Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Medford or
1033 am PST sun Dec 28 2014

Discussion...updates have already been sent this morning, mainly
to raise probability of precipitation on the west side this morning and to end the Winter
Weather Advisory for the Cascades. A weakening cold front is
moving across our area this morning. Cool, moist onshore flow
continues in its wake, and this is resulting in a lot of shower
activity west of the Cascades. Satellite and upstream observations
show the cloud deck breaking up, and we expect this to move over
US this afternoon with less shower activity and some breaks in
cloud cover. Light snow will likely continue in the Cascades,
particularly north of Highway 140, but further accumulations will
be light, thus the Winter Weather Advisory has been canceled.

An Arctic cold front is on track to move through our area Monday.
This will produce another round of snow in the mountains and snow
may affect a wider area than today's front. Model quantitative precipitation forecast is not
impressive, but we will be looking closely at this today. -Wright


Aviation...for the 28/18z taf cycle...showers will continue across
the area today through Monday morning. Areas of MVFR ceilings with local
IFR ceilings are expected through this morning. Low ceilings are expected to
lift to VFR around 18z-21z. Partial terrain obscurations are
expected through the day, and will be near total, at times, in the
Oregon Cascades and Siskiyous, along higher portions of the Umpqua
Divide, and Coos County Coast Range. Late this evening through the
overnight MVFR ceilings are expected to return to the airfields,
lowering to IFR and isolated LIFR in showers and patchy fog
overnight into early Monday morning. Widespread mountain
obscurations late tonight into Monday morning as a secondary cold
front pushes into the area. Btl/cirrocumulus


Marine...updated 840 am PST December 28, 2014... northwest seas
will build today bringing 10 to 11 foot combined seas to primarily
areas north of Cape Blanco. Another cold front will move in from the
north on Monday bringing showers and gusty northwest through
northeast winds. Freezing temperatures are expected near the shore
north of Cape Blanco Monday...Tuesday and Wednesday nights and near
the shore from Cape Blanco south on Tuesday night. Moderate north to
northeast winds and steep seas are expected Monday night through
Wednesday. Btl/cirrocumulus


Previous discussion... /issued 401 am PST sun Dec 28 2014/

Discussion...confidence is increasing regarding the very cold
air expected to flow into the forecast area early this week. The
system at the leading edge of the Arctic air mass will produce
some precipitation, particularly along the Cascades, but with
snow levels dropping rapidly Monday, some lower elevation snow is
possible by Monday evening. Cold air is expected to then remain
over the region through midweek, before gradually warming through
the remainder of the forecast term. Have relied on a general blend
of the GFS and European model (ecmwf) throughout the week, with a lean towards some
of the higher resolution models, which seem to better handle
terrain driven features, during the short term.

Rain and snow showers are expected to diminish through this
morning, but not after a brief increase in coverage and intensity
early this morning as a brief shot of energy moves overhead. Snow
levels will generally remain above 3000 feet, and with most of the
precipitation falling along and west of the Cascades, the Winter
Weather Advisory in place for the higher Cascade passes appears
to be on track. Will let the advisory continue as is, as another
inch or two is possible within the advisory area before it
expires late this morning. Showers will continue, but diminish in
intensity through the remainder of the day and into tonight.

The next wave will drop south tonight, riding the boundary
between the cold air mass currently in place and the truly frigid
Arctic air expected to settle into the region through the middle
of the week. This second system will be north the weak side, and will
not have as much moisture associated with it as the previous one,
but with such cold air, snow levels will drop significantly Sunday
night through Monday. By the time precipitation comes to an end
late Monday, snow levels are likely to drop to well below 1000
feet in most locations. This means that low elevation snow is
appearing more and more likely for the west side, particularly in
the Cascade foothills, the higher elevations of the coastal
ranges, and the Siskiyous and Umpqua Divide. Most valley floors,
such as the Umpqua and lower Rogue Valley may six some snow mix
into the light rain, but no accumulation is expected there.
However, the upper Rogue Valley, with a slight elevation
advantage, could see snow at the valley floor, although area Road
surfaces would likely be to warm to allow accumulation. Have
issued a Special Weather Statement to cover the event, as
snowfall amounts are expected to remain below advisory criteria,
but some local travel impacts are possible. For the east side,
this whole event will amount to little more than cold temperatures
and some light snow, as most moisture will be wrung out of the
system as it crosses the Cascades.

Monday night through Thursday, high pressure should act to clear
most of the skies, and with the Arctic air and fresh snowfall in
place, conditions will be optimal for radiational cooling
overnight. The result will be some of the coldest temperatures
seen so far this season, with east side locations dropping into
single digits or even below zero for overnight lows, and much of
the west side dropping into the teens and low 20s. High
temperatures Wednesday will even struggle to get above freezing
for all but the coast, but even there, overnight lows are expected
to drop to below freezing. Freeze headlines will most likely be
needed for the coast this week. Unfortunately, this same scenario
also typically produces strong inversions and freezing fog within
many of the area valleys, and this will be an ongoing concern for
much of the week as well.

Model spread is rather large in the extended period, but there are
some signs that a trough will pass closely enough to the forecast
area to help scour the valleys and perhaps bring another round of
precipitation over the weekend. However, uncertainty remains
high, especially given the potential for upstream blocking, so no
major changes were made to the previous forecast trends. -Bpn


Mfr watches/warnings/advisories...


Pacific coastal waters...Small Craft Advisory for hazardous seas
until 10 PM PST this evening for pzz356-376.
Small Craft Advisory for hazardous seas until 10 PM PST this
evening for pzz350-370.


National Weather Service Glossary of Abbreviations