Area Forecast Discussion
Issued by NWS Medford, OR

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000
FXUS66 KMFR 280423
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
923 PM PDT Wed Mar 27 2024

.DISCUSSION...A cold front is making its way through Modoc and
east Lake counties this evening, with the frontal precipitation
band starting to make its way out of the region and winds coming
down behind it. A large low- pressure system well offshore of
southern BC/northern WA is providing plenty of moisture and lift
for numerous showers this evening, however. At the moment, these
are most prolific along the coastal counties into Douglas. Cold
air aloft behind the front has destabilized the atmosphere enough
that there are thunderstorms mixed in with showers. Any one
thunderstorm can produce gusty winds, small hail, periods of heavy
rain, and dangerous lightning
- seek shelter indoors if you hear thunder.

Chances for thunderstorms will continue tonight into Thursday,
with the highest chances along the coast (25-35% chance for any
one area to see a thunderstorm this evening and early tonight
decreasing slightly to 20-30% Thursday morning into the evening)
extending into inland areas west of the Cascades late Thursday
morning for much of the day (15-20% chance).

Besides thunderstorms, cold air behind the front will result in
snow levels dropping tonight into the early hours Thursday to
around 3,000 feet, before rising again in the afternoon to
3,500-4,000 feet or so. This would mean some snow accumulations
along I-5 over Siskiyou Summit tonight and between Weed and
Dunsmuir in northern CA Thursday morning are possible. Total
amounts are expected to be generally light (less than half an inch
for the Mt. Shasta City area and an inch or two over Siskiyou
Summit) but if travelling tonight or Thursday morning take extra
care and be prepared for winter weather. Light, intermittent snow
over lower elevation areas east of the Cascades is also expected
overnight.

Additionally, heavier snow over the mountains will continue, with
travel conditions expected to remain hazardous. Winter Weather
Advisories for the Cascades as well as western and east-central
Siskiyou County above 3,000 feet continue through late Thursday
evening. For more details on the forecast, please see the
discussions below. -CSP


&&

.AVIATION (28/00Z TAFs)...Showers are continuing behind a
cold front, bringing a mix of MVFR/low end VFR conditions and
terrain obscurations in showers. Otherwise, conditions will remain
VFR across most of southern Oregon and northern California.
Meanwhile, gusty winds will persist today with a wind shift expected
as the front passes through the region.

There is about a 20% chance of thunderstorms over the coastal
waters, along the coast and inland west of the I-5 corridor. The
best chance for storms will will be in the evening with small hail
and gusty winds a realistic threat in these storms.

By tonight, showers will remain in the region with the heaviest
precipitation along the coast. MVFR ceilings will likely persist
overnight for locations along the coast. Farther inland, it seems
VFR ceilings remain more likely. -Schaaf/BR-y


&&

.MARINE...Updated 700 PM Wednesday, March 27, 2024..Gusty west to
southwest winds along with very steep and hazardous seas will
continue this afternoon with showers and isolated thunderstorms
expected through this evening. Southwest winds will increase this
evening and remain elevated through the day Thursday. Winds will
likely reach gale force north of Cape Blanco on Thursday, and the
likelihood is enhanced due to shower and thunderstorm activity
expected Thursday. Any of the stronger storms could potentially
produce brief periods of strong gale force gusts with winds reaching
40 to 50 kt. This potential exists south of Cape Blanco as well, but
the threat seems more isolated than north of Cape Blanco. Due to the
more widespread potential and borderline sustained wind speeds, have
opted to issue a Gale Warning for Thursday morning through the
evening for all areas north of Cape Blanco.

Meanwhile, a heavy west swell will build into the waters tonight
into Thursday resulting steep to very steep seas. Seas will be
steepest (16 to 19 ft at 13 to 14 seconds) north of Cape Blanco
where moderate to strong south winds will lead to higher wind driven
seas combined with this heavy swell. South of Cape Blanco, seas will
be more swell dominated and less steep, but still high as seas peak
around 15 to 17 ft.

