Event Summary
     National Weather Service, Raleigh NC

February 10, 2008 High Wind and Fire Weather Event
Updated 2008/02/25

Event Headlines -

...A strong, dry cold front and high winds caused hazardous weather across much of North Carolina...
...Widespread wind damage was reported as winds gusted to over 45 mph...
...Over 300 wildfires burned more than 9,000 acres of land across North Carolina on February 10, with the 9,000 acres representing over 46% of the typical number of acres that burn in a year...
...A variety of online resources are available for meteorologists to maintain awareness of the weather's impact including power outage maps from power companies...

Event Overview -

A very strong mid-level jet, channeled between a mid-level ridge over the Western U.S. and an anomalous low drifting across the Great Lakes Region, and a surface cold front surged southeastward through the Upper Midwest during the early morning hours of February 10th. A cold frontal passage across the Mid-Atlantic States the previous day left dewpoints in the upper teens and lower 20's throughout the Piedmont and Foothills of Virginia and North Carolina. Temperatures on the 10th ranged from the upper 50's to mid 60's across the area, creating relative humidities in the 15 to 23 percent range. When the mid-level jet nosed across the Appalachian Mountains during the mid-morning hours, a developing deep mixed layer reaching up to near 750 mb allowed winds in excess of 40 mph to be mixed down to the surface. Low relative humidities (< 20%) and strong winds (sustained > 20 mph and gusts >30 mph) are known to cause conditions favorable for fire development and growth. The National Weather Service in Raleigh issued a Red Flag Warning from 9am to 9pm on the 10th, as well as a Wind Advisory that lasted through the day until 9pm. High Wind Warnings were issued by NWS offices in Blacksburg, VA and Greenville-Spartanburg, SC for even higher wind gusts of 60mph or higher. By mid-afternoon, many measured wind gusts of 40+ mph and a few above 50 mph, had been reported, and hundreds of fires had developed across Central North Carolina.

Synoptic Overview -

On Saturday, February 9, 2008, an anomalously low (around 400 meters) 500 mb low center was developing over the Great Lakes region. Behind the upper-level low, a strong jet was digging southeastward into the Upper Midwest. The strong winds extended down into the mid-levels of the atmosphere, where over 110 knots was observed at 500 mb, and over 70 knots at 700 mb. A strong cold front, seen in the strong 850 mb temperature gradient, was also pushing through the Upper Midwest, as 850 mb temperatures fell to less than 30 below zero in some locations near the Canadian border. Model forecasts from 00 UTC on February 10 (Saturday evening) predicted a rapid strengthening of the wind field, particularly below 500 mb. Winds at 700 mb were forecast to strengthen to nearly 90 knots over Virginia, with over 70 knots over the NWS Raleigh County Warning Area (CWA) during the day on Sunday (February 10).

Upper-air analysis of the 12 UTC February 10 RAOB data verified the forecast strengthening, as 500 mb winds increased to over 180 knots and 700 mb winds up to 90 knots from Iowa to Indiana. In addition to the strong winds moving into the Mid-Atlantic region, deep mixing in the boundary layer was also poised to take place. The upper-air sounding from Greensboro (GSO) from Saturday evening revealed a mixed layer extending to near 650 mb, or over 10,000 feet. With little change in the synoptic pattern, a similar deep mixed layer was expected during the day on Sunday. The 12 UTC sounding on Sunday February 10 shows the residual mixed layer above the surface inversion. As the strong winds rounded the base of the upper-level trough and crossed the Appalachian Mountains, wind speeds of 60 to 70 knots were located at the top of the mixed layer. As temperatures during the day on Sunday climbed into the upper 50's to mid 60's across Central North Carolina, the low-level inversion eroded, and the high winds began to be realized at the surface.

Most locations across North Carolina started reporting wind gusts of 20 knots or higher during the early morning hours (around 14 UTC or 9am LST). The mixed layer reached to near 700 mb on Sunday afternoon, where winds were in excess of 75 knots. Wind gusts steadily increased throughout the day as deeper mixing and the passage of the strongest mid-level winds occurred. By early afternoon (around 18 UTC), wind gusts over 40 mph were reported at many locations, with a sites gusting to over 50mph (see the official Public Information Statement for a full list of maximum wind gusts). There were numerous reports of damage to homes, and downed trees and power lines closed many local roads and interstates.

