Area Forecast Discussion
Issued by NWS Eureka, CA

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000
FXUS66 KEKA 210001
AFDEKA

AREA FORECAST DISCUSSION
National Weather Service Eureka CA
401 PM PST Mon Feb 20 2017

.SYNOPSIS...
Wet and unsettled weather will persist through much of the week. A
few thunderstorms may also develop, with some small hail possible,
mainly on Tuesday. Colder air filtering into the region aloft will
allow some higher elevation snowfall to develop as well.

&&

.DISCUSSION...
.SHORT TERM...
(Tonight through Tuesday evening)

...A few strong thunderstorms MAY develop tonight and Tuesday...

Quite an interesting forecast is shaping up for the the next 36
hours. A pair of storm system will pass close by to the northwest,
pulling a warm front northward as it does so this evening. The area
remains in a moist southwest flow aloft, and this will only get
reinforced through the short term, as we remain in that flow
pattern. This first storm is also fairly robust in strength, with
water vapor imagery showing a distinct circulation and front/trough
axis. Lightning activity is also fairly decent along this feature,
and a secondary system to its southwest. This pair of circulations
is what looks to have the most impact to our weather.

So lets get down into the details. The first disturbance aloft,
which has an associated surface low, will cross the region this
evening. A secondary vort max will cross the area tomorrow morning.
Strong lift aloft will be maintained as they cross the region. The
SPC mesoanalysis, model data, and area point soundings all show
increasing instability through the night due to moisture advection.
With strong low-level helicity and 0-6km bulk shear in place, we`ll
have to watch the warm front this evening, as 0-1km helicity values
peak around 300 m2/s2 along the front. This will occur right on the
edge of the instability surge (warm frontal passage). However, the
data shows only ~200-400 J/kg of MUCAPE, while the bulk shear values
range from around 50 knots across the outer waters to near 100 knots
across the southeastern third of the forecast area. The latter would
be insufficient instability to pose much of a low-level rotation
concern, but, across the outer waters this could be a much
different story. Typically, while there`s no "hard rule" for
instability/shear combinations, I`d like to see MUCAPEs below 500
J/kg in environments of 50 knots of less of shear, and at least
~750 MUCAPE (if not 1000) above that. The reason being is that
while kinetic energy (wind shear in this case) can compensate for
low instability (thermodynamics), these values tend to be the
upper limits before the updraft gets torn apart in the weaker
thermodynamic environment. As the trailing surface front/trough
crosses the region overnight, a convective line may develop.
Should this occur, increasing instability will allow these storms
to attain greater vertical depth, making downward momentum
transfer (strong wind gusts) more of a concern, with a few gusts
being on the strong to maybe marginally severe side.

Following the passage of the first storm, cooler air aloft will
overspread the region during the day Tuesday. This can be seen
nicely in the model data and soundings, as mid-level (700-500mb)
lapse rates fall into the 7.8 to 8.5 deg. C/km range. This is
largely due to a strong cold pool aloft moving overhead, with models
showing 500mb temps down to around -34 deg. C. This, combined with
diurnal heating and increasing instability may lead to some small
hail with the stronger showers or storms Tuesday. Please use caution
if you encounter and hail while traveling on the area roads.

As for any flooding concerns, the latest trends in the data has been
to shift the higher precipitable water values south of the forecast
area. Although some higher rain rates in stronger or training
convection may pose some local hydro concerns, the overall limited
areal and temporal resolution to such were sufficient to cancel the
flood watch. Should higher rain rates develop, relatively short-fused
advisories will be issued to address these potential concerns.

.LONG TERM...
(Wednesday through next Sunday)

The unsettled weather pattern may continue. Mainly light showers are
anticipated through the first half of Thursday, before things dry
out briefly, before another system dropping south brings more
widespread precipitation back to the area. Colder air aloft will
continue to filter into the region through at least the start of the
weekend, with falling snow levels. These levels will drop as low as
1000 to 1500 feet by Thursday morning, before rising some by the
weekend. As such, higher elevation snowfall will develop, with many
locations seeing at least a dusting of new snowfall. Some passes may
be impacted as well, some of which may approach advisory level.
This is something else we`ll be keeping an eye on with the later
model runs. /PD

&&

.AVIATION...
A potent storm system will produce strong winds and periods of
moderate to heavy rain through tonight. There will be a slight
potential for low level wind shear this evening before the strong
southerly winds mix down to airport elevations. Southerly gusts
around 40kt to 50kt are expected, primarily from 06z-12z at both
KACV and KCEC. Winds will rapidly shift around to the west by
early Tuesday morning. Current indications are for westerly winds
to be strongest near the coast around the Crescent City around
daybreak. Brief westerly wind gusts up to 50 kt will be possible.

In addition to the strong winds, the atmosphere will grow
increasingly unstable tonight through Tuesday. Isolated storms may
affect all the air terminals with gusty and erratic winds, low
ceilings and vsbys due to heavy rain. Lower freezing levels on
Tuesday will increase the risk for accumulating hail on the runways.

&&

.MARINE...
Yet another surface low and frontal system will deepen and
approach the N portion of the area from the SW tonight. Winds over
the S waters have already increased to near gale force, so have gone
ahead and issued a Gale Warning for that area which runs thru 3 AM.
Lingering marginally hazardous seas over the N waters has led to a
continuation of the Hazardous Seas Warning for that area until
midnight tonight when wind speeds increase to gale force and
necessitate a Gale Warning. Wind gusts over the N waters are
expected to reach at least 45 knots over the offshore waters, with
the Gale warning there running thru daybreak. Short period seas will
increase in kind. Winds are forecast to drop off quickly during the
day Tuesday from S to N. Short period seas will be a bit slower to
respond. /SEC

&&

.HYDROLOGY...The river forecast for the Navarro River at Navarro was
dropped enough below flood stage this morning that the decision was
made to cancel the Flood Warning. However, updated info this
afternoon has led the RFC to bring the river back about a foot above
flood stage by daybreak Tuesday. Have gone ahead and re-issued a
Flood Watch for the river. As more up-to-date precipitation data is
used in the model, the 9 PM update should hone in on a number which
will aid in the decision to upgrade to a warning or cancel.
Residents along the river should closely monitor the latest
forecasts and warnings this evening.

Elsewhere, the Eel River at Fernbridge is still forecast to rise
above monitor stage on Tuesday morning. The Russian River at Hopland
is forecast to crest around 14 feet Tuesday morning. While a Flood
Advisory is not presently anticipated, we will continue to closely
monitor. /SEC&KAR

&&

.PUBLIC WIND FORECAST...

Winds are forecast to ramp up again this evening and overnight from
SW to NE as another low pressure system deepens offshore and moves
toward the OR/CA border. This wind event is expected to be a bit
stronger than last night`s, and have issued a fairly widespread Wind
Advisory with a smaller High Wind Warning. Winds will drop off
rapidly from S to N during Tuesday morning. /SEC

&&

.EKA WATCHES/WARNINGS/ADVISORIES...
CA...Wind Advisory from 7 PM this evening to 9 AM PST Tuesday for
     CAZ101>113.

     High Wind Warning from 7 PM this evening to 9 AM PST Tuesday for
     CAZ101-102-104>108-110-111.

NORTHWEST CALIFORNIA COASTAL WATERS...Wind Advisory from 7 PM this evening to 9 AM PST Tuesday for
     PZZ410.

     Gale Warning from midnight tonight to 6 AM PST Tuesday for
     PZZ450-470.

     Hazardous Seas Warning until midnight PST tonight for PZZ450-470.

     Gale Warning until 3 AM PST Tuesday for PZZ455-475.

&&

$$

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