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Area forecast discussion 
National Weather Service Eureka California
433 am PDT Tuesday Jul 7 2015

Synopsis...a disturbance off the California coast will bring a
chance of showers and thunderstorms to portions of interior northwest
California through the week. There will be a slight chance that
storms may impact coastal areas this evening through Thursday.

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Discussion...morning satellite imagery shows an upper low off
the central California coast continuing to slowly track eastward.
This low is acting to deepen the marine layer of northwest cal, lifting
the clouds from the immediate coast, causing mostly cloudy
conditions instead of fog. However, fog will be present in the
coastal hills this morning. This pattern should repeat over the
next few days due to this low. As a side note, sea surface
temperatures are running between 57 and 60f. These types of SST
can make it challenging for coastal fog to form as well.

The main threat with this low will be thunderstorms and heavy
rain. Models continue to remain in good agreement with an unstable
atmosphere over northwest California through the week. However, models
are not handling the low well as data in and around this low is
very limited. As it moves into a more data rich region, we should
get a better idea as to the structure of the low. That said,
satellite interpretation shows a dynamic low approaching with a
deep moist layer. Models are showing MUCAPE values in the 1700 to
2100 j/kg with precipitable waters over an inch. Lifted indice's are negative, low and middle
level shear is in place, and northwest cal looks to be in an area of
diffluence aloft. This is all the needed ingredients to generate
strong convection. With deep layer moisture, these storms should
be very wet. Freezing levels are looking to be a touch high for
large hail to develop, but small hail is possible in the stronger
storms. If the freezing level drops more than models indicate, we
could see hail approaching severe size. Also, as larger storms
collapse, there could be wind gusts approaching 50 miles per hour. The
strongest storms are more likely on Wednesday, but today could see
a few roll across the area. Strong storms are possible again on
Thursday, but it looks like the dynamics should be shifting a bit
east of northwest cal by then. A low in the this region is our classic
thunderstorm pattern.

In terms of motion, being on the northern periphery of the low we
will have easterly flow aloft which should cause the storms to
track from east to west. If some of the stronger storms hold
together, there could be an isolated storm that drifts over the
coast. Models currently indicate that this should be over del
norte and Humboldt counties, but Mendocino County could see a
storm or two as well. Along those lines, a shift in the storm to
the north or south will change the location of the best
instability and where the storms initiate. While the forecast
continues to gain in confidence of thunderstorms occurring, there
remains skepticism as to where they will occur. Keep an eye on
the forecast.

This low will bring in a cooling and moistening air mass. Daytime
high temperatures will be below normal for much of the week, but the
overnight lows will likely be higher than normal due to the moist
air mass. As this low exits the region early in the weekend, an
upper trough will dig in from the northwest. This will keep a slight
chance of thunderstorms in the forecast, over the Trinity
Mountains, through the weekend and into early next week. This will
be an interesting pattern to watch as it unfolds. Bfg

&&

Aviation...10z satellite imagery shows areas of stratus/fog along
the Redwood coast and mostly clear skies inland. Along the
coast...conditions should improve to VFR late this morning as the
stratus/fog Burns off, with VFR persisting into the afternoon.
Inland...moisture and instability will generate showers and strong
thunderstorms this afternoon. Tempo reductions are expected in
some of the stronger showers/storms.

&&

Marine...the buoys this morning are reporting calm to light winds
and mixed seas ranging from 4-5 feet. The 04z ascat pass showed calm
to light winds up to 5 kt, which was in line with the forecast. The
models are in great agreement showing an upper level low (which was
located near 38n 130w at 930z) very slowly moving towards the coast
and then over central California by Thursday. This is causing a weak
pressure gradient across the coastal waters, which will result in
light winds and low seas this week.

&&

Fire weather...fine fuels will gradually moisten over the next
few days as a combination of scattered showers, cloud cover,
cooler temperatures and high relative humidity values blanket the region.
Although a fair amount of lightning is possible over the next few
days...due to the expectation of wetter fuels...fire weather
watches or red flag warnings will not be necessary. Jt/bfg

&&

Eka watches/warnings/advisories...
California...none.
Northwest California coastal waters...none.
&&

$$

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