In addition to the wind, fire danger was also a concern statewide. A series of dry cold fronts swept across North Carolina during the first full week of February 2008, under a relatively flat, zonal mean flow across the Central US. This kept dewpoints in the low to mid 30s throughout the week. With nearly all of North Carolina in the midst of a persistent, exceptional drought, not only was the air dry, but fuel moistures (the percent water content of vegetation) were also very low. These conditions placed North Carolina under a level of high fire danger.

Many fires began in the early afternoon, as relative humidities fell to around 20% and winds increased. The image below shows widespread fires across the Deep South and Southeast U.S. at 18Z on Sunday afternoon.

Maximum Wind Gust Map

Maximum wind gust map

Wind Gust Data from RAWS Sites

Remote Automated Weather Stations (RAWS) are instrumentation platforms that monitor the weather and provide weather data that assists land management agencies with a variety of projects such as monitoring fire danger, monitoring air quality, and providing information for research applications. There are more than two dozen RAWS sites in North Carolina. The instrumentation array is rather simple; a photograph of the RAWS site at the Duke Forest is available here.

Most of the stations owned by the wildland fire agencies are placed in locations where they can monitor fire danger. RAWS units collect, store, and forward data to a computer system at the National Interagency Fire Center (NIFC) in Boise, Idaho, via the Geostationary Operational Environmental Satellite (GOES). The GOES is operated by the National Oceanic and Atmospheric Administration (NOAA).

One advantage of the RAWS dataset is the availability of the data both internally to the NWS and on the internet with various web pages providing access to the data in various formats. The three images below display the wind gusts observed at three RAWS locations across central North Carolina on Sunday, February 10.

The RAWS locations shown below are from Duke Forest (Orange County), Rocky-Mount (Nash County), and Caswell Game Lands (Caswell County). The maximum wind gust for each hour is shown on the graph which uses LST (Local Standard Time) which is the same as Eastern Standard Time. The wind gusts gradually increase during the morning and into the early afternoon hours before peaking between 200 pm and 600 pm. Wind gusts in excess of 40 mph were observed for between 4 and 6 hours.

Remarkable Number of Wildfires

The NC Division of Forest Resources reported that there were 302 fires and 9387.5 acres burned during the 24 hour period on February 10. This compares with the typical yearly values, based on averages during the 1997-2006 period, of 4931 fires and 20008.7 acres burned. That means that over 46% of the typical number of acres that burn in a year, burned on a single day.

The image below is from the USDA Forest Service MODIS Active Fire Mapping Program which is a product of the U.S. Forest Service's Remote Sensing Applications Center (RSAC). The embedded image may be difficult to view, but clicking on it will display a larger image that more clearly shows the location of recent fires across the Carolinas and Virginia. The red dots show the location of fires during the 12 hours ending at 500 am EST on February 11 with the orange dots showing the location of fires during the 24 hour period ending at the same time.

Factors Contributing to Extreme Fire Behavior

Various studies have produced a list of weather factors that result in extreme fire behavior by influencing burning conditions at the time of fire initiation or that impact ongoing fires. The critical weather factors contributing to extreme fire behavior and large fire growth include:
  • Dew point Depression of 10 degrees C at the 850 mb level.
  • Temperature lapse rate of 7 degrees C at the 950 - 850 mb levels.
  • Position and/or location of frontal passages possessing strong winds, and of intense upper level troughs at 500 mb level.
  • Strong winds above the nocturnal inversion.
  • Low level jets just before and just after cold front passage.
  • Pre-frontal and post-frontal (cold) passage associated with Byram (reverse wind speed and low level jet) & Brotak (surface at 9 kts and upper air at 34 kts wind profiles.
  • Haines Index of 4.

A Mitigating Factor to Dry Conditions

Although the conditions on February 10th supported an outbreak of wildfires, a look back at the moisture profile, specifically within the boundary layer, suggests that conditions could have been much drier. Instead, relative humidites hovered around 20% throughout the day.

