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000
FXUS66 KMFR 131807
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
945 AM PDT Sat Apr 13 2024

...Updated AVIATION Discussion...

.DISCUSSION...Satellite imagery shows an upper level low spinning
offshore of the central California coast. This system will cause
an active weather day across our forecast area today with numerous
showers and also the potential for thunderstorms. Current radar is
showing a fairly large area of showers extending from western
Modoc/eastern Siskiyou County northward into Klamath County to
near the Cascades. Another area of showers is affecting the
immediate coast and coastal waters. These areas will continue to
move off to the northwest through the remainder of the morning
hours. Model guidance is showing broad east- southeasterly mid-
level flow, which will continue to advect moisture into the area
through this evening. Right now, clouds are prevalent across the
area, but we do expect some cloud thinning late this morning,
especially over the East Side, allowing for stronger surface
heating. 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, probably in the 20-22z (1-3pm)
time frame. These will move NW at a pretty good clip with storm
motions of 35-45 mph. Convection allowing models (CAMs) this
morning 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, by far the CAMs are highlighting areas
from northern Modoc into western Lake/Klamath and over to eastern
Douglas County (where thunder probabilities exceed 50%). Over
there, the Storm Prediction Center has 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, a tornado could not be
ruled out (5% chance). This appears most likely in the 22-01z
(3-6pm) time frame.

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. Wet snow could mix in down to
as low as 3000 feet by Sunday morning. The forecast for those
areas remains unchanged with a winter weather advisory in effect
for at least some minor winter impacts, especially over higher
passes by Sunday morning. See previous discussion for those
details.

The upper low should finally get a kick eastward on Sunday
afternoon as precip probabilities decrease (to 15-30%) west of the
Cascades. It`ll remain wet/showery in NE California and over the
East Side into Sunday evening, however, and there`s still a
chance of thunderstorms. Upper ridging will dry things out Sunday
night. -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


&&

.PREV DISCUSSION... /Issued 639 AM PDT Sat Apr 13 2024/

DISCUSSION...A large upper low pressure system, the current
driver of our weather, is currently positioned offshore of the CA
Bay Area. This low will send waves of showery (on-and-off)
precipitation into our region as it slowly spins southeast. Per
radar imagery this morning, we`ve currently got one elongated band
along the coast and another moving into southwest Siskiyou County.
The low will be in a better position today to lead to more
widespread showers across the region today compared to yesterday
(Friday).

Along with that, unstable conditions will also result in a chance
for afternoon and evening thunderstorms across the majority of
the region today. Chances are highest (20-40%) from the east
Cascade foothills into Klamath and Lake counties in OR and Modoc
and eastern Siskiyou counties in CA. Conditions conducive to
stronger thunderstorms (CAPE and shear) means there`s marginal
risk (2-5%) for severe thunderstorms over northern/eastern Klamath
and northern/western Lake counties. This includes the potential
for strong, gusty winds and hail in these stronger thunderstorms.
Thunderstorms could occur at any point in the afternoon into the
late evening, but the highest chances in the area are in the late
afternoon/early evening.

Thunderstorm chances decrease west of the Cascades, but there`s
still a 10-20% chance for a thunderstorm at any one point (higher
towards the Cascades, lower as you head west towards the coast).
With the low positioned so that the storm motion is out of the
southeast, this is the kind of pattern that results in rain
showers and thunderstorms actually making it into the Rogue
Valley.

Potential for showers will continue Sunday, though the area where
showers are expected will slowly shrink to the east as the low
progresses southeast. Snow levels will gradually lower today into
the overnight period, generally from the southwest to the
northeast, from 5,500-7,500 feet to 4,000-6,000 feet. Light snow
is expected down the valleys across Modoc and eastern Siskiyou
counties, with heavier snow in the mountains of wider Siskiyou and
Modoc above 4,000-4,500 feet. 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. -CSP

PREVIOUS LONG TERM DISCUSSION...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

&&

.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/MAS/MAS