Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Diego California
906 am PDT sun Oct 4 2015
Synopsis... low pressure aloft will drop south over Southern
California today...and move east on Tuesday. Clouds and scattered
showers will be widespread today...then become more scattered on
Monday. A few thunderstorms are possible...and rainfall may be
locally heavy. Turning dry and warmer for the remainder of the week
as high pressure over the subtropical eastern Pacific expands across
the southwest. Temperatures will be well above normal again later in
the week through next weekend. Cooler next week as the high weakens.
Discussion...for extreme southwestern California including Orange...
San Diego...western Riverside and southwestern San Bernardino
Clouds were widespread across the region this morning...from the
mountain crests...westward to well offshore. Radar showed narrow
bands of showers...mainly west of the mountains...but extending onto
the San Bernardino County mountain slopes. Some of the showers are
heavy...but were moving rapidly NE so amounts of rain were generally
modest through 8 am PDT. The 12z Miramar sounding was close to
saturation surface-7k feet...capped by a 2.5 degree c inversion. Precipitable water was
about one inch. Winds were veering with height from south 15kt near
the surface...to west-southwest around 50 knots at 18k feet. The freezing level was
The forecast looks on track today for continued shower development.
Instability parameters are weak this morning...but should increase
later in the day...especially over northern portions of the County Warning Area
where there is a chance of thunder. Showers will be numerous and
widespread today...becoming more more scattered this evening and
overnight into Monday. Expect a general decrease in stability
overnight...especially over the coastal waters and near shore where
isolated heavy showers may develop into Monday morning.
A 554 dm upper low was centered just north of Point Conception this
morning. Water vapor imagery showed the center dropping slowly south
with enhanced clouds swirling around the NE quadrant where
thunderstorms were occurring. The upper low will drop southeast and be
centered over the la basin by 12z Monday morning. This will keep all of
socal under the circulation...with abundant cloud cover and showers
at times along and west of the mts. The lower deserts will see
sunshine...and the upper deserts varying degrees of cloud cover as
the upper low approaches. The center of the upper low will be over
tuscon by 18z Wednesday...which will allow a quick rebound of heights/
thickness over socal. This trend will continue into the weekend when
temperatures will be well above normal under sunny skies.
041512z...mainly broken-overcast clouds based around 2500 feet mean sea level merging
with higher layers to near 12000 feet mean sea level. Occasional -shra and brief
+shra. Local visibility 1-3sm in showers and fog. Confidence in the
low clouds bases rising to above 3000 feet mean sea level at ksan increases
through the day today ... brief periods of broken bases below 3000
feet mean sea level will still be possible even into the afternoon. The risk of
thunderstorms is too low at this time to put into the tafs.
There is also a slight chance of thunderstorms for the San
Bernardino County mountains and high deserts during the 04/18z-
05/06z time-period...with cumulonimbus tops to 20000 feet mean sea level. Southwest winds
around 25 knots with local gusts to 40 knots along with moderate-strong
uddfs and wind shear are possible in the mountains and adjacent
deserts. Mountains obscured into Monday.
Marine... 812 am...gusty south-southwest winds 15-20 knots this
morning with locally higher gusts possible...however...higher gusts
would likely be brief and cover a small area...so a Small Craft
Advisory issuance is likely not needed. Seas will be choppy and
chaotic due to the gusty winds and the 4-5 foot short period wind
Hydrology... bands of showers have been developing and moving
eastward over the area this morning. Individual showers were small
and moving rapidly NE...so rainfall generally has been under one-
quarter inch...with a few exceptions...through 9 am PDT. Higher
resolution models show these bands will solidify as the main cold
front sweeps from the coast to the coastal slopes during the day.
Amounts from one-quarter to one-half inch should fall in the heavier
showers based on the latest local WRF/hrrr guidance...more on the
south slopes of the San Bernardino mts.
Behind the main windshift/front....colder air will continue to
filter in and destabilize the atmosphere...closer to the upper low
over northern areas....isolated thunderstorms are more likely.
Elsewhere the showers could still be locally heavy...including the
coastal strip and western valleys. The higher resolution models are
still advertising bulls eyes of one-half inch or more into Monday
morning...but they are few and far between...and predicting where
and when they will eventually materialize is a low confidence game.
The showers are expected to gradually decrease west of the mts later
Monday...but continue over San Bernardino County including the high
deserts into early evening.
Total rainfall amounts are expected to range from one-half to
locally one inch or more near the coast...to 1 to 2 inches in the
mountains with local amounts exceeding 3 inches possible on the
south slopes of the San Bernardino County mountains. Rainfall
amounts in the upper deserts will be around one-third inch with only
a few hundredths of an inch...at most...in the lower deserts.
Rainfall today has the potential to cause local Urban and Small
Stream flooding...with a smaller possibility for flash flooding in
recent burn areas if isolated thunderstorms or bands of showers with
heavier rainfall were to track over those areas.
Snow levels may lower to around 8000 feet for tonight into Monday
with several inches of snowfall possible on the highest peaks of
the mountains in San Bernardino and Riverside counties...mainly
above 8500 feet.
Skywarn activation is not requested. However weather spotters are
encouraged to report significant weather conditions.