Thursday will be similar to Wednesday with dangerous heat indexes and afternoon showers. What is a pileus cloud?
Wednesday afternoon a stationary thunderstorm dropped close to four inches of rain in parts of Mt. Pleasant (all in a couple of hours). This thunderstorm was situated in an area of weak steering and had nothing to push it along. Therefore, it sat still and slowly trained over the Wando River. While growing and bubbling, the cumulonimbus cloud grew taller and taller into the atmosphere before a thin cloud appeared on top. It was a pileus cloud that soon over-topped the entire top of the storm. What exactly is a pileus cloud?
While the picture above is not yesterday's pileus cloud, it is a good example of one. They are characterized by a thin wispy cloud that covers the cloud like a cap. Pileus clouds are similar to lenticular clouds in which moist air is pushed upwards until it reaches its dew point. The water vapor then condenses and forms a cloud. Lenticular clouds are often formed by mountains, but pileus clouds are formed by strong updrafts in a thunderstorm. When a cumulonimbus cloud rapidly expands upward, it is pushing very moist air into the space above the cloud. At a certain height the vapor condenses and forms the pileus cloud. Pileus clouds are often short lived as the cumulonimbus below grows through it, or the storm weakens and updrafts are not sustained. Rarely, pileus clouds can even become iridescent and reflect a rainbow of light.
There are updrafts in all thunderstorms, but pileus clouds don't occur in all storms. This is because the atmosphere must be moist enough that the air being pushed into it causes condensation.
Temperatures will be much higher than Wednesday for most locations. Highest temperatures will occur 3-4pm in the Varnville-Walterboro corridor. Highs there will be 97-98 degrees. Most other inland areas will top out from 95-96 degrees, but the Charleston Metro and area around the lakes may only reach the lower 90s. The barrier islands will be cooler around 90 degrees. Low temperatures in the morning will be in the mid 70s (upper 70s at the coast). Coolest areas will be near Orangeburg where it will be closer to 72-73 degrees. It is possible the barrier islands will stay above 80 degrees all night, offering no heat relief.
The day will be mostly clear as drier mid level air enters the area overnight. Convection will be focused towards the southern Low Country and the sea breeze. Highest cloud cover will be in the late afternoon for intracoastal areas, mid afternoon for inland areas, and the evening for the coast. Rain chances are highest in the Southern Low Country at 20-30%. Most areas will remain rain free, but scattered storms are possible in the evening.
Heat indexes will be over 100 degrees for the ENTIRE Low Country outside the immediate coast. The heat index will be slightly cooler than yesterday because the humidity will be slightly less. Dew points will make it into the lower 60s during the afternoon, which will bring some relief (still uncomfortable). Heat index will be lowest at the beaches and lakes, but the highest in the southern intracoastal corridor. This includes Bluffton and Beaufort where a 107 degrees heat index is possible.
*Written information is not official and should be used to make official decisions*