744
FXUS66 KMFR 310619
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
1019 PM PST Thu Jan 30 2025

...Updated the aviation section...

&&

.AVIATION...31/06Z TAFs...Along the coast and just offshore, MVFR
ceilings are predominant with an approaching front, but patches of
IFR are likely. The front will bring rain with moderate to strong
winds and wind shear at the coast, including North Bend overnight
through Friday morning. A showery air mass will follow on Friday
afternoon through Friday night, with winds diminishing slightly but
continued MVFR.

Inland, VFR conditions are expected to continue into the late night
hours of tonight. A front will then move inland across the area from
around 10Z through Friday afternoon, resulting in lowering ceilings
(mainly to MVFR), mountain obscuration, and precipitation. The
incoming system will bring snow to the Cascades and portions of the
east side, with snow levels around 4500 feet to 5000 feet msl during
Friday into Friday night. -DW

&&

.MARINE...Updated 800 PM Thursday, January 30, 2025...Rain is about
to reach the waters. Winds will continue to increase into Friday
morning as a cold front approaches. Small craft conditions will
continue for the remainder of this evening. Thereafter, late tonight
into Friday morning, gales and very steep wind driven seas will
accompany the front.

Late Friday morning, behind the front, the gale force winds shift
southwest and gradually diminish. However, they will still remain
strong enough for Small Craft Advisory conditions late Friday
morning into Friday evening. A colder air mass aloft will move over
the northern Waters Friday afternoon with 500mb temperatures between
-25 and -27C, therefore could not rule out an isolated thunderstorm
or two, mainly north of Cape Blanco.

Though light to moderate west to northwest winds will diminish for
the weekend, steep seas are expected through Saturday night. This is
due to a fresh northwest swell train. Low pressure will persist
early next week with swell dominated seas expected to diminish
slightly.

-Petrucelli/DW

&&

.PREV DISCUSSION... /Issued 927 PM PST Thu Jan 30 2025/

DISCUSSION...

Plenty of high clouds are spreading over the forecast area with
some echos along the coast as well. Surface observations have not
reported any precipitation as of writing this AFD, although that
should change within the next few hours as EKA`s radar KBHX shows
plenty of precipitation off their coastline. Snow will begin to
accumulate in the mountains above 5000 feet overnight with the
most accumulation occuring in the Cascades. Overall, this will be a
high impact winter storm in the high Cascades due to snow
accumulation and winds, but lower impact for all other areas
through Friday as snow levels are generally 5000 feet through the
day.

Read the previous discussion below for more information on Sunday
into the start of next week when we`ll be in a much cooler
airmass and under threat for valley snow.

PREV DISCUSSION... /Issued 203 PM PST Thu Jan 30 2025/

DISCUSSION...Marine layer clouds had pushed into the coast this
morning, and cirrus clouds have begun to overspread the region
this afternoon. These are big flashing warning signs that things
are about to change for southern Oregon and far northern
California. That change will be a shift from a dry, high pressure
dominated pattern, to a wet and active pattern that could produce
snow all the way down to the coast by early or mid next week.

The ridge that has controlled our weather for so long now is
finally moving to the east, and releasing its hold on the West
Coast. A trough originating from Alaska will take advantage of
the absent ridge, digging south along the western coast of North
America through the next several days, eventually taking up
position off the coast of Oregon by early next week. This will
bring a front with a rich moisture plume into the region, which
after producing a good shot of rain in our area over the next 48
hours, will then stall out to our south. Where exactly this front
stalls will be of great importance to the forecast Sunday into the
first part of next week, determining where the rain will be,
where the cold air will be, and how low in elevation the snow will
be.

Until then, we will deal with the passage of the front itself
beginning tonight. Rain chances increase sharply along the coast
this evening, and will then spread inland overnight with snow
developing over the mountains and in the Mount Shasta region. A
Winter Storm Warning is in effect for the Cascades, where the
heaviest snowfall and snowfall rates will be. Two periods of
moderate to heavy snowfall are expected, one between 10 am and 4
pm Friday and another between 10 pm and 4 am (or so) Friday night
into Saturday morning. It is during these time periods that
snowfall rates could reach 1"/hr at times. All told, snow amounts
near and above 5000 feet in the Cascades/Siskiyous will be 1 to 2
feet with some higher amounts possible. Gusty winds will accompany
the snow, so expect some blowing and perhaps drifting of snow
along with reduced visibility. Travel through the Cascades could
become treacherous for the Friday morning commute. Elsewhere, Winter
Weather Advisories are in effect for the Mount Shasta region and
western Siskiyou County above 5000 ft, and for portions of the
East Side and the Warner Mountains for late tonight through
Saturday morning. Lower elevations in these areas (generally
4000-4500 feet), at least where most roadways are and where people
live, will have a couple to several (2-8") of snow with higher
amounts across the higher terrain. This includes Highway 89 near
Snowman Summit and Dead Horse Summit. Snow levels could fluctuate
downward to include I-5 near Black Butte Summit or even around
3500 feet near Mt Shasta City if precipitation intensity is higher
Friday morning, so be aware of that risk. Since winds will also
accompany the snow, we rolled the wind potential into the Winter
Weather Advisories. Wind gusts of 45-55 mph will be common tonight
through Friday. This in combination with the snow could cause
difficult driving conditions due to blowing snow and lowered
visibilities.

