New Insight On The Plains' Biggest Rains -- Rocky Mountains Affect Midwest Flooding

Most summertime floods across the Great Plains are caused by mesoscaleconvective systems (MCSs). These giant complexes often emerge fromshowers and thunderstorms that form over the Rocky Mountains. Tucker andCrook used the Pennsylvania State University/NCAR mesoscale model tosimulate convection (showers and thunderstorms) and to test howdifferent modes of mountain convection affect the likelihood of MCSformation downstream. In the model, they found that an MCS was mostlikely to form when a mass of rain-cooled air descended from themountains, colliding with moist air on the plains and forcing it upward.

Although forecasters have seen this process unfold many times, it isstill unclear whether a given day's mountain storms will be the rightkind to trigger an MCS. Sometimes the initial storms lead to an MCS thatcan travel as far as Illinois; other times, the storms dissipate shortlyafter they move off the mountains. Tucker and Crook's modeling suggeststhat the strength of the rain-cooled outflow from the mountain storms iscritical to downstream MCS development. Several factors play into theoutflow strength, including the fall speed of ice crystals within themountain storms.

Fine-scale modeling for better prediction

Even today's most sophisticated forecast models cannot peg mountainconvection well enough to assess how it might trigger storm complexesdownstream. However, under a new NSF grant, Tucker and Crook are using afiner-scale model built by NCAR scientist Terry Clark to look moreclosely at mountain convection and how it relates to the larger-scaleatmospheric flow. Since the large-scale flow is routinely forecast bycomputer models, this new work could allow forecasters to betterpinpoint a given day's mountain convection and where it might triggerlarge storm complexes on the plains. Tucker and Crook's work issupported by the University of Kansas and NSF.

One downpour leads to another: NCAR team pinpoints culprit

A typical MCS peaks in strength during the overnight hours anddissipates the next day. However, it may be followed by a second MCS thefollowing night. Sometimes a slow-moving sequence of MCSs will extendover several days, causing torrential rains over a large area. If such amultiday sequence could be forecast, valuable lead time might be gainedon flooding threats.

NCAR scientists Christopher Davis, Stanley Trier, and colleagues havegained new insight on a type of low-pressure center that connects oneMCS to the next. This low is called a mesoscale convective vortex (MCV).With a core only 30 to 60 miles wide and 1 to 3 miles deep, an MCV isoften overlooked in standard weather analyses. But Davis and Trier havefound that MCVs play a key role in helping storms regenerate over two ormore days.

Looking closely at satellite, upper-air, and radar observations from1998, Davis and Trier found evidence of 17 separate MCVs over thecentral and eastern United States. Previous studies had found only twoor three MCVs per year. The vortices appear most likely to persist whenlower- and upper-level winds are relatively light. This allows thecirculation to maintain its integrity for up to 12 hours after thestorms dissipate. If other conditions are favorable, a new round ofstorms may cluster around the vortex. For example, one MCV triggeredheavy rains in Texas on May 27, 1998; flooding in Arkansas early on the28th; and additional flooding the following night in Mississippi. An MCVthat moves into tropical waters, such as the Gulf of Mexico, can serveas the nucleus for a tropical storm or hurricane.

Currently, it's difficult to spot and track mesoscale convectivevortices from upper wind observations alone, due to their small size.However, a technique developed by NCAR's John Tuttle calculates windsusing cloud movements observed by satellite in order to spot MCVs andother features. This promising technique, along with better observationsand models, could make it practical for forecasters to use MCVs as aguide to predict locations of heavy rain. Davis and Trier's work issupported by NASA and the U.S. Weather Research Program, which isexamining forecast tools for heavy precipitation.

NCAR is managed by the University Corporation for Atmospheric Research,a consortium of more than 60 universities offering Ph.D.s in atmosphericand related sciences.

-The End-

Writer: Bob HensonNote to Editors:Visuals: Images are available at ftp://ftp.ucar.edu/communications.Filename(s): mcsradar.tif, downdraft.tif, sat.tif, satwind.tif. Captionsare at the Web address below.

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