632
FXUS66 KMFR 280357
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
857 PM PDT Mon May 27 2024

.DISCUSSION...Isolated showers and even a couple of thunderstorms
popped up in southern Siskiyou, Modoc and Lake counties this
afternoon and this evening ahead of an upper trough offshore. Only
about 5-10 cloud to ground flashes occurred, according to various
lightning detection networks. Most were in NE Lake County, but
there were also a couple in southern portions of the Modoc. These
are dissipating this evening as the sun sets and instability
wanes. Overnight, expect a clear to partly cloudy sky for most,
though low clouds and patchy fog will return the coast north of
Cape Blanco and also down around Brookings. A bit deeper marine
push (compared to last night) is possible Tuesday morning.

The upper trough will move onshore on Tuesday with more in the
way of clouds, but also some sunny intervals. For many areas,
including here in the Rogue Valley, this trough will move through
dry and only result in some cooling and an uptick in afternoon WNW
breezes. These winds could become gusty at times (peak gusts of
20-25 mph west side), but especially over the east side where peak
gusts of 25-35 mph are likely. In terms of precipitation, shower
chances will be highest (20-40%) across NW sections in northern
Coos and Douglas counties. Instability will peak again Tuesday
afternoon and evening from eastern Siskiyou to Modoc and southern
Lake counties. Similar to today, these areas stand the best chance
at showers/isolated thunderstorms. The upper trough axis will
shift east of the Cascades Tuesday night, but onshore flow west of
the Cascades will cause some cloud cover along the coast and also to
bank up against the mountains. Inland, these clouds should
dissipate Wednesday morning yielding a nice, sunny afternoon,
though some will likely remain along the coast. Warmer weather is
expected Thursday and Friday. -Spilde

&&

.AVIATION...28/00Z TAFs...VFR conditions prevail across the region
this afternoon with the typical gusty afternoon breezes. Scattered
cumulus with isolated showers have developed east of the Cascades,
and there remains a 10 to 20% chance of thunderstorms there through
this evening. Cloud bases are fairly high, so expect gusty outflow
winds in the vicinity of these showers/thunderstorms.

Marine stratus will make a return later this evening and overnight
as an upper level trough approaches the Pacific Northwest. Expect
the return of IFR/LIFR conditions along the coast between 03-06z,
with ceilings lifting to MVFR late tonight/early Tuesday morning as
the marine layer deepens and pushes inland. Marine stratus is
expected to make a farther inland intrusion than previous nights, so
we expect some MVFR cigs to make into the Umpqua Basin around 10-
12z. Clouds are likely to develop south of the Rogue-Umpqua Divide
and into the Illinois and Rogue Valleys, ceilings should remain just
above MVFR and confidence is low on how much the clouds will fill in
and impact the Medford Airport. While conditions should generally
improve later Tuesday morning, cloud cover will likely linger
through the day as a weak front pushes through the area. /BR-y


&&

.MARINE...Updated 830 PM Monday, May 27, 2024...Relatively calm
conditions will continue into Tuesday. Northerly winds will turn
westerly on Tuesday as an upper trough passes to the north, and
swell will increase slightly, but remain unimpactful.

A thermal trough will develop under upper level ridging on Wednesday
afternoon, driving conditions through the rest of the week. North
winds will return and bring wind-driven steep and very steep seas to
all marine waters. Sustained winds will be in the 20 to 35 kt range
at their highest, with the strongest winds expected south of Cape
Blanco. Chances for gale gusts south of Cape Blanco will be very
high (50-90%) from Wednesday afternoon through Friday afternoon, and
there is a lesser (10-50%) chance of gales over the outer waters
north of Cape Blanco as well. A Gale Watch is in place from
Wednesday evening through Friday evening for all waters south of
Cape Blanco where the chances are highest for gales. Meanwhile,
steep seas and conditions hazardous to small craft are likely for
all areas north of Cape Blanco during this time.

Conditions could improve next weekend as a pattern change disrupts
the thermal trough, and winds ease. -TAD/BR-y


&&

.PREV DISCUSSION... /Issued 242 PM PDT Mon May 27 2024/

SHORT TERM...Satellite image shows clear skies west of the
Southern Oregon Cascades, with cumulus buildups along and east of
the Cascades and over the higher terrain in northern Cal.
Regarding the thunderstorm potential later this afternoon and
evening east of the Cascades and northern Cal. It still exist, but
it could end up bring more of the exception and not the rule.
Meaning a few isolated storms are possible, but odds are we`ll
just have some building cumulus in these areas. Any storms will
dissipate early this evening with dry weather likely for tonight.

