Area Forecast Discussion
Issued by NWS Eureka, CA

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000
FXUS66 KEKA 191338
AFDEKA

AREA FORECAST DISCUSSION
National Weather Service Eureka CA
518 AM PDT Tue Sep 19 2017

.SYNOPSIS...Showers and cooler temperatures will persist through
the week due to an unseasonably cold upper level low. High
pressure will rebuild this weekend leading to a warming and
drying trend into the early portion of next week.

&&

.DISCUSSION...Showers have been increasing in coverage overnight
across the northern and western most portions of our area. Another
impulse in the brisk westerly flow aloft will bring an uptick
in showers through the early morning hours, before another lull
develops by mid to late morning. A surface low and frontal
boundary will develop offshore this afternoon. Southerly flow and
warm air advection will result in slight drying for most of the
area this afternoon. The exception will be across Del Norte
county where upslope flow will produce pre-frontal showers.

Rain will start to increase again late this afternoon into this
evening as a frontal system moves toward the north coast. We will
see a period of gusty southerly winds in advance of the front this
evening, primarily over the ridges and coastal headlands, followed
by a rapid shift to the northwest early Wednesday. The southerly
downslope flow in the lee of the King range may delay the arrival of
rain for the Eureka area. The high resolution models capture the lee
side drying the best.

The front will move across the area overnight through early
Wednesday morning and provide a period of moderate to heavy rain.
Totals of a quarter to half an inch are expected for Del Norte and
Humboldt counties. The west slopes and higher elevations will no
doubt see greater quantities due to the enhancement from
topography. A few hundredths to tenth or two will be possible for
Mendocino and Trinity counties.

Cold air aloft and greater instability will follow behind the
front on Wednesday. On and off showers will probably continue
through the day on Wednesday for Del Norte and Humboldt counties
as a post frontal trough embedded in NW flow approaches the north
coast. The potential for low topped convection will increase by
Wed evening with 500mb temperatures plummeting to -25C over the
coastal waters. The upper trough will dig SW and away from land
during this time frame and prevent thunderstorms from reaching
the coast.

Colder and drier air will settle over the area on Thu. There will
be a threat for morning frost in the interior valleys. The greatest
threat will be across Trinity county, however some spots in
northern and eastern Mendocino county may also see early morning
frost. The threat for early morning frost in the valleys will
continue on Fri and perhaps into Sat even as the air warms aloft.

The chance for showers will continue on Thu. The models were
still not in agreement. The NAM12 and GFS dry us out, while the
ECMWF continues to indicate the potential for showers. There will
probably be a fair amount of cumulus with daytime heating thanks
to the cold air aloft. Thus, a few terrain driven showers may
sprout up over the mountains.

The cold weather will moderate this weekend as high pressure
builds back over our region. An offshore flow will result in a
warming and drying trend this weekend that will last into the
early portion of next week.

&&

.AVIATION...An approaching frontal boundary will bring mid level
cloud cover and southerly winds today with rain first impacting
the north coast later this evening. VFR ceilings will lower
through the afternoon as the band of rain approaches the region.
Expect MVFR to IFR as the rain impacts the coast this evening with
periods of LIFR possible with heavier showers. Conditions will
improve behind the boundary on Wednesday. /KML

&&

.MARINE...One of the first large swells of the season will bring
10 to 13 ft waves around 16 seconds to northwest California waters
today and then gradually subside through the rest of the work week.
The small craft advisories for large seas remain in effect,
although, they may need be adjusted to end earlier as the forecast
suggest that seas will fall below criteria later Wednesday. Moderate
winds, on the other hand, will continue over the next several days
as multiple passing frontal boundaries maintain a weak pressure
gradient across the coastal waters. Today, an approaching front will
bring west to southwesterly winds through tonight. Gusts to 20 kt
will be possible nearshore this evening. Then increasing northerly
winds will return late this week as high pressure offshore builds
towards the west coast. /KML

BEACH/SURF CONDITIONS...While tomorrow`s approaching swell is
not particularly large by the standards of northwest California
beaches, it is the first significant swell of the season, and
this has a few special implications that may not otherwise apply
during peak winter season. Area beaches currently feature a
summertime profile shaped by an entire season`s worth of smaller,
steeper waves. As larger, longer period waves move into this
environment, the gradual slope of the ocean floor may allow waves
to run farther up the beach than normal, which will make some
typically safe areas hazardous to beachgoers. While this effect is
much more dramatic with larger waves than we`re expected this
week, localized areas with favorable beach orientation and shape
may allow this to occur. As a result, beachgoers should be extra
vigilant when venturing near the surf zone, particularly Tuesday
and Wednesday, as sneaker waves will be possible. /BRC

&&

.EKA WATCHES/WARNINGS/ADVISORIES...
CA...Beach Hazards Statement from 8 AM PDT this morning through
     Wednesday evening for CAZ101-103-104-109.

NORTHWEST CALIFORNIA COASTAL WATERS...
Small Craft Advisory til 3 AM PDT Fri for PZZ450-455-470-475.

&&

$$

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