Some interesting observations about dangerous heat index anomalies along parts of southeastern U.S. coast

In excessive heat regimes during summer in the southeastern U.S., depending on the synoptic-scale pattern, sea breeze penetration is often notably slower than usual, with arrival - at least a few miles inland - occurring much later in the day than on the majority of summer days. There doesn't necessarily have to be a strong offshore flow for this to occur. Real-time data have revealed significant spikes in heat index values in the first 1 to 2 hours following the initial arrival of the sea breeze discontinuity. With a slowly-penetrating sea breeze transition zone, the dew point may rise abruptly into the upper 70s or lower 80s with sea breeze arrival, while the temperature falls more gradually. Short-term spikes in heat index values to dangerous levels of 115 degrees or higher occur at times, until the marine layer becomes well established and the temperature drops further.  

 

NWS Cooperative station Edisto Island Middleton was commissioned in January 2004, and is located about 5 miles from the immediate coast. The cooperative observer ("Co-op") site has been equipped with a variety of instrumentation. The data for this write-up was obtained from a Davis “Vantage Pro 2” weather station that continuously monitors temperature, dew point, wind, pressure, precipitation and other variables, and computes a “heat index” (or apparent temperature) as well. Customarily, observations are logged and archived at 30-minute intervals, but can be set to archive as frequently as every minute.  The Davis’s inside receiving unit records and displays maximum daily values of the measured elements and the derived indices.

 

Observations during the past five summers (2004 through 2008) suggest the existence of  an occasionally occurring mesoscale phenomenon, previously neither well-recognized nor appreciated, which is associated with periods of well above-normal summer temperatures (“heat waves” or “hot spells”). This effect may be most pronounced in a relatively narrow “sub-zone” - oriented along and seaward of the sea breeze front - on the order of only a few miles wide embedded within the broader “coastal zone” along the southeastern U.S. coast.

 

For purposes of this discussion, the “coastal zone” will refer to the strip along the coast that is directly impacted by the sea breeze on most days in summer. Average distance of sea breeze penetration along the Southeast coast varies with the orientation and geography of the coast line. During unusually hot periods, the sea breeze seems to become more “subdued” and less dramatic, both in effect and the distance it is able to penetrate inland. On some days, it appears to start developing later than normal, and moves inland very slowly. This may allow temperatures only a few miles from the coast to climb nearly as high, or perhaps as high, as temperatures much further inland. The sea breeze typically arrives at Middleton between noon and 2 p.m. during the summer. During periods of excessive heat, what is often observed and revealed in the observations at the Middleton Co-op site 5 miles inland is a late-arriving sea breeze (4 or 5 p.m., near the time the maximum temperature would occur with no sea breeze), accompanied by a sudden significant increase in dew point temperature. For a period ranging from 30 minutes to an hour or more, the dew point temperature increases more rapidly than the air temperature drops.  This results in some anomalously high (but short-lived) heat index values. 

 

Along the lower coast of South Carolina, the average daily sea breeze penetration is 30 miles or more, greater than in most other areas for two main reasons. First, in summer, the dominant synoptic-scale feature affecting the area is the Bermuda High, resulting in a prevailing southwesterly low-level synoptic-scale flow, generally parallel to the SC coast. On a typical day, therefore, early in the process of daily sea breeze formation (i.e., shortly after the temperature differentials that force the marine layer inland start to become pronounced) areas near the beach may start to feel a breeze off the water as the surface southwesterly flow gradually backs to the south or southeast. The sea breeze often starts at the immediate coast well before noon on days when the prevailing winds are southwest, or have only a light offshore component. Geographically, the lower South Carolina coast is also characterized by numerous barrier islands (part of the “Sea Islands” that extend roughly from just north of Charleston, SC to near Jacksonville, FL), and by extensive areas of low-elevation coastal marshlands and waterways. This allows for the sea breeze to progress inland relatively unimpeded (compared to heavily forested areas that would offer a little more resistance). 

