WELCOME BACK: Procedure continued from STUDY GUIDE.
Over Thanksgiving and the past holiday weekend a storm system affected the continental US from the southern plains to the Northeast. The storm system brought rain along the Gulf Coast on Thursday with freezing rain along the northern edges of the precipitation area as it swept northeastward toward New England. The cold front associated with the storm crossed the Gulf and the southeast as the low-pressure system became occluded. As noted in the Monday, 27 November 2000, Daily Weather Summary, thunderstorm activity broke out in the warm sector, between the warm and cold fronts. On Saturday morning, 25 November, some minor damage was sustained in southern Florida from an unconfirmed tornado.
Image 1 is the surface weather map for 12Z 24 NOV 2000 (Friday morning) as the storm was becoming occluded. The primary low-pressure center was in eastern Texas with a secondary Low in southwestern Louisiana at the point of occlusion, where the occluded, cold, and warm fronts meet. The [(clockwise and outward) (counterclockwise and inward)] flow around the Lows brought the warmer, more humid Gulf air northward on the east side of the storm over the [(warm) (cold)] frontal surface. At map time the precipitation, as shown by the radar echoes, was located in a broad area generally more to the [(north) (south)] of the Lows and warm front.
The low-pressure centers tracked to the northeast with the cold front trailing across the eastern US. Image 2 is the satellite Infrared Image for 1215Z 26 NOV 2000 (Sunday morning). At this time the primary low-pressure center was near Lake Michigan while the secondary low center at the point of occlusion was in eastern Virginia. This satellite image shows the cloud patterns associated with the storm system at that time. Bright white shading on infrared satellite imagery denotes cloud tops that are very [(warm) (cold)]. This temperature range generally relates to cloud tops that are therefore relatively [(low) (high)]. Thunderstorms extend to great heights and are therefore often seen as bright "blobs" within the larger cloud pattern.
The line of brighter cloudiness from northern Florida into the Gulf of Mexico probably [(did) (did not)] contain thunderstorms. Another line of bright blobs extended along the cold frontal position off the Atlantic Coast east of the Carolinas. These thunderstorms were occurring over the warm waters of the [(Gulf Stream) (Labrador Current)].
Image 3 is the satellite Water Vapor Image for 1215Z 26 NOV 2000, the same time as the infrared satellite image. (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 white in water vapor imagery. However, water vapor imagery does not "see" humidity contained in the lower troposphere, that is, in the lowest few kilometers of the atmosphere.
Compare the water vapor image with the infrared image for the same time. Where bright shading indicates middle and upper tropospheric clouds, the water vapor image shows [(bright) (dark)] shading or cloudiness and high moisture.
Water vapor images can also display patterns indicating broad-scale atmospheric circulations in the middle troposphere. Dark areas indicate relatively dry air, often the result of sinking motions involving cold, dry upper tropospheric air. Lighter shading can indicate rising motions that involve warmer, more humid lower tropospheric air. Recall from the hand-twist model of a Low that shows Lows have [(counterclockwise) (clockwise)] circulation and accompanying [(upward) (downward)] motions. At the time of the image the primary Low at the surface was located near the southern end of Lake Michigan. The upper level Low circulation is shown by the bright counterclockwise swirl over Indiana.
The jet stream at this time extended from northern Mexico and south Texas to northern Florida to Virginia. This area is seen on the water vapor image as a band of relatively [(bright white) (dark)] shading. Additional circulation features can be seen over north-central Nebraska and across southern Canada.
Surface weather reports as shown on weather maps and meteograms, upper air data in constant-pressure maps and on Stüves, combined with 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 warning 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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