Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Eureka California
500 am PDT Friday Jul 31 2015
Synopsis...hot temperatures will continue across Trinity County
today while other portions of northwest California begin to cool.
Thunderstorms will remain a threat through early next week due to
monsoonal moisture streaming in from the south.
Discussion...the strong ridge that has been bringing hot
temperatures to interior northwest California has shown little signs of
shifting eastward this morning. Models are actually showing the
500mb heights building slightly today over Trinity County. As a
proxy, this can be seen through a very compressed marine layer at
the coast. Meanwhile, an upper low continues to take shape off
the central cal coast today and will slowly force the ridge
eastward tonight and through the weekend. This pattern is acting
to draw sub-tropical moisture into northwest cal. The head of this
moisture struck yesterday with thunderstorms amid very hot temperatures.
While models are not very robust on widespread convective activity
today, the same factors at play on Thursday could bring storms
again today. As such, have continued to carry a slight chance of
thunderstorms over Trinity, eastern Humboldt, and portions of eastern del
norte and Mendocino counties. The mountains will act as a focus
for thunderstorm development, but some thunderstorms may drift off over the
valleys. Some models indicate the convective activity will
continue overnight. To remain in good coordination with our
neighbors, I did add a slight chance of thunderstorms in over the
mountains of eastern Trinity and NE Mendocino. For more on today's
tstms, see the fire weather discussion below.
As low pressure moves closer to northwest cal, several vorticity maxes are
expected to cross the area coincident with a continued influx of
sub-tropical moisture. Combine this with the heating of the day
and the ingredients will remain in place for at least an isolated
thunderstorm each afternoon into nex week. Models are showing
Saturday to be the most vigorous day for convective activity. This
is due to falling heights, cooling aloft, warm temperatures remaining at
the surface, and several short waves passing by. With that, have
gone with a more widespread chance of thunderstorms for northwest cal. The GFS
has been showing showers making it to the north coast. While this
is not model consensus, I did bring showers to the coast, though
not thunderstorms. We'll need to see how this plays out over the next few
At the coast conditions will be much more seasonable over the next
several days with some morning low clouds and fog followed by
partial afternoon clearing. The marine layer will deepen through
the next several days as low pressure approaches from the west.
Temperatures across the region will begin a slow slide toward
normal through the next few days. However, with monsoonal moisture
streaming in, the overnight lows will remain on the high side. Hot
temperatures are not expected to return for at least the next week. Bfg
Aviation...shallow marine layer will meander near the coastline
today and tonight bringing periodic LIFR conditions in fog and low
overcast to the air terminals. Slight improvement may occur by
noon as the land mass heats up. The fog and low clouds will roll
back onto the coastline by middle to late afternoon.
Inland...convection will spark up again with daytime heating
resulting cb's and thunderstorm activity. Southeast flow aloft may spread
some of the convective debris out to the coast this evening.
Marine...winds and seas will remain below our criteria for
advisories for small craft the next several days. Over most of
the area winds will remain light with speeds not expected to
exceed 10 knots. Winds will swing back around to the south across
most of the area on Sat as weak low pressure develops west-
southwest of cape mendo. High resolution models indicate winds
spiking up to 15 knots around the cape...thus beefed up the wind
forecast in that area for Sat. This low should slowly meander
southward sun and Monday as high pressure builds toward the coast.
For the next few days seas will be made up of a short period northwest
group around 4 feet at 8 seconds and a longer period group from the
SW centered near 14 seconds. More dominating short period waves
will build Monday and Tuesday as northerly winds increase in the outer
Fire weather...convection will remain a threat into next week.
The main threats will be this afternoon and Saturday. For more
information see the general discussion above. The main concern
today will be possible dry thunderstorms. Monsoonal moisture is
streaming in aloft while the 500mb heights build slightly over
yesterday. Building heights will maintain hot temperatures at the
surface and should help to provide the trigger for convection.
However, hot temperatures may also help to dry any precipitation that falls from
convection before it reaches the ground. Relative humidity has been on the
increase, but the heat should be able to overcome this today. As a
result, I have headlined the fire weather forecast for this
potential. I did not have enough confidence in this assessment to
hoist a warning or watch. It will need to be monitored through
the day. Saturday looks to be more widespread convection, but
should also be more wet due to lower evaporation rates near the
surface. We'll look for the possibility of watch for Saturday as
we pass today's activity. Bfg
Northwest California coastal waters...none.
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