760
FXUS66 KEKA 032128
AFDEKA

AREA FORECAST DISCUSSION
National Weather Service Eureka CA
228 PM PDT Thu Jul 3 2025

.SYNOPSIS...Cooler interior temperatures and a deeper marine layer
are expected through Friday. Temperatures will generally trend
warmer this weekend and into early next week. There is a slight
chance of thunderstorms over NE Trinity County this evening.

&&

.DISCUSSION...

Satellite imagery and web cameras showed variable high level
clouds moving across the area today. Along the coast stratus has
been less extensive and appeared more disrupted along the North
Coast. In contrast, south of  Cape Mendocino, stratus was
actually expanding along the Mendocino coast. Stratus should
rebuild and push deeper into the river valleys tonight as a short
wave trough progresses across the area. A stronger and higher
marine inversion will likely keep cool and humid marine air
trapped against the coastal terrain through Friday, especially
around Humboldt Bay and Eel delta. Interior temperatures cooled
considerably today in response to stronger onshore winds. Temps
are forecast to remain below seasonal averages on Friday; in the
80`s to lower 90`s that is.

Waters vapor imagery showed a cyclonic circulation center just
offshore at the base of a 500mb trough steadily moving eastward
toward the North Coast this afternoon. Model based soundings
continue to depict steep mid level lapse rates with sufficient
CAPE and instability for storms across NE Trinty County this
evening. Some of the convective allowing models even hint at
storms firing up with 500mb vort passage around or shortly after
midnight. HREF thunder probabilities are just north of the area in
Siskiyou County, however. We cannot completely rule out a stray
lightning strike, especially with shallow moist convection
developing over NE Trinity this afternoon with daytime heating.
Confidence is by no means high there will be any strikes with only
a few HREF composite cores over 40dBz skirting along the border
with Siskiyou and Shasta Counties.

Another short wave trough will approach over the weekend and eventually
develop into a semi-closed 500mb circulation or cut-off low either
offshore to our SW or closer to the northern California coast by
late Sunday into Monday. Deterministic model guidance has robust
SE or E flow at 500mb (40-50kt of bulk shear) by Sunday night into
Monday. This pattern (close low offshore) and 500mb SE wind direction
is conducive for thunderstorms in our forecast area this time of
year. Moisture appears quite sparse. Convective parameters and
model derived soundings are not very supportive for any storms.
Mid level lapse rates are forecast to become steep, however. With
below normal PWATS across our more convectively active areas in
the interior, see no compelling reason to deviate from the NBM
guidance and add thunderstorms to the forecast at this time. This
will need to be watched. Once within range of the CAMS, we should
have a better handle on this potentially high impact but low
probability event.

Interior temperatures are forecast to warm up next week as the
trough ejects N-NE in response to an upstream kicker. By mid to late
next week the four corners ridge will expand westward and
northwestward. There is still uncertainty with the rate and
magnitude of the warming with upstream troughs possibly keeping the
hottest air to our south. Multiple ensemble systems continue to
consistently show high probabilities (>80% chance) for 500mb
heights 588DM or more with lower probabilities (30-50% chance)
for 594DM or more. 100+ degree heat for our typical hot valleys in
the interior is highly probable. There is greater uncertainty for
coastal valleys of western Humboldt (Eel river valley) and
Mendocino (Anderson valley). Late next week will likely be the
hottest days so far this summer.

&&

.AVIATION...

A deep marine layer continues to compress into the north coast
this afternoon with IFR ceilings at the coastal terminals. Greatest
chance for brief MVFR scattering in CEC late this afternoon as
northerly winds increase offshore and scour out the cloud cover.
Gusts exceeding 10 to 15 knots possible at both coastal terminals
prior to 00Z. UKI already experiencing 15 to 20 knot gusts, with
gusts up to 30 knots possible late in the afternoon. HREF shows
>80% probability for <1K ft ceilings redeveloping at CEC and ACV
after 03Z. A weak shortwave will provide additional instability
overnight and potential for coastal drizzle and LIFR viz/ceilings
after 09Z Friday morning.

&&

.MARINE...

Northerly winds will continue in the outer waters with gale force
gusts south of Cape Mendocino beginning to wind down late this
evening. Winds and steep waves will decrease first in the northern
waters, extending into the southern waters through Friday.
Moderate to fresh breezes will continue in the outer waters
through Friday as a long period southwesterly swell fills into the
waters, with locally stronger gusts nearshore and downwind of the
Cape. Northerlies expected to strengthen again late Saturday into
early next week as high pressure builds back into the region. NBM
and global ensembles increasing confidence (50 to 70% chance) in
potential for gale gusts in the outer waters Sunday through
Monday, especially in the northern waters.

&&

.FIRE WEATHER...

Isolated lightning strikes will be possible (15% chance) over NE
Trinity (zone 283) this evening as a dry trough progresses across
the area. Lightning chances decrease to 10% or less after
midnight. Greatest lightning activity is expected in NE California
and southern Oregon. Otherwise, gusty diurnally driven winds are
forecast to continue through Friday. The lowest RH`s are once
again forecast for lower elevations of eastern Trinity and
southern Lake County, around 20-25%. Gusts up 25 mph are expected
for the Weaverville basin and southern Lake County. Any new fire
starts or hold over fires from lightning starts could spread
rapidly before winds dies down overnight.

High temperatures are forecast to warm over the weekend into
early next week while minimum RH`s become drier. Overnight
recoveries are generally expected to remain good except for a few
exposed ridges where low RH`s may persist through the night.
Another trough will approach over the weekend and poses a low risk
for convection. Strong southeast flow aloft should develop Sunday
into Monday, however the air will be dry at mid levels. Steep mid
level lapse rates do increase on Sunday and Monday, however deep
column water vapor content is forecast to remain below normal. The
cut-off low is forecast to finally eject N-NE by mid next week as
a massive ridge over the four corners region expands over the
area. Much hotter temperatures and drier humidity are highly
probable (80% chance) in the interior mid to late next week.

&&

.EKA WATCHES/WARNINGS/ADVISORIES...
CA...
None.
NORTHWEST CALIFORNIA COASTAL WATERS...
     Small Craft Advisory until 3 AM PDT Friday for PZZ455.

     Small Craft Advisory until 3 PM PDT this afternoon for
     PZZ470.

     Gale Warning until 9 PM PDT this evening for PZZ475.

&&

$$

NOTE: The full forecast discussion is now regularly issued between
10 am/pm and 1 pm/am. Additional updates will be issued as needed.

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For forecast zone information
see the forecast zone map online:
https://www.weather.gov/images/eka/zonemap.png