FXUS66 KMFR 281149

Area Forecast Discussion
National Weather Service Medford OR
349 AM PST Tue Feb 28 2017

Updated AVIATION and MARINE Sections

.Short term...Deep, moist onshore flow aided by a strong
northwesterly jet aloft continues to produce a lot of shower
activity over the west side this morning. Snow is coming down at a
pretty good clip as the flow is forced over the mountains,
especially along the Cascade Crest where the low level flow is
most orthogonal to the terrain. Radar shows persistent training of
showers in the Cascades, and webcams show a snowy scene from
Diamond Lake down to around highway 140 at Doak Mountain. Hi-res
models show this continuing through at least dawn and then showers
begin to diminish as jet dynamics weaken and the airmass starts
to dry out. The winter weather advisory was expanded to include
the area from around Highway 140 north in the Cascades. Snow
showers are certain occurring further south, but the flow is much
less favorable for heavy accumulations, so although roadways may
be slick, Interstate 5 at Siskiyou Summit was not included in the

As a general rule, we'll probably see less shower activity today
compared to yesterday as upper ridging starts to develop off shore
and heights over us rise. Very weak ridging then develops over the
PacNW Wednesday and Thursday, and the emphasis is on weak. The
ridge will do little more than deflect precipitation to our north,
and clouds will likely have little trouble getting over the ridge
into our area. Precipitation will begin to encroach on our area
from the northwest late Thursday as the next system approaches.

Little was changed in the extended forecast, so the previous
discussion follows for reference. Suffice it to say, our respites
from stormy weather remain short, and there is little sign of any
significant break from this active pattern. -Wright

.Long Term...Friday through Monday...The operational models show
general agreement in the forecast period. However the GFS suggest
the upper low in the Gulf of Alaska will have a greater influence
in our area next Sunday and Monday by way of lower snow levels,
persistent precipitation and cooler temperatures. In contrast, the
ECMWF and Canadian suggest the upper low will have less influence
resulting in milder temperatures and higher snow levels. Despite
the differences, the storm track will be close enough to bring
precipitation to the forecast area, especially along and west of
the Cascades. It's all going to come down to the timing
differences with individual fronts from Friday through Monday.


.AVIATION...28/12z TAF CYCLE...Showers focused over the terrain
this morning, especially the Cascades, will gradually decrease in
coverage this afternoon and tonight. Overall, expect widespread
terrain obscuration with areas of MVFR ceilings in showers today.
MVFR will prevail along the coast and into the Umpqua Valley tonight
while terrain obscurations will decrease. Mostly VFR is expected
south of the Umpqua Divide, in northern California and over the east
side, though areas of MVFR ceilings and visibility are still
possible. -Spilde


.MARINE...Updated 230 AM PST Tuesday 28 February 2017...
14 second period west swell and a shorter period fresh northwest
swell will combine to keep seas steep through this morning, but will
gradually ease below small craft advisory levels for all areas by
early this afternoon. While showers will diminish over the next day
or so, they probably won't go away completely. The main storm track
will remain to the north of the waters through Thursday, then the
next series of more significant fronts will affect the area Friday
into early next week. Latest 00z guidance has beefed up the seas a
bit over the weekend and early next week, peaking during that time
period at 12-14 feet at 11 seconds. -Spilde


OR...Winter Weather Advisory until 10 AM PST this morning for
Winter Weather Advisory until 10 AM PST this morning for ORZ025.


Pacific Coastal Waters...Small Craft Advisory for hazardous seas
until 10 AM PST this morning for PZZ350-356-376.
Small Craft Advisory for hazardous seas until 1 PM PST this
afternoon for PZZ370.

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