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000
FXUS66 KMFR 122334 CCA
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
435 PM PDT Fri Apr 12 2024

...Updated AVIATION discussion...

.SHORT TERM...An upper low will remain positioned off the
northern California coast through Saturday then gradually shift
inland over California Saturday night and Sunday. This low will
bring cooler and showery weather. Unstable conditions will also
result in chances for afternoon and evening thunderstorms through
Saturday. This is followed by rain and snow Saturday night and
Sunday. Thunderstorms chances are highest today and Saturday for
areas east of the Cascades with a chance for strong gusty winds,
hail, and brief heavy downpours with cells. Of note, there is a
marginal risk (2-5%) for severe thunderstorms today over eastern
Lake County and on Saturday over northern/eastern Klamath and
norther/western Lake counties.

Current radar shows a band of light rain showers inland along and
near the Cascades and into central Siskiyou County. East of the
Cascades, there are showers with isolated thunderstorms. High
resolution models show showers increasing across much of the area
this afternoon, with isolated to scattered thunderstorms expected
to develop east of the Cascades. A few isolated thunderstorms are
possible from Cascades west as well. Models continue to support
enough CAPE and shear to support a marginal (2-5%) chance for
severe storms in far eastern Lake County with a potential for
strong winds and hail. This is mainly for areas east of Lakeview
and east of Silver Lake. However, the NAMNest model also indicates
a potential for strong storms slightly further west today, into
western Lake and far eastern Klamth Counties. So, we will be
monitoring this closely. The chance for strong thunderstorms for
eastern Lake county is mainly from 1pm through 7pm today.

Tonight, the focus for precipitation shifts to the coast and
northern California areas. However, precipitation will increase
in Saturday morning, mainly from the Cascades east and Siskiyous
south, then spread across most the area Saturday afternoon and
evening.Models support a risk for thunderstorms on Saturday across
most inland areas, 10-20% chances west of the Cascades and 20-40%
chances east of the Cascades. Additionally, across
northern/eastern Klamath and northern/western Lake counties
Saturday afternoon/evening, models show this area under the exit
region of an upper level jet and indicate sufficient CAPE and
shear to support a marginal risk (2-5%) for severe thunderstorms
with strong wind and damaging hail possible.

Widespread precipitation continues Saturday evening into Sunday.
Snow levels will also lower from 5500-7500 feet Saturday down to
around 4000-5500 feet Saturday night and Sunday morning with
light to moderate snow expected in the mountains and down to some
valleys across eastern Siskiyou and Modoc counties.

.LONG TERM...Sunday through Friday, April 14-19, 2024...An upper
level low spinning offshore near the San Francisco Bay area will
move onshore Sunday morning, then shift into the Great Basin Sunday
afternoon/evening. This will maintain a cool, active weather pattern
across southern Oregon and northern California. Models continue to
show an axis of steady precipitation forming on the northern
periphery of the low in NorCal by Sunday morning with a colder air
mass moving in there. Snow levels beneath this precip axis will drop
to around 4000-4500 feet. As such, we expect precipitation to change
to snow in some areas, with at least minor snow impacts, especially
for portions of southern Siskiyou and Modoc counties. Given recent
mild conditions, this should preclude much more than minor winter
impacts, but we`ve issued a Winter Weather Advisory for down there
for elevations above 4000 feet in Zones 80/82 and above 4500 feet in
Zones 83/85). Snow amounts by Sunday morning (11 am) look to be in
the 2-6 inch range, with some of the higher passes, like Cedar Pass
on Highway 299 east of Alturas possibly receiving a little more than
6 inches. Other passes that are expected to be impacted by wet snow
Sunday morning -- Highway 97 near Grass Lake, Highway 89 near
Snowman Summit/Pondosa and Highway 139 south of Tionesta to Adin.
The higher pass on Sawyers Bar road west of Etna and Highway 3 south
of Callahan could also see some impacts, but I-5 should be OK.
Farther north (and farther away from the upper level cold pool),
snow levels are actually expected to remain above 5000 feet, so snow
amounts look fairly minimal and confined to the higher mountains.

