Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Medford or
858 PM PST Wednesday Dec 11 2013
Discussion...clear skies will result in one more night with
temperatures well below normal across the forecast area. A ridge
axis has pushed just east of the County Warning Area...and even though
temperatures are already below freezing except at the
coast...readings are trending about 5 degrees higher than last
night. Temperatures will bottom out in the teens in the Illinois
and rogue valleys...20s in the Umpqua basin...and single digits
over much of the east side. These cold lows could play a critical
factor in what transpires tomorrow in terms of precipitation type
as a frontal system moves into the region...spreading
precipitation from the coast in the early afternoon to inland
valleys by later in the afternoon.
Models have not been extremely helpful in depicting surface
temperatures across west side valleys in this stagnant air mass with
lingering snow cover. Therefore...confidence is still rather low on
whether a period of freezing rain occurs in west side valleys
tomorrow. We now know Roseburg is trending less likely to see
freezing rain tomorrow because they have bottomed out at 29
degrees with fog and low clouds settling in there...and won't take
much to reach above freezing tomorrow. That leaves the Illinois and rogue
valleys...and potentially the Scott and Klamath River valleys in
Siskiyou County...as the focus areas for the potential for
freezing rain. Fortunately...if fog clears Thursday
morning...a mostly full day of sunshine is expected for west side
valleys...and this could bring temperatures above freezing before
any precipitation starts. Timing for precipitation still looks
like late afternoon through the evening...when surface
temperatures could go from just above freezing to freezing...and
rain could fall through a warm layer then freeze on contact with
roads. Latest Storm Prediction Center plumes suggest a mean value of 0.1 inches of
precipitation in west side valleys like the Rogue Valley. A
Special Weather Statement at spsmfr covers this potential.
Conditions dry out Friday morning and remain that way through
Saturday at least...with low temperatures finally climbing to near
30 degrees west of the Cascades and 20 degrees east of the Cascades.
Please see the previous discussion below for more details on
freezing rain potential.
Aviation...conditions are mainly VFR across the area this evening. However IFR
ceilings/visibilities in low clouds and freezing fog will develop in the
west side valleys later this evening between 05z-10z...lowering to
LIFR between 09-13z. IFR/LIFR ceilings are expected to impact west side
valleys including at krbg and kmfr. IFR ceilings are forecast to
gradually lift around 18z and clear to VFR around 20z.
A weak front is also expected to approach the coast Thursday
morning and begin to move inland Thursday afternoon. This will bring
rain to the coast and into the Umpqua valley beginning in the late
morning and into the afternoon. Rain and mountain snow is expected
to spread into other inland areas in the late afternoon and evening.
MVFR ceilings are expected to develop in rain along coastal areas
Thursday afternoon. Otherwise expect mainly VFR conditions for
inland areas Thursday afternoon with increasing mountain
obscurations. One potential concern with this system is that cold
air in many inland valleys may allow for a period of light freezing
rain before conditions change to rain.
Previous discussion... /issued 243 PM PST Wednesday Dec 11 2013/
Discussion...high pressure remains over the region and will
provide another night similar to last night. Temperatures will
start out this evening up to several degrees warmer than
yesterday however. With minimal cloud cover and efficient
radiational cooling, temperatures will again plummet and areas of
fog and freezing fog will again make an appearance for the morning
hours in the western valleys.
The main concern of the forecast is for the potential of freezing
rain across the valleys of mainly the west side during the
afternoon and evening Thursday. Confidence in the event overall is
low due to a warming trend in place (albeit subtle in the western
valleys), some mixing of the surface air as the front moves
through, and the overall rarity of freezing rain in this region.
Nonetheless, it's a possibility of a high impact event and an Special Weather Statement
has been issued.
Overall precipitation amounts will be low, but there will be
enough for 1-2 inches of snow in the Cascades from Highway 140
northward to around Crater Lake. A few other areas should get
dusted above about 4500 feet across Siskiyou and Klamath counties.
Precipitation may linger a bit into Friday morning in areas north
of Medford, but the event will taper off by Friday afternoon.
Thereafter high pressure will build back in and dominate the
weather through at least the middle of next week with stagnation
headlines likely necessary again. Temperatures overall will not
be as cold from Friday forward, with lows generally in the 20s
western valleys and upper single digits to teens east. Highs will
trend back toward normals of 40s west, but in the higher
elevations and east side the ridge will provide some nice weather
with temperatures reaching 5 to 10 degrees above normal.
We'll keep an eye out for another northerly surge of cool air to
arrive some time Wednesday through Thursday. Currently this system
doesn't look too exciting but may bring some light precipitation including
a light snow threat to lower elevations above roughly 2000 feet.
Aviation...IFR ceilings/visibilities in low clouds and freezing fog will
return to the west side valleys after sunset...lowering to LIFR
after 08z. With a weak trough moving inland Thursday...IFR ceilings
may be slow to lift and may not rise to VFR until Thursday
or...air stagnation advisory until noon PST Thursday for
Pacific coastal waters...Small Craft Advisory for winds from 10 am Thursday to 1 am PST
Friday for pzz350-356-376.
Small Craft Advisory for hazardous seas from 10 am Thursday to 7
am PST Friday for pzz350-356-376.
Small Craft Advisory for winds from 7 am Thursday to 1 am PST
Friday for pzz370.
Small Craft Advisory for hazardous seas from 7 am Thursday to 7
am PST Friday for pzz370.