Winds diminish Thursday night into Friday, and seas will gradually
subside, though likely remain steep into the afternoon. Conditions
could improve briefly during the afternoon Friday before the pattern
changes and winds become northerly Friday evening, then increase on
Saturday. These northerly winds are likely to stick around into the
first half of next week, resulting in periods of steep wind driven
seas, especially south of Cape Blanco. /BR-y


&&

.PREV DISCUSSION... /Issued 604 PM PDT Wed Mar 27 2024/

SHORT TERM...Through Sunday morning (3/31)...Active weather will
continue through tonight and into Thursday as a low pressure
system spins over the Pacific Ocean. As of this afternoon, the
main band of precipitation is over Jackson eastern Douglas
counties. Heavy snowfall continues over the Cascades with
scattered showers getting past the higher terrain and moving over
east side areas. Gusty winds continue in the Shasta Valley and
Lake County, with speeds of 50 mph observed by local stations.
Partly cloudy skies persist behind the band, with radar picking up
light activity over Coos and Curry counties. In general,
widespread precipitation and winds should both calm overnight,
especially for low lying areas. Wind Advisories for the Shasta
Valley until 3 PM and for Lake County and the Warner Mountains
until 8 PM will continue but are unlikely to be extended beyond
those times.

With the low pressure system associated with today`s fronts content
to continue spinning over the Pacific, southwesterly flow aloft will
continue to support elevated precipitation chances across the area.
For most areas in lower terrain, any further precipitation should be
unimpactful. The highest rainfall totals will continue along the
Oregon coast, with coastal cities getting 0.5 to 1 inch while higher
elevations will get up to 2 inches of rainfall. Chances for
thunderstorms will also develop along the Oregon coast tonight,
peaking at 25 to 30% early Thursday morning. Chances will continue
through the day Thursday, decreasing slightly (20-25%) by Thursday
afternoon and lesser chances (15-20%) developing inland. These
chances will remain into Thursday night.

Current snow levels of 4000 to 5000 feet will drop to about 3000
feet overnight, then rise to 4000 feet through the day Thursday.
While dropping snow levels are expected to bring snowfall to areas
east of the Cascades, amounts look to be negligible over all but the
highest terrain. Expected daytime highs in the low to mid 40s should
help to prevent accumulation in most areas.

Higher terrain west of the Cascades may continue to see impactful
snowfall given the moist flow aloft and dropping snow levels. While
the 1 inch per hour rates forecast over the Cascades today are
unlikely without a cold front to give extra uplift, persistent
snowfall could still bring high totals to the Cascades and the Mount
Shasta area as well as western Siskiyou County. For this reason,
existing Winter Weather Advisories for the Cascades north of Highway
140 and the Mount Shasta area will be extended through Thursday at
11 PM. An additional Winter Weather Advisory is in place from
tonight at 11 PM through Thursday at 11 PM for areas of western
Siskiyou County expected to get steady snowfall through the day
Thursday.

Impactful activity looks to sharply decrease Friday as the low
pressure system starts to move to the south over California.
Moderate precipitation chances (25-50%) will remain over most areas,
although only light rain or snowfall is expected where showers do
form. Lower single-digit precipitation chances are expected for the
Rogue and Shasta Valleys as well as the Klamath Basin. As flow aloft
shifts to southeasterly late Friday into early Saturday, some slight
(20-40%) chances for precipitation will continue over Siskiyou and
Modoc counties. The path of the system could either increase or
decrease these chances, but right now any activity through
Saturday and Sunday morning is expected to be light if any occurs
at all. -TAD

Extended Discussion...(Sunday through Tuesday)...There`s sufficient
evidence supporting dry and milder weather Sunday through Tuesday of
next week.

Most locations will be dry Sunday. However residual wraparound
moisture from a departing upper low southeast of our area could
bring a few showers in portions of southeast Modoc County Sunday
morning. Sunday afternoon will be dry for all areas.

