Special Weather Statement, NWS Eureka, CA

AREA FORECAST DISCUSSION
National Weather Service Eureka CA
525 AM PDT Sat Aug 19 2023

.SYNOPSIS...Interior temperatures will remain slightly above
normal through the weekend, then cool below normal early next
week. The threat of thunderstorms across mostly the interior will
persist today through Monday.

&&

.DISCUSSION...The persistent upper low off the central CA coast
continues to meander in the base of a larger scale trough that
extends S from British Columbia. However, the large upper high
over the S Plains remains entrenched. The smoke from wildfires
will keep temperatures across Del Norte and extreme NE Humboldt
counties fairly cool (68-76F for interior Del Norte and upper 80s
to mid 90's for NE Humboldt). The rest of the interior has been
above normal (mid to upper 90's). More cloud cover will knock
temperatures down some on Sunday, followed by greater cooling
and below normal temperatures early next week.

Scattered to isolated thunderstorms are expected again this
afternoon over the interior. Stronger south and southeast flow aloft
will support faster storm motions today compared to the last couple
of days. A couple of the convective allowing models depict decaying
storm cells approaching the Humboldt Bay area late this afternoon.

High precipitable water values (over 1 inch) will persist
today, however the lower atmosphere remains dry. With inverted-v
thermodynamic profiles expect high based storms with potential
for dry strikes and gusty outflow winds outside the main
precip cores.

The upper low will continue to tap into residual monsoonal moisture
from the S, as well as producing vorticity maxima that rotate
counter-clockwise around the broader circulation. This will
continue the threat of at least isolated afternoon and evening
thunderstorms on Sunday across mainly the interior and triggered
by daytime heat. There is a low potential for some nocturnal
storms late tonight into early Sunday morning for southern Lake
and southern Mendocino as the cut-off low begins eject eastward.
The exact coverage and timing storms will be complicated by
ongoing smoke in the area and model variability.

As we get into early next week, moisture from the remnants of Tropical
Cyclone Hilary is expected to spread N from S CA. Precipitable
water ramps up to 200% of normals, over 1 inch. Looking at the
ensemble distribution from the NBM, RAW ensemble probabilities
from the GEFS and ENS, there is a chance (30-50%) for some rain
on the order of 0.10 to 0.25 of an inch. On the limbs of the
distributions, there is a 2% chance for an inch or more in 12
hours. The chance for 0.01in or zero in 12 hours ranges from
41-75%. The bulk of the rain will be shunted east of our area.
Thick cloud cover may also inhibit the potential for storms and
deep convection in the interior.

&&

.AVIATION...Coastal marine stratus persist along the Mendocino
coastline, while low clouds redeveloped along the North Coast
overnight and is expected to bring a period of LIFR conditions to
ACV this morning. HREF suggest stratus eroding back to the coast
this afternoon, and bring mostly VFR condition at the coastal
terminals. LIFR ceiling will be possible to reach UKI this morning
with the southerly winds, but this will be for a brief
period..before VFR conditions redevelop. Expect another day with
thunderstorms activity across the interior mountains this afternoon
and early evening. This will spread mid-high level clouds toward the
coastal areas.  Additionally, active wildfires across NW California
will continue to create skies obscuration and hazy skies over mainly
Del Norte, Humboldt and Trinity counties. Winds from S-SE around 5
kt becoming W-SW around 5 to 15 kt in the afternoon along the coast.

&&

.MARINE...Strong to near gale N-NW winds have developed across the
outer waters, with gale force gust primarily over the northern outer
waters. Very steep, hazardous waves have developed in response to
the winds and will continue to propagate into the northern inner
waters through this morning. The Crescent City and Eel River Buoys
reported swells up to 11 feet early this morning. However, very
steep seas remains across the outer waters through late tonight.
Another round of gales and very steep seas is expected to develop
across the coastal waters late Sunday night and Monday, mainly
across the outer waters and northern inner waters.

&&

.FIRE WEATHER...We hoisted a red flag warning for abundant lightning
from 1 PM this afternoon to 8 PM this evening for fire weather
zones 204, 211, 212, 277 and 283. Storm coverage and lightning
activity level will be similar to the last couple of days with a
mix of wet and dry storms and the potential for more fire starts,
especially outside the main precip cores. In addition, gusty and
erratic outflow winds will fan existing fires. South-southeasterly
flow aloft will increase today compared to yesterday and mean
storm motion will be slightly faster. A few storms could spread
out toward the coast as well (20% chance), per a couple of the
convective allowing models. Temperatures will remain slightly
above normal through the weekend 90-100F. Areas with thick and
more persistent smoke will see below normal temperatures. Winds
for the remainder of the weekend will generally remain diurnally
driven under 10 mph, with stronger westerly breezes expected over
exposed ridges and in channeled terrain. Northerly and easterly
breezes with localized gusts to 25 mph over exposed terrain are
expected Sunday night.

An upstream trough will kick out the pesky cut-off low early next
week, namely on Tuesday. Remnant moisture from Hurricane Hilary
will spread northward late Sunday into Monday with PWATS ramping
up to 200% of normal. Expect scattered showers and possible
thunderstorms Sunday night and Monday, with the potential for some
wetting rain, mostly across the interior.

By Tuesday, westerly flow will increase aloft, however there are
significant differences from the GFS and ECMWF on how quickly
PWATS diminish and how fast the upstream trough progresses
eastward toward the California coast. Temperatures should
diminish below normal with generally stronger onshore winds and
a deeper marine air mass pushing into the river valleys. Some
showers will be possible on Thu if the faster GFS pans out. The
ECMWF is drier with broader ridging and warmer temperatures.

&&

.EKA WATCHES/WARNINGS/ADVISORIES...
CA...
     Red Flag Warning from 1 PM this afternoon to 8 PM PDT this
     evening for CAZ204-211-212-277-283.

NORTHWEST CALIFORNIA COASTAL WATERS...
     Hazardous Seas Warning until 9 AM PDT this morning for
     PZZ450.

     Gale Warning until 3 PM PDT this afternoon for PZZ470.

     Hazardous Seas Warning from 3 PM this afternoon to 3 AM PDT
     Sunday for PZZ470.

     Hazardous Seas Warning until 3 AM PDT Sunday for PZZ475.

&&

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

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