Area Forecast Discussion
Issued by NWS Medford, OR

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194
FXUS66 KMFR 250618
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
1017 PM PST Fri Feb 24 2017

Updated Aviation Discussion.

.SHORT TERM...Showers will continue this evening and tonight,
especially over western and southwestern portions of the area as
a low just off the coast of Southern Oregon shifts southward. The
best area of showers this evening has been in Curry, Josephine,
Jackson counties in Oregon and western and central Siskiyou County
in Northern California. Models continue to show that these areas
will continue to see numerous showers this evening and overnight.
East of the Cascades, expect more isolated to scattered activity.
Snow levels have lowered this evening to between 1000 to 1500
feet elevation. So light snow is expected in showers down to
western valley floors, especially in Josephine, Jackson and
Siskiyou counties where showers are more numerous. Accumulating
snow may be very light though for most western valley locations
with moderate accumulations possible in showers above the valley
floors. Of note, given the southwestern trajectory, areas in the
Illinois Valley and valleys in Western Siskiyou county may see
accumulations of 1-2 inches in heavier showers. Otherwise, the
main impacts are expected over the elevations above 1500 feet
including the passes on I-5. A winter weather advisory remains in
effect for many western locations with a winter storm warning for
mountains in western Josephine and Curry county. Shower activity
is forecast to decrease over Oregon and become scattered over
Northern California by early Saturday morning.

In addition to shower activity, earlier this evening, isolated
thunderstorms were observed just off the Southern Oregon coast.
Models indicate that this potential for thunderstorms along
coastal areas and over the coastal waters is expected to continue
overnight and then diminish early Saturday morning.

As this storm shifts south on Saturday, expect a break in the
active weather with mainly isolated showers Saturday afternoon
and evening. However a stronger and more significant storm remains
on track to enter the region Sunday and remain over the area
Sunday night. This storm will bring additional low elevation snow.


&&

.AVIATION...25/06z TAF CYCLE...As a disturbance off the coast shifts
southward, showers will decrease in the early morning hours. Low
level moisture lingering in western valleys in Southern Oregon is
expected to bring areas of MVFR cigs/vis and local IFR in inland
valleys through Saturday morning. Confidence is low on timing and
amount of low clouds at KMFR due to the possibility of additional
showers tonight through around 12z.  Mountains, from the Cascades
west will be mostly obscured through 12z then expect decreasing
obscurations in the morning. Across western and central Siskiyou
County in Northern California, expect a mix of VFR and MVFR in snow
showers through Saturday morning. Elsewhere, east of the Cascades,
VFR conditions are expected to be the predominate condition with
local MVFR in showers through the overnight period and early morning
period, with partial mountain obscuration. VFR is expected all areas
by 19Z as skies become mostly clear. -BTL/CC


&&

.MARINE...Updated 300 PM PST Friday, 24 February 2017...
Low pressure centered west of the northwestern Oregon coast this
afternoon will sink southward over the area through Saturday
morning. This will bring breezy southerly winds that will become
northerly late Saturday afternoon. There is a slight chance of
thunderstorms as the core of the upper level low moves overhead
tonight into the pre-dawn hours of Saturday. Showers tonight will
taper off Saturday morning as high pressure builds in. Saturday
night through Monday another cold low pressure system will move
through from the northwest. Fresh swell will build Sunday through
Monday brining combined seas in the 9 to 12 foot range. BTL


&&

.PREV DISCUSSION... /Issued 607 PM PST Fri Feb 24 2017/

DISCUSSION...

..Short Term...Tonight through Monday Night...The main focus of
efforts today revolved around the two systems that will bring
lower elevation snow to our area. Also of concern will be the cool
temperatures tonight, especially along the coast where a Frost
Advisory is in place through tonight.

The first system is beginning to arrive as this is being written,
with showers moving into the coast and snow levels dropping down
to sea level at times. Snow levels should stay above 1000 to 1500
feet for much of the time, but lower snow levels will not be
uncommon. The main burst of precipitation is expected later this
evening and tonight, as the trough and associated surface low
tracks south just offshore of the forecast area. The system is
tracking further west than in previous forecast cycles, and
normally, this would result in less precipitation amounts, but
moisture with the system has increased, and the end result is a
wider area where snowfall is expected to impact travel. The
advisory area now includes the entire coastal range from Douglas
to Curry and Josephine Counties and down into Siskiyou county, and
the foothills of the Cascades in Douglas and Jackson counties.

After a brief break Saturday afternoon and night, the next system
arrives Sunday morning. This low will track almost directly over
the forecast area, and should produce more widespread and
somewhat more intense snow. Snow levels will once again drop to
near 1000 feet, and suspect that snowfall will be rather
substantial above 2500 to 3000 feet. This would bring impactful
accumulations to nearly all passes, and perhaps even down into
some of the West Side valley floors. Have issued a Winter Storm
Watch for the Cascade Foothills, where the heaviest amounts are
likely, but will leave the more advisory-level areas covered
under the current SPS.

precipitation should wane, and cooler temperatures should
continue, as ridging builds in Monday behind the departing low.
High pressure will then remain in control as we head into the
extended term. -BPN

Long Term...Tuesday through Friday...The operational models are
in pretty good agreement in this time period. The pattern will be
rather quiet with little or no chance of precipitation. A
northwest flow continues Tuesday with increasing 500 mb heights.
The models still show some QPF, but they are not as bullish as
they were yesterday. Even then suspect they are still overdone
given 500mb heights will be on the rise as the upper trough moves
east of our area. However the ECMWF shows weak pva moving in the
northwest flow during the day, so we`ll keep a slight chance
mention. Confidence is high it will be dry from next Wednesday
through the end of next week as the storm track shifts well north
of the area. Of note, the GFS shows more of a dirty ridge
scenario, but still keeps all of the QPF north of our area, but it
could mean more cloud cover and slightly cooler afternoon
temperatures.

We could be dealing with an increasing subsidence inversion next
Wednesday through Friday which means the valleys could stay cool and
the ridges milder. -Petrucelli


&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Frost Advisory until 8 AM PST Saturday for ORZ021-022.
     Winter Weather Advisory until 10 AM PST Saturday for
     ORZ021-023-025.
     Winter Storm Watch from Sunday morning through late Sunday night
     for ORZ023-025-026.
     Winter Storm Warning until 10 AM PST Saturday for ORZ022-024.
     Winter Weather Advisory until 10 AM PST Saturday for ORZ024-026.

CA...Winter Weather Advisory until 10 AM PST Saturday for CAZ080.

Pacific Coastal Waters...None.

$$

CC



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