WEATHERAmerica Summary, Saturday, May 18, 2024

[Larry Cosgrove]

Thunderstorm Threats Shift Northward; Mexican Heat Ridge Is The Big Player

The slow strengthening and northward relocation of the Mexican and Sargasso heat ridges has begun. The polar and subtropical jet streams have merged and also are translating to higher latitudes. While this cuts back on severe weather potential in much of Texas and the Deep South, the throat zone for intense convection will occupy the Great Plains into the Midwest/Great Lakes (and occasionally the Mid-Atlantic) for much of May.

A critical danger for severe weather, including large hail and tornadoes, will stretch from North Texas into the Upper Midwest on Tuesday and Wednesday. I doubt that the worst of the storms get much further south than the Interstate 20 corridor with this feature, since the aforementioned heat ridge will shunt the vigorous forcing and lifting, along with classic surface convergence more north than what was seen on this past Thursday and Friday. The rest of the Lone Star State and the Deep South will have capped stability under the subtropical high. And be very warm in the process.The tendency for some of the numerical models, especially the GFS series, to overdom cold advection should warn the forecaster to not embrace the American operational scheme with its continued push for record cold in the eastern half of the USA. In fact, warmer than normal air may persist to the right of the Appalachian Mountains.

I suspect that in terms of temperature, the cool West vs warm elsewhere scenario will remain. But the high sun and lack of cold polar air drainage can only mean that cool forecasts will be turned to "near normal", and the warm outlooks become hot. The formerly moist southern rim will dry out, and quickly this month. That is the summer pattern (JJAS) in a nutshell, with the only breaks in the heat along the Gulf Coast and Atlantic shoreline with the increase of tropical cyclone threats after August 15.