An upper ridge will build in on Monday with the upper ridge axis
over our area Monday afternoon. This will lead to dry and milder
weather. Skies will be clear, although could not rule out low clouds
in portions of the Umpqua Basin, Illinois Valley and Grants Pass
area due ample ground moisture from the active weather today through
Friday.

Dry weather will continue through Tuesday with the ridge axis
shifting east and at the same time flattening out as an upper trough
approaches from the west. There are some differences among the
operational models regarding the strength and timing of the upper
trough. The GFS is showing a stronger upper trough, but the ensemble
mean shows a solution that resembles the operational ECMWF which is
weaker. The majority of the ECMWF individual ensemble means lean
towards dry, but some members show very light precipitation amounts.
The individual GFS ensemble means show a mixed bag of dry and wet.

For now, we`ll keep a slight chance of precipitation along the
coast, portions of Douglas County and Cascades on Wednesday.

Beyond next Wednesday, the evidence is pointing towards troughiness
over the Pac NW the latter part of next week, leading to more cool
and unsettled weather. Stay tuned. -Petrucelli

AVIATION (28/00Z TAFs)...Showers are continuing behind a
cold front, bringing a mix of MVFR/low end VFR conditions and
terrain obscurations in showers. Otherwise, conditions will remain
VFR across most of southern Oregon and northern California.
Meanwhile, gusty winds will persist today with a wind shift expected
as the front passes through the region.

There is about a 20% chance of thunderstorms over the coastal
waters, along the coast and inland west of the I-5 corridor. The
best chance for storms will will be in the evening with small hail
and gusty winds a realistic threat in these storms.

By tonight, showers will remain in the region with the heaviest
precipitation along the coast. MVFR ceilings will likely persist
overnight for locations along the coast. Farther inland, it seems
VFR ceilings remain more likely. -Schaaf/BR-y

MARINE...Updated 200 PM Wednesday, March 27, 2024..Gusty west to
southwest winds along with very steep and hazardous seas will
continue this afternoon with showers and isolated thunderstorms
expected through this evening. Southwest winds will increase this
evening and remain elevated through the day Thursday. Winds will
likely reach gale force north of Cape Blanco on Thursday, and the
likelihood is enhanced due to shower and thunderstorm activity
expected Thursday. Any of the stronger storms could potentially
produce brief periods of strong gale force gusts with winds reaching
40 to 50 kt. This potential exists south of Cape Blanco as well, but
the threat seems more isolated than north of Cape Blanco. Due to the
more widespread potential and borderline sustained wind speeds, have
opted to issue a Gale Warning for Thursday morning through the
evening for all areas north of Cape Blanco.

Meanwhile, a heavy west swell will build into the waters tonight
into Thursday resulting steep to very steep seas. Seas will be
steepest (16 to 19 ft at 13 to 14 seconds) north of Cape Blanco
where moderate to strong south winds will lead to higher wind driven
seas combined with this heavy swell. South of Cape Blanco, seas will
be more swell dominated and less steep, but still high as seas peak
around 15 to 17 ft.

Winds diminish Thursday night into Friday, and seas will gradually
subside, though likely remain steep into the afternoon. Conditions
could improve briefly during the afternoon Friday before the pattern
changes and winds become northerly Friday evening, then increase on
Saturday. These northerly winds are likely to stick around into the
first half of next week, resulting in periods of steep wind driven
seas, especially south of Cape Blanco. /BR-y

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory until 11 PM PDT Thursday above 5500 feet
     for ORZ027-028.

CA...Winter Weather Advisory until 11 PM PDT Thursday above 3000 feet
     for CAZ080.

     Winter Weather Advisory until 11 PM PDT Thursday above 5000 feet
     for CAZ082-083.

PACIFIC COASTAL WATERS...Small Craft Advisory until 11 PM PDT
     Thursday for PZZ356-376.

     Hazardous Seas Warning until 5 AM PDT Thursday for PZZ350-370.

     Gale Warning from 5 AM to 11 PM PDT Thursday for PZZ350-370.

&&

$$


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