An upper trough will approach from the west Tuesday and could bring
a few showers north of Cape Blanco, north of the Umpqua Divide and
northern Cascades. Additionally, instability will be on the increase
along with ample low level moisture, and strong enough trigger east
of the Cascades that could bring another round of isolated storms.
The general consensus shows convection breaking out and orientated
from southwest to northeast from Mount Shasta northeast to southeast
Klamath and central Lake County. Afternoon temperatures for the
interior will be cooler and winds will be stronger east of the
Cascades Tuesday afternoon and evening due to a tighter pressure
gradient and stronger winds aloft (near 700mb). However, we`re
not expecting winds to reach advisory criteria.

Isolated storms will once again diminish and end early Tuesday
evening with not much more than a few showers in northern Coos and
Douglas County Tuesday night.

The upper trough axis shifts east of the area Wednesday with a dry
and stable northwest flow. Therefore it`s likely to remain dry for
the entire forecast area, with gusty breezes in the afternoon and
early evening hours for the interior westside valleys and east of
the Cascades.

Wednesday night, a thermal trough will set up along the south Oregon
coast resulting in increased east to northeast breezes near and at
the ridges for the coastal mountains in southwest Oregon and western
Siskiyou County. It will remain dry with cooler overnight
temperatures east of the Cascades and wind sheltered locations for
the interior west side. -Petrucelli

LONG TERM...Models and ensembles show high confidence for a high
pressure ridge building into the area on Thursday and remaining
in place on Friday. This will bring warming trend and dry weather
across the area. Temperatures on Friday will be roughly 8 to 15
degrees warmer than the normals for this time of year in late May,
although remain a limited heat risk to those sensitive to heat.
High temperatures are forecast to be in the upper 70s to mid 80s
across inland valleys Friday. Models also indicate a high
potential for gusty northeast to north winds along the coast and
over the coastal mountains during this period (Thursday and
Friday), as surface thermal trough develops. This is likely to
bring a warming and drying trend to the southern Curry coast,
especially near Brookings. Guidance the National Blend of Models
(NBM) supports temperatures in the mid 70s at Brookings Thursday.
T

The pattern may  begin to change Saturday night into Sunday with
ensembles showing a front and a shortwave upper level trough moving
into the area. There is low confidence on the strength of this
system and how much moisture it will bring. Currently, the National
Blend of Models favors light rain, with best chances along the coast
and into Douglas County on Sunday and Sunday night. NBM shows the
probability of precipitation of 0.25 inches or more is only 20-30%
along the coast and into Douglas County for Sunday and Sunday night,
with lower chances 5-15% across other parts of southern Oregon and
northern California. Temperatures are expected to trend closer to
seasonal norms though on the weekend.

-CC

AVIATION...27/18Z TAFs...Stable weather will continue to support
generally VFR conditions across northern California and southern
Oregon. A marine stratus layer north of Cape Blanco is quickly
breaking up and should be cleared out within the hour. The usual
afternoon winds will develop across the area and will calm later
this evening. Marine stratus will return tonight and could remain
in place through the end of the TAF period. -TAD

MARINE...Updated 230 PM Monday, May 27, 2024...Calm seas will
continue through today and Tuesday. Northerly winds will turn
westerly as an upper trough passes to the north, and swell will
remain unimpactful.

A thermal trough will develop under upper level ridging on Wednesday
afternoon, driving conditions through the rest of the week.
Northerly winds will return and bring wind-built steep or very steep
seas to all marine waters. Sustained winds will be in the 20 to 35
kt range at their highest, with the strongest winds south of Cape
Blanco. Chances for gale gusts south of Cape Blanco will be very
high (50-90%) from Wednesday afternoon to Friday afternoon, and
there are lesser (10-50%) chances of gales over outer waters north
of Cape Blanco as well. A Gale Watch is in place for waters south of
Cape Blanco from Wednesday at 5 PM through Friday at 5 PM to
communicate the most active period of the thermal trough, but more
complete hazard timing and areas will take more time to develop.

A pattern change near the weekend will break the thermal trough up
and is forecast to bring calm seas by Saturday. -TAD

&&

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

PACIFIC COASTAL WATERS...Gale Watch from Wednesday afternoon through
     Friday afternoon for PZZ356-376.

&&

$$

MAS/MNF/MAP/CC/TAD