Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Eureka California
333 PM PDT Friday Aug 28 2015
Synopsis...an unseasonably strong front will rapidly move across
northwest California tonight...accompanied by rain and high elevation
gusty winds through early Saturday morning.
Discussion...a stronger than normal system for late August is
taking shape off the northwest California coast. The storm system
is taking on a classic comma shape on satellite and will continue
to deepen as it moves north-northeast overnight to off the Oregon
coast. The low was analyzed by opc at 1007 mb at 18z (11 am) and
is forecast by the models to deepen to near 990mb overnight. This
will result in increasing southerly winds over the coastal waters
and aloft with gusty conditions expected ahead of a cold frontal
passage tonight. Wind gusts 20-30 miles per hour on coastal headlands and
gusts 30-40 miles per hour on ridge tops will be possible late tonight/very
early Saturday morning. At this time conditions are anticipated to
remain below advisory criteria...though considering this is the
first reasonably gusty south wind event of the year...a few
smaller tree branches breaking and falling will be possible in
the areas that experience the strongest winds.
Abundant moisture is also feeding into this low pressure system and
cold front with precipitable water values over an inch and a half
expected to impact the northwest California coast. This will allow
for a band of precipitation to quickly move through the region just
ahead of and with the frontal passage overnight into early Saturday
morning. Rainfall is currently anticipated to range from three
quarters of an inch in the higher elevations of del Norte County to
a tenth or two in southern Humboldt/Trinity counties to nearly dry
or completely dry in southern Mendocino County. The primary
benefit of this rainfall will be to put a damper on wildfire
activity across the interior. Otherwise most of the rain will
quickly be absorbed into the ground and very little impact is
expected with respect to stream flows and water supply. The
biggest hazard is expected to be slippery roadways caused by
built up oil over the last several week dry period.
Rain showers will come to an end on Saturday as the front moves
inland leaving a cooler air mass over the region and broad onshore
flow in place. The best onshore flow is anticipated to be over del
Norte County where upsloping may allow light showers/drizzle to
persist into Saturday afternoon and then redevelop Sunday morning as
a secondary much weaker front impacts the Pacific
northwest/extreme northern California.
Models are in general agreement that a long wave trough will remain
in place over the West Coast for much of the next week resulting in
below normal temperatures and higher humidity. This trough will also
allow shortwaves to swing around the trough possibly bringing brief
shots of shower activity to the region middle to late next week. Models
may be in agreement on the long wave pattern...but that is basically
where the agreement ends with both the European model (ecmwf) and GFS indicating very
different solutions on when chances for additional showers may be
possible middle to late week. Thus...for now...most areas have been
kept dry in the forecast after Sunday except for a slight chance of
showers in northern del Norte County Tuesday into Wednesday. Rpa
Aviation...variable middle and high level clouds will continue to
spread across the region this evening. VFR ceilings will
persist...but this will change late tonight as a frontal system
reaches the coast...resulting in a brief period of moderate to heavy
rain and possible IFR ceilings particularly at cec. Conditions at uki
are more uncertain...but a brief period of light rain and MVFR
ceilings will be possible there by early Sat morning.
Marine...seas continued to be mixed today but generally under 4 feet
with two distinct wave groups centered around 8 and 15
seconds...respectively. A frontal system will approach tonight and
south winds will increase...likely into the 15-25kt range. There is
a bit of uncertainty regarding just how strong the winds will get.
For now leaned toward the higher resolution models which indicate
25kt late tonight. This front will also result in a brief period of
moderate and perhaps heavy rain tonight...followed by some showery
activity tomorrow morning. Peak wind speeds will likely be reached
during the overnight hours tonight across the northern outer waters
and begin to decrease by tomorrow evening. This should allow steeper
short period seas to develop...but multiple swell groups will also
be present in the waters. The first of which that may be moderately
significant will be a westerly long period swell set to build into
the waters overnight tomorrow and into the day on Sunday. While not
large...this will build to around 4 feet at 17 seconds Sunday...at a
time when winds and short period seas should be on the decline.
While not a huge concern for boaters...this could present a moderate
sneaker wave threat for area beaches depending on how quickly winds
subside on Sunday. Sunday night...an additional northwesterly swell
will build into the waters...although this will likely come in with
a shorter period of around 12 to 13 seconds. Still...forecasted
heights of around 6 to 7 feet will make this swell something to
watch out for early in the week.
Fire weather...a low pressure system will swing a cold front
through northwest California tonight bringing a period of rain
showers with wetting rainfall likely north of Mendocino County.
Rainfall amounts are still expected to range from 0.10-0.20 in
southern Humboldt to as much as three quarters of an inch in favored
terrain with upslope flow in the mountains of del Norte County. The
front will also bring some gusty ridge top winds immediately in
advance of the frontal passage. Gusts between 30 and 40 miles per hour will be
possible on the higher peaks of the interior and coastal ranges.
As the front moves east of the area Saturday morning the majority of
the rain showers will come to an end...though on and off rain may
linger in del norte and southern Humboldt into Sunday as another
weak front moves into the Pacific northwest and extreme northern
A cooler than normal and moist atmospheric pattern will persist for
much of the next week as a long wave trough axis remains in place
over the West Coast. Rpa
Northwest California coastal waters...
Small Craft Advisory from 1 am to 9 am PDT Saturday for pzz450.
Small Craft Advisory from 10 PM this evening to 9 am PDT
Saturday for pzz470.
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