Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Medford or
854 am PST Wednesday Nov 26 2014

Discussion...there is widespread cloud cover this morning from
both valley low clouds and middle and upper level clouds ahead of the
next frontal system. This mornings model run is consistent with
the previous one and brings rain to the marine waters after
midnight tonight with rain spreading into the Coos County coast
early Thanksgiving day. By the end of Thanksgiving day rain is
expected to spread into the Klamath basin and western Siskiyou
County. Snow levels will remain above 6000 feet so travel across
the major passes should not be an issue. It looks like the Goose
Lake basin will remain dry until Friday.

Winds at the upper slopes and ridges will be breezy this afternoon
especially with the stretch of Highway 31 between Summer Lake
area and Paisley in Oregon. Stronger winds are expected Friday and
Saturday as 700 mb winds approaches 50 knots. In addition south bound
traffic in the Shasta Valley could encounter moderately strong
headwind on Friday as pressure gradient between Redding and
Medford is forecast to approach 5 mb.

We will be adjusting the snow level and timing of the next event
in the afternoon forecast package. /Fb


Aviation...based on the 26/12z taf cycle...IFR to LIFR low
clouds and fog at the coast and in the valleys this morning will
lift into IFR ceilings then dissipate during the morning. Low clouds
near North Bend/koth have already dissipated with increased mixing
and stronger winds aloft ahead of a cold front. All areas will
become VFR by noon PST with the exception of the Roseburg area and
possibly the Rogue Valley east of Medford, where IFR could hang on
into the early afternoon. MVFR ceilings will form in west side
valleys again this evening then the cold front is expected to bring
rain with a mix of IFR and MVFR ceilings early Thursday morning.


Marine...updated 500 am PST Wednesday Nov 26 2014...west swell will
subside into this morning. But south winds have begun to increase
ahead of a frontal system. Tonight through Friday, seas will build
to 10 to 14 feet with south wind waves and a swell that is slightly
south of west. Forecast confidence is high into Friday then
diminishes but the pattern is likely to remain active with frequent
periods of rain accompanied by Small Craft Advisory level winds and
seas. Model differences become significant on Friday with
differences in the timing and strength of a surface low approaching
the coast. This low is expected to back winds to northerly with the
northerly winds reaching the northwest portion of the waters as
early as Friday afternoon per the NAM solution or as late as late
Friday night per the GFS. The northerlies will spread across the
waters and persist for most of the weekend with continued moderate
to high seas as southerly wind waves transition to a fresh southwest
swell that will be accompanied by dominant northerly wind waves and
a west-northwest swell at 13 to 15 seconds. Another low is then
likely to approach northern California from the southwest and shift
the coastal winds back to southerly for early next week. /Dw


Previous discussion... /issued 349 am PST Wednesday Nov 26 2014/

Discussion...Thanksgiving day travel is upon US...and weather impacts in
southern Oregon and northern California will include the rather
typical late-Fall weather elements of fog and freezing fog. Per
webcams and observations...fog this morning is focused in the Umpqua
basin...Illinois Valley...coastal valleys like the Coquille...and
portions of the Rogue Valley. Additionally...freezing fog has been
observed in Klamath Falls and near additional caution
should be used when traveling in these areas this morning. It is
likely that fog will expand to other valley areas this
morning...with visibility improving by 10am in most places. Low
clouds will likely linger into the early afternoon in valleys this
morning...mostly due to building ridging overhead and stronger
inversions near the surface. Elsewhere...only high clouds are
expected ahead of an approaching frontal system.

