On a given year, the RGV receives anywhere from 20 to 30 inches of rain (figure 1, above). September is the wettest month of the year for deep south Texas. On average, the Rio Grande Valley receives anywhere from 3 to 7 inches during the month of September, or about 1/4 of yearly average (figure 2). However, 2011 was one of the driest years for the Valley, on record. Figure 3 shows the departure from normal for the year 2011. Notice that some areas of south Texas were more than 15 inches below normal! There are a couple of theories as to why 2011 was so dry for not only south Texas, but for the majority of the state as well. The top theory was because how strong La Nina had been in 2011. This blog will not go into further detail as to why Texas was so dry in 2011.
Since September of 2010 (figure 4), there have only been 3 good months of above average rainfall for deep south Texas, including this month (February). June of 2011 was relatively wet for south Texas because of deep, tropical moisture that surged into the southern part of the state (figure 5).
It is not uncommon that the RGV gets drenched from tropical moisture during hurricane season, especially since this has been the case recently over the past few years. The month of September, though towards the end of hurricane season, is nevertheless included during this tropical moisture period. Now we come to the other 2 months that received above average rainfall, December 2011 and February 2012 (figures 6, 7). These two months were very similar with the overall mid-and-upper level patterns. I will briefly explain the pattern and why it rained so much.
General Setup: The main reason why the Valley got soaked in the month of February (as well as December 2011) was because of the constant southwesterly flow in the mid-levels of the atmosphere. Throughout the month of February, the main jet over the northern latitudes would split into what is called the “sub-tropical” jet. On three separate occasions during the month, the split flow developed into closed lows that dug into western and eventually north-central Mexico (figure 7). Being on the east side of these lows, south Texas was in the “lifting” zone, where positive vorticity and positive vorticity advection (PVA) were present. As if PVA wasn’t enough, the mid-level lows would induce cyclogenesis on the lee side of the Sierra Madre Mountains, at the surface. This, in turn, would provide more lift at the surface for south Texas that would spark showers and thunderstorm throughout the month of February.
*All images courtesy of NWS/NOAA/SPC/DoC*