Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Eureka California
450 am PDT Monday Aug 3 2015
Synopsis...an upper level low off the coast will bring isolated
showers and thunderstorms today and Tuesday. The best chance is
in the afternoon over the inland areas. Weak ridging will bring
dry conditions Wednesday and Thursday with another system Friday
into the weekend bringing additional showers and thunderstorms.
Discussion...an upper level low continues to spin off the California
coast with shortwaves rotating around it. This shows up well in
water vapor. A shortwave has been bringing some showers and
thunderstorms to southern Oregon this morning...but these have
remained north of the area. Another weak shortwave looks to bring
a few showers and thunderstorms to the area this afternoon.
Coverage does not look very expansive...but there is a slight
chance for thunderstorms in northern Trinity County...eastern del
norte and northeastern Humboldt counties. The marine layer is
fairly deep once again and the coast is expected to remain mostly
cloudy this afternoon. Tonight a few showers may linger in del
Norte County...but confidence is low on these.
Tuesday another shortwave moves through the area from the
southwest. Expanded the thunderstorms a bit farther south in the
afternoon and evening. Moisture may be a bit limited...but hard to
rule out a thunderstorm or two. The marine layer is expected to
remain fairly deep and may not clear at the coast in the
Wednesday the upper level low retrogrades a bit and a weak period
of ridging will bring some slightly warmer temperatures. There is
some weak offshore flow...but this is confined to the hills of del
Norte County and looks weak. The coast will likely stay mostly
cloudy...although the marine layer may compress down a bit.
Thursday the upper level low starts to approach the area again.
Offshore flow starts to increase north of Cape
Mendocino....although it remains fairly light. This may be enough
the dry things out a bit more and bring some gusty east winds over
the ridges at night. Also...this may bring some more clearing to
the coast in the afternoon. At this point thunderstorms are
expected to hold off until Friday...but this will need to be
watched. Moisture does look to increase on Thursday...but models
arent showing any quantitative precipitation forecast and instability is limited. Friday the
upper level low moves overhead and thunderstorms look more likely.
Storm movement will likely be slow as the low moves overhead. The
GFS is slightly faster with this upper level low than the European model (ecmwf) or
Gem. Closed upper level lows tend to slow down more than forecast
so would favor the slower solution.
For the weekend models are in decent agreement on an upper level
trough settling over the area. This may bring some thunderstorms
to the area...but with west to northwest flow it is not a
particularly favorable pattern for thunderstorms. This will need
to be watched. Mkk
Aviation...stratus has once again pushed well into the coastal
river valleys overnight. Ceilings should fall below 1000 feet by
daybreak with perhaps intermittent mist at both kacv and kcec. The
marine layer does not appear to be deep enough to push into Ukiah
from the west this morning. It may creep in from the south though
confidence is not high that will occur. High resolution models
continue to indicate high boundary layer humidities this morning
easing up during the afternoon at the coast. Suspect there will be some
improvement at the coastal airports this afternoon...though
confidence is not high it will last very long at kacv or kcec.
Profiles indicate a deeper moist layer pushing onshore again by this
evening and then into the river valleys overnight as an upper trough
offshore starts to move eastward toward the coast. The chance for
low clouds getting into Ukiah Muni Tuesday morning look slightly better
as the marine layer deepens.
Marine...broad low pressure and loose gradients will remain over
the area today and tonight. High pressure over the NE Pacific will
start to build eastward toward the Pacific northwest coast by early Tuesday
morning...creating a tighter pressure gradient over the northern
outer waters. The GFS...European model (ecmwf) and nam12 indicate northerly winds
reaching 20kt in the northern 1/3rd of pzz470 by Tuesday. This is not
quite advisory level...however northerly winds will continue to
increase in the outer waters Tuesday night through Wednesday. The GFS
continues to indicate low end gales on Thursday...with gusts to 35-40kt
possible over the northern outer waters. It is still several days
out and the models can change. Confidence is high winds will
increase. The exact magnitude is still sketchy.
Seas have been around 3 feet at 8 seconds this morning. Seas will
remain low today with heights around 3 feet with periods around 8
seconds. The nwps and enp show quite a mix of longer period waves
all of which are fairly small...about 1 feet or less. The dominating
wave group has been centered near 8 seconds...so will lean toward
the shorter period waves in the forecast. The nwps and enp has wind
driven waves reaching 5 to 7 feet at 7 seconds by middle morning Tuesday in
pzz470. 7 feet at 7 seconds satisfies our steepness criteria for
hazardous seas...so will hoist a Small Craft Advisory for pzz470
starting Tuesday morning. Wind-driven seas should continue to increase
each day through Thursday. Based on our local criteria...a warning for
hazardous seas may be necessary in the outer waters if seas reach 10
feet or more with periods of 8 seconds or less. We are following the
nwps guidance which has seas reaching 13 feet on Thursday in pzz470. The
enp was a couple of feet less...but still near criteria.
Fire weather...there is a slight chance for thunderstorms this
afternoon. The best chance is inland areas north of Highway 299.
Tonight similar conditions are expected with decent recoveries in
the valleys and moderate to poor recoveries over the higher
elevations. Tuesday a decent shortwave moves in from the
southwest. This brings a chance for slightly elevated winds and
additional thunderstorms. For now moisture looks limited...but
lapse rates look good so this will need to be watched.
Wednesday some weak ridging will start to warm temperatures a bit
and lower relative humidity. Thursday will like be slightly warmer and drier as
well...although there is an outside chance for a thunderstorm
Thursday night. Friday there is a better chance for thunderstorms
in the inland areas. Cooler temperatures and more moisture is
expected for the weekend as an upper level trough approaches the
Northwest California coastal waters...
Small Craft Advisory from 8 am Tuesday to 5 PM PDT Friday for
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