Area Forecast Discussion
Issued by NWS Eureka, CA

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000
FXUS66 KEKA 132151
AFDEKA

AREA FORECAST DISCUSSION
National Weather Service Eureka CA
251 PM PDT Thu May 13 2021

.SYNOPSIS...Above normal interior temperatures are forecast to
continue through the weekend. Coastal areas will remain cool with
occasional low clouds and fog through Friday. An upper level trough
will bring a chance for thunderstorms to eastern Trinity county
Friday afternoon and evening. The upper low may also produce some
showers with isolated thunderstorms Friday night through Saturday
for portions of Humboldt, Mendocino and Lake counties.

&&

.DISCUSSION...Stratus and fog has once again persisted over much
of the coast through the day today. The shallow marine air mass
will likely deepen tonight, with greater intrusion of low clouds
and fog into the interior valleys by Friday morning. Time-height
sections show a deeper moist layer developing tonight through
Friday morning as a shortwave trough approaches the north coast.
Drizzle appears likely given the increasing northerlies winds
offshore and marine air mass convergence upstream of Cape Mendo.
There is certainly a chance for a few hundredths of an inch.

The forecast becomes more interesting and active as we head into
the afternoon and evening hours of Friday as the above mentioned
shortwave trough develops into a closed mid level low. The threat
for storms across will increase during peak heating across
initially NE Trinity county. Moisture will be limited with PWATS
generally less than 0.70in. Any rainfall will most likely not be
heavy. BUFKIT profiles indicate surface CAPE over 500J/kg with
equilibrium levels just above 30kft by late afternoon/early Friday
evening. The primary threat appears to be frequent cloud-to-
ground lightning strikes and perhaps gusty outflow winds due to a
really dry lower atmosphere. Gusty easterly winds in the wrap
around flow around the low may fan any new fire starts Saturday
across eastern Trinity. Warp easterly flow aloft and instability
associated with upper low appear sufficient to warrant a continued
convective threat through the day on Saturday for Trinity county.
Due to the easterly flow, storms will likely develop and
propagate farther west, perhaps over South Fork Mtn on Saturday.

What is really uncertain and most difficult to forecast is the
potential for nocturnal storms late Friday night over Humboldt
and northern Mendocino counties. Convective allowing models and
HREF neighborhood thunderstorm probabilities indicate possible
storms over portions of Humboldt and Mendocino counties Friday
night. The lower atmosphere will be completely decoupled from the
mid level circulation. Robust near surface northerly flow will
likely prevail offshore and severely limit any thunder potential
for coastal areas. BUFKIT soundings indicate sufficient elevated
CAPE developing and spreading westward late Friday night, however.
Thus an isolated rumble of thunder southern Humboldt and NW
Mendocino is not completely out of the question, about 5 to 10%
chance during the late evening and overnight hours.

Deterministic models as well as the ensemble guidance has the
upper low slowly meandering southeastward into the Desert SW on
Sunday. Latest deterministic and ensemble guidance suggest a
slower transition, with cyclonic N-NE flow aloft and lingering
instability. The chance for more showers and storms across the
interior on Sunday decreases, however it is not zero.

A ridge aloft is forecast to fold-over into the Pacific NW by
Monday as the cold core aloft tracks eastward across the Desert
SW. A broad trough aloft will approach by Tuesday, resulting in
generally cooler temperatures for the interior through mid next
week. GEFS and ECMWF ensemble probabilities for a tenth of an inch
or more of rain in 24-hours start to increase around Tuesday and
continue through mid week for Del Norte/northern Humbodlt
counties. The probability for a tenth of an inch or more during
any 24 hour period from Tuesday through Thursday is 10% or less
for our wettest mountain spots in Del Norte county. Thus, the
chance for significant rainfall is not looking very promising at
this point.

&&

.AVIATION...IFR to LIFR ceilings and visibilities have plagued the
immediate coast once again today, and while portions of the coast
may see brief periods of clearing this afternoon, these will likely
quickly be stomped out by the north Pacific`s conveyer belt of
stratus. Expect conditions to deteriorate once again to LIFR this
evening as daytime mixing over land slows down. VFR will continue
through the TAF period for interior areas.

&&

.MARINE...Northerly winds continue through the weekend, strengthening
to gale force across portions of the outer waters by Friday
afternoon...with local wind speed maximums in the usual spots
downwind of Cape Blanco and Cape Mendocino. Wind speeds in the
northern outers look to reach sustained gale force, while winds in
the southern outers may only have a burst of gale force gusts.
Have hoisted a gale warning for the northern outers and trimmed
the gale watch for the southern outers to reflect this thinking.
Winds begin to weaken by Sunday, and a trough approaching early
next week may help to further weaken the incessant
northerlies...although model dissonance continues to leave details
unclear. Seas will remain primarily dominated by the local
northerly winds, although some small, mid-period NW swells along
with small, long period SW swells will continue through much of
the forecast period.

&&

.FIRE WEATHER...The weather will be mostly dry for the next week.
However, isolated thunderstorms are possible Friday afternoon and
evening across portions of Trinity County as an upper level
disturbance moves into the area. Due to the fairly dry atmospheric
column, rainfall rates from individual storms are expected to be
fairly low. However, storm motions are also expected to be slow. Fuel
moisture levels in that area are marginal for rapid fire development
and spread should lightning strikes occur. As a result, will
continue to only headline the potential for isolated thunderstorms at
this time. /SEC

&&

.EKA WATCHES/WARNINGS/ADVISORIES...
CA...None.
NORTHWEST CALIFORNIA COASTAL WATERS...
Small Craft Advisory until 9 PM PDT Saturday for PZZ450-455.
Small Craft Advisory until 3 PM PDT Friday for PZZ470.
Small Craft Advisory until 3 AM PDT Saturday for PZZ475.
Gale Warning from 3 PM Friday to 9 PM PDT Saturday for PZZ470.
Gale Watch from late Friday night through Saturday afternoon for
PZZ475.

&&

$$

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