Event Summary
     National Weather Service, Raleigh NC


August 2007 Heat Wave Event
Updated 2007/08/15






Event Headlines

...On Friday, August 10th, a record high temperature of 104 degrees was set at the Raleigh-Durham International Airport breaking the old record of 99 set in 2001. This is one degree below the all-time record high for Raleigh-Durham of 105 degrees. A record high temperature was also set at the Piedmont Triad International Airport when the temperature reached 100 degrees. This broke the old record of 97 set in 1977...
...On Thursday, August 9th, a record high temperature of 104 degrees was set at the Raleigh-Durham International Airport breaking the old record of 100 set in 2001. This is one degree below the all-time record high for Raleigh-Durham of 105 degrees. A record high temperature was also set at the Piedmont Triad International Airport when the temperature reached 101 degrees. This broke the old record of 99 set in 1930...
...On Wednesday, August 8th, a record high temperature of 102 degrees was set at the Raleigh-Durham International Airport 8th breaking the old record of 101 set in 1999. A record high temperature was also set at the Piedmont Triad International Airport when the temperature reached 100 degrees. This broke the old record of 96 set in 1980...
...On Monday, August 6th, a record high temperature of 97 degrees was set at the Raleigh-Durham International Airport breaking the old record of 97 set in 1999. A record high temperature was also set at the Piedmont Triad International Airport when the temperature reached 98 degrees. This broke the old record of 97 set in 1977...
...There were also several record high minimum temperature records set at the Raleigh-Durham International Airport and the Piedmont Triad International Airport...


Event Overview

Central North Carolina experienced an extended period of excessively hot weather and humid conditions during the second week of August, 2007. A large, expansive ridge of high pressure in the mid and upper levels of the atmosphere that stretched from the southern Plains across the mid Mississippi valley into the and Southeast was responsible for the heat wave. At lower levels, the 850 MB ridge was centered over the northern Gulf of Mexico.

Once the ridge became established over the southern U.S., the conditions for a significant heat wave were in place. Numerous studies have identified three synoptic conditions that lead to and contribute to the build up of excessive heat, they are:
  • the presence of an upper level ridge that results in sinking air and adiabatic warming
  • the advection of warm air
  • strong solar insolation (sunshine)
Once the heat wave gets underway, it can become self sustaining, often until a significant external force acts upon. High pressure ridges generally produce sinking air or downward vertical motion which inhibits the formation of clouds and precipitation. When air is forced to descend, it is compressed which results in warming and hotter temperatures. During this kind of pattern, the upper level ridge builds (shown with rising 500 MB heights), the lower levels of the atmosphere warm (shown with rising 850 MB temperatures), and the surface temperatures rise a few degree higher each day.



Table of Temperature and Precipitation Data from August 4 - 10
RALEIGH DURHAM INTL AIRPORT
Observed Daily Data
Month: Aug 2007

Day   MaxT  MinT  AvgT   HDD   CDD   Pcpn 
 4      96    70  83.0     0    18   0.00
 5      98    72  85.0     0    20   0.03
 6      97    74  85.5     0    21      T
 7      99    75  87.0     0    22   0.00
 8     102    76  89.0     0    24   0.17
 9     104    78  91.0     0    26   0.00
10     104    75  89.5     0    25   0.05
11      85    73  79.0     0    14   0.00

PIEDMONT TRIAD INTL AIRPORT
Observed Daily Data
Month: Aug 2007

Day   MaxT  MinT  AvgT   HDD   CDD   Pcpn 
 4      94    68  81.0     0    16   0.00
 5      93    74  83.5     0    19   0.00
 6      98    75  86.5     0    22   0.00
 7      96    77  86.5     0    22   0.00
 8     100    77  88.5     0    24      T
 9     101    80  90.5     0    26   0.00
10     100    76  88.0     0    23   0.05
11      87    75  81.0     0    16   0.00

FAYETTEVILLE
Observed Daily Data
Month: Aug 2007

Day   MaxT  MinT  AvgT   HDD   CDD   Pcpn 
 4      99    68  83.0     0    18   0.00            
 5     102    73  87.5     0    23   0.00            
 6     102    79  90.5     0    26   0.00            
 7     103    76  89.5     0    25   0.00            
 8     105    79  92.0     0    27   0.00            
 9     107    80  93.5     0    27   T
10     105    78  91.5     0    27   0.09            
11      89    74  81.5     0    17   0.00


Maximum Temperatures and Heat Indices During the Heat Wave

Maximum temperature map



Maximum heat index map



Use of Ensemble Anomaly Data

Computer models will, at times, attempt to forecast rather extreme events – anything from a major winter storm or flooding rains, to tropical cyclones and extreme heat. Sometimes these occur, but often they do not. How can a forecaster get increased confidence that a model forecast of an extreme event is correct? Anomaly fields can help forecasters put certain model-forecast weather events in perspective. These plots were used with success operationally early on during the August 6-10, 2007, heat wave.

