Area Forecast Discussion
Issued by NWS Eureka, CA

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000
FXUS66 KEKA 212226
AFDEKA

AREA FORECAST DISCUSSION
National Weather Service Eureka CA
326 PM PDT Fri Jul 21 2017

.SYNOPSIS...High pressure will continue to build across the
region through the weekend. Temperatures will warm significantly
across the interior. Coastal clouds and patchy fog will continue
to develop each night and early morning, burning off by late
morning. Isolated thunderstorms may develop across interior
portions of Del Norte and Humboldt counties, and across northern
Trinity county for Sunday and Monday.

&&

.DISCUSSION...It continues to look like pleasant weather will
continue into Saturday, as high pressure builds across the region
and an offshore flow pattern emerges. A significant warm up is
expected for the interior this weekend. Models continue to
indicate 850mb temperatures climbing to 24C to 27C on Saturday
and Sunday. Assuming a well mixed dry adiabatic layer, this is a
good indicator for temperatures reaching 100 to 105F in the interior
valleys.

An offshore flow patten will develop tonight and continue
through the weekend. The offshore flow should result in less
stratus penetrating into the coastal river valleys. At the same
time, the inversion will lower and strengthen which means
clouds/fog confined to mainly the immediate coast. At the moment,
the northerly winds offshore appear to be sufficient to mix out
the stratus with the heating of the day. Temperatures will
continue to be pleasant and a tad above normal for the coast; mid
60s to lower 70s. You will not have to go very far inland to find
readings in the upper 70s and 80s. Farther inland, highs will be
quite toasty with highs well into the 90s, with valley hot spots
topping out around 100 to 105 degrees. Be sure to stay cool and
drink plenty of water.

By far the main forecast concern going into Sunday and Monday
will be the potential for thunderstorms and lightning activity,
mainly for the interior mountains. The models continue to indicate
an upper-level low approaching the northern California coast just
south of Cape Mendocino on Sunday. What has not been as
consistent is the rate at which the cyclonic circulation aloft
will progress eastward across the forecast area. The latest runs
have slowed down the eastward translation slightly and the threat
for storms appears to last through Tuesday.

The GFS and NAM12 continues to indicate PWATS around 1 inch
across Del Norte and far northern Humboldt counties on Sunday as
the offshore low gains a slight negative tilt. The Somes Bar
BUFKIT sounding (from the NAM) continue to show southeasterly
winds aloft with CAPE values in excess of 1500J/KG, lifted indices
near -3C and lapse rates greater than 7.5C/KM. The mid level flow
pattern does appear to be a favorable for storms over the western
portions of the forecast area. There are indications we could see
high based storms out over the coast and over the coastal waters.
The models have been incongruent regarding storms over the coast
and waters, however. The GFS has been consistently indicating the
potential for some showers Sunday afternoon and evening, while
the NAM12 and ECMWF have been dry as a bone. At this point, feel
the GFS may be overdone with the convection and will continue to
keep rain chances around 10% for coastal areas. This will need to
be watched closely. If planning outdoor activities this weekend,
keep abreast of the latest weather forecast and keep a watchful
eye on the sky.

For the middle to end of the week, drier southwest to westerly
flow will develop, with heights lowering as well. This will put an
end to thunder threat, while allowing temperatures to drop back
to more seasonal values for the interior.

&&

.AVIATION...Marine stratus (north of the Cape) continued to lurk
along the coast, while also extending farther into the coastal
mountain & river areas this morning. Status began eroding to the
coast after midday as expected...except at CEC where clouds
scattered out a little earlier. However, Cigs and Vis took a nose
dive at ACV as lower clouds advected onto the coast towards late
morning. Around 930 am, MVFR Cigs had dropped into IFR and then
hit minimals of 200 feet and 1/4 miles...lowering into LIFR.
Stratus expectations may be a little lacking overnight mainly at
the CEC airport as I concur with previous discussion that an
offshore component may limit stratus development. The 925 mb winds
clearly indicate a substantial offshore (northeasterly) wind
component overnight. MOS guidance is "somewhat" reflective of this
but still show a stratus Cig around 8 AM. South of ACV can expect
the return of stratus IAW guidance. Inland mountain/valley areas
will remain VFR. At this time, smoke from neighboring counties
will probably have no, or little impact on local flight
operations.

&&

.MARINE...Temperatures over northern California are forecast to
increase this weekend, which will aid in lowering surface
pressure, and subsequently tightening the gradient along the
coast. Northerly winds will increase as a result, with gale force
gusts forecast over the northern outer waters Saturday afternoon
through Monday afternoon. Large waves will build in response to
these winds especially for the outer waters. By midweek, model
guidance indicates the tight pressure gradient may shift slightly
westward, which would favor weakening winds across the coastal
waters. JG/KR

&&

.FIRE WEATHER...High pressure aloft will build across the region
through Saturday resulting in strong warming and drying. Easterly
ridge top winds will also increase tonight and into the morning
hours on Saturday, as a surface trough develops near the coast.
The upper elevations will likely have poorer recoveries as a
result. Heading into the afternoon hours on Saturday, interior
temperatures of 95 to 105 degrees are expected, with very low
humidities in the afternoon. Gusty easterly winds will likely
develop across the upper elevations of Del Norte and far northern
Humboldt counties Saturday night and Sunday morning.

As we head into the Sunday and Monday, a upper level low will
develop off the coast and interact with increasing instability. As
a result, a few thunderstorms may develop across eastern Del
Norte and Humboldt counties, into northern Trinity County.
Soundings continue to indicate cloud bases running near 8000 feet,
making gusty winds a concern in addition to the lightning with
any storm that develops. Drier westerly flow aloft will put an
end to our convective threat by mid week.

&&

.EKA WATCHES/WARNINGS/ADVISORIES...
CA...None.

NORTHWEST CALIFORNIA COASTAL WATERS...Small Craft Advisory from
10 PM this evening to 3 PM PDT Saturday for PZZ470. Gale Warning
from 3 PM Saturday to 5 PM PDT Monday for PZZ470. Small Craft
Advisory until 5 PM PDT Monday for PZZ475.

&&

$$

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