Sunday afternoon, shower chances remain highest from the Cascades
eastward with PoPs generally in the 40-70% range, except up above
80% in Modoc and eastern Lake counties due to their closer proximity
to the low. Snow levels in NorCal should rise some again Sunday
afternoon to above 5000 feet. In addition, there is still a slight
chance (20%) of thunderstorms over the far East Side. PoPs diminish
(to 10-30%) Sunday afternoon west of the Cascades as heights begin
to rise; some sunny breaks are probable as well. Isolated showers
probably linger into Sunday evening over the far East Side. High
temperatures Sunday will be at least a few degrees below normal in
most cases, but as much as 10-15F below normal in NorCal.

We expect things to calm down Sunday night through Monday night as
the low finally exits to the east and an upper ridge noses in. A few
sprinkles/flurries are possible (20% chance) in far northern
sections of Douglas County, but most likely it will be dry. The rest
of the region will be dry with moderating temperatures, but still
remaining at least a few degrees below normal Monday afternoon.
While it looks to remain above freezing west of the Cascades, much
will depend on cloud cover Monday night; low temperatures could be
in the mid 30s in the valleys west of the Cascades.

Tuesday onward appears to be a largely dry period, but with upper
level pattern variations that make the temperature forecast a little
less certain. Initially, an upper trough swinging through to the
north on Tuesday may make for some breezy conditions during the
afternoon, but precipitation chances remain low and mostly to our
north. It could get chilly Tuesday night with patchy/areas of frost
by Wednesday morning in some valleys west of the Cascades. The
overall trend after that is toward milder temperatures, with the
latest model clustering showing some semblance of upper ridging, a
thermal trough and a period of offshore flow. NBM indicates high
temperatures getting back to above normal levels as early as
Wednesday, but then especially Thursday/Friday with highs in the mid
to upper 70s west of the Cascades. -Spilde


&&

.AVIATION...13/00Z TAFs...MVR levels continue under widespread cloud
cover and southerly winds from a low pressure system over the
Pacific Ocean. More showers are appearing on radar, especially east
of the Cascades. The intensity of individual cells does seem to be
dropping, with most lightning staying outside of this coverage area.
Elevated chances (15-30%) of thunderstorms over Lake, Modoc, and
eastern Klamath counties is still in the forecast for this evening,
but these chances clear out quickly overnight.

Chances for precipitation (60-90%) and thunderstorms (20-40%) return
to areas east of the Cascades on Saturday, with the highest
thunderstorm chances along the border of Klamath and Lake counties
on Saturday afternoon and to the end of the TAF period. There will
be lesser chances for showers (60%) and thunderstorms (15-20%)
through the day Saturday for areas west of the Cascades. -TAD

&&

.MARINE...Updated 200 PM Friday, April 12, 2024...A low pressure
system will continue moving south over the Pacific, bringing
elevated northerly winds over marine areas. This evening, the
strongest winds will stay over waters north of Cape Blanco. Through
tomorrow, gusty winds will continue over all waters between 10 and
60 nm from shore. Light showers are expected over the water through
the weekend, with a slight (10-15%) chance of thunderstorms late
this evening into early Saturday morning over waters west of Cape
Blanco.

The combination of gusty northerly winds and westerly swell will
build steep seas in all waters from 10 nm to 60 nm from shore
through the weekend. A Small Craft Advisory is in place from 5 PM
this evening through Sunday evening at 5 PM to communicate these
conditions. Isolated periods of very steep and hazardous seas are
possible in the outermost waters, and steep seas will occasionally
develop in waters from the shore to 10 nm.

Seas are expected to calm on Monday morning as the upper low moves
further south and atmospheric stability builds. The calm period will
be short lived, as a thermal trough is expected to develop on Monday
afternoon and could remain in place through midweek or longer. This
pattern would bring more gusty northerly winds that would be limited
to areas south of Cape Blanco. Current guidance suggests fluctuating
areas of steep to very steep seas developing by early Monday
afternoon. While there is not yet enough confidence in the exact
timing and the conditions to issue any products, some amount of
chaotic seas looks likely for next week especially south of Cape
Blanco. -TAD

&&

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

CA...Winter Weather Advisory from 8 PM Saturday to 11 AM PDT Sunday
     above 4000 feet for CAZ080-082.

     Winter Weather Advisory from 8 PM Saturday to 11 AM PDT Sunday
     above 4500 feet for CAZ083-085.

PACIFIC COASTAL WATERS...Small Craft Advisory from 5 PM this
     afternoon to 5 PM PDT Saturday for PZZ350-370-376.

&&

$$

CC