Area Forecast Discussion
Issued by NWS Eureka, CA

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FXUS66 KEKA 291150

National Weather Service Eureka CA
450 AM PDT Mon May 29 2023

.SYNOPSIS...Afternoon showers and thunderstorms will occur today
and Tuesday over the interior mountains. Otherwise, temperatures
will remain near seasonal normals through Wednesday, followed by
gradual warming and drying for the latter portion of the week.


.DISCUSSION...A closed upper low about 180 miles SW of Pt Arena
has been pumping up a deep marine layer along the coast. Extensive
low cloud cover has been spreading northward onto the Mendocino
coast and pushing well into the Mendocino interior via the
Russian river valley. Marine layer deepening has also been ongoing
through the night north of Cape Mendocino with extensive low
clouds extending over the Humboldt Bay area and well into the Eel
Delta. The trend will be for low clouds to increase or persist
near the shoreline today as onshore winds strengthens under a
stout marine inversion. Meanwhile, interior areas where low clouds
have penetrated will clear out. Abundant afternoon sunshine is
expected, however thunderstorms will most likely develop again
with the daytime ground heating. Precipitable water values remain
over 200% of normal for late May. Buoyant energy over 500J/kg and
lifted indices less than -1C will be sufficient for interior
convection and thunderstorms again today. Storm will be triggered
and forced by solar insolation and orographic lifting. Easterly
winds aloft will allow some storms to move westward toward the
coast, however the cool and damp marine air environment will
likely be quite hostile to storm survival. Dangerous lightning,
hail, brief heavy rain and gusty winds, may impact outdoor activities
once again today.

The upper low will meander southward on Tuesday, however an
upstream trough will begin to approach from the west. Sufficient
instability and buoyant energy coupled with upslope southwesterly
flow and solar heating will likely yield more storms on Tuesday.
Westerly flow will likely keep the bulk of lightning activity in
Trinity and perhaps NE Mendocino counties.

The cut-off will finally eject into the Desert SW around mid week
(Wed) in response to an upstream kicker or shortwave trough
embedded in the westerlies. Increasing westerly flow aloft will
spread drier air into the area, though some cells are not
completely impossible for the higher peaks of NE Trinity on Wed.

500mb clusters and raw ensemble means continue to indicate ridging
and higher heights toward the end of the week. We should see some
warming toward the end of the week, however marine air filtering
into the river valleys will severely moderate the magnitude of the
warming for NW California. By the weekend lower 90`s will
certainly be possible for the hottest valleys in Trinity county.
Another trough will approach late next weekend and may produce more
interior showers and thunderstorms for the first weekend of June.


.AVIATION...The persisting coastal southerly reversal expands this
morning, bringing expansive stratus offshore along with a deepening
marine layer. Brief scattering may be possible along the Del Norte
coastline today, but most guidance is indicating coastal areas south
of the Klamath remaining impacted by stratus through the TAF period.
Ceilings in these areas may lift to MVFR by this afternoon, but
expect IFR to LIFR ceilings to develop once again this evening.
Meanwhile, the marine layer has deepened enough for low cloud to
spill into Ukiah this morning, with IFR ceilings at time of writing.
These low ceilings are expected to linger through the first half of
the morning, with improvement beginning a few hours before noon. Low
clouds likely return to KUKI once again late tonight.


.MARINE...Northerly winds will weaken today and shunt farther
offshore, with nearshore southerly reversals likely developing
during by this evening. Lingering steep seas keep most zones in at
least advisory-level conditions, although seas are expected to
quickly drop later this morning. Nearshore eddies break down
tomorrow, with northerlies expected to return to the inner zones
Tuesday afternoon and Wednesday while a round of Gales is possible
in the outer waters during this time. Seas remain primarily locally
generated, although a fresh but small WNW swell is expected to fill
in Tuesday afternoon and Wednesday.


     Small Craft Advisory until 9 AM PDT this morning for PZZ450.

     Hazardous Seas Warning until 9 AM PDT this morning for

     Small Craft Advisory until 3 PM PDT this afternoon for



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