An analysis of an isolated severe thunderstorm event with injuries in Summerville (and an illustration of why severe thunderstorm warnings should be taken seriously)…

Introduction

This analysis examines an isolated severe thunderstorm event that occurred in the Summerville, SC, area on July 28, 2008. High winds from the storm blew down a few trees and large limbs, causing one serious injury in the Pineridge Trailer Park on the northern side of town just inside Berkeley County.  

 

Thunderstorm Basics

A thunderstorm can develop only if sufficient moisture and instability are present. The developing stage is marked by growing cumulus clouds, resulting from a rising column of air called an updraft. The initial lift is provided by some method of forcing – usually surface heating, or air moving over terrain or “boundaries” such as fronts, sea breeze boundaries, or outflow boundaries from other thunderstorms. In this area the most common factors in summer are surface heating and the sea breeze. Outflow from other thunderstorms complicates things on many days and can lead to more intense storms. In any case, the cumulus grows into a “towering” cumulus as the updraft continues to develop. Little if any rain is likely during this stage, but lightning is possible. The developing stage lasts on the order of 10 to 30 minutes.

The thunderstorm enters the mature stage when the updraft continues to feed the storm. Latent heat is released to the environment as the moisture in the rising air condenses, providing further buoyancy. Precipitation begins to fall out of the storm, and a downdraft (a column of air pushing downward) begins. When the downdraft’s rain-cooled air spreads out along the ground it forms a “gust front," which is simply an outflow characterized by gusty winds. Usually these wind gusts are less than 40 mph. If conditions are right for a more intense storm than usual to form, the mature stage is the most likely time for hail, heavy rain, lightning strikes, strong winds, and (only if conditions are right) tornadoes to occur.  

Eventually, a large amount of precipitation (some water, some ice – the freezing level here was 15,000 feet on July 28, 2008) -- is produced and the updraft is overcome (“loaded”) by a downdraft, beginning the dissipating stage. If enough dry air is present in “middle” levels (say, 6 to 15 thousand feet), the descending air is further cooled through evaporation of some of the rain and hail, and the potential for stronger downdrafts increases because the air becomes even cooler and heavier.  At the ground, the gust front moves out a significant distance from the storm and cuts off the warm moist air that was feeding the thunderstorm. Rainfall decreases in intensity. The storm then usually falls apart fairly quickly, but its outflow may leave boundaries in place for new storms to form.

 

The storm over the Charleston Tri-County area on the afternoon of July 28, 2008, was what is called an intense single cell thunderstorm. Most single cell storms do not become severe. However, it is possible for one to produce a brief severe weather event. When this happens, it is called a “pulse” type severe storm. Their updrafts and downdrafts are stronger, and typically produce hail that may barely reach severe limits and/or brief microbursts (a strong downdraft of air that hits the ground and then spreads out).  Brief heavy rainfall and occasionally a weak tornado are possible. Though pulse severe storms tend to form in more unstable environments than non-severe single cell storms, they are usually more poorly organized and occur at seemingly random times and locations, making it impossible to forecast exactly where they will form.  

Synopsis for July 28, 2008 

Below is the surface map for July 29, 2008 at 18Z (GMT) – or 2:00 pm EDT (all times are EDT unless otherwise noted). It shows a typical summer pattern in place over the eastern part of the country. The remains of a weak cold front that aided in earlier thunderstorm development overnight are dissipating over eastern North Carolina. 

Atlantic high pressure was centered well offshore, with a moist and potentially unstable air mass in place over the southeastern states, including coastal South Carolina. A weak surface trough of low pressure was in place over inland areas. The general weather conditions and synoptic pattern were typical for the area for late July. Temperatures and dew points had been running near to slightly above normal for the previous few days. Daytime highs across the area were generally in the middle 90s on July 28, after morning lows in the low to mid 70s. No rain was recorded at Summerville (the “co-op” station 4 miles west of town is part of NWS’s cooperative observer network). A trace of rain was recorded at the Charleston NWS Forecast Office at the airport.

 

The morning upper air (balloon) sounding data indicated a good amount of potential instability. Most of the severe weather-related indices (computed from the actual sounding) indicated values not much out of the ordinary for the area for July, but suggested sufficient instability for a few strong storms to develop in the afternoon. The NWS at Charleston indicated in its late morning and early afternoon Area Forecast Discussion (AFD) that the sounding showed a strong low-level “capping inversion” (a stable layer in the lowest few thousand feet) which would likely inhibit thunderstorm development until at least the early or middle part of the afternoon…when surface heating could “break” the cap.

