Synoptic-Scale Conditions Leading to Flooding in South Carolina: A Case

Study of October 22-23, 1990(www.treehugger.com)

Erik D. Kabela Joanne Stevenson

Mini Tech 2009

Objectives• Illustrate the effects on society (public/private property

damage, evacuations, loss of life, etc) of this event.

• Link Maddox's (1979)* definition of a synoptic flash flooding event with October 22-23, 1990 event– Determine which of the 5 heavy rain event set-ups

identified by Konrad (1997)** fits with this event

• Enhance the understanding and application of pattern recognition in heavy rain and flood forecasting

*Maddox, R. A., Chappell, C. F. and L. R. Hoxit, 1979. Synoptic and Meso-α Scale Aspects of Flash Flood Events. Bull. of the Amer. Meteor. Soc. , 60 (2), 115-123.

**Konrad II, C. E., 1997. Synoptic-Scale Features Associated with Warm Season Heavy Rainfall over the Interior Southeastern United States. Wea. Forcasting, 12, 557-571.

Methods•Archives

• The State, Greenville News, Aiken County Standard, The New York Times

• SHELDUS

•Web interfaces: • NCDC - NOMADS• Plymouth State University -Plot

generation interface• NOAA - National Severe

Storms Laboratory

•NOAA – Daily Weather Maps•USGS – Water Resources Data

(Maddox et al., 1979)

Flooding Impacts•SC hit by disintegrating Hurricane Klaus and Tropical Storm Marco (Oct. 8-14)

•Up to 15” of rain•Damage of $3-8 million, several thousand evacuations, 120 dam failures, 5 deaths

•Cold front moved through (Oct.22)•Day and a half of nearly continuous rainfall on already stressed hydro system•11 counties receive Presidential Disaster Declarations (PDDs)

• >1,400 registrations for temporary housing, recovery of lost property, or repairs

• >$10 m of aid administered by federal government

(NOAA,http://www.hpc.ncep.noaa.gov/tropical/rain/marcoklaus1990.html)

Pre-Exisiting Conditions

Flood Stage

Heavy Rain/Flood Definition

• According to Konrad (1997)– At least 2 inches in a 6 hour period

• Break in between hourly precipitation could not exceed 2 hours

36-Hour Total Precipitation

Components of a Maddox Synoptic-Type Flood Event

• Quasi-stationary or slow moving weak cold front

• Ample low-level moisture– Usually greater than 60°F surface dew point

• 500mb strong short-wave trough• Ample mid-level moisture

– Dew point depression of <= 6°C

Surface Map – Oct. 22, 1990

500mb – Oct. 22, 1990

Maddox Ideal vs. Oct 23 00Z Surface Map

Td = 70

Td = 70

Maddox Ideal vs. Oct 23 00Z 500mb Map

Comparison to Konrad – Type 2Key Contributions Include:• Slow moving cold front

– Type 2 typically occurs during the transition season

• Ample low- and mid-level moisture• Low-level convergence

– Located NW of heavy rain• Upper-level divergence• Strong 500mb Vorticity Max• Strong mean 850-200mb winds (>21 m/s)

from the SW

• Strong instability using the K-Index• 850mb warm-air advection• Rain cells develop in “echo train” pattern

Ample Mid and Low-Level Moisture

700Mb mixing ratio of greater than 7 g/kg

Surface precipitable water of greater than 1.5 inches (38.1 mm)

Low-level Convergence

• Low-level convergence max to the NW of the heavy precipitation

200mb Divergence

• Upper-level divergence promotes upward motion

• Heavy Rain area located over the left exit region of jet

500mb Vorticity Maximum

• Heavy rain located to the right of vorticity max

850 – 200mb Winds

• Mean 850 to 200mb winds blowing parallel to surface boundary

• Mean 850 to 200mb wind speed ≈24m/s

Instability with the K-Index

• K-Index between 28 and 32– Corresponds to roughly 40-

60% chance of showers and thunderstorms

850mb Warm-Air Advection

• 850mb WWA promotes low-level rising motion

Conclusions• Pre-existing stream conditions lead to

increased potential for flooding on Oct 22-23• Flooding event of Oct. 22-23, 1990 fits

classification of Maddox synoptic-type flood• Flooding event further broken down into a

Konrad Pattern 2 heavy rain event• Provides a basis for using climatological

studies for use in everyday forecasting• Can prove useful for operational forecasters

as well as emergency managers