Debris Flow in California, July 7, 2018

Area Forecast Discussion
National Weather Service San Diego CA
914 AM PDT Sat Jul 7 2018

.SYNOPSIS...
High pressure aloft will continue the hot weather this weekend,
though temperatures will be a few degrees lower each day in most
areas, with a very gradual cooling trend next week. Monsoonal
moisture aloft could spark a few thunderstorms through Monday, mainly
in the mountains, but any precipitation looks spotty at best. There
will be a better chance of more significant rainfall on Tuesday as
moisture increases with a passing easterly wave.

&&

.DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE...
SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO
COUNTIES...

Mid and high clouds were traversing the northern parts of Southern
California this morning, mingling with the smoke plume from the
fire near Forest Falls. The night was warm to say the least. Some
foothills did not fall below the 90s all night. Temperatures at 8
am were ridiculously over 100 degrees in those foohills and many
inland valleys. The air mass is obviously very warm and today will
be very hot again, but we have a few factors working in our favor:
1) Today's atmosphere is not subsiding and compressing as much as
it was yesterday, 2) The offshore flow and easterly winds that
drove compression and astronomical temperatures yesterday, is much
weaker today, and 3) Dew points are up, especially inland, meaning
the temperatures will not skyrocket as much today, but our dry
heat will not feel quite as dry either. So today won't be as
extreme as it was yesterday, but extreme enough for our heat
warnings and advisories to be fully warranted today. Be careful
and serious about staying cool again today. The strong high
pressure aloft will slowly weaken through Monday. That will allow
the sea breeze to redevelop and a weak and intermittent coastal
eddy to continue. And then there's the monsoon moisture moving in
to the region. We had that yesterday, but today there is some
instability to go along with it, so those cumulus clouds you've
noticed lately will be able to grow taller. Surface heating should
be able to initiate isolated convection and thunderstorm formation
today, probably in the mountains. Rainfall will be isolated and
only very briefly heavy. There could be gusty winds and lightning
striking the ground with any thunderstorms. Sunday will be a
little cooler and the chance of thunderstorms looks even more
marginal. On Monday and Tuesday models show an increase in
moisture and through a deeper layer, along with an easterly wave
that would provide some lifting energy. It could be we get a
couple thunderstorms Monday in the mountains, followed by more
clouds, showers and thunderstorms Monday night through Wednesday.
These could happen anywhere, including west of the mountains, and
even during the night and morning. These easterly waves are so
ill-defined so confidence is not high. For Thursday and beyond,
the models differ on the moisture pattern, some solutions dry it
out a little while others keep it coming heading into next
weekend. We'll keep riding with the chance each afternoon mainly
in the mountains. Temperatures will come back down to near average
levels for mid July next week. The air mass is still quite summery
and warm, but we'll have less subsidence and more cloud cover to
keep those temps in check.

&&

.AVIATION...
071525Z...SCT clouds mainly based 15000 to 20000 feet MSL through
Sunday morning. Isolated to widely scattered TSRA possible
mountains, deserts, and Inland Empire this afternoon and evening.
Strong up/downdrafts and gusty outflow winds are possible near
storms. Any storms will likely diminish by 08/03Z this evening. Risk
of thunderstorms reaching KSAN is very low, so confidence in keeping
thunderstorms out of the KSAN area this afternoon and evening in the
TAF is high. Probability of thunderstorms near KONT later today into
the evening is still high enough to keep in the KONT TAF.

&&

.MARINE...
No hazardous marine weather is expected through Wednesday.

&&

.FIRE WEATHER...
Elevated fire weather conditions will continue today due to hot
and dry conditions, but should not reach critical conditions.
Local east wind gusts 15-20 MPH this morning on the coastal
mountain slopes/foothills will turn back onshore by early
afternoon and may be gusty. RH will be higher today and maximum
temperatures about 5-10 degrees lower. Isolated thunderstorms are
possible, mainly over the mountains, which could produce dry
lightning strikes. Expect an incremental improvement in
conditions, higher humidity, lower temperatures Sunday and Monday.

&&

.SKYWARN...
Skywarn activation is not requested. However weather spotters are
encouraged to report significant weather conditions.

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Excessive Heat Warning until 9 PM PDT this evening for Orange
     County Inland Areas-San Bernardino and Riverside County
     Valleys-The Inland Empire-Santa Ana Mountains and Foothills.

     Heat Advisory until 9 PM PDT this evening for San Diego County
     Valleys.

PZ...NONE.

&&

PUBLIC/FIRE...MM
AVIATION/MARINE...Small