The next few months for the Texas Hill Country are expected to be warmer and drier than average. The Climate Prediction Center (CPC), a division of NOAA, predicts an enhanced probability for above normal temperatures and a slight probability for below average precipitation. No severe drought is predicted. The cause of this warmer and drier weather is La Nina. If La Nina starts weakening early next year as predicted, the second half of winter should turn slightly wetter than average as the ENSO (El Nino Southern Oscillation) conditions revert to normal. Currently, NOAA has a La Nina Advisory active, which means that La Nina conditions are currently being observed and are expected to continue.
ENSO (also simply called “El Nino”) is a quasi-periodic climate pattern that affects the Pacific Ocean on a 3-7 year pattern. It is characterized by sea surface temperature (SST) variations. A SST warming is called El Nino and the opposite cooling is called La Nina. These SST variations in turn affect areas of air pressure, with El Nino bringing higher pressures to the west Pacific and the cold phase La Nina bringing lower air pressures. ENSO is the cause of extreme weather worldwide, including droughts and floods. Developing nations that depend on weather-sensitive activities such as fishing and farming are particularly affected by ENSO.
The La Nina part of ENSO is a natural cooling of the eastern equatorial Pacific Ocean. The average winter temperature from November through March for the Hill country is 1.5-2 degrees warmer than average. The precipitation can be up to 5" less than average.
The current CPC synopsis is that La Nina in the Northern Hemisphere is expected to last at least into next spring. La Nina conditions in September showed a SST average of between 1.3 and 1.8 degrees C cooler. Half of the long-term ENSO models predict La Nina to become a strong episode (defined by a three month average Nino-3.4 index of minus 1.5 degrees C or cooler). La Nina impacts for the rest of the year are a below average precipitation across the southern tier of the US. La Nina can also contribute to a more active hurricane season by decreasing vertical wind shear over the Caribbean and Atlantic. A decreased vertical wind shear means hurricanes have a greater chance of developing or strengthening, as vertical shear tears hurricanes apart.
La Nina influences are felt the most during the winter months and aren’t really noticeable until November due to other weather patterns affecting the area. Moisture from tropical systems can trickle through the area, and during October, upper level disturbances and frontal boundaries feed off of moisture to bring heavy rainfall, which is the opposite of what La Nina alone would bring. El Nino means “the boy child” in Spanish and refers to the birth of Christ because periodic warming in the Pacific near South America is usually noticed around Christmastime.
Last winter was an El Nino winter, with El Nino rapidly breaking up in the spring. This, in turn, cooled the SST in the Equatorial Pacific Ocean, and continued cooling of SST there led to La Nina conditions this year.