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fxus66 kmfr 221113 

Area forecast discussion
National Weather Service Medford or
313 am PST Thu Feb 22 2018

Short term...focus this morning has been on a developing winter
storm over our area. As expected, snow spread into our area from
the north this morning. Webcams show that Roseburg is already
snow-covered, and it is now snowing heavily in the I-5 passes
from there south to Grants Pass. Given radar and satellite trends
as well as upstream observations, the Winter Weather Advisory was
upgraded to a Winter Storm Warning for west side locations above
1500 feet. We'll probably see at least 3-6 inches in these
locations. We're still expecting 1-3 inches in the valleys, so the
winter weather advisories remain in effect for valley locations.
We also issued a Winter Weather Advisory for the Cascades and
Siskiyous, including I-5 at Siskiyou Summit. Snowfall won't be
excessive by their standards, but it is very cold up there, and
snow will make for difficult travel conditions much of today.

A big difference with this storm compared to previous storms is
that the ambient environment is much colder, and this will allow
snow to stick much more easily, as we've already seen up north. It
has even snowed down to the beaches occasionally already, and
we'll likely see some accumulations at elevations down to 100 or
200 feet.

Radar continues to fill in over the rogue and Illinois valleys,
and we expect to see snow increase in these areas through dawn.
The morning commute will be a slippery one for much of the west
side. Snow will diminish to flurries by late morning. No changes
were made beyond today, and much of the previous discussion
follows for reference. -Wright

Previous short term issued 412 PM PST Wed Feb 21 2018: snow
showers will taper off Thursday evening with some clearing
expected Thursday night into Friday morning. With new snow on the
ground and a cold air mass overhead, temperatures could make a
significant plunge by sunrise on Friday morning. However, if cloud
cover continues, as has occurred with the past several events, it
will be more difficult for temperatures to drop significantly.
Current thinking is that clouds will clear enough to allow for
good radiational cooling, and this should produce below freezing
lows all the way down to the coast, with single digits across the
east side, and low 20s to teens possible in the west side valleys.
Have issued a freeze watch for the coastal areas, but any early
season plant growth in the area is under the threat of freeze
damage if the proper precautions are not taken.

After a brief break Friday, the next trough arrives Friday night
into Saturday. This wave appears weaker and slightly warmer than
the previous ones, so snow levels will be slightly higher, and
precipitation amounts should stay on the lower side. The wave
will pass by Saturday afternoon, but moist onshore flow should
keep showers ongoing into the extended period. -Bpn

Long term...sun, Feb 25th through Thu, Mar 1st...and beyond...
this long term period of the forecast will begin with a trough of
low pressure centered over The Rockies and high pressure extending
from just east of the Hawaiian islands north-northwestward to the
Bering Sea. This will continue to result in a northwest flow storm
track across the forecast area. By the end of the period, gefs
500mb geopotential height anomalies suggest the low pressure
troughing is likely to retrograde westward. This puts the trough
axis just inland from the West Coast, west of the Cascades.
Although the ridge over the Pacific is not expected to move much,
the net effect of the change in the overall pattern during this
time period is likely to allow for the weather systems moving in
to pick up more moisture off of the Pacific before they move
inland. Since temperatures are very likely to remain on the colder
side of late February normals, we expect to continue to see our
snowpack grow, likely faster than it has recently.

More specifically, on Sunday the 25th through Monday the 26th, a
major shortwave trough is expected to move through, with water
amounts mostly in the 0.50" to 1.25" range from the Oregon Cascades
and Marble Mountains of California westward. Snow levels are
expected to begin around 3500 feet, falling to the 1500 to 2000
foot range as precipitation tapers off. Notably, the higher snow
levels than we've seen lately also mean that the atmosphere will
be able to hold more water, so mountain snowfall is likely to be
more significant during this time period. Preliminary amounts for
this frontal system appear to be 5 to 10 inches above 4kft, with
10 to 15 inches possible above 6kft.

Tuesday and Wednesday look to be a relative break in the
precipitation action. However, due to timing differences in the
models and a cold air mass lingering over the forecast area, some
light rain and snow showers will still be possible, especially
along and near northwest facing slopes in the afternoon hours.

The next major shortwave trough in the northwest flow is expected
to arrive on Thursday, with snow levels around 2.5kft. With the
upper level trough likely to have reoriented along or near the
West Coast, we'll probably see a little more southwest flow
enhancement to the precipitation with that one. Thus, it appears
March is most likely to come in lion-like.

Overall, long range guidance is indicating March will, most
likely, be colder than normal with above average precipitation
for our forecast area. For early March, the latest gefs mean and
cfsv2 indicate that the pressure pattern is likely to shift
westward enough to bring in more southwest flow weather systems
during the first week of the month as low pressure gears up in the
Gulf of Alaska. Currently, it appears this will be followed by a
shift to west flow track weather systems for the 2nd week of the
month. Btl


Aviation...22/06z taf cycle...VFR conditions will prevail
initially, with obscuration of terrain along the Cascades and
Siskiyous. Late tonight into Thursday a front will move in from
the northwest bringing snowfall down to elevations between sea
level and 1000 feet. MVFR to IFR cigs/vsbys in snow showers are
likely until 18z, with showers lingering into the evening in some
areas. Conditions will improve to VFR over most if not all of the
area Thursday afternoon, with any lingering lower ceilings clearing
Thursday evening.


Marine...updated 800 PM PST Wednesday 21 Feb 2018...a weak low
will move onshore from the north/northwest late tonight. North
winds will increase over the waters behind the low Thursday to
just below gale force, and waves will become steep to very steep.
High pressure will build over the region Friday resulting in
lighter winds and calmer sea conditions that will last into
Saturday. A front will move onshore Sunday, and it will bring high
and steep northwest swell with it. Even heavier swell will move
in Monday.


Mfr watches/warnings/advisories...
or...Winter Weather Advisory until 1 PM PST this afternoon for
Freeze watch from this evening through Friday morning for
Winter Weather Advisory until 1 PM PST this afternoon for
Winter Storm Warning until 1 PM PST this afternoon above 1500
feet in the for orz021>026.
Winter Weather Advisory until 1 PM PST this afternoon for

California...Winter Weather Advisory until 1 PM PST this afternoon for

Pacific coastal waters...hazardous seas warning until 10 am PST
Friday for pzz350-356-370-376.
Small Craft Advisory for hazardous seas until 10 am PST Friday

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