One hypothesis to why dewpoints did not fall during the day is the advection of moisture across the Appalachians at the top of the boundary layer, which would be mixed to the surface. If this moisture was not forecast well by models, then dewpoints would subsequently be higher.

During the day as the surface heats up, vertical mixing of air causes the boundary layer, or layer of the atmosphere influenced by the surface, to grow. Warm air rises toward the top of the boundary layer, and cooler air at the top of the boundary layer is displaced downward. This redistribution of air also causes moisture and momentum to be redistributed. Eventually, the profiles of heat, moisture, and momentum become nearly uniform, and the boundary layer can be considered the "mixed layer".

The mixing of moisture and momentum were significant on February 10. As has already been shown above, the mixing of wind from the top of the mixed layer to the surface was the catalyst for the high winds and damage that were reported across the region.

In addition to winds, moisture also played role in the fire weather that day. Before the event, forecasters knew the mixed layer would be exceptionally high on the 10th. To facilitate the forecasting surface dewpoints and relative humidities, forecasters have a tool at their disposal that simulates the mixing that is forecast to occur on a particular day. In the days preceding this event, the tool indicated that dewpoints would crash into the single digits and relative humidities would fall well below 20%.

On the morning of the 10th, the 12 UTC sounding from Greensboro showed a very dry, residual mixed layer above the surface. The same sounding from Roanoke, VA showed more moisture in this layer. The visible satellite loop below shows that indeed there were clouds present both along the Appalachians in Virginia and upstream.

click to enlarge

As the mid and upper-level wave crossed the region, moisture was eventually advected across the Appalachians and across North Carolina. High-based statocumulus were present across much of central and western North Carolina. as mountain wave activity increased. The 00 UTC sounding later in the day revealed more moisture in the mixed layer at Greensboro. The addition of moisture at the top of the mixed layer moistened the air being mixed down to the surface during the day, and ultimately kept dewpoints and relative humudities from crashing to an even more dangerous level.

The Red Flag Program

Specific details about NWS Raleigh's fire weather program can be found in our Fire Weather Operating Plan. The Fire Weather Operating Plan specifically defines the critical weather parameters for collaboration of a Red Flag Warning. These parameters were agreed upon by the National Weather Service Offices that produce fire weather forecasts in NC, and the local user agencies, which include the U.S. Forest Service and the N.C. Forest Service. The critical weather parameters for collaboration of a Red Flag Warning include:
  • Sustained winds of 20 MPH or greater and/or gusts of 30 MPH or greater
  • Minimum relative humidity values of 25 percent or less
When the weather parameters (as determined by the NWS) are forecast to reach the critical thresholds, the NWS contacts the N.C. Forest Service (NCFS). The NCFS is responsible for monitoring fuel moisture levels. When the fuel moisture values are expected to reach critically dry levels (as determined by the NCFS) and the critical weather parameters are expected to be met, a Red Flag Warning or a Fire Weather Watch is issued. The User Agency (NCFS) makes the final decision on whether or not to issue the Red Flag Warning or Fire Weather Watch after all coordination has taken place.

There are times in which the NCFS has preferred to have a Fire Danger Statement issued in lieu of a Red Flag Warning, even when the critical fire weather parameters are forecast to be met. This may be in part due to marginal fuel moisture levels, time of year, and other issues within the agency.

Remote Sensing of Wildfires

click to enlarge Remote sensing instruments, specifically satellite and radar, provide imagery which can be quite useful for fire detection. Smoke plumes from fires can be viewed with weather surveillance radars and visible channel satellite imagery while the shortwave infrared (3.9 micron) satellite channel can literally sense heat associated with fires.

The sensitivity of the WSR-88D radar used by the NWS allows forecasters to view non precipitation echoes such as ground clutter, birds, insects, and smoke plumes. On Sunday February 10, several WSR-88D radars across the Mid Atlantic detected several smoke plumes across the Carolinas and Virginia. The image to the right is a regional base reflectivity image from 2312Z that shows several smoke plumes across the region. There are a handful of smoke plumes being blown eastward across southern Virginia along with smoke plumes originating in Franklin, Harnett, and Hoke Counties in North Carolina.

