Area Forecast Discussion
Issued by NWS Eureka, CA

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FXUS66 KEKA 221316

National Weather Service Eureka CA
516 AM PST Wed Jan 22 2020

.SYNOPSIS...Occasional light showers are expected today through
Friday, mainly north of Cape Mendocino. Widespread rain, possibly
heavy, is expected this weekend.


.DISCUSSION...A warm front will generate periods of light rain
(in the hundredths of an inch) for the northern portion of the
forecast area today. Upslope southerly flow into the hills and
mountains of Del Norte and far northern Humboldt counties will
likely yield higher amounts of rain, with isolated 12-18 hour
totals over the higher terrain possibly reaching an inch by late
this evening. Generally, dry weather is expected for much of the
forecast area tonight as the warm front lifts northward and 500mb
heights increase. The exception is Del Norte county where light
showers may persist through early Thursday morning.

A period of relatively dry weather is expected through most of
the day on Thursday before the next front brings another round of
light to moderate rain, late in the day. Rain is more likely to
arrive overnight Thursday with the passage of the shortwave
trough. Friday may also be mostly dry, however rain with the next
warm front may begin in the afternoon. This next frontal system
will be wetter and may provide a bout of heavy rain over the
weekend as integrated water vapor transport values increase. It
appears to be a weak atmospheric river with GEFS mean IVT values
under 500 kg/m/s. The duration and coverage of the heavy rain is
still not certain. Latest guidance indicates a break in the steady
rain on Saturday as well as a shorter duration heavy rain. GEFS
probabilities of an inch or more in 6 hours increase to 10% to 20%
in the evening or overnight hours Saturday. Before and after that
it is zero. Thus, the period of moderate to heavy rain appears to
be centered on the 12 hour period from 4 PM Saturday to 4 AM
Sunday. It may speed up or slow down, but either way the duration
of the heavier rain appears too short to warrant additional
highlights and products at this time. A weather story graphic
may be the best approach at this time to increase awareness.

Current indications are for snow levels to remain high and above
all NW CA passes through Saturday, then fall to 4000-5000 feet
after trough passage on Sunday. Rain and snow showers are likely
on Sunday in the cooler/more unstable air mass behind the front.
Winter weather products may be necessary for the high mountain
passes; specifically South Fork and Scott Mountains.

An active weather pattern will persist Monday through Wednesday
next week, with occasional bouts of rain and mountain snow about
every other day or so as transient shortwave troughs move over
a dominating northeast Pacific ridge.


Shower activity is expected to slowly increase again to
the north of Cape Mendocino this morning as a weak low- level
shortwave approaches the coast. This should bring locally gusty
southerly winds to Del Norte county, however wind intensity quickly
diminishes toward the south. However, widespread MVFR to possibly
IFR ceilings are expected across Del Norte, Humboldt and Trinity
counties by this afternoon as the system passes overhead. Conditions
then improve late tonight as the pulse dissipates and moves farther
inland. To the south, low clouds which have formed overnight should
mix out by mid to late morning, revealing a VFR day for most of
Mendocino county. /TDJ


.MARINE...Southerly winds persist across the waters today as a
weak low- level shortwave approaches the coast, reaching marginal
SCY levels in the northern outers. Winds should remain relatively
southerly and fluctuate around low SCY levels for the foreseeable
future as a series of weak upper- level shortwave troughs pass
overhead, with a persistent local wind speed maximum in the northern
outer waters with each passing trough.

Seas are expected to quickly diminish today, likely dropping below
10 ft for the inners and southern outers from tonight through
Thursday morning. Short period southerly seas linger in the northern
outers in response to the southerlies today and tonight, and keep a
persistent local maximum in this zone as each of these series of
systems approaches the coast for the next week or so. Another long
period west swell is expected to fill in late this week, elevating
seas through at least the first half of this weekend. /TDJ


Small Craft Advisory until 9 AM PST this morning for
PZZ450-455-475. Small Craft Advisory until 9 AM PST
Thursday for PZZ470.



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