The First Warning

While developing the daily synoptic weather map is the responsibility of the HPC, predicting, monitoring and warning the public of severe weather events is the domain of the Storm Prediction Center (SPC) in Norman, Oklahoma. Severe weather may take many forms, but the SPC largely focuses on tornadoes, winds greater than 50 knots, hail equal to or greater than 3/4" in diameter, heavy rain, excessively hot or cold temperatures and winter storms.

Each day, the SPC prepares Convective Outlooks for the current day and each of the following two days to identify those areas where severe thunderstorms might erupt during the specified forecast period. In addition to these longer-range forecast products, the SPC issues short-term (1-6 hours) forecasts, referred to as mesoscale discussions (MCD), when an area is likely to be affected by an outbreak of severe weather.

MCDs are issued as severe weather threatens and are sequentially numbered for easy retrieval. As shown in this sample, MCDs provide a chart of the affected area annotated with the particular weather warnings, along with a technical explanation of the atmospheric conditions responsible for the severe event.

The SPC issued a total of seven MCDs for the December 22-23, 2004 winter storm, the first (#2556) at 6:35pm CST on Tuesday, December 21st. The discussion warned that:

The warning graphic that accompanied Mesoscale Discussion # 2556 issued at 6:35pm CST on December 21, 2004 by the Storm Prediction Center.

As identified in the chart issued with MCD # 2556 (above), the cause of this winter storm was a clash between cold, relatively dry air from Canada and warm, moist air from the Gulf of Mexico. This showdown was taking place along the cyclone's cold front, identified on the chart by blue triangles on a solid blue line.

The chart (below) of surface temperatures at 6PM CST on December 21st shows the extent of the cold air plunging into the country's mid section behind the system's cold front. The freezing line (0°C) stretches across the middle of Missouri and Illinois just north of the cold front. While the temperature in central Illinois and Missouri hovered just above freezing, temperatures in central Tennessee and northern Mississippi ranged from 12°C to 16C°. This equated to nearly a 16°C (20°F) temperature difference in just 250 miles.

Surface temperatures in Celsius at 6PM CST on December 21, 2004 (0Z on December 22, 2004) (full image). Image provided by the NOAA-CIRES Climate Diagnostics Center, Boulder, Colorado.

However, the relatively warm temperatures over Louisiana and Mississippi were only part of the story. The MCD #2556 warning graphic included a green arrow and a notation stating "increasing low-level moisture advection". Advection is the horizontal transfer of any atmospheric property, such as moisture or temperature, by the wind. The MCD text also made note that:

...moist flow from the western Gulf of Mexico continues to stream northward in advance of the surface low over northeastern Texas...

Both the MCD graphic and discussion were referring to moist air being transported northward by the winds at 850mb (approximately 1,500 meters above the surface). The chart (below) from 6PM CST on December 21st shows the 850mb winds blowing in a northerly direction at approximately 20 meters per second (40mph). Increasing wind speed over the next two days provided the storm with an ample supply of warm, moist air.

850mb wind speed in meters per second at 6PM CST on December 21, 2004 (0Z on December 22, 2004) (full image). Image provided by the NOAA-CIRES Climate Diagnostics Center, Boulder, Colorado.

The geographic area covered by MCD #2556 was dwarfed by the region covered by MCD #2570 issued just after midnight on December 23rd.