Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Eureka California
355 PM PDT Tuesday Sep 1 2015
Synopsis...temperatures will remain near or below normal for the
next several days as a trough sits over the West Coast. Little to
no rain is expected...although a few light showers will be
possible north of Cape Mendocino Wednesday.
Discussion...mostly clear skies have dominated today ahead of the
next advancing frontal boundary in association with an upper low
moving into the Pacific northwest. Moisture will be more limited
with this system...so only a few hundredths of an inch of rain is
anticipated primarily in northern Humboldt and del norte counties
later tonight through Wednesday as the front washes out over the
area. Models have trended slower with the advance of the front so
precipitation has been delayed 6-hours with this forecast update.
Drier air will quickly build in behind the dissipating front leaving
northwest California dry Wednesday night through Thursday. There is
a slight possibility that storms could be triggered over the Trinity
horn Thursday afternoon as another upper level wave moves overhead
..but overall dry and mostly sunny conditions are forecast.
Model guidance has been trending farther south with an upper level
shortwave that attempts to cut off over southern Oregon or far
northern California Thursday night into Friday. The NAM is the
farthest south with the low...moving directly over northwest
California possibly triggering showers/thunderstorms over the interior
mountains Friday afternoon and evening...while the GFS and European model (ecmwf) are
more progressive moving the upper low/trough through southern Oregon
leaving northwest California mostly dry. For now opted to introduce
a slight chance of showers over the higher peaks of eastern Trinity
County Friday afternoon and otherwise leave the forecast dry.
Regardless of the precipitation forecast...temperatures will be much
cooler starting Wednesday through Saturday behind the initial
frontal passage and within the West Coast trough. Highs across the
interior will likely only peak out in the upper 70s and lower 80s
late in the week and possibly through Saturday.
By late in the weekend the upper low will move east transitioning
the trough axis over Nevada. This will allow northwesterly flow to
develop aloft and east-northeasterly flow to develop near the
surface as the thermal trough strengthens across the interior and
high pressure builds off the coast. Temperatures will warm back to
near normal and easterly winds are anticipated during the overnight
periods Sunday into early next week. Rpa
Aviation...VFR conditions and light to moderate winds will continue
at kcec and kacv through the early evening. VFR to MVFR conditions
and light winds to calm conditions will occur at kcec and kacv
overnight with the lower conditions due to stratus and/or patchy fog
reducing ceilings and/or visibilities. VFR conditions and light to
moderate winds will prevail at kcec and kacv Wednesday. VFR
conditions and light winds to calm conditions will continue at kuki
Marine...winds and seas have subsided. Light to moderate
northerly winds will prevail across the waters through tonight.
Winds and seas will increase Thursday and Friday as a high
pressure system strengthens across the northeastern Pacific and
interacts with an inland low pressure trough. Winds and seas will
be higher across the outer waters. Small craft advisories will be
necessary across the southern outer waters Thursday through this
weekend and across the northern outer waters Thursday night
through this weekend due to increasing winds and short period
seas. Small craft advisories are likely across the near shore
waters Saturday due to increasing short period seas.
Fire weather...moisture within the valleys mixed into several
higher elevation sites this afternoon resulting in an increase in relative humidity
through the day...particularly in del Norte County. As the
atmosphere fully decouples again this evening the dry air should re-
establish itself on many of the Mountain Ridge tops before the
moisture associated with an advancing front moves into the area.
Much of Humboldt...del norte...and potentially western Trinity
counties will be impacted by this front bringing increasing cloud
cover and higher humidity later tonight through Wednesday. Farther
east and south cloud cover will remain minimal and drier air will
remain in place.
Another push of dry air will quickly move in behind the front
Wednesday night. Model soundings indicate that subsidence will not
be as substantial Wednesday and Thursday nights...likely
resulting in less drying on the ridge tops and higher nocturnal
inversions. There will also be the potential for some valley clouds
to form each night where moisture pools best in Humboldt and del
A system on Friday may bring some showers or possibly thunderstorms
to the interior...but models remain very inconsistent on exactly how
deep a trough will dive Thursday night into Friday. Thus confidence
in showers/tstorm formation is very low at this time and chances for
showers have been limited to far eastern portions of Trinity County
Friday afternoon in the more climatologically favored locations.
Regardless of the precipitation potential...temperatures will run
several degrees below normal beginning Wednesday through Saturday.
Dry and warm conditions will return Sunday into early next week as
light offshore flow develops. Rpa
Northwest California coastal waters...none.
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