850 mb SREF plot The image to the right (from the Pennsylvania State University’s “Eyewall” website at http://eyewall.met.psu.edu/ensembles/) shows the mean 850 mb temperature (green contours) from the Short Range Ensemble Forecast (SREF) modeling system (see http://www.hpc.ncep.noaa.gov/ensembletraining/ for details). An ensemble mean solution is essentially the average solution from several different independent solutions (often called the ensemble “members”). The SREF includes members with differing initial conditions as well as members from different model packages. The result is the Short Range Ensemble Forecast (SREF) system. By using ensemble mean forecasts, rather than just using one particular model (which may or may not be correct by itself), the forecaster is essentially seeing the solution produced by the greatest number of model members, which oftentimes makes it the most likely solution. Those member solutions that are “outliers”, or greatly different from every other member solution, are dampened out or eliminated by viewing the ensemble mean. Ensembles do not and are not intended to produce a specific YES or NO forecast. Instead, they should be used to see a range of possibilities and the most likely outcome.

Also shown on the above map is the mean 850 mb temperature’s departure from the normal value for this time of year (look at the scale on the left). This departure from normal is given in terms of standard deviations above normal, which in very simple terms is a measure of how unusual a certain value (in this case, 850 mb temperature) is as compared to what it should be this time of year (see http://www.robertniles.com/stats/stdev.shtml for an expanded yet basic explanation of standard deviation). The higher the number of standard deviations above or below normal, the more unusual the forecasted event is. In this case, the 850 mb temperatures forecast by the SREF were near 22 C, 2-3 standard deviations above the normal – in other words, highly unusual, or anomalous.

After viewing this data, the forecaster might assume one of two things: (1) the model is forecasting an unusual event and is totally wrong, or (2) the model is forecasting an unusual event and is correct. Even with the filtering out of individual member solutions that differ too greatly from its fellow member solutions, the SREF mean temperature was still a large departure from normal. This gave forecasters the confidence to believe that this very unusual solution was probably correct.

Forecasters were also able to look back to past extreme heat events to see how this particular event would stack up. The map to the left (taken from the Hydrometeorological Prediction Center’s Reanalysis data, at http://www.hpc.ncep.noaa.gov/ncepreanal/) shows the atmospheric pattern in place at 850 mb on August 18, 1988, when RDU last reached its all-time record high of 105 degrees. On this day, the 850 mb temperature (yellow contours) were 2-3 standard deviations above normal (image; scale on the left) – similar to the SREF mean forecasts. By using the forecast anomalies, and by comparing them to a past event, forecasters had increased confidence to forecast record-breaking high temperatures during the heat wave. Indeed, high temperature records were broken on August 9 and 10, both with highs of 104°F.





Synoptic Scale Pattern and Model Guidance

Under construction



Heat Wave Definition and Impact

There is no official definition of a heat wave, and the term is generally relative to the area or location in which the hot weather occurs.
  • The definition recommended by the World Meteorological Organization is when the daily maximum temperature of more than five consecutive days exceeds the maximum temperature normal by 5 °C (9 °F).
  • The definition from the American Meteorological Society Glossary of Meteorology is a period of abnormally and uncomfortably hot and usually humid weather. To be a heat wave such a period should last at least one day, but conventionally it lasts from several days to several weeks.
  • In the United States definitions often vary by region; however, a heat wave is frequently defined as a period of at least three consecutive days above 90 °F.
  • The National Weather Service criterion for the issuance of a Heat Advisory is when the heat index is expected to reach 105 °F for three hours. An Excessive Heat Warning is issued when the maximum heat index is expected to exceed 110 °F.
Severe heat waves have produced significant agricultural damage, power outages due to increased use of electricity for air conditioning, and injuries and deaths. Typically many heat related deaths go unreported, but heat waves are responsible for more deaths annually than the more dynamic natural disasters such as lightning, floods, hurricanes, and tornadoes. The National Weather Service, estimated that between 1936 and 1975, about 20,000 U.S. residents died of heat. In North Carolina, there were two reported heat related deaths in 2005 and one in 2001.

More information on heat waves, their impact, and how to protect yourself is available on our with our Heat Awareness and Safety web page.

The image below shows the average number of annual fatalities for various weather phenomena across the United States and its territories on a 10 and 30 year basis through 2006. The statistics for the image below are compiled by the Office of Services and the National Climatic Data Center from information contained in Storm Data, a report comprising data from NWS forecast offices in the 50 states, Puerto Rico, Guam and the Virgin Islands.

