High Surf Advisory 4, NWS Medford, OR

Area Forecast Discussion
National Weather Service Medford OR
930 AM PST Sat Jan 20 2018

.DISCUSSION...Morning update. Showers continue to taper off as a
weak ridge moves across the region. Current forecast looks on track
and will not update this morning. Sven

.Previous SHORT TERM...The focus this morning has been on a strong
front that will bring a variety of weather on Sunday including wind,
rain, and snow.

Right now, we have some showers roaming around, especially along
the coast and in the coastal mountains. But precipitation
should continue to decrease today as very weak short wave ridging
slides through. The flow does remain onshore, so while inland
areas will probably be dry by this afternoon and evening, coastal
areas will continue to see some showers.

The front in question for Sunday currently stretches from a
large occluded low centered off the US/Canadian border near
50N/145W off to the southwest to near the Hawaiian Islands. A
baroclinic leaf (representing a wave of low pressure) can already
be seen forming along the front around 160W. Model consistency
remains high in bringing this frontal system to our area Sunday.
This front looks very dynamic, and strong winds will occur ahead
of and along the front in the usual areas (Shasta Valley and east
side), but winds will also affect much of the coast. Normally
winds are confined to the capes and headlands along the coast, but
the orientation of this front suggests that winds will become
onshore just as it arrives Sunday morning, and this is one
indicator that high winds can affect more of the coast than usual.

Confidence was highest in the wind aspect of this front, so the
high winds watches that were in effect have been upgraded to high
wind warnings. The Medford-Redding gradients are expected to be
around -9 MB, and while winds are not particularly well aligned
from the southeast, high resolution guidance shows some wind
getting into the southern end of the Rogue Valley around Ashland
and perhaps all the way to Medford by Sunday afternoon. We held
off on a wind advisory for the Rogue Valley for now, but this may
be needed if model trends continue.

Confidence in precipitation amounts with this front is quite a bit
lower. Guidance varies significantly with the GFS/NAM generally
showing lower amounts whereas the European is much wetter.
Obviously, these differences have the most impact on snow
potential in the mountains. Snow levels will start out low
(around 3000 feet) as precipitation arrives Sunday morning the
rise to around 4000 feet Sunday afternoon before falling again
Sunday evening. The Cascades seem in line to receive a moderate
snowfall, and an advisory was issued for the Cascades and the
Siskiyous. This includes I-5 at Siskiyou Summit where we could see
up to 4 inches of snow Sunday.

Around Mt. Shasta, the forecast is trickier. With 850 MB
temperatures of -3 to -5 C and 1000-700 MB thicknesses well below
2884 meters, thermodynamics are certainly favorable for snow at
Mount Shasta. However, as mentioned, models vary a lot on the
amount of precipitation. There are also a few things working
against heavy snow impacting Mount Shasta City. First, winds aloft
are not aligned well from the south. In fact, 700 mb winds are
almost due west, and when winds are not aligned, it doesn't favor
the strong upslope that we need to get heavy precipitation rates.
Second, road temperatures are expected to be warm. Metro forecasts
show road temperatures remaining above freezing throughout the
event. Finally, the bulk of the precipitation falls during
daylight hours, which would also tend to limit accumulations. I
can certainly still see how we get heavy snow, especially if the
EC is correct, but there remains enough uncertainty to prevent me
from upgrading the watch to a warning at this time. We'll let the
next shift have a look at the 12Z guidance before making that
decision.

This front doesn't seem to linger like the last one and scoots out
of our area by Monday morning. Expect cool conditions Monday with
residual showers mainly along the coast. No changes were made to
the extended forecast, and the previous discussion follows for
reference. -Wright

.LONG TERM...Previous extended discussion issued 307 PM PST Fri
Jan 19 2018: Tuesday, Jan 23rd through Friday night, Jan 26th...
The next major shortwave trough in our ongoing storm series is
expected to arrive on Tuesday and persist through Friday. This
trough of low pressure is currently on track to be very
significant in terms of precipitation amounts, duration, and
mountain snow, and will also bring with it the possibility of
lower elevation snowfall Wednesday night through Friday. After
this trough moves through model guidance has generally been
indicating high pressure ridging for next Saturday the 27th into
Tuesday the 30th, followed by a progressive trough from the Gulf
of Alaska clipping us as it focuses to our north around the last
day of the month into the first days of February.

Precipitation probabilities, amounts, and snow levels have been
adjusted for Tuesday through Wednesday to be mostly in line with
model consensus, but with a slight lean (60/40) toward the colder
and wetter 12Z ECMWF. While both models indicate significant
precipitation amounts, the ECMWF has been more consistent and
seems to hit the upslope areas more appropriately given the
expected flow directions. Snow levels have been raised some for
the early part of this weather system, up to about 5500 feet for
most areas late Tuesday. This is an indicator that the frontal
system will be capable of holding a significant amount of
moisture. By Thursday and Friday snow levels are expected to be
teetering in the 1,000 to 2,500 foot range. BTL

&&

.AVIATION...21/12Z TAF CYCLE...Along the coast and west of the
Cascades...A shortwave moving through the area is generating
showers, and this is keeping valleys mixed out with mainly VFR cigs
and vis. Expect conditions to lower to MVFR with any heavier
showers and once showers pass through, IFR conditions will be
possible around sunrise for inland valleys, but should be short
lived if they do develop. Once conditions improve, BKN to SCT VFR
cigs will prevail through the afternoon with scattered showers
continuing mainly along the coast.

East of the Cascades, a mix of MVFR/IFR conditions will persist
through the morning...improving to VFR by this afternoon. /BR-y

&&

.MARINE...Updated 230 AM PST Saturday 20 Jan 2018...Westerly swell
will gradually diminish this morning, decreasing briefly to 11 to
14 feet with small craft conditions expected through this
evening. Another strong front will move through the waters late
Saturday night into Sunday. Storm force winds are expected north
of Gold Beach early Sunday morning and gales are expected
elsewhere late Saturday night into Sunday afternoon. Seas will
build again Saturday night, peaking at 20 to 23 feet Sunday
morning. Conditions will improve late Sunday night into Monday,
but seas will remain elevated and active weather continues through
the week. /BR-y

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...High Wind Warning from 7 AM to 7 PM PST Sunday for ORZ030-031.
     Wind Advisory from 7 AM to 7 PM PST Sunday for ORZ030-031.
     High Wind Warning from 4 AM to 1 PM PST Sunday for ORZ021-022.
     Winter Weather Advisory from 4 AM to 4 PM PST Sunday for
     ORZ027-028.

CA...Winter Storm Watch from Sunday morning through Sunday evening
     for CAZ080-082-083.
     High Wind Warning from 7 AM to 7 PM PST Sunday for CAZ081.

Pacific Coastal Waters...Small Craft Advisory until 10 PM PST this
     evening for PZZ350-356-370-376.
     Storm Warning from 10 PM this evening to 1 PM PST Sunday for
     PZZ350-356-370-376.
     Gale Warning from 10 PM this evening to 1 PM PST Sunday for
     PZZ350-356-376.
     Hazardous Seas Warning until 4 AM PST early this morning for
     PZZ350-356-370-376.