A longwave trough is a synoptic region of cooler air that is displaced toward the south in the Northern Hemisphere. In the upper troposphere a trough will have lower heights compared to surrounding regions at the same latitude on each side of the trough. The average number of longwave troughs in the Hemisphere is about 6 and they are generally located along the polar jet stream. The amplitude of an average longwave trough is about 1,700 miles. Typically, longwave troughs will move to the east at about 15 knots. In some cases, troughs can stall for awhile or even retrograde (move westward; opposite of normal). The amplitude of any particular longwave trough is highly variable. The winds on each side of the trough help determine if the trough will dig (become more amplified) or lift (become less amplified). Refer to this website below for more information on trough amplitude: http://theweatherprediction.com/habyhints/256/ Go to the UNISYS website (300 mb link below) and notice how the troughs are moving through each time frame. Typically they will slide toward the east but they can also stall or retrograde: http://weather.unisys.com/nam/300.php |