086
FXUS66 KMFR 111126
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
426 AM PDT Sat May 11 2024

.DISCUSSION...Our summer like pattern continues today. Skies are
clear across the region this morning thanks to east to
northeasterly (offshore) flow brought on by a thermal trough. The
thermal trough moved inland yesterday and winds are lighter this
morning compared to 24 hours ago, but dry easterly flow is still
in place and is keeping the marine layer far offshore. Satellite
imagery does show the marine layer encroaching on the outer waters
from the northwest, but we don`t expect a full on "return" of the
marine layer until this afternoon for areas north of Cape Blanco.

With the thermal trough inland today, we expect temperatures to
trend cooler along the coast while remaining hot for areas inland
west of the Cascades. Expect upper 60s along the coast, while inland
areas reach well into the upper 80s. It`s possible for Medford to
see it`s first 90 degree day of the year today, with guidance
showing about a 60% chance of high temperatures reaching 90 degrees.
Temperatures will trend a few degrees cooler in the Umpqua Basin,
while trending warmer for areas east of the Cascades. This warming
trend will peak for west side locations today, and peak on Sunday
for the East Side. High temperatures will be around 15 degrees above
normal, reaching the warmest values of the year so far. One change
added to the forecast for today was the introduction of isolated
showers across southwestern Siskiyou County late this afternoon.
High resolution CAMs are showing some very isolated activity in this
area, which lines up with some weak instability and sufficient
moisture noted on the GFS. It`s more likely that we`ll see some
cumulus buildups in this region, but isolated showers and maybe even
a thunderstorm are possible today.

The pattern takes a more appreciable shift on Sunday as high
pressure aloft gets flattened and weakened by an incoming trough
from the northwest on Sunday night. This will largely be a dry
trough, but it will bring a marine push with cooler onshore flow and
a brief cooling trend through early next week. Just ahead of this
trough, a weak disturbance is expected to move through the area
Sunday afternoon and models continue to show enough instability and
moisture to warrant a chance (20-30%) of thunderstorms for portions
of the area. Thunderstorm activity looks to be focused across
northern California and into the East Side, starting in southwest
Siskiyou County, stretching northeastward to the Medicine
Lake/Klamath Falls area into central and northern Lake Counties.

The upper level ridge rebounds on Monday, rebuilding over the
eastern Pacific and leaving the region in dry and mostly stable
northwest flow Monday into at least midweek. Models do show some
weak instability across southern Siskiyou and Modoc counties on
Monday, but moisture doesn`t look to be sufficient for thunderstorm
development. Temperatures will remain cooler than what is
anticipated this weekend, but still above normal for early-mid May.

Uncertainty increases for the latter half of the week with models
remaining split on the upper level pattern. Even cluster analysis
leads to uncertainty with about 60% of the ensemble envelope
indicating ridge (with a 50/50 split on strength and position) while
about 40% indicate a trough. Even if the trough solutions come to
fruition, the location of this trough is not favorable for
widespread precipitation and tends to lead to very little if any
precipitation for our area. So while confidence is higher for drier
conditions to persist into next weekend, there is lower confidence
on whether temperatures will trend cooler or remain well above
normal. /BR-y

&&

.AVIATION...11/12Z TAFs...VFR conditions continue over most of
northern California and southern Oregon this morning, with the
exception of some attempts of fog and marine stratus at North Bend.
Ceilings and visibilities are moving between MVFR and IFR, and could
continue to do so through the early morning. Sunrise should quickly
break up any stratus development this morning.

VFR levels will continue through the day across the area. Guidance
agrees that a more consistent marine layer will develop tonight,
with IFR or lower levels possible along the Oregon coast. There is a
possibility of development in Roseburg near the end of the TAF
period, but there`s low confidence in that possibility. Other areas
will stay VFR through the TAF period. -TAD

&&

.MARINE...Updated 230 AM Saturday, May 11, 2024...Calm seas continue
this morning and will remain through this afternoon. Winds are
expected to pick up over the outer waters under a weak upper level
disturbance. Additionally, a thermal trough will start to develop
and bring gusty northerly winds south of Cape Blanco. The winds from
these two conditions will rebuild steep seas in waters beyond 10 nm
from shore, as well as waters south of Cape Blanco. A Small Craft
Advisory has been extended for these waters and is now in place from
Saturday at 5 PM through Monday at 5 AM. Steep seas aren`t
impossible in waters north of Cape Blanco and from 0-10nm from
shore, but are not forecast to be a constant condition in this area.

The thermal trough looks to strengthen on Monday morning, with areas
of steep and very steep seas as well as gale winds as expected
outcomes. Probabilistic modeling suggests a 20-50% chance of waves
exceeding 12 feet in waters south of Cape Blanco from Tuesday
morning to Thursday morning, so chaotic seas of some variety are
expected late into the week. The timing and area of these conditions
would still benefit from more information, but further hazard
products after the existing Small Craft Advisory are certain.

Currently, steep seas look to persist into next weekend but there`s
uncertainty in long-term conditions. Different outcomes suggest
different amounts of unsettled seas continuing past Thursday, but
future model runs will help to refine forecasts past the middle of
this week. -TAD

&&

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

PACIFIC COASTAL WATERS...Small Craft Advisory from 5 PM this
     afternoon to 5 AM PDT Monday for PZZ356-370-376.

&&

$$

MNF/TAD