Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Medford or
918 PM PST Wednesday Dec 17 2014
Discussion...the remnants of an occluded front are moving east
this evening with a slow moving band of light precipitation
roughly now just east of the I-5 corridor through southern Oregon
and Siskiyou County. Precipitation has been widespread on the west
side with amounts in the last 3 hours largely below 0.10 inches.
Meantime...showers in Klamath and eastern Siskiyou County have
been isolated to scattered and will be greatest in number during
the next 3 hours. The snow level is around 4500 feet and snow
amounts over the Cascades and east side are expected to be not
much more than a dusting...up to a half of an inch. Showers and
snow showers will taper off overnight into Thursday morning.
The 00z GFS has arrived and indicates light rain to arrive at the
Coos County coast during Thursday afternoon...in advance of the
next front. Otherwise, there will be a mix of clouds across the
area including morning valley low clouds and patchy fog. Frontal
precipitation will be focused on the Thursday night period with
moderate to heavy precipitation for the Curry County and northern
California coast into Josephine and Siskiyou counties. The snow
level is forecast to begin the evening at around 6000 to 6500 feet
then fall to around 4000 to 5000 feet as precipitation amounts
diminish and precipitation transitions to showers on Friday
morning...diminishing during the day. Thus, the snow level will be
above the major passes during the majority of the event.
A strong warm front is still indicated to arrive Saturday with
widespread precipitation at least through Saturday evening but
also rising snow levels that will be well above the passes.
This front will linger through the weekend with moderate to heavy
precipitation late Saturday night into Monday mainly from Roseburg
Aviation...based on the 18/00z taf cycle...VFR conditions will
prevail initially with areas of higher terrain obscured...except for
western and south central Siskiyou County...where areas of MVFR ceilings
will prevail through Thursday morning. Widespread MVFR ceilings will
return to the west side valleys overnight...and with increasing
winds and the MVFR decks fog formation should be limited. All areas
will clear to VFR by Thursday afternoon. /Sven
Marine...updated 645 PM PST Wednesday Dec 17 2014...weak low pressure
systems will move through the area tonight. Southwest swell
dominated seas will remain mostly at or above 10 feet today.
Thursday into Thursday night a stronger frontal system will likely
bring gale force winds and very steep southwest seas. Long period
west swell will begin to arrive in the predawn hours Friday morning.
It will peak Friday night at around 20 feet at 20 seconds. Series of
large long period swell continue to move into the region through
early next week. A warm front will move through Friday
night...followed by a cold front Saturday. Offshore high pressure
will build Saturday night into Sunday. /Sven
Previous discussion... /issued 206 PM PST Wednesday Dec 17 2014/
Discussion...another weak upper trough is near the coast and this
will move inland this evening. Current radar shows two areas of
scattered showers. The first is in northeast Siskiyou and Klamath
County and the second is moving towards the coast. This second
feature will move inland this evening and bring scattered showers
over northern cal and southwest Oregon. The latest rap models
depicts the placement of the precipitation well and will follow this
for this evening. The upper trough will weaken as it moves east and
shortwave ridging begins to build in overnight. This should shut off
the shower activity over most of the area, although some residual
showers could linger in southeast Siskiyou and Modoc counties.
We'll catch a dry break on Thursday with the ridge axis over the
area, but this will be short lived. The models show a stronger front
approaching the area Thursday afternoon bringing increasing south
winds over the marine waters which are expected to get close to
gales. Precipitation could also reach the coast as early as Thursday
afternoon. The front will move inland Thursday night bringing
moderate to heavy precipitation over the usual suspects...the Coast
Range...coastal mountains and Cascades. However the models are in
good agreement the front will be a quick mover and so we won't get
significant precipitation amounts. Snow levels will start out high
Thursday night, but will drop to around 4500 feet early Friday
morning. So it's a good bet the mountain passes will get
accumulating snow starting late Thursday night through Friday
morning. Current thinking is Crater Lake could pick up at least 6
inches or more and a few inches are possible over Siskiyou Summit
and Highway 140. Keep in mind the details on this will change, so
stay tuned for the latest updates.
The above mentioned front will quickly move through Friday and we'll
catch a relative break in precipitation Friday afternoon and evening.
But again this will be short lived. The models are in pretty good
agreement showing a strong warm front arriving at the coast late
Friday night and moving over the area Saturday. This front has a lot
of juice with it and heavy precipitation will likely impact the
coast...Coast Range...coastal mountains and Cascades. Moderate to
heavy precipitation will also impact much of the Westside valleys. Of
note the ivt transport (which is GFS based) puts the highest values
in central and northwest Oregon. Even the gefs ensemble mean is
similar to the operational GFS. Additionaly, the upper flow is
almost due west which will squeeze out even more moisture along the
Coast Range. Also winds will increase ahead of the front and there
is a chance for gale force winds over the marine waters and moderate
to strong winds over the coast and headlands. If that weren't enough
there is concern for snow east of the Cascades as the warm front
arrives. Model soundings support the idea that enough cold air will
be present near the surface and this in combination with overrunning
precipitation could bring a period of snow Saturday morning.
Strong westerly flow continues Saturday night with the heaviest
precipitation west of the Cascades, but snow levels will Jump Up to
around 7000 feet Saturday afternoon and remain high Saturday night.
Long-term forecast...Sunday through Wednesday. A broad long wave
ridge will be in place at the beginning of the extended period with
with generally northwest flow aloft. Short waves riding over the top
of the ridge will weaken it from Sunday night on...but it will
remain in place through Tuesday. The flow aloft will become more
This pattern will force the main storm track to the north of the
Medford County Warning Area...but the area will still catch the southern end of
several systems which will move onshore during this interval. The
strongest of these systems will move onshore Saturday afternoon and
evening...and will be over the County Warning Area Sunday. Reinforcing short waves
will keep the precipitation going through most of Monday. The bulk
of the associated precipitation will fall along the coast...Coast
Range...and Umpqua basin. Amounts will be significant but not
excessive...no flooding problems are anticipated. Snow levels will
be above 7000 feet.
Monday night into Tuesday look to be mostly dry with nearly all the
action to the north of the area.
The ridge will break down Wednesday and a long wave trough will
begin to dig offshore. This change in pattern will support a front
moving onshore Wednesday. This will be the wettest system of the
extended period if the current ec/GFS model solutions pan out. Snow
levels may also fall as low as 4000 feet behind the front Wednesday
night...so snow may return to the passes around that time. -Jrs
Pacific coastal waters...Small Craft Advisory for winds from 4 am
to 10 PM PST Thursday for pzz350-356-370-376.
Hazardous seas warning from 11 am Thursday to 4 am PST
Friday for pzz350-356-370-376.
Gale Warning from 10 am to 10 PM PST Thursday for
Small Craft Advisory for hazardous seas from 4 am to
10 am PST Thursday for pzz350-356-370-376.