 

Further south along the Georgia and northern Florida coasts, the coast line runs generally north-south (and the marshlands don’t extend as far inland). The prevailing southwest flow in summer therefore has a greater offshore component there, so the developing sea breeze is opposed or “held off” longer in these areas on a typical day. The offshore component of the prevailing flow also helps limit the distance the sea breeze moves inland during the day. The average daily penetration is closer to 20 to 25 miles along much of the Georgia coast and the east coast of Florida. (It’s also less on the upper South Carolina coast, and into lower North Carolina, than on the lower SC coast.)

 

During periods when strong high pressure settles over the region, mostly clear (but often hazy) skies and well-above normal temperatures may persist over the Southeast for several days or longer (or, infrequently, for weeks with occasional short breaks). The hot air mass, and the high percentage of sunshine, gradually warm the coastal waters. Surface water temperatures may eventually climb into the middle or even upper 80s off the southern South Carolina coast. 

 

During heat waves, with high pressure centered over the Southeast, the onset of the sea breeze becomes typically later, as the land/sea temperature and pressure differentials seem less effective in developing a distinct sea breeze, or at least the process is retarded. When the high pressure system is situated so that the synoptic-scale low-level flow is more northwesterly (perpendicular to the coast), the sea breeze can be significantly delayed, and if the offshore component is higher than 10 mph, Middleton (5 miles inland) may not feel the sea breeze until after 6 p.m.

 

With the very warm coastal surface water temperatures, early morning minimum air temperatures over the near-shore waters likely remain in the lower 80s to even mid 80s (unfortunately, observational data is sparse here). Minimum readings a few miles inland will typically be in the 75 to 80 degree range. The anomalously warm sea surface temperatures may allow air temperatures even at the beach to stay at or slightly above 90 degrees until well after sea breeze onset. On such days, the sea breeze brings little relief even at the immediate coast.

 

Late in the day, however, the sea breeze may be reinforced by slightly cooler air in the marine layer from further offshore, where the deeper waters are slower to warm than the quite shallow waters closer to shore (the ocean floor slopes very gradually off this part of the coast). Once the sea breeze finally does fully develop, the pressure gradient along the coast may force the sea breeze to blow with stronger force than usual late in the afternoon when air temperature differences between the coast and the interior become greatest. 

 

The observed spikes in heat index values seem to be most noticeable within a zone probably only a few miles wide, where air with very high dew points (80 degrees or higher) in the marine layer only a short distance behind the sea breeze “front” meets the hotter, somewhat drier air over the interior. The sea breeze front is not a true discontinuity, but rather a narrow zone of transition. In these scenarios it appears to be more diffuse than is normally the case. Mixing of the high dew point marine layer with the hotter, drier air inland is occurring in this narrow zone which is progressing slowly inland and becoming gradually more diffuse. The highest heat index values appear to occur between a few and perhaps 10 to 15 miles from the coast. The effect seems less pronounced as far inland as the Charleston airport (where the official Charleston readings are taken). Typically, the temperature 20 to 25 miles inland has started to drop by the time the later-than-normal sea breeze arrival occurs there (perhaps after 7 p.m.), and the extremely high dew points that were able to make it several miles inland have “mixed out” somewhat with the “drier” air over the interior by that time.

 

The conclusion? A heat index value of 115 degrees or higher (for any period of time) meets the NWS criteria for an excessive heat warning. Yet, there is a paucity of readily available observational data in the area, and the phenomenon may occur without ever being noticed or detected. There are potentially dangerous consequences for people who stay outdoors in these conditions, and especially if they are engaging in any strenuous activity. There are usually large numbers of people at the beach or on the Sea Islands during such weather conditions. During excessive heat regimes, a later-than-usual arriving sea breeze may bring a temporary increase in the apparent temperature, or at the very least may maintain heat index values, for a couple of hours or longer, near the same levels as prior to sea breeze arrival.