The 2nd named storm of the season
Our well announced wet pattern is in full swing over the state with a plethora of watches and warnings.
An El Nino pattern will be with us, on and off, through April. A warm episode El Nino has been underway since last summer, but is only now producing the signature effects for California. The Oceanic Nino Index (ONI) is a primary indicator of whether or not an episode, warm or cold, is happening. Sea surface temperature anomalies in the equatorial Pacific (degrees Celsius) are used to determine that number, and to a lesser degree winds at high levels and low levels, and the outgoing long wave radiation. But the bread and butter of an El Nino analysis is that temperature difference from climatological normal at the ocean surface. When the ONI is +0.5 or greater for a 90-day period and has been so for 5 consecutive months, we say El Nino is occurring. The same is true for a La Nina cold event, but the values must be -0.5 or lower. For comparative purposes, the 1997-98 Great El Nino topped out at an ONI of +2.5. It was this El Nino that delivered 14.73” of rain in one water year, the heaviest on record for Bakersfield. The 2009-2010 El Nino is now up to +1.5 and may not be finished intensifying.
This upcoming week looks to be very wet with 5 defined storm periods. The first has already moved through the region Sunday the 17th with Bakersfield having received a quarter inch of rain. System number 2 arrives Monday with an occluded front that is forecast to pass through Kern County late in the afternoon. System number 3 affects our area Tuesday afternoon through about midnight. It will feature a cold front and moderate rainfall. Systems number 4 and 5 should be the most powerful of these 5.
This series of weather systems is also known as a “family of storms” and was first identified as a recurring weather phenomenon in published material by Norwegian meteorologists of the early 20th century. In a family of storms scenario, the parade of moderate storms is usually punctuated by the strongest one at the end. These are promulgated by a blocking ridge somewhere at higher latitudes and they are made more dynamic due to their negative tilt (oriented on a northwest to southeast axis). Systems 4 and 5 are schedule to blow through from Wednesday afternoon through late Friday. The first of these two will be windy and very wet while the second one brings in noticeably colder air. Any problems on the Grapevine will be from system 5, expected to be here with plenty of cold air advection Thursday morning through the day on Friday. There may also be convection Thursday which could bring thunderstorms and locally very heavy rainfall.
I have analyzed the prospect of systems 4 and 5 against the objective parameters of a named storm and it appears we may have one. One of the primary considerations for naming a storm is the intense rainfall that must exceed the 3-hour flash flood guidance. Currently, that guidance is fairly high at around 2.50 inches in the central valley. But after several days of rain from systems 1 through 3, the south valley may be primed (so to speak) for potential flash flooding. That goes without saying in the Los Angeles basin where 10-20 inches of rain is possible in upslope areas. Residents are already evacuating from locations below the Station Fire burn area.
Here in Kern County I am predicting a total of 1.25 to 3.00 of rain for the entire week. Systems 4 and 5 will officially be named “Storm Ares”, named after the Greek god of war. It must be stressed that Storm Ares will have a strong pressure gradient with a major rain shadow over the Bakersfield area. That means, due to downsloping winds from the south, rainfall tallies will be minimized for a period of time in Bakersfield. Therefore the lower amount in my prediction (1.25”) applies to Bakersfield proper. So at least one additional inch of rain following Sunday is forecast. Snowfall in the Kern County mountains will range from 3-6” above 3,500 feet Thursday and Friday to 15-30” above 4,000 feet late in the period. More than a foot of snow could fall on the Grapevine and in Tehachapi along highway 58. Above 6,000 feet, I am expecting significant snow of 35-60” for the entire week. And even more than that in the high Sierra- perhaps up to 10 feet of new snow from this procession of storm systems.
So a very busy weather week is in store for California. The California Storm Index (CSI) preliminary estimate is that this will be a Class 1 event. A calculated wind index of 68 and rain index of 49 are both above the minimum threshold of 35 for naming the storm. As explained in a previous blog, the CSI uses a variety of parameters to quantify potential storm threat. This will be the second storm of the season, following Storm Poseidon which hit California in October 2009. It provided a abundant rain, wind and mountain snow for most of the state, although Bakersfield only received .08” with a major dust storm. Dust does not appear to be a problem with Storm Ares. But rainfall that may threat life and property is the forecast at this time. Stay tuned for further updates.
