889
FXUS66 KEKA 221222
AFDEKA

AREA FORECAST DISCUSSION
National Weather Service Eureka CA
511 AM PDT Wed May 22 2024

.SYNOPSIS...Slightly below normal temperatures are forecast
through the weekend. Gusty north and northwest winds are forecast
for portions of the area today through Thursday before more
typical breezes for late May return this weekend.

&&

.DISCUSSION...Northwesterly flow aloft with generally below
normal 850mb temperatures (especially across the northern half of
the forecast area) will result in below normal temperatures for
the next several days (til at least Saturday). A decaying front
associated with a shortwave trough over the Pacific NW this
morning has induced a push of shallow moist air with low clouds
and fog pushing into interior Del Norte and Humboldt Counties.
High resolution ensemble continues to indicate drizzle possible
this morning with a few hundredths of an inch (10-20% chance it
will measure) for coastal northern Humboldt, north of Cape Mendo.
HREF means indicate over a tenth in 12 hours for higher terrain of
SW Humboldt, but this is probably way overdone. Surface ridge will
build behind the front this afternoon and pressure gradients will
tighten. This is a distinctly onshore flow and clouds may end up re-
packing up against the coastal terrain. Ridge folding over into SW
Oregon should promote drying and probably clearing late today into
this evening. Granted time-height sections indicate RH`s over 90%
and with northerly winds offshore over the waters, this is a good
sign that low clouds will reform in the delta and spring back over
over Humboldt Bay.

The slight airmass cooling, drying offshore winds and clearer
skies inland will probably result in cooler temperatures for the
interior valleys tonight. Probabilities for 36F or less increase
to 30% or more for valleys of Trinity and eastern Humboldt below
3000 feet and frost may form. Confidence is not high enough at
this time to hoist a frost advisory with probabilities less
than 50%.

Diffuse trough aloft will remain over the area on Thu and Fri,
and temperatures in the interior will probably remain near or
below late May normals which are about 80F for Willow Creek,
Weaverville and Ukiah. Another shortwave trough will drop down in
NW flow aloft Fri-Sat and once again may result in light drizzle for
the North Coast as the depth of the moist layer increases. GFS does
indicate some light precip for the interior mountains - mainly
Trinity - with a deeper shortwave trough scenario. NBM probabilities
for 0.01in/6hr are 5% or less and not worth including in the
forecast at this point.

Above normal 500mb heights and above normal 850mb temperatures
will likely result in above normal interior temperatures early to
mid next week. How warm and the rate of warming are not certain.
Temperatures will probably start to rebound on Sunday. National
Blend of models indicates probabilities greater than 60% for 80F or
more in many of the inland valleys by Monday. By Thu, probabilities
for 90F or more increase to 20-40% for the inland valleys. Large
scale flow pattern does not appear to be one that would promote deep
offshore wind flow and suppression of the marine layer. NBM GUIDANCE
indicates a distinct onshore wind regime at the surface. Thus it is
impossible for coastal areas to warm up with N-NW breezes and chilly
48-55F air spreading onto coast. DB

&&

.AVIATION...A weak cold front descending into NorCal is reinforcing a
stratus layer along the coast north of Cape Mendocino. CEC and ACV
ceilings and visibility`s quickly lowered to IFR/LIFR conditions
amidst moist NW flow. Poor conditions expected to persist through
the morning - stratus may struggle to lift until gusty NW winds
resume this afternoon. Gusts 20-30 mph possible at coastal terminals
through late this evening in the wake of the frontal boundary,
although the jet core will remain south of the cape. Gusts 15-20 mph
likely to develop at UKI in the afternoon as well beneath clear
skies. Marine stratus expected to redevelop around the Humboldt Bay
area this evening as winds diminish, while HREF indicates a weaker
signature for low level ceilings at CEC. JM

&&

.MARINE...A resurgence of near gale to gale force wind gusts will
develop in the southern waters and downwind of Cape Mendocino this
afternoon as a transient ridge strengthens the coastal jet. The
broad wind field will reinforce elevated winds and short period seas
in the northern and southern outer waters through Thursday, with the
highest wind gusts confined to the lee of the Cape. Calm winds and
mild seas in the northern inner waters will remain below advisory
level through the end of the week. The sea state is forecast to
diminish Friday before another upper trough digs into the region. JM

&&

.EKA WATCHES/WARNINGS/ADVISORIES...
CA...
None.
NORTHWEST CALIFORNIA COASTAL WATERS...
     Small Craft Advisory until 9 PM PDT Thursday for PZZ455-475.

     Small Craft Advisory until 9 AM PDT Thursday for PZZ470.

&&

$$

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