The sea breeze was also expected to assist in the thunderstorm development once the capping inversion was broken sometime after noon. The sea breeze was not well defined or particularly strong on this day and was actually rather diffuse, unlike what we see on many days. A general southwesterly flow was already in place over the area, and the sea breeze resulted in little noticeable change while moving slowly inland except for a gradual shift of wind from the southwest to due south. By the time the sea breeze had penetrated as far inland as Summerville, some convection had already started to form inland, and outflow boundaries from a few storms were already interacting with the sea breeze, limiting any further effect it may have had. The sea breeze was never as highly visible on the radar this day as it often is, though some of the outflow boundaries were quite distinct.  

The Charleston NWS pointed out in the early afternoon AFD that computer-generated numerical model forecast soundings were showing quite a bit of drier air at mid and upper levels, which was expected to “mix down” during the day. Therefore, they were expecting the potential for a “few strong storms given the mid-level dry air and robust potential instability.” The public zone forecast called for only a 20 percent chance of thunderstorms for the area, which ended up being a good forecast for the Tri-County area, since most places did not receive rain.

Here is an excerpt from the AFD issued on the 28^th at 1:43 pm (a repeat of the wording from the AFD issued at 10:49 am):

.NEAR TERM /UNTIL 6 PM THIS EVENING/...

MORNING SOUNDING SHOWS A STRONG 5 DEGREE CAP IN PLACE AND MID-

LEVELS ARE QUITE WARM WITH 700-500 MB LAPSE RATES BELOW 5C. THIS

IS THE RESULT OF THE 700 MB RIDGE AXIS SHIFTING EAST WITH MOST OF

THE SHORTWAVE ENERGY BEING DEFLECTED TO THE NE OF THE AREA. IN THE

LOWER LEVELS...A VERY WARM AND MOIST AIRMASS IS IN PLACE WITH

DEWPOINTS EXPECTED TO SURGE ALONG THE COAST WITH THE SEABREEZE.

INLAND QUITE A BIT OF DRIER AIR IS SHOWING UP IN MODEL SOUNDINGS

AND ON WATER VAPOR IMAGERY WHICH COULD PARTIALLY MIX DOWN DURING

THE DAY. MODIFIED 12Z CHS SOUNDING RESULTS IN CAPE VALUES IN

EXCESS OF 3500 WITH ABOUT 10 KT OF 0-4 KM SHEAR...INDICATING

MULTICELLS WILL AGAIN BE POSSIBLE. HOWEVER...THE LACK OF A CU

FIELD THUS FAR AND CONVECTIVE TEMPS IN THE LOWER 90S SHOWS THAT IT

WILL PROBABLY BE UNTIL THE SEABREEZE BEGINS TO PUSH INLAND BEFORE

ANY CONVECTION CAN FIRE. THE BEST COVERAGE EXPECTED IN SOUTHEAST

GEORGIA DUE TO THE HIGHER LOW-LEVEL DEWPOINTS AND LOWER CONVECTIVE

TEMPS. WILL MAINTAIN CHANCE TO SLIGHT CHANCE POPS THIS AFTERNOON.

WE MAY SEE A FEW STRONG STORMS GIVEN THE MID-LEVEL DRY AIR AND

ROBUST POTENTIAL INSTABILITY. HEAVY RAIN IS ALSO A THREAT.

INCREASING THICKNESSES WITH THE UPPER RIDGING AND MOSTLY SUNNY

SKIES WILL RESULT IN A WARM DAY ACROSS THE AREA. HIGHS IN THE MID

90S EXPECTED INLAND WITH LOWER TO MIDDLE 90S CLOSER TO THE COAST.

DEWPOINTS COULD SURGE INTO THE UPPER 70S WITH THE SEABREEZE AGAIN

WHICH WILL PROBABLY PUSH HEAT INDICES IN A THIN STRIPE ALONG THE

SC COAST ABOVE 105 FOR 1 TO 2 HOURS LATE IN THE AFTERNOON. HEAT ADVISORY LOOKS GOOD. 

The figure below shows the Day 1 Convective Outlook (AC) graphic issued by the NWS Storm Prediction Center (SPC). Other areas in the country were considered to have a more significant risk of severe storms on July 28, especially parts of the Dakotas. The Carolinas were in the “general thunderstorm” area only…and were mentioned in the discussion part of the AC issued at 740 am on the 28th: 

   ...DEEP SOUTH INTO THE CAROLINAS...

