Area Forecast Discussion
Issued by NWS Medford, OR

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000
FXUS66 KMFR 220503
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
903 PM PST Wed Feb 21 2018

.DISCUSSION...Current observations are showing a brief quiet
period before the next system digs into the Pacific Northwest from
the northwest. High resolution models are showing precipitation
reaching the coast in a couple of hours with the precipitation
reaching the Rogue Valley around 4 AM, and spreading quickly into
western Siskiyou County and the east Side. The precipitation
should turn showery in nature later this morning. Overall amounts
of 1 to 3 inches are expected across much of southern Oregon and
northern California with isolated mountain areas seeing up to 6
inches.

Snow levels will be near sea level for nearly everywhere except
for the coast. Thus, winter weather advisories have been issued
for most of the West Side for tonight. This includes Interstate 5
from Weed, California through the Lane and Douglas border. It also
includes highway 140 near lake of the woods and highway 199.
Overall, the snowfall could be enough to impact morning commutes
tomorrow. As mentioned yesterday, please take it slow tomorrow
morning. Allow extra time to reach your destination, and make sure
there`s extra space between you and the car ahead of you. See the
WSWMFR and the previous discussion (listed below) for more
information.

Otherwise, have made no changes to the forecast. -Schaaf

&&

.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 CIGS 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.


&&

.PREV DISCUSSION... /Issued 412 PM PST Wed Feb 21 2018/

Short Term...Tonight through Saturday Night...Even before the
last significant band of snow showers had dissipated this
morning, attention turned towards the next wave expected tonight
into tomorrow morning. Until then, some light snow showers are
possible along the Cascades, Siskiyous, and area foothills, but
with warmer daytime temperatures, expect very light if any
accumulations on area roads.

The overall pattern of ridging to the west and deep troughing
over the central US will not change anytime soon. This places the
forecast area squarely under north to northwest flow aloft,
through which numerous waves will traverse, each producing another
burst of precipitation and a reinforcing shot of cold air.

The next such wave arrives tonight. While moisture inflow is not
overly impressive, the system is robust enough and temperatures
will be more than cold enough to produce accumulating snows down
to 500 feet overnight and into Thursday morning. The most
concerning aspects of this event are twofold: First, the
temperature and moisture profiles show an area of significant lift
right in the most favorable snow crystal growth zone, and second,
this stronger burst of snow is likely to arrive around or after
sunrise, right during the morning commute. With more traffic on
the roads during this time, and a greater potential for impacts,
have issued a Winter Weather Advisory for nearly all of the area
west of the Cascades, including the entire stretch of Interstate 5
from the Douglas/Lane county line south to Weed in Northern
California. For more details on this system and the expected snow
amounts, see the winter weather message at PDXWSWMFR.

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


&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory until 1 PM PST Thursday for ORZ028.
     Freeze Watch from Thursday evening through Friday morning for
     ORZ021-022.
     Winter Weather Advisory until 1 PM PST Thursday for
     ORZ022-024-026.
     Winter Weather Advisory until 1 PM PST Thursday for
     ORZ021-023-025.

CA...Winter Weather Advisory until 1 PM PST Thursday for CAZ080-081.

Pacific Coastal Waters...Hazardous Seas Warning from 4 AM Thursday to 10 AM PST Friday
     for PZZ350-356-370-376.
     Small Craft Advisory for hazardous seas until 10 AM PST Friday
     for PZZ350-356-370.
     Small Craft Advisory for hazardous seas until 4 AM PST Thursday
     for PZZ376.

$$

BMS/JRS/BPN



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