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A Bit of History

Learning the flood types via Synoptic and Meso-  Scale Aspects of Flash Flood Events R.A. Maddox, C.F. Chappell and L. R. Hoxit BAMS , 1979, 115-123 Meteorology 415-December 7, 2006. A Bit of History.

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A Bit of History

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  1. Learning the flood types viaSynoptic and Meso- Scale Aspects of Flash Flood EventsR.A. Maddox, C.F. Chappell and L. R. HoxitBAMS, 1979, 115-123Meteorology 415-December 7, 2006

  2. A Bit of History • Dr. Robert A. Maddox is arguably the foremost convective heavy rain authority in the United States • Recommended flash flood readings if you are interested: • Maddox, et al., Meteorological Aspects of the Big Thompson Flash Flood of 31 July 1976 • Hoxit, Maddox, et al., Meteorological Analysis of the Johnstown , Pennsylvania, Flash Flood of 19-20 July 1977

  3. Basic Premise • Maddox decided that flash floods could be differentiated into four basic types • 2 of the four are typical summertime, weakly forced events • 1 is a spring/fall, stronger forcing event • 1 is a mainly topographic event

  4. Items Common To All Floods • All had convective elements for the convective storms • All had surface dew points very high • Deep tropospheric moisture • Weak to moderate shear through cloud depth • Months • Most in warm season (April to September) • Time of day • Many are at night, especially frontal and mesohigh

  5. SYNOPTIC • Need strong dynamics, so typically more of a spring/fall event • Still need some good thermodynamics--without the strong thermo, there can’t be convective elements!

  6. More on Synoptic Type • The synoptic cases typically have the strongest winds aloft. This should not be a surprise since these types of storms need very strong dynamics to overcome some deficiencies in thermodynamics. • Favored regions are near the fronts…in this pattern, often will get the heavy rain bands near and just ahead of the fronts within broader area of rain.

  7. FRONTAL • Stationary or very slow moving E-W oriented front acts as a trigger and a focusing mechanism • Heavy rain occurs on the cool side of the front as high theta-e air is forced to ascend isentropic surfaces. • Winds aloft are nearly parallel to the front • Unusually strong low-level jet perpendicular to the front. • PWATs average 1.6 inches, above the mean for warm season events

  8. More on Frontal Type • Vvery warm, humid air, perhaps aloft, intersects front • Wweak winds aloft allow for slow movement of cells • Sstorms continually form in the same region and train or back-build

  9. MESOHIGH • Very similar to frontal events • The warm, humid air will intercept an outflow boundary on the leading edge of the mesohigh, lifting the fuel and acting as a trigger • Heaviest rains occur on the cool side of the boundary • Winds aloft nearly parallel to the boundary • Weakest winds of any type of Maddox event • Tend to happen more at night than at any other time • A result of the nocturnal low-level jet being crucial

  10. More on Mesohigh Type • Primarily a summertime phenomenon • Result of largest difference in temperature sustaining the mesohighs longer • Winds are usually north to northeast on the south side of mesohigh with westerlies aloft and southerlies between the two—significant veering! • PWATs average 1.64 inches…above the mean for the warm season events

  11. More on Mesohigh Type • Typically a weak shortwave moving through the ridge near where the storms form, allowing for weak winds aloft but persistent upwards motion

  12. WESTERN • Typically occur along the east slopes of the Rockies, although later studies have shown these types to occur on the east slopes of the Appalachians as well • Low-level southeasterly flow will interact with old boundaries near the mountains and upsloping will play a major role

  13. More on Western Type • Again near mean 500-mb ridge position, although a short-wave is likely passing through the ridge • Very weak winds aloft • Do not need quite as much moisture since the upslope will help to generate anomalous precipitation amounts • Not all topographically aided events are western type

  14. Conclusions • Flash floods are almost always convective, this is expected since a flash flood needs very heavy bursts of rain • This can include embedded rain bands • Abundant moisture is a necessity • Deep moisture is a bonus (high surface dew points under nocturnal low-level jet)

  15. More Conclusions • Weak wind shear (direction and speed) • Shortwave passing through a mean-ridge for upward motion with weak winds aloft • Regeneration in same areas • Frequently nocturnal

  16. Forecast Clues • Did the system produce heavy rain bursts upstream? Does the pattern look identical? • Are moisture variables all extremely high • Are winds weak aloft? • Is there evidence of old boundaries floating through the forecast area? • Is there a trigger?

  17. References • Forbes, G.S. Hydrometeorology Course Notes, 1995. • Maddox, R.A., L.R. Hoxit, C.F. Chappell and F. Caracena, 1978: Meteorological Aspects of the Big Thompson Flash Flood of 31 July 1976. NOAA Tech. Rep., ERL 388-APCL 41, Boulder, Colo. • Maddox, R.A., C.F. Chappell and L.R. Hoxit, 1979: Synoptic and Meso-a Scale Aspects of Flash Flood Events. Bull. Am. Meteorol. Soc.,60, 115-123

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