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Organized areas of thunderstorm activity reinforce pre-existing frontal zones, and can outrun cold fronts. This outrunning occurs within the [[westerlies]] in a pattern where the upper level jet splits into two streams. The resultant [[Mesoscale Convective System|mesoscale convective system]] (MCS) forms at the point of the upper level split in the wind pattern in the area of best low level inflow. The convection then moves east and toward the [[equator]] into the warm sector, parallel to low-level thickness lines. When the convection is strong and linear or curved, the MCS is called a [[squall line]], with the feature placed at the leading edge of the significant wind shift and pressure rise which is normally just ahead of its radar signature.<ref>{{cite web|author=Office of the Federal Coordinator for Meteorology|date=2008|url=http://www.ofcm.gov/slso/pdf/slsochp2.pdf|title=Chapter 2: Definitions|page=2-1|publisher=[[NOAA]]|accessdate=3 de mayo de 2009}}</ref> This feature is commonly depicted in the warm season across the [[United States]] on surface analyses, as they lie within sharp surface troughs.
A macroburst, normally associated with squall lines, is a strong downburst larger than {{convert|4|km|mi}}.<ref>{{cite web|author=Dr. Ali Tokay|url=http://userpages.umbc.edu/~tokay/chapter13.html|title=Chapter #13: Thunderstorms|accessdate=9 de julio de 2008|date=21 de abril de 2000|publisher=[[University of Maryland]] Baltimore College|urlarchivo=http://web.archive.org/web/http://userpages.umbc.edu/~tokay/chapter13.html|fechaarchivo=22 de noviembre de 2015}}</ref> A wet microburst consists of precipitation and an atmosphere saturated in the low-levels. A dry microburst emminates from high-based thunderstorms with [[virga]] falling from their base.<ref name=autogenerated1>Fernando Caracena, Ronald L. Holle, and Charles A. Doswell III. [http://www.cimms.ou.edu/~doswell/microbursts/Handbook.html Microbursts: A Handbook for Visual Identification.] Retrieved on 2008-07-09.</ref> All types are formed by precipitation-cooled air rushing to the surface. Downbursts can occur over large areas. In the extreme case, a [[derecho]] can cover a huge area more than {{convert|200|mi|km|abbr=off}} wide and over {{convert|1000|mi|km|abbr=off}} long, lasting up to 12 hours or more, and is associated with some of the most intense straight-line winds,<ref>{{cite web|author=Peter S. Parke and Norvan J. Larson|date=23 de noviembre de 2005|url=http://www.crh.noaa.gov/dlh/science/event_archive/summer_archive/1999blowdown/1999blowdown.php|title=Boundary Waters Windstorm|accessdate=30 de julio de 2008|publisher=[[National Weather Service]] Forecast Office, [[Duluth, Minnesota]]}}</ref> but the generative process is somewhat different from that of most downbursts.
[[Image:Undular bore waves over Arabian Sea.jpg|250px|thumb|Satellite image of an undular bore.]]
{{See also|Wind shear|Undular bore}}
Gust fronts create low-level [[wind shear]] which can be hazardous to planes when they takeoff or land.<ref>{{cite journal|author=Diana L. Klingle, David R. Smith, and Marilyn M. Wolfson|url=http://ams.allenpress.com/perlserv/?request=get-abstract&doi=10.1175%2F1520-0493(1987)115%3C0905%3AGFCADB%3E2.0.CO%3B2|title=Gust Front Characteristics as Detected by Doppler Radar|accessdate=9 de julio de 2008|journal=Monthly Weather Review|pages=905-918|volume=115|number=5|date=May 1987}}</ref> Flying [[insect]]s, a subset of [[arthropods]], are swept along by the [[prevailing winds]].<ref>{{cite web|author=Diana Yates|date=2008|url=http://news.illinois.edu/news/08/0707birds.html|title=Birds migrate together at night in dispersed flocks, new study indicates|publisher=[[University of Illinois]] at Urbana - Champaign|accessdate=26 de abril de 2009}}</ref> As such, fine line patterns within [[weather radar]] imagery, associated with converging winds, are dominated by insect returns.<ref>{{cite web|author=Bart Geerts and Dave Leon|date=2003|url=http://www-das.uwyo.edu/wcr/projects/ihop02/coldfront_preprint.pdf|title=P5A.6 Fine-Scale Vertical Structure of a Cold Front As Revealed By Airborne 95 GHZ Radar|publisher=[[University of Wyoming]]|accessdate=26 de abril de 2009}}</ref> At the surface, clouds of dust can be raised by outflow boundaries. If squall lines form over arid regions, a duststorm known as a [[haboob]] may result from the high winds in their wake picking up dust from the desert floor.<ref>{{cite web|author=Western Region Climate Center|date=2002|url=http://www.wrcc.dri.edu/ams/glossary.html#H|title=H|publisher=Desert Research Institute|accessdate=22 de octubre de 2006}}</ref> If outflow boundaries move into areas of the atmosphere which are stable in the low levels, such as over colder pockets of ocean or through the cold sector of [[extratropical cyclone]]s, they can create a phenomenon known as an undular bore, which shows up on satellite and radar imagery as a series of [[transverse wave]]s in the cloud field oriented perpendicular to the low-level winds.<ref>{{cite web|url=http://www.arpal.org/Pubbl/paper/eumetsat-mauritania.pdf|title=Outflow from convective storm, Mauritania and adjacent Atlantic Ocean (13 August 2006)|author=Martin Setvak, Jochen Kerkmann, Alexander Jacob, HansPeter Roesli, Stefano Gallino, and Daniel Lindsey|accessdate=3 de julio de 2009|date=19 de marzo de 2007|publisher=Agenzia Regionale per la Protezione dell'Ambiente Ligure|urlarchivo=http://web.archive.org/web/http://www.arpal.org/Pubbl/paper/eumetsat-mauritania.pdf|fechaarchivo=22 de noviembre de 2015}}</ref>
== Véase también ==
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