At the end of the weekend, a storm system developed over northern New Mexico along a wandering frontal system and began moving northeastward toward the mid-Mississippi River Valley. A cold front stretched southwestward from the low-pressure center. Around the Low center and along the cold front thunderstorms were found across the south-central portion of the country. Details of the weather conditions with this system can be found in the Monday, 16 November, Daily Weather Summary.
Image 1 is the surface weather map for 12Z 16 NOV 2009, Monday morning, from NOAA's National Centers for Environmental Prediction (NCEP). The map shows the storm centered over Arkansas with one Low within a small circular 1012-mb isobar (mostly obscured on the map) along the AR-MO border. A second L (also obscured) in central Arkansas marked the transition from a stationary front curving to the northeast and the cold front to the southwest.
Note the temperature and dewpoint at Lake Charles, in southwestern Louisiana located in the warm sector of the storm system and at San Antonio, in south-central Texas behind the cold front. The air in the warm sector of the system was generally [(cool and dry)(warm and humid)] compared to that over central Texas. You might particularly note Amarillo, in north Texas where it was 29 °F!
The cold front was moving generally toward the [(northeast)(southeast)(southwest)].
The radar echoes denoting precipitation were generally located [(well behind the cold front)(along the cold front)].
Bright red (on-screen) specks within the radar shading areas, particularly in northeast Louisiana and eastern Missouri, denote very heavy rainfall intensities. This [(is)(is not)] consistent with the existence of thunderstorms.
The dewpoints across the western Gulf Coast area, particularly ahead of the cold front, [(do)(do not)] indicates that there was abundant near-surface moisture for precipitation formation.
The direction of movement of the cold front toward the Gulf Coast area [(does)(does not)] indicate that there was a lifting mechanism for that humid air.
Image 2 is a Water Vapor Image at 1215Z 16 NOV 2009, near the time of the surface map conditions. Water vapor imagery displays conditions mainly in the middle troposphere, approximately 700 to 400 mb. Clouds are bright white while humid, cloud-free areas are medium gray. Dark areas denote relatively dry air in the mid-troposphere. The water vapor image shows generally [(dry)(humid and cloudy)] conditions along and ahead of the surface cold front.
Generally, where the Image 1 surface map showed radar shadings of precipitation, there [(were)(were not)] clouds and humid air at mid-tropospheric levels above as shown on the water vapor image.
Image 3 is the 300-mb map for 12Z 16 NOV 2009 displaying conditions in the upper troposphere at the time of the surface map and water vapor image. The wind speeds plotted on the 300-mb map indicate there [(was)(was not)] a jet stream of 70-knot or greater winds over the south-central 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 [(ridge)(trough)] over the central portion of the country.
The surface map low-pressure center was located to the [(west)(east)] side of the axis of this upper air pattern.
Examine the pattern of upper tropospheric wind directions shown from northeast Arkansas to Illinois and Indiana. These wind directions show the 300-mb wind flow to be [(spreading apart)(coming together)] over the that area. This pattern of flow is consistent with divergence in the upper troposphere. Such flow at upper atmospheric levels characteristically induces upward vertical motions across that region.
Thunderstorm cells need sufficient moisture, lifting mechanisms to "trigger" their formation, and supportive upper atmospheric conditions to develop. Evidence of these necessary conditions for thunderstorms [(can be seen)(do not exist)] on the images in this Current Weather Studies investigation. If these conditions become very well organized, severe thunderstorms can occur.
With continued movement of the storm system over the day, the Storm Prediction Center had alerted areas along the Gulf Coast and over the central Missouri region to the possibility of continued thunderstorm activity. However, conditions did not seem unstable or well-enough organized for severe weather outbreaks.
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 damaging winds, one thunderstorm threat, see the Monday, 16 November 2009, Supplemental Information...In Greater Depth 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, 2009, American Meteorological Society
