351
FXUS66 KMFR 031537
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
837 AM PDT Fri May 3 2024

.DISCUSSION...The forecast for northern California and southern
Oregon remains on track for today. An approaching low pressure
system will bring rain to the Oregon coast later this afternoon,
with 80-100% chances for rain over Coos, Curry, and western
Douglas counties. Lesser 50-80% chances are forecast for Josephine
and eastern Douglas counties through tonight. The system will move
eastward through the weekend, with winter weather impacts expected
over the Cascades and western Siskiyou County and Advisory-level
winds for parts of Lake County on Saturday.

Please see the previous discussion for more details about the
short- and long-term forecasts. -TAD

&&

.AVIATION...03/12Z TAFs...Areas of LIFR/IFR/MVFR ceilings are
present in the Coquille Valley up to North Bend and over into the
Umpqua Valley near Roseburg. The lower conditions will lift to VFR
this morning, but drop back to MVFR with local IFR cigs/vsbys as a
potent cold front moves in with periods of moderate to occasionally
heavy precipitation this afternoon. VFR will prevail elsewhere
inland through late this afternoon/early this evening.

The front will spread moderate precipitation across the rest of the
west side tonight, reaching east of the Cascades Saturday morning.
Expect widespread MVFR conditions and local IFR in rain with higher
terrain obscured in rain/snow. A rapid drop in freezing levels will
occur overnight (~09-15Z) from west to east and especially around
12Z Saturday morning near the Cascades. Rain will change to snow
down to as low as 2500 feet with brief heavy snow in the mountains,
reducing visibility to LIFR at times. Breezy south winds are
expected with the front too, with peak gusts in the 40-50 mph range.
Despite the increasing winds, low level wind shear is not a concern
at this time.

Conditions should improve a bit behind the front with precipitation
becoming more showery, but still with a good amount of MVFR
ceilings/visibility. -Spilde

&&

.MARINE...Updated 800 AM Friday, May 3, 2024...A cold
front will move through today into this evening with moderate to
heavy rain, gusty south winds and steep seas. Following the front,
seas will remain steep, dominated by fresh short period west swell
Friday night into Saturday morning. Showers continue with a slight
chance of thunderstorms (20%) on Saturday. West seas trend less
steep Saturday afternoon through Sunday. Another front arrives on
Monday with the potential for gusty south to southwest winds
followed by building west-northwest seas Monday night into Tuesday.
After that, we expect high pressure to build in mid-late next week
with a thermal trough strengthening along the coast. This will bring
a return of stronger north winds and steep seas, highest south of
Cape Blanco. -Spilde


&&

.PREV DISCUSSION... /Issued 501 AM PDT Fri May 3 2024/

DISCUSSION...Satellite imagery is showing some marine stratus at
the coast and in the Umpqua valley of Douglas County with some
higher clouds beginning to move toward the coast. This is part of
a more dynamic low pressure system that will be working its way
through southern Oregon and northern California today into
Saturday night. This low is not particularly strong, but it could
bring some relatively significant precipitation to the area.
Precipitation will begin at the coast and spread inland throughout
the day. With the low tracking in right overhead, initial amounts
may be impacted by terrain influences (specifically downsloping
causing less precipitation to reach the ground for inland valleys
and upsloping at the coast and coast range to increase the
precipitation). However, as the low moves across the area, the
terrain influences will likely be overwhelmed, and this shot of
precipitation could be relatively heavy for this time of year
(especially early Saturday morning). A few thunderstorms may be
embedded in the precipitation on Saturday, especially with a
strong cold pool aloft as evidenced by quickly lowering snow
levels.

In fact, snow levels will be coming down quickly from 8000 feet
today to around 3000 to 3500 feet tonight. The snow levels could
fall in alignment with the heavy precipitation which could
facilitate a quicker changeover from rain to snow. This could lead
to a period of inch per hour snowfall rates of wet snow in the
Cascades and in Western Siskiyou County. Specifically, the HREF is
indicating a 60 to 80 percent chance of these inch an hour
snowfall rates from 4 AM to 8AM, which could lead to decent
accumulations. The NBM shows a 40% chance of seeing 6 inches of
snow in the Cascades Saturday night. Have issued winter weather
advisories for the Cascades north of Highway 140 and Western
Siskiyou County above 4000 feet (mainly for Sawyers Bar Road near
Mt. Etna). Other areas may see some snow, but accumulations are
not expected to be as high.

In addition to the precipitation, winds will be a factor as well
with this low. There will be a point where winds will largely be
from the south along the front ahead of the low pressure system.
Statistical guidance is suggesting 40 mph winds at Weed in the
Shasta Valley on Saturday night, which is reasonable due to the
decoupling of the atmosphere likely ahead of the increase in winds
(which is leading to the higher precipitation rates). However,
after daybreak on Saturday, the winds will pick up from the south,
and could reach 50 mph. This will largely be confined to areas of
Lake County, Oregon and eastward. Have issued a wind advisory for
the higher elevations of Lake County including the Winter Rim and
Highway 31.

Showers associated with this low will linger through Saturday,
although the heaviest precipitation and the gustiest winds will
largely have concluded by Saturday night. Temperatures, however,
will be relatively chilly and any wet roads along and east of the
Cascades and in northern California may refreeze.

The flow pattern becomes more zonal for Monday and another impulse
will pass through the area bringing additional showers and
possibly some more thunderstorms to the area. Precipitation
amounts should be relatively light even if snow levels continue to
hover between 3000 and 3500 feet which is unusual for May.

The pattern then shifts Wednesday to a ridge of high pressure
which will allow southern Oregon and northern California to warm
up and dry out. This pattern will likely last into the weekend.

-Schaaf

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory from 2 AM to 11 PM PDT Saturday above
     4000 feet for ORZ027-028.

     Wind Advisory from 5 AM to 5 PM PDT Saturday for ORZ030-031.

CA...Winter Weather Advisory from 2 AM to 11 AM PDT Saturday above
     4000 feet for CAZ080.

PACIFIC COASTAL WATERS...Small Craft Advisory until 11 AM PDT
     Saturday for PZZ350-356-370-376.

&&

$$