Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Medford or
255 PM PST Friday Dec 26 2014

Short term...we've got a fairly significant change coming in our
weather over the next few days, with a shift toward much colder
weather over our entire area by early next week. This will likely
be the coldest weather we've seen yet this winter with highs only
in the 30s in west side valleys by Monday and lows well down into
the 20s. The east side will, of course, be even colder with highs
below freezing and lows in the single digits at best. Snow should
be confined to elevations above 2000 feet for the most part, but
there will likely be some travel impacts, which I will get to in a
moment.

In the meantime, we don't expect much difference in the weather
tonight and much of tomorrow from what we've experienced over the
last 24 hours. Skies have cleared out this afternoon, and boundary
layer moisture has actually increased, probably as a result of
evaporation from the ground. Dewpoints are higher than 24 hours
ago, and this yields higher cross over temperatures and increases
the likelihood of fog tonight and tomorrow morning. Tomorrow
should be relatively quiet early on, and fog may be a bit more
persistent than today, but we should also break out by early
afternoon.

The change toward colder weather is being precipitated by a shift
in the long wave pattern. The ridge that has been off our coast
much of the fall and early this winter will retrograde (move west)
just enough to allow troughs/cold fronts to come south along the
coast and pull cold, Continental air further west than usual. The
first of these shortwaves will move into the pacnw Saturday
afternoon. As an upper jet and cold front drop south along the
Cascades, we'll see light snow develop around Crater Lake and then
shift south through the mountains tomorrow night. Snow levels will
remain around 3500 to 4000 feet, so we won't see any snow in the
west side valleys, but some light snow is possible on the east
side. To determine snowfall amounts, I used a combination of the
usual models (nam, GFS, ec) but nudged them up with the wetter MM5
solutions. Since this snow is going to be largely orographically
produced, it seemed to make sense that the high resolution models
would do well. The first wave should produce 3-6 inches in the
Cascades, especially around Crater Lake, with a few inches
possible down at Siskiyou Summit. This will probably produce some
travel impacts, but not too bad.

We will likely get a bit of a break Sunday, but then an even
stronger trough moves south Sunday night into Monday. This system
will have some dynamics and much colder air with it, but it will
be lacking in moisture. We're looking for similar snowfall amounts
in the Cascades (3-6 inches), and it may get cold enough to see a
mix of rain/snow down into west side valleys on Monday. I don't
think we'll see low elevation impacts from snow, but travelers
should keep an eye on it. -Wright

Long term...Tuesday through Friday. The 12z GFS and European model (ecmwf) models
have differences in timing and strength but are in agreement on the
general evolution of the upper level pattern through the long term.
The weather Tuesday through Thursday is forecast to be dry and cold.
I have blended between the GFS and the colder European model (ecmwf) solution.

The cold low will continue to drop south into and then through the
Great Basin with a strong ridge positioned offshore. Moderate/breezy
northeast winds will develop on Tuesday with the strongest winds
over the east side and higher terrain. This will usher in a modified
Arctic air mass with a cooling trend continuing into Wednesday
morning. This will result in the coldest temperatures registered in
our area so far this season. Besides the cold, this is a typical
freezing fog pattern for the west side valleys...especially with the
abundant ground moisture and relatively warm soil temperatures. With
the cooler air mass aloft there may be sufficient mixing to clear
things out briefly in the afternoons...especially on Tuesday. The
inversion is expected to gradually strengthen through middle-week and
reach peak strength as the upper low slowly moves into the Desert
Southwest on Thursday. Thus, valley fog and freezing fog would have
a higher probability of lingering through the afternoon.

A pattern change is indicated for the tail end of the extended
period. Model differences increase. For now, the forecast leans
toward the weaker GFS solution. A weak cold front on Friday will
arrive from the northwest and bring at least a chance of
precipitation in southwest Oregon with the probability in the slight
chance to chance category elsewhere in our area. With its maritime
trajectory, this cold front will actually bring slightly warmer air.
The very early projection is that the snow level will be around 3500
to 4000 feet. /Dw

&&

Aviation...for the 26/18z taf cycle...along the coast...VFR
conditions will continue through Saturday morning with a ceiling
developing after 12z early Saturday. This ceiling will be ahead of a
cold front with rain and a mix of IFR and MVFR ceilings for Saturday
afternoon and night.

Inland valleys west of the Cascades, including at Roseburg and
Medford, will improve to VFR this afternoon then IFR/LIFR will
return after 05z this evening and continue through Saturday morning.

East of the Cascades...VFR ceilings over lake and Modoc counties
will dissipate this afternoon and persist into Saturday afternoon
with skies remaining mostly clear until high level cirrus clouds
increase Saturday morning. /Dw

&&

Marine....updated 215 PM PST December 26 2014...under high
pressure...light north winds with a moderate northwest swell will
result in seas reaching a minimum early this evening. Northwest
swell will increase slightly tonight then northwest winds develop
late Saturday ahead of a weak front with a further increase in seas
into Sunday. On Sunday...steep seas are expected, building to around
9 to 12 feet. On Monday...a cold trough is expected to move down from
the north over Oregon with high pressure offshore. This pattern is
forecast to bring an increasing surface pressure gradient and allow
areas of moderate north to northeast winds will bring clean swell to
the coast and wind waves that increase as the continue offshore.
Steep seas and gusty winds may continue through Wednesday before
decreasing. -Sven

&&

Mfr watches/warnings/advisories...
or...none.

California...none.

Pacific coastal waters...none.

$$

National Weather Service Glossary of Abbreviations