January 12, 2011 Cold and Wet 30 days in L.A.

January 12th, 2011 by William Reid

My posts here have been primarily concerned with storm chase accounts, storm photography, and other photography, but not much with regard to local weather and climate.  I would like to start sharing some of my weather and climate thoughts and research with the world via my web site, and this is the first one here on Stormbruiser.

Southern California was on the receiving end of very wet weather during the second half of December, thanks to a large and cold upper low which set up shop west of the Pacific Northwest.  This low steered abundant moisture into California, and many stations in the central and southern parts of the state recorded record amounts of precipitation.

At my little weather station here at the condo in Westlake Village, I measured 7.95″ in December.  Normal December rain for this area is about two and one-half inches.  Nearly eight inches of rain is an impressive amount for one month in this part of the world.  This is especially so considering that this area (the Conejo Valley and the San Fernando Valley) found itself “skipped over” by the heaviest rain activity.  During the wet week from December 17 to 22, areas to our northwest got hit hard early in the period and areas to the southeast got hit especially hard later in the period.  Westlake Village and vicinity had near-continuous light-to-moderate rain for about five days, but the heavy flood-producing precipitation bands avoided this area.

Locally, this event pales in comparison to many heavy rain events in the past.  For instance, I measured about 20 inches of rain when I lived in nearby Agoura Hills in February, 1998.  The wettest day here in Westlake Village was December 19th, with 2.27″.  Some mountain stations in Southern California received close to 25 inches of rain in just a few days during this December event!

This post is written largely to show how cool and cloudy and wet the 30-day period from December 14th to January 12th, 2011, was.   Since it looks like we have a week of warm and sunny weather in our near future, let’s look at the numbers for the previous 30 days.  Here in Westlake Village (WLV), ave max temp was a chilly 57.5F, and the ave min temp was 40.9F.  The cloudy and wet nights kept the minimums up quite a bit.  But, an ave max under 60F for 30 consecutive days is cold for these parts.  For comparison, at Pierce College in Woodland Hills, the average daily maximum for Dec and Jan is about 67F.  A long-term average maximum for Dec/Jan for Agoura and Westlake Village is not available, but is probably about 65F.  So, the ave max temp was about 7 degrees colder than normal from Dec 14 to Jan 12.  The ave min temp was likely near normal here at my WLV station.

Pierce College’s weather station opened in July 1949, and its coldest average maximum for any month was 59.6F in December, 1971.  The previous 30 days at Pierce in Woodland Hills had an ave max temp of 59.0F, slightly colder than ave max temp for December, 1971.  (A few daily maximums for Pierce were estimated at the end of the recent 30-day period, as the weather station there is currently being upgraded.)  So, yes, we have cherry-picked a particular 30-day period and are comparing not-too-fairly with just calendar months; but it is clear that the recent cold spell has been fairly exceptional and noteworthy.  By the way, it is too bad that Pierce was not recording temperature in January, 1949, as that month was no doubt much colder in Woodland Hills than any other during the past 60 years.  A station called “Thomas Ranch”, near Nordhoff and Corbin in Northridge, had an average maximum of just 51.5F in January 1949.  That month featured heavy snow in the western San Fernando Valley, and the early afternoon temperature on January 11th was 31F with snow (according to thermograph charts at Thomas Ranch).

Here is a set of data for area stations Westlake Village/Van Nuys AP/Los Angeles (USC) for December 14th, 2010 to January 12, 2011, a 30-day period:

Station                   WLV/VNY/L.A.

Ave Max Temp    57.5/58.8/61.8

Ave Min Temp    40.9/45.3/48.7

Ave Temp             49.2/52.1/55.3

Max Temp             68 / 68 / 72

Min Temp              28 / 35 / 37

Total Prec  (in)      8.02/7.79/10.50

Rainy Days              16 / 15 / 15

The rainy day total above includes a couple of trace amount days at each station.  During these 30 days, I observed only four clear or mostly clear days.  The coolest maximum at WLV was 47F on January 2, and VNY reached only 49F that day.  The normal max and min at L.A./USC for these 30 days are approximately 67.5F/48.5F, so the departures downtown were about -6/0 for our recent 30-day cool and wet spell.

Coldest ave max temp for any month at Los Angeles is 55.2F for January 1937.  January 1949 was a close second at 55.8F.  The recent 30-day period at L.A. had an average max temp of 61.8F, about the same as for January 1979.

Finally, I’d like to point out how cloudy and wet the first half of our wet season has been.  Rainfall amounts since July 1 are 11.91″ here at WLV, 10.56″ at VNY, and 12.28″ at L.A./USC.  These amounts are just a few inches below the normal totals for an entire year (L.A. has an annual normal near 15 inches)—-not bad, considering that January, February and March have the highest monthly normal precipitation amounts.  In October, there was an unprecedented run (for October!) of wet days here in WLV.  Measurable rain fell on 11 consecutive days, and on a total of 14 days in October 2010.

Since July 1, here at WLV, there have been 42 days with rain (through January 12th), 34 of these with measurable rain.  At Van Nuys, rain has fallen on 37 days (26 with measurable); and at Los Angeles, rain fell on 34 days (26 with measurable).  All but a couple of these wet days were since October 1.  According to my oldish Weather Almanac, the average number of days annually with measurable precipitation at Los Angeles is only 35.  I’m ready for some warm and sunny weather, but certainly would not mind experiencing several more stormy periods this winter and spring to make the 2010-2011 rain season a big one.

Note—-the data I have presented here are taken from preliminary data sources provided by the NWS, and should not be considered official.  “Normal” and “average” data are from the Western Regional Climate Center.

Last note—I have the “comments” section turned off here at Stormbruiser.  If you would like to contact me, or have a comment or local data which might be a nice addition to the above article, please e-mail me

bill@stormbruiser.com

Posted in 2011, Weather and Climate Blog

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