WELCOME BACK: Procedure continued from STUDY GUIDE.
Over the past Thanksgiving holiday weekend a vigorous storm system developed in the central US. The storm's movement northeastward across the lower Mississippi Valley produced thunderstorms that spawned severe weather from Friday evening through Saturday. The severe weather included wind damage, large hail and tornadoes that killed a dozen people with many others injured. The details can be found in the Monday, 26 November 2001, Daily Weather Summary.
Image 1 is the surface weather map for 00Z 24 NOV 2001 (Friday evening). The major low-pressure storm center was in north-central Kansas. An occluded front curved to eastern Kansas where it joined with a warm front and a cold front. The meeting of these three fronts is termed a "triple point". The warm front stretched southeastward to southern Alabama and on to just off the North Carolina coast. The cold front extended from the triple point southwestward to western Texas. The [(clockwise and outward) (counterclockwise and inward)] flow around the Low brought warm, humid Gulf of Mexico air northward into the warm sector of the storm (between the cold and warm fronts). At Jackson, Mississippi, just south of the warm front, the temperature was _____ °F, dewpoint 63 °F, and wind from the [(southeast at 10 knots) (northeast at 15 knots)].
Shaded areas denote radar echoes where precipitation was likely falling at map time. The shading scale to the lower left of the map indicates intensity of precipitation. Generally, extensive areas of precipitation were located
Thunderstorms need (a) abundant moisture, (b) a lifting mechanism to "trigger" their formation, and (c) supportive atmospheric conditions to develop. Severe thunderstorms are likely where these conditions are very well organized. The relatively high dewpoints for November [(did) (did not)] indicate there was abundant water vapor in the near-surface air associated with this storm system. The fronts associated with this storm system that were moving across the country, [(did) (did not)] provide a lifting mechanism for the formation of the thunderstorms.
Image 2 is the satellite Water Vapor Image for 0015Z 24 NOV 2001, approximately the same time as the surface map. (Water vapor images are available from the homepage, Satellite section.) Satellite water vapor sensors are tuned to the infrared radiation that is absorbed and emitted by water vapor in the atmosphere, particularly between the levels of about 700 and 400 mb, roughly from 3 km to 7 km altitude in the middle troposphere. Thick clouds appear bright white in water vapor imagery. In cloudless regions, humid areas (relatively high dewpoints) appear generally medium gray while relatively dry mid-levels (low dewpoints) appear dark. Although areas can be clear in visible and infrared imagery, extensive water vapor in the mid-troposphere can even make the same areas appear medium gray in water vapor imagery. However, water vapor imagery does not "see" humidity contained in the lower troposphere, that is, in the lowest 3 kilometers of the atmosphere.
The dark shading in an arc from southeast New Mexico across north Texas indicates there was relatively [(humid) (dry)] mid-tropospheric air along that band. A broad band of medium gray shading from northern Mexico to eastern Oklahoma and Arkansas indicates that relatively humid air [(is) (is not)] flowing into the storm system's warm sector at middle levels of the troposphere. The bright white patches from Nebraska curving around to Missouri and those from the western Carolinas around to northern Florida generally [(do) (do not)] coincide with the radar echoes indicating the areas of precipitation.
The bright white arc in the central US indicates the circulation of the storm system centered in Kansas. If a sequence of water vapor images in animation were available, it would show atmospheric circulation patterns very well. Particularly over ocean regions, water vapor imagery is often the first indicator of developing storms.
Image 3 is the 300-mb constant pressure map for 00Z 24 NOV 2001, the same time as the surface map and water vapor satellite image. Wind speeds of 70 knots or more traditionally are used to indicate the presence of a jet stream. At map time there [(was) (was not)] evidence of a jet stream across the US. The general upper-air flow pattern indicated that the axis of a [(trough) (ridge)] was located west of the low-pressure system's center on the surface map.
On upper-air maps, the wind directions generally tend to be [(parallel) (perpendicular)] to the contour lines. The orientation of the contour lines from Missouri to Louisiana are generally [(spreading apart) (coming together)] along the direction of the wind flow. Spreading of upper-level flow indicates divergence at that level while coming together indicates convergence.
The upper level divergence at 300 mb removed air from the region that was replaced by the convergence and rising of air in the circulation of the Low at lower levels, and the lifting of the humid air by the frontal systems, all combining to generate the wide-spread severe thunderstorms that swept across the southern US over the weekend. Additionally, the 300-mb map shows a dashed 90-knot isotach (line of constant wind speed) across central Texas. This locally higher wind speed area was a jet streak. The severe weather area is favored in the diverging air ahead of this streak. It showed as the darker area in the dark sweeping curve of the jet stream on the water vapor image.
By 12Z 24 NOV, the cold front extended from western Tennessee southwestward to Louisiana, having swept across Missouri, Arkansas, and into Mississippi where the tornadoes and hail occurred. These maps and satellite image show the ingredients for the severe weather outbreak that happened.
Surface weather reports as shown on weather maps and meteograms, upper air data on constant-pressure maps and on Stüves, and radar summaries and satellite imagery (visible, infrared, and water vapor) complement each other to produce a three-dimensional picture of the atmosphere that evolves over time. From this present understanding, forecasts can be made and necessary watches and warnings for threatening weather given.
Hold this activity until you have completed all applications for this week. Instructions for faxing your LIT mentor will appear at the end of this week's Activity B.
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