Maximum temperature and heat index map





Historically Significant Event

This event will go down as one of the hottest periods in central North Carolina history.
  • Raleigh had two days with highs of 104 which is one degree shy of the all time record high. Raleigh also had 3 consecutive days with highs of 102 or more and 4 consecutive days with highs at or above 99.
  • Greensboro had highs 0f 100 or more for 3 consecutive days with one morning in which the low temperature did not fall out of the 80s.
  • Fayetteville had 6 consecutive days with temperatures of 102 degrees or more and 3 days with highs of 105 degrees or more. In addition, Fayetteville also had 5 days with daily average temperature approached 90 degrees.
As noted previously, the potential for a heat wave can be initially identified by examining the upper air pattern. In particular, forecasters can determine if the pattern is a significant anomaly and compare it to past events with a historical database. Examining this event in this context shows that this event featured a large high pressure ridge across the southern U.S. The ridge was of fairly significant amplitude, with 5 day mean heights in excess of 5950 meters. The climatological average of 500 MB heights for August 6 through August 10 during the period of 1968 to 1996 shows that the there is typically a ridge of high pressure over the southwestern U.S., a broad trough in the eastern U.S. and a ridge in the subtropical Atlantic with maximum heights around 5900 meters. Comparing the observed mean over the August 6 through August 10 period with the climatological normals from 1968 to 1996 yields an objective anomaly chart which shows that the 500 MB heights were 45 to 60 meters above normal across the mid Mississippi Valley and western Ohio Valley. An anomaly of 45 to 60 meters is certainly very significant as these values are generally in the 2 to 3 standard deviation range. The anomaly location corresponds well with the location of the ridge axis where there is typically a broad trough.


Examining the temperatures at 850 MB (at around 5,000 feet) is another signal of the potential for extreme heat. A long used technique to forecast maximum temperatures is to use the 850 MB temperature and warm it dry adiabatically to the surface. The technique has various assumptions and limitations including that it is most applicable near sea level, in generally fair weather (cloud free with limited wind), and flat topography. But the implication here is that the 850 MB temperature can be used as a surrogate for the surface temperature when the skies and weather are fair. During this event you can see that there was a large area of very warm temperatures in the southwestern U.S. and another maximum of warm temperatures over the southeastern U.S. where the 5 day mean value from August 6 through August 10 was in excess of 22 degrees C. The climatological average of 850 MB temperatures for August 6 through August 10 during the period of 1968 to 1996 shows that the 850 MB temperatures typically range in the 16 to 17 degree C range. Comparing the observed mean over the August 6 through August 10 period with the climatological normals from 1968 to 1996 yields an objective anomaly chart which shows that the 850 MB temperatures were a significant 5 to 6 degrees C above normal (around 10 degrees F above normal).



The State Climate Office of North Carolina produced the graphs below which detail the number of hours that the heat index exceeds specific thresholds at Raleigh-Durham and Greensboro.

Graph of the number of hours of specific heat levels at Raleigh-Durham, NC from 1972 through 2005




Graph of the number of hours of specific heat levels at Greensboro, NC from 1972 through 2005





Drought Impact

At the beginning of August, 2007 moderate drought conditions were occurring across central North Carolina. Rainfall across central North Carolina was generally isolated and insignificant during the first two weeks of the month. Through August 11, rainfall amounts were much below normal across central North Carolina with deficits generally ranging between 1 and 2 inches. The lack of significant rainfall by itself would add to the severity of the drought but multiple days of extreme heat across the central part of the state significantly exacerbated the drought conditions.

Drought is defined in a number of ways that reflect the various perspectives and interests of those it impacts. A NOAA Public Fact Sheet on Drought provides three commonly used definitions:
  • Meteorological drought occurs there is a prolonged period of below average precipitation. Meteorological drought usually precedes the other kinds of drought.
  • Agricultural droughts are droughts that affect crop production or other agricultural interest. This condition can occur independently of any change in precipitation amounts when soil conditions result in a shortfall of water available to crops. Typically, though, it results from an extended period of below normal precipitation.
  • Hydrological drought occurs when water reserves available in sources such as aquifers, lakes and reservoirs fall below the statistical average. Like an agricultural drought, this can be triggered by more than just a loss of rainfall.
The multiple days of extreme heat certainly had an impact on drought conditions. The extreme heat should have produced greater evaporation across local bodies of water and the soil. Local municipalities and water management agencies reported record water usage during the days with the hottest temperatures and this will obviously have an adverse effect on reservoirs. Hotter temperatures should result in greater transpiration by plants, further reducing the moisture in the soil. Finally, the stress from extreme temperatures may have resulted in the death of some plants or crops.

Drought Monitor conditions across North Carolina just before the peak of extreme heat on August 8-10, 2007

Drought Monitor conditions across North Carolina


Map of 7-day average stream flow compared to historical stream flow

7-day average streamflow compared to historical streamflow



Air Quality

High pressure over the southeastern United States produced light winds, hot temperatures and reduced mixing during the heat wave which enhanced ozone and particle formation. The high humidity also increased particle production. These conditions limited pollutant dispersion and resulted in air quality levels that were unhealthy for sensitive groups.

The image below is from the www.airnow.gov web site which shows a schematic of the surface weather features and anticipated air quality issues on Thursday, August 9, 2007.

Air Quality Outlook



References

Chen, F., and C.E. Konrad, 2006: A Synoptic Climatology of Summertime Heat and Humidity in the Piedmont Region of North Carolina. J. Appl. Meteor. Climatol., 45, 674–685.



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