The fall season is advancing with warm air beating a retreat from ever more frequent and cooler cold air masses. As a result, the energy clashes in autumn weather systems may provide occasional displays of severe weather. Over the end of last week and the past weekend, episodes of thunderstorms were found across the eastern portion of the country. Early Saturday was a particularly devastating example of severe weather including fatal tornadoes spawned by thunderstorms. Details of the weather conditions at that time can be found in the Monday, 17 November, Daily Weather Summary.
Image 1 is the surface weather map for 12Z 15 NOV 2008, Saturday morning. At map time, a complex pair of frontal systems stretched over the eastern third of the country. One stationary front spread across the eastern Great Lakes to the Ohio River Valley. A more active frontal system was to the southeast of that with a warm front from the Atlantic westward to Maryland becoming a stationary front to a pair of low-pressure centers along the Appalachians. Finally a long cold front reached southwestward from the Lows over the Gulf of Mexico to northern Mexico. This long frontal system was the major driving force in the severe weather outbreak on Saturday.
The temperature (68 °F) and dewpoint (65 °F) at Charleston, South Carolina typified the air in the warm sector of the storm system. This air was generally [(cool and dry) (warm and humid)], especially compared to stations in the middle of the country.
Winds in the warm sector between the fronts were generally from the [(north) (south)].
These conditions of temperature, dewpoints and winds [(would) (would not)] favor warm air and moisture advection along the coastal region toward the stationary and warm frontal system.
Precipitation as indicated by the radar echoes [(was) (was not)] spread widely over much of the northeastern part of the country. Fairly bright shadings suggested many areas of relatively heavy precipitation, likely including thunderstorms.
Of particular note is a relatively narrow band of precipitation along the Virginia and North Carolina coasts. The thin lines and speckles of red shadings within the band would likely denote [(light and widely spread) (intense and concentrated)] rainfall. Precipitation reflectivity values in the scale along the left margin of the map are correlated with rainfall intensities, reds and magentas are often associated with severe thunderstorms and flooding rains.
Further to the south and west of the precipitation band was the cold front. The cold frontal symbols indicated the movement of the front was to the [(east and southeast ) (north and northeast)].
With abundant Atlantic moisture near the surface flowing toward the stationary/warm front, parallel to an advancing cold front, there [(was) (was not)] evidence for both sufficient low-level moisture and lifting mechanisms that are necessary ingredients for thunderstorm formation.
Image 2 is the satellite Water Vapor Image for 1215Z 15 NOV 2008, at nearly the same time as the surface map. Dark shadings denote relatively dry air in the middle troposphere while light gray shading shows regions of relatively high water vapor content. From the water vapor image there [(was) (was not)] evidence of a band of mid-tropospheric humid air extending from the Gulf of Mexico across central Florida to the Atlantic and curving into the warm sector of the surface frontal system.
Bright white blotches with sharp boundaries on the water vapor image indicates high cloud tops such as would occur in towering thunderstorms. From the water vapor image there [(was) (was not)] evidence of thunderstorm cloud development in the Atlantic east of South Carolina and along the North Carolina and Virginia coasts where the radar echoes existed.
Darker bands to the northwest of the Florida moist band are generally associated with upper-troposphere jet streams feeding drier air into storm systems at upper levels. The low-level moisture and upper-level dry air is a typically unstable situation that enhances thunderstorm growth.
Image 3 is the 300-mb map for 12Z 15 NOV 2008, the conditions in the upper troposphere at the time of the surface map and satellite view. The wind speeds plotted on the 300-mb map indicate that probably there [(was) (was not)] a jet stream of 70-knot or greater winds over the coterminous U.S. at this level.
Note the pattern of 300-mb wind directions across the U.S. The upper air wind pattern is generally one of a deep [(ridge) (trough)] over the east-central portion of the country.
The surface map low-pressure centers are located to the [(west) (east)] side of the axis of this upper air pattern.
Examine the wind directions shown for Greensboro in west-central North Carolina and Cape Hatteras in eastern North Carolina. These two wind reports show the 300-mb wind flow to be [(spreading apart) (coming together)] over the Carolina area. This pattern of flow is consistent with divergence in the upper troposphere. Such flow at upper atmospheric levels characteristically induces upward vertical motions in that region.
Thunderstorm cells need sufficient moisture, lifting mechanisms to "trigger" their formation, and supportive atmospheric conditions to develop. If these conditions become very well organized, severe thunderstorms can occur. Evidence of these necessary conditions for thunderstorms [(can be seen) (do not exist)] on the images in this Current Weather Studies investigation.
As a result of this weather system, at approximately 08Z of 15 November NOAA's Storm Prediction Center received 10 reports of tornadoes across eastern South and North Carolina, including 3 fatalities. The band of heavy precipitation shown on the Image 1 surface map produced those thunderstorms that spawned the tornadoes. It is the responsibility of the Storm Prediction Center to put out weather watches for such severe weather conditions for the entire U.S. When such conditions are actually occurring or are detected by radar, a local National Weather Service Office puts out a specific weather warning for their area. For the conditions and terminology used by the National Weather Service for winds, see the Monday, 17 November 2008, Supplemental Information file. We will look at tornado detection and warning in the next Current Weather Study.
Place the answers to Current Weather Studies investigations 11A and 11B on the CWS Answer Form (provided from the DataStreme Atmosphere website on Wednesdays).
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©Copyright, 2008, American Meteorological Society