The animation below shows the base reflectivity product from the Raleigh, krax WSR-88D radar between 1603Z and 2356Z on Sunday. Note the smoke plumes that develop and then spread east in the strong westerly low level flow. Persistent plumes can be seen from fires in northern Hoke, eastern Harnett, and western Franklin counties. A Java Loop krax base reflectivity imagery that can be stopped, controlled and zoomed is also available.

click to enlarge Meteorologists often use long wave (10.7 micron) infrared satellite imagery in weather forecasting. But the properties of the 3.9 micron channel, however, make it valuable for detecting hot spots associated with fires. Blackbody radiance in the 3.9 micron channel increases more rapidly with temperature than the radiance in the 10.7 micron channel. Therefore, the 3.9 micron channel is more sensitive to sub pixel hot spots than the 10.7 micron channel, and is resultantly better suited for fire detection.

The image to the right is the 3.9 micron channel satellite imagery from 2301Z over southern Virginia and North Carolina. Two numerous dark colored pixels can be seen in the imagery. They indicate pixel temperatures of 20 deg C or more. The dark colored areas correspond very well to the location of fires at the time of the satellite image.

A web page has been developed that allows visitors to interactively view data from the trained satellite analysts in the Satellite Analysis Branch (SAB), within the Satellite Services Division (SSD), to manually integrate data from various automated fire detection algorithms with GOES and polar (Advanced Very High Resolution Radiometer (AVHRR), Moderate Resolution Imaging Spectroradiometer Fire Algorithm (MODIS) and Defense Meteorological Satellite Program/Operational Linescan System (DMSP/OLS) images. The result is a quality controlled display of the locations of fires and significant smoke plumes detected by meteorological satellites.

The image below is from the NOAA Satellite Services Division Fire Detection and Smoke web site. The image was grabbed at around 21Z on February 10.

Impacts Assessed from External Resources

Wind gusts in excess of 45 mph affected nearly all of central and western North Carolina on February 10 with many locations experiencing wind gusts in excess of 50 mph. Power outages, many from downed trees, were reported across much of North Carolina. Two of the largest electric utilities in North Carolina are Duke Energy Corporation and Progress Energy Corporation and these companies provide online information on the number and location of power outages. Meteorologists can carefully use this information during particular weather events such as ice storms and high wind events to infer at some of the possible impact of weather.

Progress Energy Power Outage Map
Duke Energy Power Outage Map

The images below were captured at around 430 pm on February 10 from the websites of Duke Energy and Progress Energy. The images show the possible impact of the high wind event on some electric customers in central and western NC.

Other External Resources

The links below are to various online resources for fire weather information. Some of these links contain materials that may be useful both during and after an event.

Archived Text Data from the Event

Select the desired product along with the date and click "Get Archive Data."
Date and time should be selected based on issuance time in GMT (Greenwich Mean Time which equals EST time + 5 hours).

Product ID information for the most frequently used products...

RDUAFDRAH - Area Forecast Discussion
RDUAFMRAH - Area Forecast Matrices
RDUHWORAH - Hazardous Weather Outlook
RDUNOWRAH - Short Term Forecast
RDUPFMRAH - Point Forecast Matrices
RDUPNSRAH - Public Information Statements (snow/ice reports among other items.)
RDUWRKDRT - Soil Temperature Data from the NC State Climate Office
RDUWSWRAH - Winter Storm Watch/Warning/Advisory
RDUZFPRAH - Zone Forecast Products



Many of the images and graphics used in this review were provided by parties outside of WFO RAH. The surface analysis graphic was obtained from the Hydrometeorological Prediction Center. Upper air analyses were obtained from the Storm Prediction Center, the National Center for Environmental Prediction, and the Ohio State Weather Server archive. Satellite data was obtained from National Environmental Satellite, Data, and Information Service. Some radar imagery was obtained from the National Weather Service web site. The skew-T imagery was obtained from the University of Wyoming. Wildfire statistics for North Carolina are courtesy of the NC Division of Forest Resources. Power outage maps are courtesy of Duke Energy Corporation and Progress Energy Corporation.

Case study team -
Barrett Smith
Phil Badgett
Jonathan Blaes

For questions regarding the web site, please contact Jonathan Blaes.

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