   VERY MOIST LOW-LEVEL AIRMASS WILL CONTRIBUTE TO MODERATE

   DESTABILIZATION ACROSS THIS REGION...AND THE SUBSEQUENT DEVELOPMENT

   OF SCATTERED SHOWERS AND THUNDERSTORMS DURING THE AFTERNOON.

   GENERALLY WEAK FLOW ALOFT SHOULD LIMIT OVERALL POTENTIAL FOR

   ORGANIZATION...ALTHOUGH A FEW STRONGER/PULSE-TYPE STORMS MAY PRODUCE

   LOCALLY GUSTY WINDS AND/OR HAIL NEAR OR BRIEFLY EXCEEDING SEVERE

   LEVELS.

     

 

 

 




















Even when conditions or forecasts for a specific area do not meet SPC’s criteria for issuance of a severe thunderstorm or tornado watch, those areas still may be included in a “slight risk” risk area in the AC. Otherwise, expected or developing conditions which may be potentially significant may be mentioned in SPC-issued “mesoscale discussions” (MCD). Regardless of whether any SPC products mention a specific area, NWS policy is for the local forecast offices (WFOs) to handle isolated occurrences in forecasts, statements and warnings as necessary. Though the Tri-County area was not singled out as being subject to any kind of enhanced risk, it is understood by NWS personnel, and by a sizable portion of the general public, that on the majority of days in the summer in this area, isolated strong storms are possible and a warning or two may be required. A watch is neither required nor desirable in these marginal cases.   

Evolution of the event

Widely scattered thunderstorms developed in the middle and late afternoon across the CHS county warning area (CWA), which comprises 20 counties in South Carolina and Georgia, and extends from the South Santee River (near Georgetown) to the Altamaha River in Georgia (near Brunswick). Thunderstorm coverage was expected to be greater in Georgia than in southern South Carolina, which proved to be the case.  A few warnings were issued across the CWA during the afternoon of July 28.  In South Carolina, isolated damaging wind events were reported in Hampton and Dorchester counties. 

The storm that produced damage near downtown Summerville (which was actually in Dorchester County) and in southeastern Berkeley County in the Pineridge Trailer Park on North Main Street in Summerville, began developing before 3 pm. Around 3 pm, the storm was centered 10 or 12 miles north of Summerville. It drifted slowly toward the southeast during the next hour and intensified. By 4:15 pm, the most intense part of the storm according to radar was about 2 miles to the east northeast of the trailer park. 

The Charleston National Weather Service issued a severe thunderstorm warning which was effective from 4:17 pm until 5:15 pm, warning of the possibility of wind gusts exceeding 60 mph. The warning verified with a report of wind damage (tree down across road) near downtown Summerville, a few miles southwest of Pineridge. The official time of that event was logged as 4:35 pm, based on a report from the public.  

Radar data also indicated that the storm top (“echo top”) was highest between 3:53 pm and 4:14 pm. Here are two radar images of the storm cell, showing the base reflectivity and the "echo tops" at that time. The reflectivity is an indication of the intensity of the storm, and can help meteorologists determine the location of the potential downdraft core. 

 

  

The storm height (top) had reached 50,000 feet by 3:53 pm, then dropped to 45,000 feet before spiking again briefly to about 52,000 feet at 4:14 pm. When the top spiked the second time (4:14 pm), the core of the most intense “reflectivity” (the returned energy computed by the radar) was only around 2 miles to the east northeast of the Pineridge trailer park (hereafter referred to as Pineridge). During the 15 minutes after that time, the top descended noticeably to about 35,000 feet by 4:30 pm. This was likely an indication that the top “collapsed” during this time – radar evidence that the strongest downdrafts were occurring, and the potential for wind damage was highest. 

The public reported a tree down across the road…near or just east of the Summerville downtown area…which was reported to the NWS. It was judged to be evidence of 50 knot (58 mph) wind gusts or higher and therefore verified the NWS warning. The event is recorded in the National Oceanic and Atmospheric Administration (NOAA) publication “Storm Data.” It is available from NOAA’s National Climatic Data Center in Asheville, NC, and from the NWS Storm Prediction Center in Norman, OK. The official reported time of the event (4:35 pm EDT) fits the radar data quite well. Based on this fact and on the available radar data in general, it is consistent with the timing of the event at Pineridge. Any strong downburst winds should have occurred there before reaching downtown Summerville, since Pineridge was closer to the core of the storm. 

