January 21- 25, 2024 Heavy Rainfall & Severe Storms

An active weather pattern brought several rounds of heavy rainfall across portions of South Central Texas between January 21st and 25th. The bulk of this rainfall fell along the Interstate 35 corridor and east into the Coastal Plains. Small creeks, streams, and low water crossings were flooded. Over a foot of rainfall fell across some spots in Fayette County! Downstream river flooding was observed along the Colorado and Guadalupe Rivers. Isolated severe storms also produced large hail. This StoryMap provides a recap of the extraordinary rainfall that occurred and impacts to the region.

An active weather pattern began to shape up across the southwestern United States leading up to January 21st with a deep upper level troughing axis set up to the west of our region. The main trough axis remained to our west through January 25th which allowed for several smaller disturbances to dislodge from the main parent trough axis and move into South-Central Texas for several days between January 21 and January 25th. At the same time, the atmosphere had well above normal moisture levels with precipitable water values over 1.5-1.6” across areas mainly along and east of the I-35 corridor and this is where the bulk of the rain would fall. The prolonged period of upper level support for convection combined with the above normal moisture levels across the region allowed for multiple rounds of moderate to heavy rainfall to occur across most of South-Central Texas.  

The first round of moderate to heavy rainfall occurred in the evening hours of January 21st and lasted through the overnight and morning hours of January 22nd. This activity developed as the first piece of upper level support arrived to the area. Most areas of South-Central Texas received rainfall from this initial event but the I-35 corridor from San Antonio northward into Georgetown received up to 2-4 inches of rainfall with some 1-4 inch totals noted in the Coastal Plains northward into Fayette and Lee County. 

The second and third rounds of rainfall occurred between the afternoon of January 23rd and lasted through the evening hours of January 24th. Another piece of upper level energy moved into the area which allowed for prolonged periods of heavy rainfall across portions of the Coastal Plains. Multi-hour heavy rainfall near Fayette and Gonzales County brought upwards of 8-10 inches of rain to that portion of the area which caused numerous flooding issues of roadways, creeks, and rivers. 

The last round of moderate to heavy rainfall occurred during the evening period of January 24th lasting into the morning hours of January 25th. The main upper trough that had been to our west for this multi-day rainfall event would finally be pushing east which would put an end to the prolonged period of rainfall to South-Central Texas. With this last event light amounts of rain would be noted for the Hill Country and areas to the west with another 1-3 inches of rainfall seen along and east of the I-35 corridor. 

Totaling it all together, the 5 day rain event led to rainfall totals generally less than an inch for the western Hill Country, Edwards Plateau, and Rio Grande Plains. The I-35 corridor picked up 3-8 inches of rain and the Coastal Plains had 5-15 inches of rain. A maximum of 14.45 inches of rainfall recorded in Fayette County!

The heavy rainfall occurred over portions of the Edwards Aquifer recharge zone, and led to a 9 foot rise of the J-17 index well in San Antonio. Unfortunately, the heavy rainfall fell downstream of larger lake reservoirs. Significant rises on the Highland Lakes, Canyon Lake, and Medina Lake did not occur.

In addition to the period of heavy rainfall, there was enough instability in the atmosphere to allow for some isolated strong to severe storms to occur during this event, mostly at the times the activity was not as widespread. This is pretty typical when storms are more isolated, they can tap into all the instability available for the individual updraft versus numerous competing updrafts when activity is more widespread. During the early morning hours on January 23rd, up to ping-pong (1.5”) sized hail fell near Horseshoe Bay along the Llano/Burnet County line and up to egg sized hail (2”) fell near Poteet in Atascosa county in the overnight hours of January 24th.

San Antonio's 2nd and Austin's 5th wettest January on record observed in 2024.

Near to record values of vertical moisture content for January were observed during the event.

Rainfall for January 2024 was 4-5 inches above normal along the Interstate 35 corridor, and over 8 inches above normal across portions of the Inland Coastal Plains!

Drought conditions improved by one to two categories as a result of this heavy rainfall event.

Only 2% of all 2 inch diameter or greater sized hail reports across South Central Texas have occurred in the month of January

Through the first half of Meteorological Winter (which runs December 1st through February 29th) most of south-central TX residents had been waiting patiently for the "wetter than normal" winter conditions that are typical in El Niño winters in the region. The phenomenon, defined by warmer than normal surface water temperatures in the tropical Pacific Ocean, typically helps force a stronger southern stream jet over the southern US, which in turn leads to an active storm track across the southern and southeastern US in these winters. Thus, most of the past several strong El Niño events (see image below) had produced wetter than normal winters to varying degrees. But it's important to remember that these are 3-month averages, and the day-to-day weather variability looks different within each of these winters. For example, well above normal rainfall occurred in all three months of the 1991-1992 winter but especially in Dec 1991. In contrast during the 1957-1958 winter, December '57 was rather dry but above normal rains in Jan-Feb '58 brought the seasonal averages back above normal. 

In some cases, a very wet few days can be enough to overcome months of dryness, tipping the seasonal scales in favor of the typical El Niño teleconnections even if the majority of the rest of the season looked a lot different. This may end up being one of these cases. El Niño definitely helps set the stage for these kinds of Winter months in Texas (several other of the top 5 wettest Januarys locally were El Nino years including 1991 and 2007). But it's not quite appropriate to look at a wet week like we experienced here and say "El Nino did this", since it never acts alone. Other modes of climate variability, such as the Madden-Julian Oscillation, the Pacific Decadal Oscillation, and others can work in tandem to help drive large-scale weather patterns. As a result, we should not draw direct links between single subseasonal-to-seasonal climate forcings and individual weather events. For a deeper look at El Niño's impacts on seasonal climate vs daily weather and an analogy some might find apt, check out the  climate.gov  ENSO (El Niño/Southern Oscillation) blog published Jan 25th  here. 

In October of 2023, the National Weather Service released experimental flood inundation mapping services that take real-time and forecast conditions into account. This was a large leap forward in flood forecasting however, as is typical in south central Texas, drought withheld opportunities to demonstrate its potential usefulness. 

One such opportunity arose during this event on the Colorado River at La Grange. In this particular example, the flood inundation map displayed shows what is known as the River Forecast Center (RFC) Flood Inundation Map (FIM). The West Gulf River Forecast Center in Fort Worth, TX issued a forecast of the Colorado River to rise above the minor flood stage of (26 ft). The blue shaded area shows the peak (max) extent of the flooding.

Looking closer at the inundation depiction near the US HWY 77 bridge, you will notice that the river is not overtopping the northern bank due to it’s higher elevation however, the lower southern bank is shown being overtopped. 

Video posted by KXAN shows how this inundation looked in real time. The video begins looking at the north bank before panning to the southern bank. In the video you can see that the low southern bank is in fact covered with flood waters while the higher second bank has yet to be overtopped. 

This is but one positive example of the potential advantages these new flood inundation mapping services can provide. A parting word of caution, these maps should not be utilized as a stand alone product as there are known deficiencies. This very same example shows one such deficiency, false inundation of bridges. The Highway 77 bridge is shown to be inundated in the map however, from the video, you can see that not only is the bridge not flooded the river would need to rise several more feet before even threatening the bridge. 

If you are interested in learning more about these new services feel free to visit:  weather.gov/owp/operations  and scroll down to the Flood Inundation section.