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000
FXUS66 KMFR 132027
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
127 PM PDT Sat Apr 13 2024

.SHORT TERM...Well-defined low pressure spinning offshore of the
north-central California coast early this afternoon. This system
will cause active weather across our forecast area this afternoon
through Sunday evening with numerous showers and also the
potential for thunderstorms. Current radar is fairly benign at the
moment with a corridor of showers extending from the Mt. Shasta
region NW through Jackson and into Douglas/Coos counties. Several
lightning flashes have recently accompanied a cell in NE Jackson
County near Butte Falls and is crossing into SE Douglas County.
Model guidance continues to show broad southeasterly mid-level
flow, which will continue to advect moisture into the area through
this evening. Outside of the showers, high clouds are prevalent
across the area, including over the East Side. This may be
delaying convection somewhat, but surface heating will continue
this afternoon and this will allow the atmosphere to become more
unstable. Steep mid-level lapse rates, modest CAPE and ample
shear supported by the left exit region of a 60-80kt upper level
jet will allow convection to develop in the next couple of hours.
The convection will move NW at a pretty good clip with storm
motions of 25-35 mph. Convection allowing models (CAMs) are
showing at least a slight chance (>15%) of thunder across a large
portion of the CWA (all but the immediate coast and SW Siskiyou
County). But, the highest thunder probabilities (>50%) extend from
northern Modoc into western Lake/Klamath and over to eastern
Douglas County. The Storm Prediction Center has highlighted these
areas as a marginal risk for severe thunderstorms. The stronger
cells in those locations will have the ability to produce strong,
potentially damaging wind gusts, hail and brief heavy rainfall.
Given the strong shear expected and recent model soundings also
showing a decent rotational component, if a supercell were able to
get going, even a tornado cannot be ruled out (2-5% chance). Time
frame of greatest risk: 22-02z (3-7pm). While any thunderstorm
could produce gusty winds and small hail, the most likely areas
for severe thunderstorms will be across northwest Lake/northern
Klamath and into eastern Douglas counties.

This convection should morph into an area of showers and embedded
thunder this evening, eventually pivoting southward across SW
Oregon and becoming a steadier rain area in NorCal overnight. This
area of steadier precip will change to snow in some areas tonight
into Sunday morning as snow levels drop from 5500 feet this
evening to around 4000 feet tonight. How low the snow levels get
will depend somewhat on precipitation intensity. If precip rates
are consistent enough, a changeover to snow may also occur around
Mt Shasta City (after midnight) and wet snow could mix in down to
as low as 3000 feet by Sunday morning. The overall forecast in
those areas remains unchanged, however, with a winter weather
advisory in effect for at least some minor winter impacts,
especially over higher passes by Sunday morning. 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 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. I-5 should be OK, though wet snow
could bring a bit of accumulation on non-paved surfaces.

Most areas will be damp/wet Sunday morning, but the upper low
should finally get a kick eastward Sunday afternoon. Precip
probabilities (highest in the morning 50-80%) west of the
Cascades decrease to 15-40% in the afternoon (highest south end of
Rogue Valley and in the foothills). It`ll remain wet/showery all
day into Sunday evening in NE California and over the East Side
and and there`s still a chance of thunderstorms. Upper ridging
will dry things out Sunday night.

-Spilde

.LONG TERM...PREVIOUS...Sunday through Friday, April 14-19,
2024...(Issued 435 PM PDT Fri Apr 12 2024)

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...13/18Z TAFs...Generally MVFR levels are present across
northern California and southern Oregon this afternoon. Moisture
wrapping around a low pressure system lingering near the California
coast will continue to support active weather. Currently activity is
mostly over Jackson, Douglas, and northern Klamath counties. The
system is expected to move inland through the day, which will focus
most activity on Siskiyou and Modoc counties as well as areas around
the Cascades through the end of the TAF period.

Elevated thunderstorm chances will develop over much of the area
today, with the highest chances (35-45%) along the border of Lake
and Klamath counties. Other east side areas will have a 25-35%
chance of thunderstorms, while lesser 10-20% chances will reach
areas west of the Cascades. The west side chances will disappear
overnight while 15-25% will continue through the end of the TAF
period east of the Cascades. -TAD


&&

.MARINE...Updated 820 AM Saturday, April 13, 2024...A low pressure
system is currently situated near the Bay Area. Light showers are
expected over the Pacific through the weekend, but the threat for
thunderstorms has shifted onshore, and is less than 10% over the
Pacific.

North winds will become more northwesterly throughout today and will
increase in speed. The combination of gusty winds and westerly swell
will result in steep seas with conditions hazardous to small craft
for all areas beyond 10 nm from shore through the weekend.

Conditions improve by Monday morning as the upper low moves further
south and atmospheric stability builds. Improved conditions will be
short lived, however, as a thermal trough develops on Monday
afternoon and could remain in place through midweek or longer. This
pattern would bring another round of gusty north winds, along with
steep to very steep seas, especially south of Cape Blanco by Monday
afternoon. -Miles/Smith


&&

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

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

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

PACIFIC COASTAL WATERS...Small Craft Advisory until 5 PM PDT Sunday
     for PZZ370-376.

&&

$$

MAS/MCB/TAD