The next frontal system will usher in a narrow band of significant
moisture on Thursday...bringing rain initially to the coast and
Coast Range during the morning...then expanding eastward to affect
most areas in southern Oregon along and west of the Cascades. Models
are in good agreement that most areas east of the Cascades and in
northern California should remain dry. Thursday night the boundary
waves north some...likely bringing the rain back north of Medford
overnight. It should be noted that snow levels during these periods
of precipitation should be above 6000 on the plus Side
Mountain passes should not be affected...but unfortunately...snow
pack will not be enhanced or initiated in most areas. An
additional significant note is on winds...with solid 50kt west to
southwest 700mb flow through the weekend. Initially...only expect
high terrain areas east of the Cascades to be impacted by gusty
winds...with surface pressure gradients remaining relatively have issued a Wind Advisory to highlight gusty
southwest Crosswinds along Highway 31 near Summer Lake this
afternoon. I would not be surprised if winds remain gusty over
high terrain and portions of the east side through Friday and have
increased winds to account for this. Additionally...the south to
north pressure gradient may increase enough on Thursday to bring
gusty winds to the Shasta Valley. Future shifts may need to
consider a Wind Advisory Thursday and/or Friday for that area too.

Back to the frontal boundary and precipitation concerns...significant
upstream troughing will shove the main rain band back southward on
Friday...followed by unstable and generally showery conditions
late Friday into Saturday across most of the forecast area. Snow
levels will fall on Saturday and it's likely that mountain passes
will see some accumulating snowfall Saturday evening into Saturday

The models have thrown an interesting wrinkle into the forecast on
Sunday...with the GFS showing strong south flow and plenty of
moisture waving back north ahead of an offshore closed low. combination with some cold air east of the
Cascades...could yield some accumulating snows there.
Additionally...strong south flow with a marginally cold air mass
can mean snows for the I-5 corridor in The Mount Shasta city area.
The 00z European model is much tamer...with mostly light
precipitation along the old frontal boundary which would result in
minimal impacts in either the wind or snow category. We have
blended the two and keep precipitation chances going through the
weekend...and hopefully this model divergence will be short-lived
and we can hone in on any Sunday travel weather impacts.

Farther out...the European model (ecmwf) does finally bring in the offshore low
midweek...spreading precipitation across the forecast area Tuesday
night into Thursday. Meanwhile...the GFS suggests ridging midweek. A
look at the GFS ensembles suggests a good spread amongst members but
most have at least low-amplitude ridging in place.

Aviation...based on the 26/06z taf cycle... southwest flow and warm
air advection will continue through today as high pressure gradually
gives way to a frontal system approaching from the Pacific. This
will create increasing winds above the nocturnal inversion on this
morning. Low level moisture and clearing skies aloft are expected
to bring IFR to LIFR fog formation in the valleys that will lift
into IFR ceilings during the morning. Due to the increasing winds
aloft, we do expect that all areas will become VFR by noon PST
Wednesday with the exception of the Roseburg area and possibly the
Rogue Valley east of Medford, where IFR could hang on into the
early afternoon. /Cc

Marine...updated 1000 PM PST Tuesday Nov 26 2014...northwest swell will
continue to subside today. However south winds and wind waves will
gradually increase ahead of an incoming frontal system. Tonight
through Friday, seas are expected to become southwest and
chaotic...building to 10 to 14 feet. Saturday through Tuesday low
pressure will approach the coast from the southwest bringing a surge
of moisture and wind across the area. Confidence is still low in the
wind and wave details during this time period...but conditions are
likely to be unsettled. -Cc/btl

Climate...a wet fall thus far has many locations at above normal
for precipitation since Sep 1. The following are percent of
normal values for various cities since Sep 1.

North Bend.........73%
Klamath Falls......118%
Mount Shasta city..127%


Mfr watches/warnings/advisories...
or...Wind Advisory from 1 PM this afternoon to 7 PM PST this evening
for orz030-031-624-625.

California...Wind Advisory from 1 PM this afternoon to 7 PM PST this evening
for caz085-285.

Pacific coastal waters...Small Craft Advisory for winds from 1 PM this afternoon to 10 PM
PST Friday for pzz350-356-370-376.
Small Craft Advisory for hazardous seas from 1 PM this afternoon
to 10 PM PST Friday for pzz350-356.
Small Craft Advisory for hazardous seas until 10 PM PST Friday
for pzz370-376.



National Weather Service Glossary of Abbreviations