Area Forecast Discussion
Issued by NWS Eureka, CA
FXUS66 KEKA 071248
AREA FORECAST DISCUSSION
National Weather Service Eureka CA
448 AM PST Wed Dec 7 2016
.SYNOPSIS...High pressure was located from WRN Canada SWD across
the Intermountain West and NRN Great Plains early Wednesday
morning. As a result, a cold continental airmass was entrenched
across NRN CA. However, a pattern change is in store for the
region as a warm front tied to an occluded cyclone positioned over
the NERN Pacific advances E Wednesday night into Thursday.
Thereafter, moist onshore flow is forecast to persist into next
week, favoring an extended period of wet rainy weather.
.DISCUSSION...Short term forecast problems are tied to the
presence of cold boundary layer temperatures. First, morning
surface observations show many sites are experiencing temperatures
between 30-36 F for coastal and inland areas. Thus, frost
advisories/freeze warnings valid until 10 AM PST appear to be
on track. Attention then turns to the previously mentioned warm
front. High pressure over the Great Basin combined with lower
pressure associated with the front will cause surface winds to
become ELY/offshore this afternoon and evening. As a result, drier
air will likely be locked in place over Trinity County, such that
the onset of moderate frontal precipitation this afternoon will
cause sufficient wet bulb cooling in the boundary layer for
snow...with accumulations of 1-4 inches expected (refer to the
latest WSWEKA for additional information).
Model guidance then shows dry midlevel air spreading east across
Trinity County during the night, accompanied by a prominent 1-4
deg C warm nose centered near ~800 mb. The dry air will aid in
reducing precipitation rates, such that drizzle may become
predominant by late night due to decreasing moist layer depth. In
addition, the warm nose will eventually favor a complete
transition to liquid precipitation falling over valley locations,
with liquid possibly refreezing at the surface due to cold air
maintenance by the ELY low-level winds. The end result will be
Wednesday evening snow transitioning to late night rain and/or
freezing drizzle. Day shift will need to re-evaluate for possible
freezing rain advisory.
Otherwise, a pre-frontal 40-50 kt low-level jet will reach
coastal locations during the late afternoon into Wednesday
evening. This jet is expected to yield windy conditions for
ridgetop locations...with gusts possibly hitting advisory
.EXTENDED DISCUSSION (FRI-TUE)...General westerly zonal flow
is expected from Friday through Tuesday. This will translate into
a very wet period for northwest CA as a tap of subtropical moisture
gets entrained into the flow from the central and western pacific. Periodic
fronts associated with parent low pressure systems migrating across the
southern Gulf of Alaska will provide the lifting mechanism and dynamics to
deliver plenty of rainy days to the north coast and and interior zones. The
first system during the extended period will arrive on sunday with a modest
amount of precipitation expected through sunday night (appx 1-2 inches according
to the GFS) but it should be noted that the Euro model is keeping Sunday dry so
obviously confidence is low for this feature. Later on Monday both extended
models come into better agreement on the formation of an Atmospheric River type
pattern shaping up with general consensus of a sustained wet
period finally ending on Thursday. This solution will likely be
quite warm with snow levels above pass levels with ranges of total
rainfall for the region of 4 to 7 inches. As this solution remains
6 to 8 days out there is plenty of time for things to change but
both extended models have been fairly consistent with this now for
the last few days so confidence in the wet week is growing.
.AVIATION...Drying easterly flow has brought VFR conditions
to all the terminals this morning. High clouds in advance of
an approaching frontal system will spread over the area this
morning. The easterly and southeasterly winds will start to increase
this afternoon as the front grows closer to the coast. Turbulent
flying conditions will be on the increase through the evening hours.
Even though the models show some light rain developing, conditions
will be too dry in the lower atmosphere for low ceilings and vsbys
at the coastal terminals. KUKI may see occasional MVFR conditions
and perhaps low level wind shear this evening. Otherwise VFR will
continue to prevail through the period.
.MARINE...The northwest wave systems have finally fallen off to
around 7 to 9 ft. Expect to see a steady decline in the NW wave
systems through the day today and not much in the way of southerly
wave systems. Attention now turns to an approaching storm that will
bring strong winds late today into tonight.
Initially the strong SE winds will be confined to the southern
waters as a warm front approaches from the SW. The strong southeast
winds will spread north of the Cape tonight as a triple point
develops just north of Cape Mendo. The boundary will steadily move
closer to the coast on Thu and winds will remain breezy to windy. A
GALE WARNING remains in effect for the outer waters. A GALE warning
has been hoisted for the inner waters south of the Cape as well.
Confidence is not high the northern inner waters will receive gales.
The offshore storm will generate a very large and steep southerly
wave system tonight, up to 15 to 17 feet. This southerly wave system
should decay quickly after winds die down on Thu. Another short
period SW wave system will arrive on Thu. Current indications are
for borderline advisory conditions due to the short periods. A
westerly wave system will start to arrive on Fri with heights that
should be sufficient to warrant an advisory. Confidence is by no
means high as the ENP has been all over the place for the last
couple of runs. Seas should slowly lay down over the weekend,
however with the wave system model being so far off lately, we may
end up seeing hazardous wave system conditions through the weekend.
CA...Frost Advisory until 10 AM PST this morning for CAZ101-103-104-
Winter Weather Advisory from 1 PM this afternoon to midnight PST
tonight for CAZ107-108.
Freeze Warning until 10 AM PST this morning for CAZ102-105-106.
NORTHWEST CALIFORNIA COASTAL WATERS...Small Craft Advisory until 8 PM PST this evening for PZZ470.
Gale Warning from 8 PM this evening to 10 AM PST Thursday for
Small Craft Advisory from 3 PM this afternoon to 6 PM PST this
evening for PZZ455.
Gale Warning from 6 PM this evening to 4 AM PST Thursday for
Small Craft Advisory from 8 PM this evening to 4 PM PST Thursday
Small Craft Advisory until 1 PM PST this afternoon for PZZ475.
Gale Warning from 1 PM this afternoon to 4 AM PST Thursday for
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