A clash between the arctic and the tropics is underway over much of the contiguous US this week. This is a fairly typical battle in autumn, especially in the northern US. Notable about this particular event, however, is that Texas is caught smack in the middle of the action. Typical fall weather for the Great Lakes or Northern New England has somehow managed to spill its way into Texas, quite literally overnight. For Texas, this week’s weather is more typical of January than of October. Long-standing climatological records are being shattered as a result.
To say that it is cold in Texas would be an understatement. Temperatures in the mid-levels of the atmosphere were outside the range of NOAA’s Climate Forecasting System 1979 – 2009 Reanalysis period Monday afternoon in northwestern Texas. In northeastern Texas, temperatures near the surface were also outside the reanalysis range, but only for the afternoon. A reanalysis is an approach to representing the previous state of the atmosphere that uses historical observations to generate data for past weather and climate for the corresponding observational period. The Climate Forecasting System was designed and implemented to optimize this procedure.
The culprit behind this winter blast is a broad cut-off low idling over Arizona. Isolated from the jet stream, the low has nowhere to go and has no means of drawing cold air to its center. On Monday the low was still quite robust and was just completing the process of cutting from the jet stream as an eastward building ridge in the Pacific Northwest was nudging it southward. The remnant jet streak around the low was therefore quite vigorous, especially over central through northern Texas.
Seemingly endless rainfall is frequently the result of such cutoff lows. The jet streak rounding the base of the cut-off low marks the difference between cold, dry, arctic air to the north and warm, wet, tropical air to the south. Amarillo, Dallas, El Paso, and even parts of northern Mexico are caught on the arctic side of the jet streak. Houston and San Antonio reside along the fringes of the tropical side. The tropical airmass is currently being transported northward by a ridge of high pressure centered just east of Florida and is gliding directly over the cold air below. The resulting temperature inversion is strongest over southwestern Texas near El Paso, where freezing rain has been accumulating in the nearby Chisos mountains of southwestern Texas.
The mid-level temperature gradient is strongest in northern Texas, central Texas, and northeastern Mexico along a line from Dallas to Monclova. Torrential rain has been falling throughout the day Monday near this temperature gradient, and will continue to do so Tuesday, as this is where the strongest forcing will occur. 2-4″ of cold rain has fallen across this corridor and up to 8″ more is expected through Saturday, when the low is finally expected to retrograde westward off the southern California coast. Its departure will likely come too late for many communities. Four river gauges are already reporting major flooding, with fourteen more reporting moderate flooding.
No major Texas city has taken the brunt of the winter blast quite like Dallas, which set four records Monday. The graphic presented below from the Dallas National Weather Service forecasting office only tells part of the story.
High temperatures over the weekend climbed into the low 80s ahead of the cold front, which are normal temperatures for the middle of October for the region. The cold front barreled southeastward across the state Sunday evening, sparking rounds of severe thunderstorms that brought tornadoes, ping-pong ball sized hail, and damaging wind gusts to the Dallas-Fort Worth Metropolitan area. Dumbfounded residents woke to survey storm damage undoubtedly wearing heavy jackets, as temperatures Monday morning were in the low 40s. The official observation was 41°F , breaking the record for the date of 42°F. Temperatures rebounded very little under the torrential rain. The high temperature never creeped out of the 40s, instead peaking at 49°F, a record low maximum representing a temperature departure of 30°F from normal. Monday was also the earliest day in the city’s recorded history that temperatures failed to reach 50°F. Perhaps if it wasn’t raining so hard, temperatures may have rebounded. As of 5pm CDT, the city recorded 2.74″ of rain, a new daily record by 4/5 of an inch.
The cold and gloom will last throughout the entire week in Texas. There may be breaks in the rain Wednesday and Thursday as the cutoff low weakens and broadens, but heavier, more persistent rain is expected to return Friday and early Saturday ahead of a separate upper-level system. Only slight relief is expected later this weekend when the cutoff low finally ejects offshore. A reinforcement of unseasonably cool air will follow the heavy rain.