Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Medford or
300 PM PST Sat Nov 28 2015
Short term...this afternoon through Tuesday...
a very dry air mass in place with diminishing winds and clear
skies will set the stage for calendar day record lows to be
broken tonight through Sunday morning. Records were set this
morning at 3 of our primary observation stations. Please see the
climate section, below, for further details.
There will be little change in the overall pattern until late
Sunday night into Monday morning, when a weak front pushes into
the forecast area and splits apart. Until then, the east to west
pressure gradient will weaken from about 12mb to 9mb across the
area and become more southeast to northwest oriented. This will
result in diminishing winds across the area and a diminishing low
level temperature gradient from west to east. Thus, less
downslope wind will result in lower temperatures for many
Monday morning through Tuesday cold temperatures will be overrun
by a weakening frontal system pushing into the area. The dry air
mass in place is most likely to weaken this system to producing
less than a tenth of an inch of water across the area.
Temperatures Monday morning, however, will supportive low snow
levels and, possibly, some freezing precipitation in the valleys
east of the Coast Range. Hayes Hill, the Illinois Valley, and the
Camas Valley area are areas of most concern. We might need a
Special Weather Statement for this if it continues to appear as if
it will hold together. After looking wetter on its 12z run, the
nam12 trended drier again with its 18z run. Expect impacts would
be minor if the system does make it inland, but, given recent cold
conditions, some areas could be a bit slick. Btl
Long term...Wednesday through Saturday...a broad long wave trough
establishes around 140w with southwest flow into the West Coast.
Through Wednesday. Both models continue to be fairly consistent at
bringing warm front and rain inland. Snow levels will generally be
rising especially from west of the Cascades. However there is some
concerns for our east side zones on how much cold air remains
entrenched in the valleys especially early on. We continue to lean
toward a gradual erosion of the cold air and potential for rain-
snow mix for the east side during the night time.
Next Thursday could be a more active day in the coastal waters and
at the coast. Both models bring a low pressure into the waters
either off SW Oregon or off northwest California. Confidence is approaching
moderate that areas west of the Cascades will see rain on Thursday.
However downsloping may keep rain out of the Rogue Valley. Thursday
could also be a very windy day as model wind at h700 is 50+ knot. As
the cold front moves inland. Given the current timing, snow levels
should stay above passes Thursday but should lower quickly behind
the front on Thursday night into Friday morning. Some drying is
possible on Saturday but the overall pattern favors quick moving
system. Both the GFS and the European model (ecmwf) show a ridge moving through on
Saturday. We have trended toward a drier condition especially from
the Cascades east. Given the fast moving jet stream, the next
front may catch up sooner that the models advertise. /Fb
a record low temperature of 11f was reached at Mount Shasta city
this morning. The official ASOS record for today had been 16f set
in 1952. The cooperative observing station recorded 13f in 1931.
Record lows were also set in Alturas and Montague this morning
with a chilly -6f and 9f, respectively.
Tonight through Sunday morning we are likely to experience the
coldest night of this current cold spell across the forecast area.
For tonight the record low at Medford is 17f, Mount Shasta 11f at
per the coop station, and Alturas -6f. Forecast low temperatures
would tie a record at Medford and break records by a degree at
Mount Shasta city and Alturas. Btl
Aviation...for the 29/00z taf cycle...VFR conditions will prevail
under offshore flow through 29/12z...except for areas of LIFR in
freezing fog/low clouds over the Umpqua basin...including krbg...
after 29/05z and until around 29/21z. Spilde
Marine...updated 300 PM PST Saturday 28 Nov 2015...westerly swell
will gradually build through tonight with long period swell arriving
on Sunday. The swell will peak Sunday night into Monday.
A frontal system will move through the waters with Small Craft
Advisory winds and seas from late Sunday through Monday morning. A
stronger front will move through the waters Tuesday night into
Wednesday with gales and hazardous seas possible. Spilde
or...frost advisory from 10 PM this evening to 9 am PST Sunday for
Freeze warning from 10 PM this evening to 9 am PST Sunday for
Pacific coastal waters...
- Small Craft Advisory from 7 PM Sunday to 10 am PST Monday for
- Small Craft Advisory for hazardous seas from 10 am to 10 PM PST
Monday for pzz350-356.
- Small Craft Advisory from 4 PM Sunday to 10 am PST Monday for
- Small Craft Advisory for hazardous seas from 10 am Monday to 10
am PST Tuesday for pzz370-376.