1. 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

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-  Scale Aspects of Flash Flood Events. Bull. Am. Meteorol. Soc., 60, 115-123