Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Medford or
259 am PST sun Dec 8 2013
Short term...high pressure associated with an Arctic air mass is
centered east of the Cascades. This air mass combined with clear
skies and light winds is bringing bitterly cold temperatures to
east side locations this morning with low temperatures expected to
range from -15 to -30 degree f in valleys east of the Cascades. West side
valleys have also lowered to single digits and low teens. Some low
clouds and fog have developed in the southern
Umpqua...Illinois...Applegate...and lower Klamath River valleys.
This may keeps low temperatures in these areas in the low teens
The very cold air mass over the region will gradually moderate
over the next couple days as the Arctic air mass gradually shifts
eastward while a high pressure ridge builds offshore and then gradually
moves into the area.
Still expect very cold temperatures across the area Sunday night and
Monday morning. However the lows Monday morning are not expected
to be as cold as this morning, especially for east side
locations. West of the Cascades expect continued cold temperatures
in the single digits to teens once more. Also high temperatures for
many inland valleys west of the Cascades will remain below
freezing today then gradually warm to near freezing Monday. By middle
week expect high temperatures to rise into the upper 30s to low
40s for most inland valley locations.
Although the upper ridge will nudge into the area, Monday night
into Tuesday expect a couple of weak disturbances to move down the
eastern side of the ridge and into the area. This is just
expected to bring an increase in middle to high clouds to the area.
Models show good agreement with the high pressure ridge axis
shifting east over the area late Tuesday into Wednesday. As the
inversion over the area strengthens and low level moisture
increases slightly, may see an increase in low clouds and fog by
Wednesday morning for western valleys, especially the Umpqua and
inti the Grants Pass area.
Otherwise, models continue to show a frontal system approaching
the area Wednesday night into Thursday and move inland by late
Thursday. Ahead of this system expect snow levels to rise. Models
continue to show variability on the timing and amount
precipitation with this frontal system though. Current runs
support light to moderate precipitation along the coast with light
precipitation extending inland and snow levels mainly above 4000
feet. Will need to monitor this storm for potential changes though.
Aviation...based on the 08/06z taf cycle.
A very cold and dry air mass is in place today...keeping most
locations VFR the next 24 hours at least. However...some low clouds
and freezing fog are occurring in the south Umpqua basin...south of
Roseburg...in the Illinois Valley...and the Klamath River valley in
Siskiyou County. These areas should lift and clear later this
morning. Humidity profiles suggest tonight will feature similarly
dry air near the surface...so expect just isolated areas to see low
clouds and freezing form late tonight into Monday morning...most
likely in the Umpqua basin and Illinois and Klamath River
valleys...and perhaps areas near Grants Pass. Sk
Previous long term discussion... /issued 755 PM PST Sat Dec 7 2013/
Long term...Wednesday through Saturday...12z operational runs of
the European model (ecmwf) and GFS as well as their ensembles are agreeable in their
solutions for the upcoming pattern middle next week into the weekend.
Since the flow aloft will shift from northerly to more westerly, the
pattern will feature air masses that are more Pacific in origin than
the polar Continental air masses we have been experiencing over the
last several days. As a result, we will see a gradual modification
in temperatures and snow levels during the period. What we will
also see is a more active Pacific storm track that will bring
multiple systems into the northwest coast. What is a little more
uncertain is the strength and timing of these systems. But, we'll
try to hone in on the details below.
Models send a weak trough east of the area Tuesday night allowing a
short wave ridge to build briefly over the area Wednesday. As a
result, have cut probability of precipitation for this period to eliminate any precipitation across
the County Warning Area. By Wednesday night, a storm system heading into British
Columbia will allow a cold front to approach the Oregon coast. This
should bring some rain to the coast, but models are now weakening
the front significantly as it moves onshore Thursday. So,
precipitation chances decrease farther inland. There may be some
light rain/snow that reaches to and just east of the Cascades, but
that's about the eastward extent. Showers may linger Thursday night
through Friday night, but models have again trended drier in this
time frame. So, have adjusted probability of precipitation downward. Another front will
arrive at the coast Saturday. Spilde
or...freeze warning until 10 am PST this morning for orz021-022.
Pacific coastal waters...Small Craft Advisory for winds from 1 PM this afternoon to 4 am
PST Tuesday for pzz376.
Small Craft Advisory for hazardous seas until 10 am PST Tuesday