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000
FXUS66 KMFR 100412
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
912 PM PDT Tue Apr 9 2024

.DISCUSSION...The center of an upper level high pressure system is
off the California coast currently with the axis extending
northward through the PacNW and into western Canada. This far
north, here in Oregon and NorCal, westerly flow aloft is
transporting some high-level moisture in from the Pacific. This is
causing some high cloudiness which made for a pretty nice sunset
this evening. While a weak short wave in the westerly flow aloft
will pass through overnight into Wednesday morning, there just
isn`t any deep layer moisture to support any precipitation. So,
we`ll remain dry. Models are showing some low-level moisture in
the Umpqua Valley, so we`ll probably see some lower clouds form
there overnight into Wednesday morning with patchy fog as well,
but these shouldn`t be as extensive as they were last night and
this morning. Another couple of nice days with cool mornings and
mild afternoons are expected Wednesday and Thursday with no
precip.

The next upper trough will head into the PacNW Thursday night
pushing the upper ridge off to the east. Models are in agreement
showing a closed upper low traveling southward offshore Friday
into Saturday, nearing the California coast (SF Bay area)
during Saturday. This will allow the flow to back to the south
and southeast, which should bring enough moisture/instability
northward for a fairly high probability of showers and even a
chance of thunderstorms (15-30%). Highest precip probabilities
Friday will be along the coast (60-80%), in the mountains and over
the East Side in the afternoon (60-80%), while interior west side
valleys have 30-60% chances of showers. By Saturday, some
guidance shows a deformation axis setting up somewhere nearby,
which could focus precip in some areas of our CWA. These are
notoriously difficult to predict with closed lows and are
dependent exactly on where the low sets up. It could mean the
difference between getting some drenching rain and nothing at all.
That said, this should at least focus highest PoPs (60-80%)
Saturday in the SE portions of the CWA (NE Cal and east of the
Cascades), but we`ll be watching if some of this activity can get
a push west of the Cascades into portions of the Rogue Valley
Saturday evening. PoPs gradually decrease the farther north and
west you are away from the low (lowest PoPs north and west of the
Umpqua Divide). The closed low cuts off from the main flow for a
while as it pinwheels onshore and into the Great Basin Sunday. So,
shower chances could persist then, especially south and east of
the Cascades. -Spilde

&&

.AVIATION...10/00Z TAFs...VFR is expected to remain the predominant
condition into Wednesday evening. But, an exception will be areas of
late night into Wednesday morning (around 07Z through 17Z) LIFR/IFR
in the Umpqua and Coquille valleys, including Roseburg. -DW


&&

.MARINE...Updated 800 PM Tuesday, April 09, 2024...Gusty
northerly winds will continue into Thursday morning, strongest
and at Small Craft Advisory strength south of Cape Blanco. More
precisely, as a thermal trough builds. winds will be at peak
strength late Wednesday afternoon from Gold Beach southward and from
shore to 30 nm out.

Conditions improve Thursday into Friday, though a low pressure
system moving south is likely to bring rain and light southerly
winds. Meantime, gusty northerly winds are likely in the offshore
waters, beyond 60 nm from shore.

Behind the low, these stronger northerly winds are likely to extend
into the coastal waters Friday night then persist through the
weekend, except for southerly winds near shore south of Gold Beach
beginning Saturday night.

-DW


&&

.PREV DISCUSSION... /Issued 259 PM PDT Tue Apr 9 2024/

SHORT TERM...Temperatures tonight will be similar to last night
with the biggest change near the coast and in the Umpqua Basin.
Models are supporting less stratus than last night in these two
locations stated, and this will help bring a slightly cooler
morning there with forecast lows in the low 40s. Otherwise lows
Wednesday morning will be similar with upper 30s and low 40s
inland west of the Cascades and upper 20s to low 30s east.
Afternoon temperatures will be on average 5 to 15 degrees warmer
than normal, meaning highs in the 60s and 70s across the board!

A tighter pressure gradient will strengthen winds Thursday with 10-
20 mph southerly winds possible east of the Cascades. This southerly
flow will bring the warmest temperatures of the week to east side
locations as forecast highs are in the upper 60s and low 70s. It
will also be another warm day for areas west of the Cascades as
they will be comparable to Wednesday highs.

