207
FXUS66 KEKA 031310
AFDEKA

AREA FORECAST DISCUSSION
National Weather Service Eureka CA
551 AM PDT Fri May 3 2024

.SYNOPSIS...A cold front will move across the area this evening
through early Saturday morning bringing a period of heavy rain.
Brief heavy wet snow is also expected above 2500 feet early
Saturday morning. Colder air will settle over the area for the
remainder of the weekend with frost and freezing temperatures
expected for the interior valleys during early morning hours.

&&

.DISCUSSION...Prefrontal light rain is expected to begin over
Del Norte County late this afternoon. Heavy rain will develop
this evening for Del Norte and northern Humboldt as a surface
cold front moves southward by midnight or shortly after. Heavy
rain rates >0.30in/hr are probable (>70% chance). Chance for
hourly rates >0.50in/hr are 10-20% for the interior mountains
Del Norte and Humboldt, except for the King Range where it is
40%. These heavy rates (>0.50in/hr) per the HREF are not expected
to sustain over a large area (an entire basin) for more than hour
or two and thus will hold off on hoisting a flood watch. Minor
nuisance flooding of low-lying areas with poor drainage and
ponding water on roads are certainly possible with these heavy
rates. It is quite anomalous for early May and will continue to
highlight the heavy rain in a weather story graphic and social
media post. Flows on the rivers are low and April was not exactly
wet with above normal precip, so soils are not saturated as earlier
this winter. There will be run-off concerns and advisories may be
necessary once the heavy rain commences by late this evening.

The cold front will move SE by early Saturday and southern Mendocino
and southern Lake Counties will also get a good soaking of rain,
0.50-0.75in over 6-12 hours, mostly Saturday morning. Threat of
widespread heavy rain will diminish rapidly by midday Saturday
after the brief influx of moisture shifts southward and an
unseasonably cold airmass settles over the area. Looking at the
HREF, snow levels are forecast to rapidly fall early Saturday with
probabilities >70% for hourly snow rates > 1in/hr. This warrants
at least a winter weather advisory for elevations above 2500 feet.
Greatest uncertainty is with snow levels. Levels could vary to as
low as 2000 feet to as high as 3500 feet. Leaned toward lower snow
levels, 2500 ft, due to heavy precip rates driving the freezing
levels lower. Impacts over the major highway passes will be offset
the high May sun angle and relatively warm road surface. Reduced
visibilities from heavy snow and slick roads will have impacts and a
winter weather advisory has been hoisted til 11 AM Saturday.

Shower activity will continue during the day on Saturday as the
cold core aloft moves overhead and onshore westerlies bring open
cellular shallow convection onto the coast. Not all that
confident in the potential for low topped thunderstorms with
surface temperatures cooling down drastically in the interior and
850mb temps dropping to -3C. High temps for the warmest valleys
will struggle to reach 60F. After the precip winds down or
diminishing subfreezing temperatures will be primary concern for
the interior Sunday morning. It is certain (100%) for the
mountains. Valleys will most likely freeze too (>70% chance),
mostly in Trinity County. Spreads are much wider with
probabilities from 30% to 70% for valleys in Del Norte, Humboldt,
Mendocino and Lake. Frost or mid 30`s are much more certain, even
for coastal areas. Fog and low clouds will probably form and
hinder the longwave cooling in some valleys, especially after 1 to
3 inches of rain.

Another shortwave trough in NW flow will probably generate more
showers Monday (highest chances Del Norte and northern Humboldt).
This trough will not be as cold and moisture will be limited.
Surface northerlies will begin to strengthen after passage of
this shortwave trough and dry conditions are most likely into mid
week (>80% chance). Majority of the 500mb ensemble cluster means
(>80%) leads to more confidence in dry weather with slow warming
of daytime temperatures. The one caveat to the "warming" will be
potential for freezing early morning temps in the interior valleys
under clearer skies and calm or light winds, assuming absolutely
no fog and low clouds. Also, the strength of E-NE winds will
factor into "how warm" and the eventual return of coastal stratus.
High temps in the low 90`s for warmest valley are not completely
out of the realm of possibilities either, increasing to 20-40% on
Friday. Interior valley high temps in the 80`s are much more
probable (>60%) by Friday. DB

&&

.AVIATION...VFR prevailed at all terminals this morning. Nighttime
fog product showed extensive low clouds and fog in the interior
valleys. The LIFR conditions (1/4SM VV002 at FOT for example) will
most likely not have long duration impacts at any of the terminals
this morning. It may spill out from the valleys around daybreak
and get close to KACV resulting in brief LIFR. Otherwise, VFR is
forecast to prevail today as multi-layered clouds with an
approaching front increase through the day. Conditions are
forecast to deteriorate to IFR in moderate to heavy rain this
evening at KCEC and KACV. IFR conditions will take longer to
envelope UKI where LLWS should return with frontal passage by 12Z
Sat. DB

&&

.MARINE...Northerly winds have diminished in response to an
approaching front. Southerlies are expected to develop this
morning and slowly increase through the day today ahead of the
cold front. Southerly gusts up to 30 kts are forecast with the
passage of the front, particularly in the northern inners this
evening, then sharply shift to west behind it`s passage. Gusts up
to 25 kts are expected with passing convective showers into
Saturday morning, slowly becoming restricted to the southern
outers by Saturday afternoon. Winds weaken Saturday night and turn
westerly Sunday. Another weak frontal passage may pass early next
week, then more robust northerlies are possible by midweek. Seas
remain low and locally generated through this weekend, and a
series of NW swells are expected to fill in early next week.
DB/TDJ

&&

.EKA WATCHES/WARNINGS/ADVISORIES...
CA...
     Winter Weather Advisory from 4 AM to 11 AM PDT Saturday for
     CAZ102-105>108-111.

     Frost Advisory until 8 AM PDT this morning for CAZ107-108.

NORTHWEST CALIFORNIA COASTAL WATERS...
     Small Craft Advisory from 9 PM this evening to 3 AM PDT
     Saturday for PZZ450.

     Small Craft Advisory from 9 PM this evening to 9 AM PDT
     Saturday for PZZ470.

     Small Craft Advisory from 3 AM to 9 PM PDT Saturday for
     PZZ475.

&&

$$

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