805
FXUS66 KMFR 010540
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
1040 PM PDT Tue Apr 30 2024

Updated Aviation and Marine Discussions.

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.AVIATION...01/06Z TAFs...Residual moisture will result in areas
of clouds lingering through the night over Umpqua Basin. Clouds
may fill in enough to produce brief IFR ceilings around KRBG
around 12Z. Clouds will also continue to bank up against the west
and north facing slopes of the Cascades and Siskiyous resulting in
mountain obscurations tonight.

Elsewhere, conditions will be VFR through the TAF period with
typical diurnal winds (light and variable at night and gusty west-
northwest in the afternoon). -Wright

MARINE...Updated 1030 PM Monday, April 30, 2024...Winds and seas
continue to diminish, and the small craft advisory will be allowed
to expire at 11 PM. -Wright

Previous: A weak surface high will move in over the waters
Wednesday with moderate north winds, highest south of Cape Blanco.
Then, another front will move through the waters Wednesday night
into Thursday with winds and wind wave dominated seas just shy of
small craft advisory levels. A break with calmer conditions is
expected Thursday night into early Friday. A series of fronts is
likely to follow Friday into early next week with stronger winds
and building seas with multiple chances for rainfall.
-DW/Petrucelli

.PREV DISCUSSION... /Issued 835 PM PDT Tue Apr 30 2024/

DISCUSSION...No updates planned for this evening. We continue to
see showers over the mountains north of the Umpqua divide this
evening as an upper level system moves by to our north. Showers
are mainly rain but we are seeing a dusting of snow over the
higher elevations such as Crater Lake. This will gradually wind
down overnight as the upper system moves east of us, and thus we
don`t expect much if any impact.

The airmass over us is chilly, so as skies clear, it`s going to
get cold - especially east of the Cascades. However, local
freezing conditions are also expected by morning in the Illinois
and Applegate valleys where freeze warnings remain in effect.

-Wright

PREV DISCUSSION... /Issued 437 PM PDT Tue Apr 30 2024/

DISCUSSION...

Short Term...Tonight through Thursday night...Showers continue
across the Cascades and points west, but so far they have been
light with very little in the way of vertical extent. With onshore
flow and the upper level trough axis passing just to our north,
this activity will likely continue into this evening, with
activity and intensity diminishing to the north. By tonight, the
area should be dry, with upper level ridging building in aloft.

Tonight, with skies clearing and winds on the weaker side, cool
overnight lows are expected. This is especially true of the
Illinois and Applegate valleys, where lows tonight are expected to
once again drop below freezing. While this is not unheard of for
this time of year, area agricultural interests may need to be
concerned given the level of development seen in area orchards and
wineries. A freeze warning has been issued for those areas, and
details can be found at PDXNPWMFR.

With the ridge overhead, Wednesday will be dry and about 10
degrees warmer than today. Mostly clear skies for most of the day
will start to give way to increasing clouds late in the afternoon
and evening as the next trough approaches from the northwest.

This next system, smaller in scale and weaker than most, will
arrive Wednesday night, then pass overhead through Thursday. Due
to the angle of the system, there will be considerable westerly onshore
flow on the front side, along with relatively little in the way of
strong winds out of the south. This type of pattern typically
produces a good shot of wetting rain across the region, especially
along and west of the Cascades, with little in the way of
downslope drying. As a result, most areas in Oregon should see
measurable rain, although northern California may not be as lucky,
as those areas will remain well south of the main moisture flow
and dynamics. Temperatures will trend towards normal for this
time of year, which means snow levels with this event will remain
at or above 5000 feet, so winter impacts will remain confined to
the backcountry and the highest passes, and even then, those
impacts will be minimal.

Most precipitation will dissipate or move out of the area by
Thursday night as the next ridge briefly builds in overhead. -BPN

LONG TERM...Friday through Tuesday, May 3-7, 2024...
The axis of a short wave upper ridge will be centered over the area
Friday morning with a surface warm front just offshore. The upper
ridge will be transient as an active jet over the north Pacific
allows another deep upper trough to carve out a closed low over the
Gulf of Alaska. Any warming that occurs Friday (compared to
Thursday) will be modest at best and where it stays sunniest
the longest (in NE California/East of the Cascades). These areas
probably have high temps 5-10F higher than on Thursday. But, west of
the Cascades, warming will be muted due to the increased cloud
cover, with temps topping out mostly in the 60s. As the warm front
pushes northward and an associated surface cold front well offshore
approaches the PacNW, rain chances increase along the coast with
rain becoming likely there (50-70% chance) by Friday evening.

This frontal system will be fairly potent in terms of precipitation
Friday night into Saturday with good moisture transport into the
region and the lack of a significant downslope component that would
otherwise lead to rain shadowing. PoPs peak at 80-100% area wide.
Don`t get me wrong, there won`t be as much rainfall in the typically
drier areas as say, the coast, but it probably won`t be as large a
difference as usual. So, the front should be an efficient rain
producer for a larger part of the area. Widespread wetting and
beneficial rainfall is expected with preliminary amounts for the
coast in the 1.25-2.50 inch range and many areas inland receiving
0.50-1.25 of an inch. A little less rain is expected for the Klamath
Basin and over the deserts east of Winter Rim where 0.25-0.50 of an
inch appears to be the most likely amount. Snow levels will be up
above 7000 feet as precipitation arrives Friday night, so no
significant winter impacts are expected initially as precipitation
begins. Snow levels will lower to around 5000 feet late Friday
night/Saturday morning as the cold front moves through, so some snow
will accumulate over the mountains (mostly above 5500 feet). Several
inches (4-8") are possible up at Crater Lake, but down on the more
traveled roadways around Diamond Lake and also over Highway 140 near
Lake of the Woods, it`ll probably just be wet or slushy (especially
due to the time of year and warmer ground temps). Model wind fields
aren`t particularly impressive with this system, with most
suggesting peak wind gusts in the 40-50 mph range over usual spots
east of the Cascades (eastern Siskiyou/Modoc and up into
Klamath/Lake counties).

The cooler, wetter weather pattern with post-frontal showers will
continue Saturday afternoon and also Saturday night into Sunday as
the deep upper trough/closed low swings through the area, but
coverage of showers will lessen with time. While showers are
possible just about anywhere and at just about any time on Sunday,
much of the time for most areas will be rain-free. Highs on the
weekend in the west side valleys will be mostly in the 50s to around
60F. East side temps will top out in the upper 40s to mid 50s.

Models are showing moist onshore flow continuing into early next
week with renewed (and increased) chances for rain showers (50-70%)
along the coast and into the Umpqua and over to the Cascades
Monday/Tuesday. But, precip chances are lower (20-40%) across SE
sections.

We may get a reprieve from the active weather as the Climate
Prediction Center 8-14 day outlook favors a return toward more
"normal" conditions (temps/precip) for the second week of May.
-Spilde

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Freeze Warning from 2 AM to 8 AM PDT Wednesday for ORZ024.

CA...None.

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

     Small Craft Advisory until 11 PM PDT this evening for PZZ350-
     370.

&&

$$