August 7th, 2019: Heat Indexes Soar in Low Country and Derechos Explained

Local Weather: After a cooler than normal start to the month, temperatures will rise alongside humidity to bring heat indexes up to 104 degrees Wednesday. Afternoon storms less likely. More below today's post.


8-7-19 TI

(As of Wednesday the Daily Blog resumes to normal schedule)

In late June 2019, Southern South Carolina saw its first major derecho since the legendary April 5, 2011 derecho that injured more than 25 people and left thousands of damage reports across the Southeast United States. While the derecho we experienced on June 21-22, 2019 was not as widespread as the 2011 derecho, the local impacts were more severe and noteworthy. A derecho is a large system of storms that travels for more than 250 miles with severe wind gusts (60mph+). Derechos are an uncommon occurrence in the US, much less the Low Country. The United States will usually see around 2-3 derecho events in a year with the majority occurring in the late spring and summer. The Low Country on the other hand will usually see a derecho every 3-4 years (6 every 20 years). A derecho forms when a front or boundary is situated in an area with ample wind shear (wind speed is different at different levels of the atmosphere) and unstable air. Thunderstorms then develop and form a Mesoscale Convective System (MCS). A MCS is basically a large line of thunderstorms where movement is based on the system as a whole and not individual storms. After the MCS develops it often persists for a few hours and rapidly moves along the atmospheric flow. They can bring heavy rain, strong winds, tornadoes, and long lines of shelf clouds. When the atmosphere remains unstable ahead of the MCS, it can persist for long distances and bring winds gusts of more than 120mph. When the MCS tracks for more than 250 miles while producing 60mph winds and occasional higher gusts, if can then be classified as a derecho (Outside of extreme circumstances derechos are not classified as such until they have passed and damage is reviewed).

Derechos most often form over the Midwest and track east into the Eastern United States. During the spring, derechos will climatologically form over the Southern Plains and Ozarks before moving eastward into the Southeast. Since the Low Country is the "end of the road" in this path, most derechos weaken past their criteria before reaching us. The April 2011 derecho formed over the Mississippi Valley alongside a strong cold front. It was a serial derecho, meaning it formed to the south of a deep low pressure and tracked eastward as a squall line. Another example of a serial derecho is the 1993 Storm of the Century. As the United States enters summer, derecho formation moves towards the Midwest and High Plains as warm moist air can make it that far north. Derechos that form here are often progressive, meaning they aren't associated with a deep low pressure system and travel along a stationary boundary as a fairly self sustaining system. The June 2019 derecho was progressive. It formed over Missouri before moving Southeast through Kentucky, Tennessee, Alabama, Georgia, and eventually South Carolina. This derecho was able to make it the SC coast due to a self sustaining cold pool within the system and a well placed northwest flow (If there was a more westerly flow, the derecho would've tracked westward from Missouri towards the Mid Atlantic).

The cold pool is an area of cool air underneath and behind a derecho that helps sustain the system. When the MCS first formed, warm air was lifted high into the atmosphere where it cooled and condensated. As the MCS grew stronger, more warm air is sucked up and the cooler air begins to sink. The sinking cold air reaches the surface as a strong downdraft that pushes outwards when it hits the ground. This cold air is pushed rapidly along the ground (this is why derechos are able to create such strong winds) and squeezed under warmer air in front of the system. The cooler air pushes more warmer air into the clouds, completing the loop and sustaining the derecho. The rapidly rising warm air in front of the derecho forms the distinctive scary shelf cloud. The warm air condensates and forms the long cloud.



High temperatures will range from 95-96 in a corridor from Varnville-Walterboro-Ridgeville. Areas further inland will top out in the lower 90s. Just inland from the coast and northern areas will see highs in the low to mid 90s. The immediate coast and barrier islands should reach 87-89 degrees as a high. Downtown Charleston may even be lucky enough to not reach 90. Low temperatures will be in the 70s across the whole area besides an isolated location or two in Hampton and Colleton Counties. Beaches will be in the upper 70s to start the day, while temperatures further inland will bottom out in the mid-lower 70s.

Cloud Cover/Precipitation:

Overnight there will likely be patchy fog near inland locations (especially where it rained Tuesday). Fog should be light and dissipate soon after sunrise. The day will be mostly clear until the afternoon when diurnal convection picks up. Rain chances are lower than normal and will be highest in northern zones (30-40%). Thunderstorms have the potential to be strong as a shortwave passes well to the north during the evening. Precipitation will be scattered.


The entire day will feature a strong southerly flow around the Bermuda High. This is pump warm humid air into the area all day. Beaches will be breezy with an onshore flow around 15kts. Waves will be 2-3 feet, and swells will be out of the south.

Humidity/Heat Index:

Wednesday will be HUMID. Dewpoints will be in the mid to upper 60s during the afternoon, leading to a very muggy feeling. Combined with temperatures in the mid 90s, heat indexes will top out around 102-103 degrees for a large portion of the Low Country (maybe head to the beach?)

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