Outlook for Winter Season 2011-2012
Recent Drought and Rainfall Trends in West Central Texas
Rainfall in September was well-below normal across most of West Central Texas. A few pockets in eastern and southern sections of the region received near to slightly above normal rainfall for the month. Most of the rainfall during the past 2 months occurred with an event in early October, when a storm system brought heavy rain with widespread coverage across West Central Texas. Rainfall for this event was mostly in the range of 2 to 5 inches across much of the region. More than 5 inches occurred in pockets east of Highway 277, especially across Brown and San Saba Counties. However, since September and October are climatologically among the wetter months of the year for West Central Texas, the amounts for the past 60 days were still below normal for most of the region.
The percentage of normal rainfall over the past 60 days (ending October 26th) shows less than 50 percent of normal across parts of the western Big Country (west and northwest of Abilene), the western Concho Valley, and across parts of the Northern Edwards Plateau. The 60-day amounts were near to slightly below normal across far eastern parts of the Big Country, and across some of the eastern Concho Valley in Runnels and Concho Counties. Above normal rainfall (100-150 percent of normal) for the past 60 days has occurred in parts of Kimble County, southern Brown County, and much of San Saba County.
The recent U.S. Drought Monitor for Texas, issued through the National Drought Mitigation Center on October 18th, showed somewhat of an improvement across the parts of Texas where the heavy rainfall occurred in early October. In West Central Texas, drought conditions in a relative sense improved slightly, from the exceptional to the severe and extreme categories, from the eastern and southern parts of the Big Country southward across the Concho Valley and into the Heartland areas.
Status with the Climate System
La Nina conditions redeveloped by the beginning of this fall season. La Nina is associated with a periodic cooling of the waters in the equatorial part of the Pacific Ocean. La Nina conditions were present during the last winter season, and continued into the spring of this year. A noteworthy item about recurring La Nina events can be seen from the historical record. The record for La Nina (and El Nino) events, dating back to 1950, shows that “back to back” occurrences of La Nina are not uncommon. The last such occurrence was with the La Nina episode which began in 1998 and ended in 2000, spanning two consecutive winter seasons.
La Nina and its Importance
La Nina (and El Nino) conditions are the best understood, in terms of their long-term impacts on weather patterns worldwide. Their developments have far-reaching effects on global circulation patterns which, in turn, affect the position and strength of jet streams. This has an important influence on the strength and track of storm systems.
Over the years, specific weather patterns have been observed, in association with La Nina (and El Nino) conditions, especially for the stronger events. This has led to a better understanding of their effects on a large scale. Their effects can be accounted for in the long-range outlooks.
The effects of La Nina typically start to become more noticeable in November, and usually have the greatest influence during the winter season months of December through February. The effects can linger into the spring season.
Climate Outlook for November, 2011 through January, 2012
The Climate Prediction Center (CPC) indicates that La Nina conditions will strengthen and will continue through the upcoming winter season of 2011-2012. From recently observed trends and climate model forecasts, a moderate La Nina event is anticipated.
The Climate Prediction Center 90-day temperature outlook, for November-January, indicates an enhanced probability for temperatures to average above normal across West Central Texas. The CPC 90-day precipitation outlook, for November-January, shows an enhanced probability for precipitation to be below normal, for all of West Central Texas.
These temperature and precipitation outlooks are essentially the same for the winter season period of December-February, for our region.
The results of local precipitation studies show that, for moderate to strong La Nina events, average winter season precipitation is below normal at stations across West Central Texas. This is consistent with what is indicated in the Outlooks from the Climate Prediction Center.
The most recent U.S. Seasonal Drought Outlook, issued by the Climate Prediction Center on October 20th, indicates that drought conditions will most likely persist across all of Texas, during the November-January time period. With La Nina in place, it is unlikely that our region will receive enough rainfall to alleviate the ongoing drought.
Implications with Patterns Influenced by La Nina
When moderate to strong La Nina events occur, the effects can influence regional weather patterns which can help to bring about the following in West-Central Texas:
- Persistence or worsening of ongoing drought conditions.
- Increased fire weather concerns, as grasses become cured and vegetation becomes dormant. An unfavorable track of storm systems can result in extended periods with lack of rainfall, and where repeat weather events occur with strong, gusty winds which are accompanied by intrusions of warm and very dry air into our region.
These adverse conditions have already occurred across Texas during much of the past year, and have been made extreme by a drought of historic proportions, along with prolonged periods of much above normal and record temperatures.
Even though there are pattern similarities with La Nina, there are unique characteristics with each new season, and no two events are exactly alike. Even when a La Nina pattern prevails overall in a winter season, certain patterns can develop which bring temporary intrusions of very cold air, or temporary wet weather periods. An example of this type of temporary pattern disruption occurred with temperatures last winter season. Although La Nina conditions were present, the development of an upper level circulation pattern, associated with a phenomenon known as the Arctic Oscillation, had an overriding effect on the overall pattern at times. This pattern allowed very cold air to plunge far south into the U.S. from Canada and the Arctic region. This occurred on several occasions during last winter season, with the most notable events in West Central Texas during the first part of February. The Arctic Oscillation pattern has episodes which typically last a few weeks and are difficult to predict more than 1 to 2 weeks in advance.
The NOAA Climate Prediction Center indicates that the La Nina conditions will strengthen and will continue through the upcoming winter season. The Outlooks November-January show enhanced probabilities for precipitation to be below normal, and for temperatures to average above normal, across all of West Central Texas. The U.S. Seasonal Drought Outlook from CPC indicates that drought conditions will most likely persist for all of Texas through January.