Since wind measuring equipment doesn’t exist at the vast majority of locations where damage occurs, professional judgment is exercised by NWS meteorologists in determining whether the evidence is sufficient to presume winds at the 50 knot threshold (or higher), which, by definition, is considered a “severe thunderstorm.” 

The NWS guideline for determining whether 50+ knot winds have occurred -- whether to train storm spotters, to classify weather events, or to measure “ground truth” data while managing warning services during an ongoing severe weather event -- is the following:  Large limbs break; shallow rooted trees pushed over. Semi-trucks overturned. More significant damage to old/weak structures. Shingles, awnings removed from houses; damage to chimneys and antennas. Dead branches and trees that are obviously weak or diseased are not considered sufficient evidence. 

In an effort to obtain critical information, NOAA and NWS established what is known as the Skywarn Program in the 1970s, in partnership with FEMA and other organizations. The backbone of the program is the training and effective use of volunteers as storm spotters. There are now about 300,000 trained storm spotters, including police and fire personnel, dispatchers, EMS workers, public utility workers and many other concerned private citizens. Their feedback provides important “ground truth” and verification information to complement radar and other resources in accurately assessing what is occurring on a real-time basis, and in knowing what actually did occur. This is a critical part of efforts to continually improve and fine tune warning techniques. 

The role of spotters is as critical as ever, even in the age of Doppler radar and other advanced technology. Information about Skywarn can be found here, which is from the Charleston NWS Forecast Office web page. Official Skywarn spotters are trained to recognize and report severe weather – and to estimate wind speeds, according to the guidelines listed in the Weather Spotter Quick Reference Guide. 

The “base velocity” radar product was examined but was of little help. The WSR-88D Doppler radar used for  Charleston’s area of responsibility is located near the rural town of  Grays in Jasper County, about 50 miles southwest of downtown Summerville. At the radar’s lowest elevation angle (0.5 degree), the center of the radar beam is “looking” at what is happening about 4,000 feet above the ground. It typically does not detect downbursts at that range, since the cold air layer spreading out from the downdraft is usually much shallower than 4,000 feet. The radar “overshoots” anything happening below the beam. There was some indication of an outflow from the thunderstorm toward the radar, but again, only at 4,000 feet or higher. Most of the radar’s velocity data for the area around Summerville/Goose Creek were “range-folded” (a radar term meaning ambiguous). The radar velocity product is therefore not relied upon by meteorologists as a critical decision-making tool for warnings of strong straight-line winds, because of its inability to measure winds close to the ground (except in areas much closer to the radar site). 

An interesting note was that on radar there was a very large precipitation gradient over the particular area in question. The “one-hour precipitation” display indicated that something on the order of ¼ inch of rain likely fell near Pineridge, while immediately southwest of there (as close as I-26), less than a tenth of an inch fell. Within 2 miles to the northeast, over an inch may have fallen. The radar data are only estimates, but show a sharp contrast in rainfall amounts across a short distance. Pineridge was on the edge of the storm. The spot near downtown Summerville where the tree was blown down appeared to have received less than 1/10 inch of rain. 

Conclusion 

High winds from the downdraft core of the thunderstorm for which the NWS warning was issued likely caused a very large limb in a trailer park to break and fall onto a mobile home, resulting in a serious and permanent injury to a resident. It is likely that isolated wind gusts of 50 knots (58 mph) or higher occurred in the area between the thunderstorm’s center (2 miles east northeast of the trailer park) to approximately the downtown part of Summerville. Radar data suggest that the strongest outflow winds pushed west-southwest through this area approximately between 4:20 and 4:35 pm. The timing of the known events correlates well with the radar data. 

 

Primary data sources for this analysis included: 

Radar Data: http://www.ncdc.noaa.gov/oa/radar/radarresources.html (NCDC Radar Resources)  

 

Archived warnings and warning polygons: http://www.ncdc.noaa.gov/oa/kml/ (Virtual Globe Archived Severe Weather Warnings)  

Storm Event Data Base: http://www4.ncdc.noaa.gov/cgi-win/wwcgi.dll?wwEvent~Storms (NCDC)  

NWS text products: (forecasts, warnings, forecast discussions, mesoscale discussions, convective outlooks, statements, etc.):  http://has.ncdc.noaa.gov/pls/plhas/has.dsselect (NCDC’s HDSS Access System) and www.spc.noaa.gov  (Storm Prediction Center, Norman, OK)

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.       

5-foot Eastern Diamondback Rattler, Little Edisto Island, SC