Area Forecast Discussion
Issued by NWS Eureka, CA

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FXUS66 KEKA 202238

National Weather Service Eureka CA
338 PM PDT Tue Apr 20 2021

.SYNOPSIS...Scattered showers and thunderstorms affecting
portions of Trinity, Lake, and northeastern Mendocino Counties
will diminish this evening. Patches of marine clouds along with
brisk northerly winds will affect the coast the next couple of
days. Drier and warmer weather is in store Wednesday through
Friday across the interior. Rain chances will increase Saturday,
with widespread rain likely on Sunday.


.DISCUSSION...An upper-level low is tracking southward along the
eastern periphery of our forecast area this afternoon. With cold
air aloft in the vicinity of the low, and strong daytime heating,
enough instability without any capping allowed for convection to
pop early this afternoon. The most vigorous activity is
progressing quickly southward near and south of the low center,
shifting quickly into northeast Mendocino and Lake Counties as we
work through the late afternoon and early evening hours. With
minimal shear and fairly quick forward-motion with these cells,
rainfall is likely struggling to exceed a quarter or even a tenth
of an inch in most spots. Lightning, brief small hail and heavy
downpours will be possible before the storms diminish this
evening. Some weaker showers are occurring as far west as Willow
Creek, but do not expect these to amount to much. Skies will clear
tonight inland, with temperatures perhaps dropping to the mid to
upper 30s in the coldest interior valleys. At the coast, breezy
conditions this evening and areas of low clouds will keep low
temperatures milder. Brisk northerly winds be present once again
tomorrow along the coast and adjacent ridges during the afternoon
and early evening, with skies clearing out in most locations.
Breezy onshore winds will linger through Friday, but will
gradually lessen each day. Warmer conditions will be found across
inland northwest California as ridging builds in, with 70s and low
80s through many of the valleys.

A major pattern change is still on track heading into the weekend,
as an upper trough amplifies over the NERN PAC. Several rounds of
rain will occur as a result, the first on Saturday for areas
mainly north of Cape Mendocino. Widespread moderate rainfall is
then expected on Sunday as stronger forcing for ascent spreads
across the region. In addition, 700 mb temperatures are forecast
to reside between -8 and -12 C Sunday and Monday, which will
support accumulating snowfall for high elevation locations such as
Scott Mountain Summit on Highway 3.


.AVIATION...Coastal terminals have improved in flight categories
since the morning TAF period. Coastal stratus along with morning
drizzle are transitioning to scattered cloud groups and a build up
of northerly winds. MVFR conditions or better are expected for this
TAF period at KCEC and KACV. KCEC will get the brunt of the gusty
north wind with no perceived threat of LLWS. Winds at KACV will
change to an east and somewhat southerly bearing late tonight. A
threat of thunderstorms linger through the late afternoon for inland
areas of Trinity, Lake, and eastern Mendocino Counties. This could
impact flight operations, particularly in the vicinity of the Yolla
Bolly mountains and Trinity Alps. Otherwise, overcasts and lower
ceilings at KACV may also impact flight conditions later tonight.
KUKI will also get gusty winds up until the later evening, when they


.MARINE...Northerlies will continue to ramp up as the pressure
gradient tightens. Gale Warnings are in effect for all zones, except
the northern inner waters. Though, localized gales will still be
possible in the zone, particularly along the Del Norte Coast and
around Cape Mendocino. Gales to near gales will persist through
Wednesday, though winds will generally be less pronounced in most of
the northern inner waters (the exception being along the Del Norte
Coast). Winds and seas are expected to wane the later half of the
work week as the pressure gradient eases. A westerly swell will
linger throughout the waters into Thursday. The combination of this
swell and building steep, short-period seas will likely lead to a
chaotic sea state through Wednesday. A southerly swell around 1 to 3
feet with a period around 20 seconds will impact portions of the
waters this weekend. Otherwise, another westerly swell will move
through the waters this weekend.


.FIRE WEATHER...Scattered showers and thunderstorms will focus on
northeast Mendocino County into Lake County late today, then
dissipate toward sunset. The storms will not linger in any one
location, so that rainfall amounts will be mostly under a quarter
of an inch, limiting the debris flow threat to the August Complex
burn scar. Otherwise, dry weather and moderating temperatures are
expected across the interior Wednesday through Friday.
Thereafter, widespread wetting rain appears probably this weekend,
especially on Sunday.


     until 3 AM PDT Thursday for PZZ450.

     Gale Warning until 3 AM PDT Thursday for PZZ470-475.

     Gale Warning until 9 PM PDT this evening for PZZ455.



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