Note - Click/tap on each image to enlarge
We are in the time of year known as the "second severe weather season" for the coastal Carolinas. What does this mean? The type of severe weather changes a bit... from unstable, lightning-rich storms of the warm season to "convective showers" of the cool season.
What is this? This shows the change in wind speed as one goes from the ground surface to 4 miles up. Over Columbus County,NC, (and in areas in yellow), there is a 40-knot change in wind speed being shown. This is representative of strong increase in wind speed from the ground surface to a level 4 miles up. This is important. Why?
Stronger wind speeds with height allow thunderstorms to "tilt," which means the updraft is kept away from the downdraft. In our summertime storms, as shown by "A" on the image, the updraft and downdraft are more-or-less right next to each other, and eventually the downdraft will "choke out" the updraft, killing off the storms. When storms have a "tilt" to them ("B"), the rain falls ahead of the updraft, and therefore the storm is allowed to survive. It also allows for forward motion of the storm.
Slide #4 is a graphical representation of how wind shear affects storms. "A significant increase of wind speed with height will tilt a storm's updraft. This allows the updraft and downdraft to occur in separate regions of the storm, reducing water loading in the updraft. The downdraft will not cut off the updraft, and actually it will even enforce it. Strong upper tropospheric winds will move mass away from the top of the updraft. This reduces precipitation loading and allows the updraft to sustain itself. Directional shear in the lower atmosphere helps initiate the development of a rotating updraft. This is one component that is important to the development of a mesocyclone and the development of tornadogenesis. Strong lower tropospheric winds and directional shear together will [...] increase the tornado threat when severe storms develop" (Haby).
The situation that will present itself with this frontal boundary is known as "unidirectional shear. The speed shear will allow the storm to move. The movement insures the storm will last longer than an airmass thunderstorm. Unidirectional shear often produces storms that form into lines. Since the storm moves, outflow produces lift that enables new storms to grow on the storm's periphery. Over time, a line a storms result" (Haby).
Next up we have the European model surface-based CAPE (Convective Available Potential Energy). CAPE is just one of the ways forecasters measure instability in the atmosphere. These numbers are actually relatively low. Thunderstorm updraft strength is determined by the amount of positive "energy" in the atmosphere... large instability = large updrafts. Our summertime storms are driven by high levels of instability, but low shear (as mentioned above).
At any rate, the instability numbers as shown here are relatively low. Typically for severe thunderstorms, you'd want to see surface-based CAPE values 1000 or higher, and the higher the better.
The cold front. (Note - the graphic above is just for reference - the temperatures are fictional values.) For any kind of storms to develop, we need to get triggered! Well, the catalyst to our being triggered is a cold front, and a strong one. As Professor Haby states, "[c]old fronts tend to be the fastest movers compared to the other front types. This fast movement increases convergence along the front and results in faster storm movement, if storms do develop. The slope of a cold front is greater than that of the other frontal types. This results in convection that is more vertical (lifting associated with warm fronts has a large horizontal component). For severe weather to be associated with cold fronts, look for the following: high dewpoints ahead of the front (60 F or greater), strong upper level winds, front movement between 10 and 20 mph, and convergence along the front" (Haby). We will have all of the above.
Ok... so now that I have blinded you with science, or utterly cofounded you, lets get to the point. Slide #7 above is the surface forecast map, wind and radar "model" for 8 PM Monday evening. The frontal boundary is expected to be traversing the Carolinas by this time, with showers and thunderstorms along this front. Since we will have low instability, these won't be "thunderstorms" with lots of lightning... these will be more along the lines of "gusty showers" or "convective showers," or ... showers with some attitude. Since we will have strong winds higher in the atmosphere, downdrafts from these cells could bring the stronger winds down to the surface, causing areas of damage. There will also be the threat for isolated tornadoes. I wouldn't be surprised to see a tornado watch posted for late Monday evening through early (predawn hours) Tuesday.
A secondary cold front is shown by this model farther to the west, but that one is dry and is more of a "reinforcing shot" bringing much colder air southward.
Slide #8 above shows is the surface forecast, wind/radar model for 8 AM Tuesday. As you can see, the main cold front is already forecast to be passing off the coast by time you head to work or school on Tuesday. The secondary boundary "dies" behind it. We'll see decreasing wind/rain threat, with clearing skies.
Slide #9 is the skinny... what are the severe weather risks with this? Given a high shear/low CAPE event, we can effectively rule out large hail. You need ice in the cloud to create the prolific lightning that we typically see during summertime storms. Since we won't be looking at a lot of instability, we won't see much hail aloft, and therefore not much lightning. Since we DO have very strong winds right off the deck, downdrafts from thunderstorms could bring damaging winds to the surface and isolated tornadoes.
Finally, in terms of picking up some much-needed rainfall, the squall line(s) could bring torrential downpours, that could produce localized poor-drainage flooding.
The timing of this threat will be between 8 PM Monday and 8 AM Tuesday.
I hope this will not only alert you to the potential for some nasty weather Monday night, but explain a little bit of the "why" behind it. Thank you for reading and feel free to share!
Meteorologist Christopher Cawley