Area Forecast Discussion
Issued by NWS Eureka, CA
FXUS66 KEKA 272258
AREA FORECAST DISCUSSION
National Weather Service Eureka CA
358 PM PDT WED JUL 27 2016
.SYNOPSIS...Hot and dry conditions will continue through the end
of the week across the interior. Coastal areas will likely see
little change, with nocturnal and morning clouds and near normal
temperatures. A slight cooling trend is expected late this
weekend through early next week.
.DISCUSSION...Uncomfortably hot temperatures continue to to
plague most of interior northwest California today as an upper
level ridge persists across the desert southwest. As of 3 PM,
temperatures have eclipsed 100 degrees in many locations, and are
actually anywhere from 1 to 5 degrees ahead of yesterday`s
temperatures at the same time. Fortunately though, overnight low
temperatures across many interior valleys were actually a bit
cooler than anticipated in many areas, with temperatures falling
generally into the 50s in the valleys thanks to efficient
nocturnal cool air drainage. Meanwhile, temperatures along the
ridges and upper slopes only cooled to the upper 60s to near 70,
likely due to the presence of a thermal belt aloft. Upon a close
examination, it appears that model data is struggling to resolve
this microscale phenomenon, particularly the NAM which appears to
be over-forecasting nighttime lows by 10 to 15 degrees in some of
these valley locations. As such, low temperatures were adjusted
to better reflect this by using primarily the GFS and manually
tweaking interior valleys, which for most interior valley
locations means a drop in forecast lows by about 5 degrees, give
or take a few degrees. However, even in these valley locations
folks can still expect a very slight uptick in lows through
Saturday morning, as continued hot days will reduce the
effectiveness of nocturnal drainage as the surface continues to
bake in the afternoon sun. Meanwhile, afternoon high temperatures
will continue to reach triple digits inland through at least Friday,
and potentially Saturday. A few showers/thunderstorms will be
possible Friday afternoon/evening across the northernmost
mountains of Trinity county, and for a more detailed discussion
please see the fire weather section below.
Along the coast, conditions will more or less remain static
through the weekend, with a shallow but persistent marine layer
resulting in overnight/morning clouds and fog each morning.
Temperatures will remain near average despite the interior heat,
as offshore winds will likely not be strong enough to erode the
marine layer along the coast.
By Saturday, a slight weakness in the upper level ridge will
deepen across northwest California, resulting in lowering
1000-500mb temperatures and a slight cooling trend that will carry
us through early next week. "Cooling trend" may be a generous
description, as high temperatures will remain generally in the
90s across interior valleys throughout this period.
.AVIATION...Current conditions across Northwest California range
from VFR to IFR with the marine layer eroding off the coast.
Current thinking is that KCEC and KACV will see a brief period of
VFR conditions with the stratus and fog returning between 03Z and
07Z. The coastal terminals will eventually fall back into IFR
conditions, likely happening after 10Z or 12Z if they continue to
follow the same trend as the past few days. Interior terminals
will stay VFR. KAR
.MARINE...A tight pressure gradient will persist through the end
of the week, as a strong area of high pressure maintains residence
across the eastern Pacific, and a "loosely" organized thermal low
has developed across the interior. The resultant tight pressure
gradient is keeping winds elevated across the outer waters, with
seas of 9 to 11 feet with periods of 9 seconds. This is mainly
affecting these waters from Cape Mendocino north, where gale
warnings are in place.
South of Cape Mendocino from 10 to 60 nautical miles from shore,
wind fields are a little bit lighter, with seas around 8 feet and
a similar wave period observed here, and with the nearshore waters
within 10 nautical miles. Thus, the current headlines in place
The numerical models have increased the winds/wave for the end of
the week, with some areas seeing a 3 to 6 foot increase compared
to the current observations. Given this has been a recent
development in the model data over the last 12 hours, we decided
to sit on the current headlines and await another model run or two
before potentially increasing/upgrading the current
warning/advisories. There will be a headline extension in the
products issued within the next 24 hours once the new trend in the
data helps us pin down temporal/headline type issues. /PD
.FIRE WEATHER...Hot and dry conditions with primarily terrain
driven winds will continue through at least Friday, with a slight
cooling trend expected late this weekend into early next week.
Otherwise, the primary item of interest will be the potential for
isolated thunderstorms Friday afternoon/evening, and to a lesser
degree Saturday afternoon/evening. Model guidance continues to
resolve a weakness in the upper level ridge developing off the
Northwest California Coast Friday, in addition to a weakly
enhanced area of vorticity. While moisture is somewhat limited and
overall forcing will be weak, this feature may be just enough to
trigger a few high-based thunderstorms across the mountains of
central Siskiyou county and perhaps northeastern Trinity County.
While confidence is low, and the focus of convection likely to
occur north of NWS Eureka`s forecast region, the rapidly drying
fuels will require that close attention be paid to this scenario
as any thunderstorms that develop will have high bases and will be
somewhat dry, increasing the likelihood of fire starts/spread.
Fortunately, wind speeds will remain mostly light and terrain
driven. The focus for convection will likely be farther east
Saturday, but given that this remains a few days away, close
attention will be paid to the setup for Saturday as well.
NORTHWEST CALIFORNIA COASTAL WATERS...Small Craft Advisory until 2 AM PDT Thursday for PZZ450.
Gale Warning until 3 AM PDT Friday for PZZ470.
Small Craft Advisory until 3 AM PDT Friday for PZZ475.
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