It will also be windy along the coast and in the Shasta Valley.
Pressure gradients peak in the -6 to -7 mb range OTH-ACV and also
MFR-RDD. The OTH-ACV gradient is typically associated with sub-
warning level winds (so we have not issued any headlines), but
wind gusts at the beaches could be in the 45-55 mph range.
Exposed headlands and capes could have some peak gusts of 60+
mph. MFR-RDD gradient is often associated with advisory level
south to southeast winds in the Shasta Valley. Peak mid-level
winds of ~50 kt also are a proxy for gusts of around 50 mph or so.
As such, a Wind Advisory is out for the I-5 corridor from Grenada
south to Weed.

The first significant measurable rainfall in almost 3 weeks (very
long stretch for late January) is expected along the coast and
inland over the west side. Amounts of 1-3 inches will be common,
with the highest amounts in the coast ranges of Curry, western
Josephine and western Siskiyou counties. Inland valleys will have
less, with 0.25-0.75 of an inch. Given the lack of recent rain,
and with more than enough capacity in area rivers and streams,
flood concerns will be confined to nuisance or urban areas, if any
at all.

After this main front pushes to south Saturday, lingering showers
will continue with relatively light precipitation amounts. The
front will stall to our south as it becomes parallel to the upper
level flow. After that, a closed low to the north along the BC
coast will wobble southward to just off the PacNW coast Sunday
into Monday allowing colder air to bleed southward. Meanwhile, a
plume of subtropical moisture with high IVT will take aim at NW
California and ride along the stalled front. Much will then depend
on how far south the front and main precipitation belt/plume
travels, where and how it sets up once it stalls out, and the
strength of any shortwaves that decide to traverse overhead at the
same time. There are still many scenarios that could play out
here, so confidence in the details is still low, but is improving
as the wide range of model solutions gradually trend closer.

Some ensemble members show the plume setting up in a zonal, west-
to- east fashion lying along a line from San Francisco to Reno,
placing us well within colder air to the north and keeping all of
the enhanced precipitation well to our south. Other solutions keep
the plume farther north with more of a southwest to northeast
orientation, somewhat warmer temperatures/higher snow levels and
the main belt of precipitation focused on Siskiyou/Modoc counties
into southern Lake/Klamath counties, with some spill over to areas
north of the Siskiyous. Other models lie somewhere in between.
Overall, the solutions have trended towards the southern
solution, and this is what is reflected in the current forecast
package.

With the front stalled to the south and a substantial pool of
colder air filtering into the region from the closed low, we will
find ourselves with a sharp temperature and snow level gradient
across the area. Be aware, this is a very difficult forecast, as
the exact placement of this gradient will have large impacts to
where the snow will fall, and any shift north or south be even
small distances will mean significant changes to the forecast.
Current thinking is that snow levels could range from over 5000
ft in Modoc County, to around 1000 ft in Douglas County by Monday,
and these snow levels are likely to gradually lower through
Wednesday, although they could wobble north and south at time.
This means that it is possible that at the same time there is rain
in Alturas, it could be snowing in Roseburg. Yes, snow levels are
expected to lower to the valley floors of the West Side Monday
night into Tuesday and, if not then, possibly again Tuesday night
into Wednesday. This will bring a couple of chances at low snow,
perhaps even with a wintry mix all the way down to the beaches at
some point next week. Keep checking back here for updates. -BPN

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory until 10 AM PST Saturday for ORZ029>031.

     Winter Storm Warning until 10 AM PST Saturday for ORZ027-028.

CA...Winter Weather Advisory until 10 AM PST Saturday for CAZ080-082-
     083-085.

     Wind Advisory until 7 PM PST Friday for CAZ081.

PACIFIC COASTAL WATERS...Gale Warning until 10 AM PST Friday for
     PZZ350-356-370-376.

     Small Craft Advisory from 10 AM Friday to 4 AM PST Sunday for
     PZZ350-356-370-376.

&&

$$