Winter Storm Warning 4, NWS Medford, OR

Area Forecast Discussion
National Weather Service Medford OR
836 AM PDT Sat Apr 9 2022

.DISCUSSION...The forecast remains mostly on track, a few
adjustments were made to the marine forecast however, the big
picture remains similar. Additionally, a few minor tweaks were
made to the cloud forecast going on right now to better match
satellite imagery.

Overall, expect today to be relatively benign if much chillier
than previous days. It is worth noting that temperatures may be a
few degrees warmer than what is in the forecast in areas that are
sunny, but the forecast should not be off by that much. More
details regarding our forecast are available in the previous
discussion below. -Schaaf

&&

.MARINE...Updated 800 AM PDT Sat, 9 April 2022...Gusty north
winds will persist today, strongest south of Cape Blanco where very
steep seas are expected. Winds and seas will lower some tonight into
Sunday but will remain steep. Another front is expected late Sunday
into Monday with very high seas possible, and confidence is growing
in the potential for gale force winds in at least some of the
coastal waters. We've issued a Gale Watch for all the coastal waters
for Sunday night. Latest data suggests the best potential for gales
is from Cape Blanco northward, but there is still significant
uncertainty in location and exact magnitude of winds. Gusty
northwest winds and steep seas could last into the middle of next
week during an active weather pattern. Keene

&&

.PREV DISCUSSION... /Issued 436 AM PDT Sat Apr 9 2022/

SYNOPSIS...One final day of weak synoptic forcing resulting in
light showers north of the Umpqua divide, from the Cascades west,
will prime the atmosphere for a low to bottom out as it approaches
the PNW coast. Sunday night through Monday, this bottoming low
will bring winter impacts in the form of wet-heavy snow from high
mountain passes down to low elevations, very gusty winds across
the whole region, freezing overnight temperatures, and a moderate-
to-high chance for wetting rains even east of the Cascades. An
active pattern is expected to continue the remainder of the week.

DISCUSSION...Model solutions have begun to converge onto a
similar solution for the time period of Sunday night through
Monday night. The discussion will focus on this time period and on
today.

Today, Saturday April 9, diffuse energy being wrapped into the
backside of a trough, which is redeveloping in the lee-side of the
Rockies will cause some of the energy to drift over the region
today. This will continue a slight chance of light rain showers
and elevated afternoon winds.

The light showers are expected north of the Umpqua divide, from
the Cascades west. Yesterday these showers resulted in 0.02 inches
in the Cascades to 0.14 inches in the north coast range, and
roughly 0.1 inch in the Coos County low lands. Light amounts were
also reported along the coast in Curry county. It seems reasonable
that similar amounts will accumulate today. Winds will again be
breezy in the afternoon west of the Cascades and on the gustier
side to the east; where blowing dust will again be possible down
wind of dry lake beds.

The wrapping of energy into the low to the east reinforces
uniform northwest flow throughout the entire troposphere. A
stacked jet of 150 kts in the upper levels will serve as the
source of energy for an area of low pressure to develop under and
intensify as it moves towards the PNW. The low will begin to
bottom out between 990 to 1005 mb Sunday night with increasing
confidence of landfall between Cape Blanco and the OR/WA state
line. However this consensus has only developed in the latest
model runs and, given the high variability in previous runs, we
are holding off on any advisory/warning products to see if this
converging solution holds.

Assuming the solutions continue to converge on a deepening low
making landfall along the central or northern Oregon coast, we can
expect an impactful period of wet, snowy, and windy weather from
Sunday night through Monday, April 11. Impacts will range from
heavy, wet snow accumulating above roughly 2500 feet in eastern
Douglas County, northeastern Jackson County, the coast range &
western Siskiyou County, and the Cascades that near or exceed one
foot. Gusty south winds at the coast from a likely southerly jet,
and unyielding gusty west winds will also be present. Combined,
the winds and snow could really drive some hazardous conditions as
the snow will be likely to stick to tree limbs and other vertical
structures, while the winds push on the now weighed down
structures. This has the potential to cause damage in the form of
downed limbs, trees, power lines and hazardous driving conditions.

Additional hazards may also be present, though are not
necessarily included in the forecast at this time because they
tend to be more isolated in nature or where they might exist is
still unknown. However, we can describe what they may be. Like,
convective showers producing accumulating amounts of ball bearing-
like graupel that can create very slick surfaces and even a low
chance for a lightning strike or two. Or, that this storm track
favors the influx of an arctic airmass that could bring overnight
lows Monday and Tuesday night 5 to 10 degrees below normal; and
below freezing in many west side valleys. There is also the
generalized impact of heavy rains and wind that won't bring much
impact to river conditions, but will make driving more hazardous.

As mentioned earlier, these impacts are conditional upon an
intensifying low moving onshore north of Cape Blanco. There still
remains some solutions that bring a weaker low south of Cape
Blanco. This would result in a drier but colder event, with
overall less precipitation due to the more offshore flow overhead
and the absence of stronger dynamics (thus weaker winds overall).
However, snow levels would be much lower, and this would result
in the potential for valley snow (although the prospects for any
of it sticking are highly questionable).

It is important to recognize the possibility of this alternative
solution because the forecast reflects a low moving onshore north
of Cape Blanco but, we do not want to advertise that this is a
sure thing just yet.   -Miles

A note from the previous discussion on the extended forecast:
The "fun" aspect of this forecast is that this is the first low in
what appears to be a generally cooler and wetter pattern that will
persist throughout the remainder of the forecast term, and with
such low confidence in the forecast for this first system, it is
difficult to nail down any confidence in the following systems
within this new pattern. As a result, have leaned leaned heavily
on the NBM guidance for the remainder of the week, keeping
temperatures cooler than normal for this time of year, and
maintaining at least a slight chance of precipitation throughout.
-BPN

AVIATION...09/12Z TAFs...VFR conditions are expected to prevail
through most of the TAF period (through this evening), though areas
of MVFR cigs are possible in West Side valleys like the Rogue and
Umpqua, at times, through this morning. Gusty are expected to return
again this afternoon. By late tonight, rain, MVFR ceilings, and
mountain obscuration will reach the coast and Coast Range as a front
pushes into the forecast area. Keene

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...
CA...None.

Pacific Coastal Waters...
     Gale Watch from Sunday evening through Monday morning for
     PZZ350-356-370-376.
     Small Craft Advisory until 11 PM PDT Sunday for
     PZZ350-356-370-376.
     Hazardous Seas Warning until 11 PM PDT this evening for
     PZZ356-376.