Irma brings potential for tornadoes to the Upstate late Monday.
With landfalling tropical systems, there is always potential for tornadoes in the spiral rain bands rotating around the common low-level center.
However, there are some differences regarding the environment in which these tornadoes form compared to the environment in which tornadoes form in classic, mid-latitude systems with which we are accustomed to dealing.
First, tornadoes from tropical systems form oftentimes when there is limited vertical instability. In common mid-latitude systems, there is warm air at the surface beneath increasing cold air aloft. Because tropical systems are warm-core systems, temperature does not decrease with height nearly as quickly, thereby reducing overall instability.
Because of this, there is rarely any lightning OR hail with these tropical-type showers rotating around the common area of low pressure. On radar, there may just be run-of-the-mill “yellows” and “oranges” (indicating heavy rain) without any menacing reds or purples, but that does NOT mean rotation and/or a tornado is not occurring.
Second, because of the limited available instability, they sometimes can develop and dissipate in minutes. This makes it more difficult to adequately issue timely tornado warnings for the National Weather Service. In mid-latitude, classic systems, storms develop a rotating mesocyclone thousands of feet off the ground eventually connecting to the ground via low-level stretching.
In contrast, tropical tornadoes develop quickly due to preferred rising motion at the ground in the presence of dramatic wind shear as well.
With Irma, its wind field will be quite expansive even as it moves inland. As a result, as wind shear and moisture increase over our area, we will have a threat for tornadoes with any organized rain band rotating through the area Monday afternoon and evening. Overall, this risk will NOT be significant, but it's one you'll want to pay close attention to if any watches or warnings are issued for your area.
As we saw with Cindy and Harvey earlier in the season, these types of environments can develop over our area. It is important to remember to treat any tornado watch or warning just as you would if it were issued in the more “common’ stormy times such as the spring months.
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