Currently an area of low pressure is near Alaska. There is agreement
among ensembles that this low will move southward and be off the
Oregon/Washington coast later Thursday. Precipitation chances will
return later Thursday for the coast and Douglas County. There is a
concern for thunder chances later Thursday as there is an increase
in lapse rates and forecasted CAPE near 100-200 J/kg. For now,
thunder chances are introduced to the forecast starting Friday
morning while precipitation chances expand to the rest of Southern
Oregon and Northern California as the long term will explain.
-Hermansen

LONG TERM...Friday morning through Monday evening.

We start off the extended period keeping an eye on a low over the
Pacific. The latest set of models is pushing this low to our south
and cutting it off from the upper level flow. Some cooler air will
still remain over the forecast area, especially west of the
Cascades. This can be seen in the snow level forecast on Friday
afternoon with snow levels around 8000 feet in Lake County and as
low as 4000 feet along the Oregon coast. With the chance of
precipitation around 50 to 70 percent at times on Friday,  some rain
or snow showers should bring no impacts to the region given the
light accumulations at elevation.

There is also the chance for thunderstorms in northern California
and east of the Cascades for Friday afternoon as a higher sun angle
mixed with a little bit of moisture is helping produce instability.
Upper level shear is about 30 to 40 knots with about 250 to 500 j/kg
of convective available potential energy, so if storms form, they
will put down some cloud to ground lightning.

The chance of precipitation has increased over the last model runs
up to 40% on average for most of Saturday as the low continues to
impact our weather. Pockets of showers seem the most likely,
although it seems they`ll be more miss than hit for many locations.
Given the low`s location, northern California and the southern
sections of Oregon are the focus for the shower activity as short
waves rotate around the center of the low, which should still be
situated to our south.

It`s also worth noting that the thunderstorm potential persists east
of the Cascades into Saturday.  The abundance of cloud cover could
really impact what happens, but the cloud to ground lightning
probability remains right around 10 to 15%.

An upper level ridge will likely build briefly on Sunday as the
upper level low departs the region.  There are probably model
solutions in the ensembles which keep the low around a little longer
with the probability of precipitation remaining around 10 to 35
percent for many locations on Sunday

Finally by around Monday evening, another upper level wave appears
to be approaching the Oregon and Washington coastlines. The NBM
keeps the probability of precipitation forecast around 10 to 30
percent for Oregon locations west of the Cascades with it drier to
the east. It seems that the wave is relatively dry with little rain
falling along the Oregon coast.

Overall, pretty minor impacts in the extended, although the
thunderstorm potential Friday afternoon and evening will have to be
watched. Even if they do form, impacts east of the Cascades are
usually minor with regards to thunderstorm activity.

-Smith

AVIATION...09/18Z TAFs...Marine stratus is beginning to dissipate
across the Oregon coast and the Umpqua Valley including at North
Bend and Roseburg airports. These MVFR and occasional IFR ceilings
will return this evening and linger overnight before breaking up
again mid to late Wednesday morning.

Other inland areas are expected to stay at VFR levels under stable
atmospheric conditions. -Schaaf/TAD

MARINE...Updated 130 PM Tuesday, April 09, 2024...High pressure
developing over the western US will continue to support generally
stable atmospheric conditions. This afternoon, northerly winds will
develop over waters south of Cape Blanco as a thermal trough builds.
These winds will have gusts of 25 to 30 knots, with the strongest
winds expected over waters at and south of Gold Beach and from shore
to 30 nm out. Steep seas will quickly build under these gusty winds,
and a Small Craft Advisory will be in place for waters south of Cape
Blanco starting at 5 PM this evening. Steep seas under gusty winds
are expected to continue through at least Thursday morning.

Active weather will return later in the week as a low pressure
system moves south over the Pacific. This will break up the thermal
trough and could bring elevated northerly winds to outer waters,
depending on the system`s path. This system will also bring slight
chances (25-50%) for precipitation over all waters on Friday that
could continue through the weekend. Northwesterly swell will
increase slightly but overall the system is forecast to be non-
impactful, with non-advisory seas forecast through the weekend.

-TAD

&&

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

CA...None.

PACIFIC COASTAL WATERS...Small Craft Advisory until 11 AM PDT
     Thursday for PZZ356-376.

&&

$$

MAS/MAS/MAS