2024 Christmas Outbreak

During the week of Christmas multiple rounds of severe weather impacted Southeast Texas. On Christmas Eve a cold front moved through the area, and while there was not a strong drop in temperatures with this frontal boundary, there was a line of strong thunderstorms that developed ahead of and along of it. Most of the reports for the Christmas Eve storms were hail-related.

There was a reprieve in severe weather for Christmas Day; however, a similar pattern was being set in place for the day after Christmas. This setup featured a little more "spin" in the lower-levels of the atmosphere, making tornado development more feasible. While multiple tornadoes were reported on December 26th, it was December 28th that ultimately dominated the headlines.

The front that triggered the severe weather outbreak on December 26th stalled along the coast the next day. The night of the 27th the front began retreating to the north as a warm front, placing the majority of Southeast Texas in a warm & moist environment ahead of yet another cold front. The dynamics in place prior to the onset of storms were much more favorable for rotating supercell thunderstorms.

By the end of Christmas Week, Southeast Texas had been impacted by three separate severe weather events that produced 11 tornadoes, 3 of which were on the stronger end of the EF-Scale. In addition to the tornadoes, these storms produced damaging wind, and hail. Unfortunately, the severe weather outbreak produced a lot of damage, resulting in several injuries, the loss of a life, and the destruction of Walt Disney Elementary School.

A robust mid-level shortwave was well defined in the 500 mb level on the morning of Christmas Eve. The axis of the shortwave moved slowly east into the state, deepening as it inched towards Central Texas. This deepening process created more curvature in the shortwave, which would lead to more convergence and upward movement of air, and with Southeast Texas located to the right of the axis we were in the prime spot for stormy weather.

At the surface, a warm front had just lifted north through the area, leaving Southeast Texas in a warm, moist, and unstable environment. Environmental analysis revealed steep lapse rates in the 700-500mb layer...this is one of the parameters we would be looking at to determine the potential for severe hail (1.0" or greater in diameter). Lapse rates were approaching 8°C/km and indicated that severe hail would be a threat for the area that day. In addition to steep lapse rates, Convective-Available-Potential-Energy, or CAPE values were in the 1000-1500 J/kg range. This would be enough energy for thunderstorms to work with, and with a warm and moist environment, there was plenty to "feed" these storms. Finally, a 25-30 knot Low-Level Jet had developed and consequently 30-35 knot bulk shear. Bulk shear at this value would be enough to keep any discrete cells organized and increase their longevity.

On the morning of the event, the Area Forecast Discussion written by the overnight shift explained that isolated to scattered thunderstorms would begin developing during the afternoon hours, mostly north of I-10. These storms would be followed by a more robust line of storms that would form along a weak cold front. These storms were said to have the potential to produce strong winds, large hail, and heavy rainfall.

On December 26th, 2024 a warm front lifted northward from the Matagorda Bay into East Texas, encompassing the entirety of Southeast Texas in a warm sector. This occurred ahead of a cold front associated with a mid-level low pressure system that was working its way from the panhandle of Texas into the Dallas/Fort Worth area. The northward progression of the warm front, and subsequently the warm unstable airmass that was left in its wake, set the stage for turbulent weather Thursday afternoon and evening.

Per SPC's Day 1 Convective Outlook issued on December 26th at 1630Z, dew points were already in the mid to upper 60s for portions of Southeast Texas (WPC's 21Z analysis with station plots revealed dew points in the low 70s across Southeast Texas within the warm sector). Typically, in scenarios when tornadic storms are possible, dew points in the mid to upper 60s indicate that enough moisture is available for tornado formation. In addition to the elevated dew points, MLCAPE (Mixed Layer Convective Available Potential Energy) values were in the 1500-2000 J/kg range across the warm sector. This meant that there would be enough energy for the storms to work with once development occurred. Long & curved hodographs indicated Bulk Shear values around 45 kts across Southeast Texas. This would allow thunderstorms to become organized into supercell thunderstorms, and with storm relative helicity values in excess of 200 m^2/s^2 there would be enough spin in the atmosphere for tornado development. Finally, Southeast Texas had additional support for enhanced upward movement of air parcels and for the transport of Gulf moisture. This was due to our placement in the jet streak aloft (left front quadrant--indicating strong convergence at the surface) and with the low-level jet that was packing winds of 35-50 knots.

Yet again, a mid-level shortwave coupled with a surface low and cold front was to blame for the severe weather that impacted Southeast Texas on December 28th. The prior day, and through the overnight hours, a warm front lifted north across Southeast Texas, causing winds to become southerly and draw warm moist air into the area. Ahead of the surface low and cold front the dynamics were becoming more favorable for a severe weather event... A strong low-level jet developed, which provided additional lift to support updraft development within thunderstorms. Overnight southerly winds sent dew point values into the mid-60s to lower 70s and brought in plenty of warm moist air to supply storms. Ample energy was available for storm development as CAPE values exceeded 2000 J/kg. Finally, 0-6km Bulk Shear values were in the 40-50 kt range, providing plenty of opportunity for storms to become organized and develop rotation.

With all of these parameters in place, damaging hail, damaging winds, and tornadoes (including stronger long-track tornadoes) were all possible. Per storm reports received that day, all three modes of severe weather were reported. Of the 6 reported tornadoes, 3 were determined to be strong tornadoes (EF-2+)

The threat for severe weather on any given day in Southeast Texas requires preparation and updates to the public and our core partners. Will severe weather occur during periods of heavy travel? Are any events going on around the Houston area or beyond that may be impacted by incoming severe weather? When the answer to this is "yes," this is where Decision Support Services (DSS) comes into play. The week of Christmas involves a lot of traveling (whether by air or on land) in addition to various events happening around the area, and thus careful planning and preparation is involved. With severe weather occurring on three separate days that week, we kept core partners, including Emergency Management, updated with daily email briefings and webinars. We put an emphasis on probabilistic forecasting by providing the probabilities of each hazard occurring. In the case of hydrological services, we provided a reasonable worst-case scenario (10% chance of exceedance) for river flooding. In addition to emails and webinars, graphics were posted to social media to help the general public to be weather aware and prepare for severe weather during the busy holiday.