Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Medford or
712 am PST Friday Dec 19 2014
a cold front is pushing through the forecast area this
morning...bringing with it moderate to occasionally heavy
precipitation and some mountain and east side snowfall as snow
levels drop to around 4000 feet this morning.
Moderate snowfall is being observed on webcams at Diamond Lake...and
along Highway 97 at Chemult this morning. Latest radar data suggests
the significant band of snow over the Cascades and Highway 97 should
diminish some before sunrise. Snow showers should continue over the
Cascades at times today with a few more inches of snow possible.
Additionally...thunderstorms are possible over Coos and western
Douglas County this morning as some strikes have already been
observed at the coast this morning in the unstable air mass behind
Attention turns to an atmospheric river event that will bring a
relatively long period of moderate to heavy precipitation to the
forecast area from Saturday through Sunday before the plume of
moisture retreats to the north. Snow levels will be between 4000
and 5000 feet when precipitation begins...so mountain passes
should get a quick shot of accumulating snows late tonight into
Saturday morning before snow levels bump above 5000 feet middle to late
Saturday morning. Therefore...travelers wanting to avoid snowfall
over mountain passes should wait until the afternoon to travel.
However...all travelers in the area will have to contend with
moderate to heavy rain on Saturday at least. Additionally...winds
will be very gusty at the coast and east side Saturday. 850 mb
winds are from the southwest...and models suggest a 7-9 mb
pressure gradient from oth- acv...which makes gusts to 50 miles per hour seem
very reasonable. Future shifts may want to bump this up even
further if data supports this. We increased winds near Summer Lake
for Saturday afternoon as 50-70kt 700mb winds traverse that area.
Additionally...have increased Shasta Valley winds simply due to
the strong south to north pressure gradient even though the 700mb
winds are from the west.
The hydrologic outlook covers the heavy rain situation well at this
time. Latest river models suggest the Coquille river at Coquille
reaches action stage...and a Flood Watch may be necessary if future
runs increase this further. The current forecast reflects a blend
between the NAM and GFS in terms of precipitation amounts...which
basically is 5 inches for the orographic-favored portions of the
Coast Range and 5 inches for the Cascades from Crater Lake north.
Interestingly...integrated vapor transport...an indicator of how
significant a rainmaker this system will be...trends have decreased
somewhat over the past few GFS ensemble runs...but still are
impressive with 5 Standard deviations above normal per the most
recent run. Snow levels will peak at 9000 feet late Sunday...and
most of the heavy precipitation with the system will fall as
rain...entering the rivers and streams. This again suggests that
there will be some hydrologic impacts with this storm.
Conditions dry out Monday into Tuesday before a cold system drops
down from the northwest and exhibits quite different behavior
than the last system...with snow levels lowering sharply and the
potential for snow as low as 3000 feet. This of course is of
importance to anyone traveling in the area or hoping for a white
Aviation...based on the 19/00z taf cycle...
showers will continue across the area today, taper off this
afternoon, and then a warm front will push in this evening through
Saturday morning. A mix of VFR and MVFR conditions will occur this
morning in showers, with mostly obscured higher terrain. Expect this
mix of MVFR and VFR to continue into the afternoon. Tonight, with the
warm front, expect primary MVFR and IFR to develop along with total
terrain obscuration. Btl
Marine...updated 515 PM PST Friday Dec 19 2014...
isolated thunderstorms with locally gusty winds will continue
this morning, primarily along and near coast and south of Cape
Blanco. Heavy west swell will build today...peaking around 20 feet
at 18 seconds Friday evening. The long period swell will remain
heavy through Saturday as another round of gale force winds
arrives with a strong warm front. Sunday into Monday this warm
front will linger along the coast north of Cape Blanco as seas
subside and winds gradually diminish. The next front is expected
to arrive from the northwest on Wednesday. Moderate to strong and
gusty north winds are expected behind it Christmas evening into
Christmas day. Btl
or...high surf advisory from 11 am this morning to 10 PM PST Saturday
Pacific coastal waters...
- Small Craft Advisory for winds from 10 PM this evening to 4 PM
PST Saturday for pzz350-356-376.
- Hazardous seas warning until 10 PM PST Saturday for
- Gale Warning from 4 am to 4 PM PST Saturday for
- Small Craft Advisory for hazardous seas until 10 am PST this
morning for pzz350-356-370-376.
- Small Craft Advisory for winds from 10 PM this evening to 4 am
PST Saturday for pzz370.