the outer waters Sunday night into Monday.
A stronger front will approach the waters Tuesday night. A stormy
pattern is likely to be in place Wednesday through Friday. Long
period moderate westerly swell may develop Wednesday night into
Friday and if it does it will make for chaotic seas that may affect
the crab fishery. -Jrs
or...frost advisory until 9 am PST this morning for orz021-022.
Hard freeze warning until 9 am PST this morning for orz026.
Pacific coastal waters...frost advisory until 9 am PST this morning for pzz356.