Gale Warning 20, NWS Medford, OR

Area Forecast Discussion
National Weather Service Medford OR
223 PM PDT Tue May 30 2023

.Updated aviation discussion for 00z TAFs.

.DISCUSSION...

.Short Term...Tonight through Friday (6/1)...The forecast area
lies between a closed low to the south off the coast of central
California and a broad trough to the northwest over the Gulf of
Alaska. The resulting pattern is one that provides plenty of
moisture flow into northern California and the East Side of
Oregon, but little in the way of steerage flow to keep any showers
and thunderstorms moving. We have already seen convection begin
in these areas, although the showers have been slow moving, and
lightning has been isolated so far. These showers and
thunderstorms are expected to continue through this afternoon and
into the evening, much like they did yesterday. We are watching
with some concern about the amount of precipitation any individual
cell may produce, especially due to the slow moving or near
stationary movement, and with precipitable waters approaching 1
inch. While flooding is not a widespread worry, it could be a
problem with any of the more recent burn scars, or in areas
where significant snow melt is occuring.

A front and associated upper level trough, originating from the flow
around the larger Gulf of Alaska trough, will pass through the
region today and tonight, kicking the southern trough into the GReat
Basin, pushing the bulk of the moisture to the east of our area, and
lowering Wednesday's temperatures by 5 to 10 degrees from those of
today. Some showers and isolated thunderstorms are possible
Wedensday afternoon in eastern Modoc and Lake counties, but overall
convective activity will be drastically less than that of the past
several days.

A second shortwave originating from the flow around the Gulf of
Alaska low will approach the area Wednesday night and pass overhead
Thursday into Thursday night. This wave has the potential to once
again fire off convection, providing the lift, instability, and
moisture needed for thunderstorm development. Models suggest that
the activity will be confined to Modoc, eastern Siskiyou,
southeastern Klamath, and southern Lake counties, but should the
timing or track of the low shift a bit, the area of concern would
likewise change. Will evaluate this further over the next several
shifts.

Temperatures will see a rebound on Thursday, warming back up towards
seasonal normals. Further warming is expected Friday as ridging
returns. -BPN

.LONG TERM...The upper level shortwave will shift east-southeast of
the area Friday night and high pressure will remain over the region
Saturday into early next week. Ensembles show southerly flow
bringing some monsoonal moisture into the region and resulting in a
slight chance for showers and thunderstorms as early as Saturday
as well as on Sunday. The chance for showers and thunderstorms may
increase early next week when an upper low is forecast to move into
southern or south-central California with additional southerly
moisture moving up into the area. Confidence is low in the details
for this period and we will watch model trends. -CC

&&

.AVIATION...31/00Z TAFs... Mainly VFR conditions will prevail across
most of the region this period. The marine layer has almost
completely eroded from the coast this afternoon, with showers and
thunderstorms ongoing in Lake and E. Klamath Counties as well as in
N. California. These will again start to die off towards the late
evening, with chances for thunderstorms tomorrow much less than
today and mostly constrained to far E. Lake and Modoc Counties.

We are expecting another marine push generally N. of Cape Blanco
later tonight with a high ceiling than last night, possibly VFR but
another round of MVFR also not out of the question. Stratus decks
could also impact some of the inland valleys again tonight, notable
the Umpqua Basin but also possibly the Rogue. There's some question
as to the extent of the cloud coverage in both of those locations,
so have leaned towards VFR for both locations through the night, but
that may change in future issuances. -CSP

&&

.MARINE...Updated 215 PM PDT Tuesday, May 30 2023...The thermal
trough pattern will continue through the week and into the weekend.
This will maintain gusty north winds and steep to very steep wind
driven seas. Gales are expected with very steep seas south of Gold
Beach through early Thursday morning with conditions hazardous to
small craft for all other areas through this afternoon. The thermal
trough weakens some late tonight into Thursday. This will bring
improving conditions for areas north of Cape Blanco, first for areas
north of Bandon this evening through Wednesday. Then, hazardous
conditions should be limited to south of Cape Blanco by early
Thursday morning and continuin.AVIATION...30/18Z TAFs... Marine
stratus is affecting the coast, with LIFR/IFR conditions south of
Gold Beach, although there is minor clearing. MVFR ceilings are also
present in the Coquille Valley and Umpqua, including KRBG. Lower
conditions are expected to improve to VFR in the late morning or
early afternoon.

Otherwise, mainly VFR conditions will prevail across most of the
region this morning. There are some lingering mid-level clouds from
"convective debris" roughly from the Scott Valley northeastward
across southern Klamath County, most of Lake County and southward
across northern California. Additionally, there are some areas of
IFR/MVFR cigs that have developed across southern Siskiyou county
and Shasta Valley where steady rain from Monday afternoon/evening
has boosted boundary layer moisture.

Much like the past several days, there are more chances for
thunderstorms this afternoon and evening in the SE part of the
region (generally along and E. of the Cascades and in N.
California). -CC/Hermanseng through Friday. The thermal trough is
expected to strengthen again by the weekend, with the return of
hazardous conditions to all areas. -CC/Hermansen

&&

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

PACIFIC COASTAL WATERS...Small Craft Advisory until 5 AM PDT Thursday
     for PZZ350-356-370-376.

Hazardous Seas Warning until 5 AM PDT Thursday for PZZ356-376.

&&