Hydrometeorological Aspects of the Kansas Turnpike Flash

Flood of August 30-31 2003

Authors: Jeffrey D. Vitale James T. Moore

Charles E. Graves,Matt Kelsch

Presented by: Brittanny Snyder Chris Iraggi

Key Points:

From 2230 UTC to 0130 UTC on August 30-31, 2003 an area 12 miles from the Kansas Turnpike was hit with 6-8 inches of rain due to a low-echo centroid (LEC) in roughly 3 hours

Results: Extreme Flash Flooding – 6 Casualties - $250,000 worth of property damage

Main Focus will be on the synoptic and mesoscale parameters that favor the development of LEC storms

Also focus on hydrological considerations that contribute to extreme flash flooding

What are the characteristics of a severe thunderstorm?

“Cold-Topped” (IR values)

Equilibrium Level is close to the tropopause

Strong updrafts due to stronger buoyancy in the storm

Reflectivity values > 50 DBZ due to the presence of hail

Generate rain by the Bergeron Process (involves ice crystals)

Peak rain rates are not very intense

What is a low-echo centroid (LEC)?

“Warm topped storm” (IR values)

Equilibrium Level is below the tropopause resulting in a shallow storm height and a warmer IR temperature

Weaker updrafts: do not produce hail

Maximum reflectivity values are lower than 50 DBZ

Occur is moist, low-shear environments favoring warm-rain processes

More efficient at producing precipitation than cold-rain processes

Rain rates: 200 mm / hr

When LEC storms form at the intersection of a strong low-level flow and a boundary, enhanced LL precipitation growth leads to intense rainfall

Efficient Ways of Forecasting Storms

Z-R relationship:

• LEC storms have warm rain characteristics and are often formed in maritime tropical air masses and have many relatively small rain drops

• Z=250R1.2

Efficient Ways of Forecasting Storms

Fig 1a. shows the regular Z-R relationship

This relationship suggested that 2-3 inches of rain would have fell within those 3 hours

Efficient Ways of Forecasting Storms

Fig 1b. Shows the tropical Z-R relationship

This relationship suggested that 4-6 inches of rain was expected during those 3 hours

We can see that this relationship was more accurate for this type of storm.

Efficient Ways of Forecasting Storms

Moisture Flux Convergence (MFC)• Used to forecast local thunderstorm formation

• Q is specific humidity• V is the total wind• Term 1=convergence term• Term 2= advection term

Flash Flooding

A flash flood is a storm-scale event that occurs when the precipitation rate is too great for the basin to accommodate

Flash flooding is tied to the characteristics of the basin in which the rainfall occurs

Severe flash floods occur in basins with < 30 square miles of the drainage area

LEC storms are slow-moving and have a larger wet footprint than severe thunderstorms making LEC storms prone to flash flooding

Actual Storm Characteristics

LEC formed 12 miles west of Emporia Kansas at 2242 UTC

Peak Intensity was reached at 0035 UTC with maximum reflectivity values around 55 DBZ

Maximum reflectivity was extended to a height of only 6 km

Storm moved NE at 4.6 m/s (EXTREMELY slow)

Stratiform precipitation surrounded storm during its entire lifetime

Dissipated at 0130 UTC only 42 km from original storm site

GOES-IR imagery showed warm cloud-tops between -30°C and -40°C during peak intensity

Actual Storm Characteristics

• Three major factors: Forcing Analysis, Moisture Analysis, and Stability/Wind Shear Analysis

Actual Storm Characteristics

Forcing Analysis: large scale lift

• Positive vorticity advection: max value of 19 x 10-5 /s

-Occurred over eastern Kansas, implyong UVM was favored down stream from the 500 hpa vorticity maxima

• Low level WAA: area of frontogenesis maximized near the storm initiation site due to a frontal zone near 850 hpa

• Strong convergence: max value of -3.4 x 10 -5 /s

-Positive convergence maximum developed and persisted over the area of LEC storm development

Actual Storm Characteristics

Moisture Analysis:

• MFC values: Deep layer (950-850 hpa) 1.6 g/kgh

Surface layer 1.5 g/kgh

-Due to the convergence associated with strong easterly 850 hpa winds

• MFC values spiked at the time of maximum storm intensity indicating convergence feedback

• MFC values indicate that the area over which the LEC formed was constantly being supplied with moisture

Actual Storm Characteristics

Stability and Wind Shear Analysis:

• Upwind (South from the Storm):

CAPE= 163 J/kg

EL=350 hpa

Tropopase= 150 hpa

NCAPE= 0.021 m/s2 indicating a profile that is tall and skinny

*Cape divided by the depth of the layer where cape is present. Distinguishes between tall and skinny, and short and fat cape profiles.

Wind speeds= veering (WAA) 15 to 25 kts with weak speed shear with height

Actual Storm Characteristics

Location of the storm:

CAPE = Decreased to 30 J/kg

NCAPE = 0.007 m/s2

EL = 480 hpa

Tropopause = 150 hpa

Wind speed = 14 kts

-A low EL height provided an upper level lid on convection during the time of the storm. Weak wind speeds and flow in this layer was the cause of slow storm movement

Hydrological Issues Interstate Highway 35, the Kansas Turnpike, lies on

top of a raised roadbed as it cuts across the Jacob Creek drainage basin

Hydrological Issues 24 hours before the LEC hit, this area was hit with 1.75 inches of

rain, increasing surface runoff

At 0130 UTC the volume of water in the creek was too great under the highway, so water ponded upstream of the highway

Hydrological Issues

At 0200 UTC water backed up onto the highway and was held back by concrete barriers

Hydrological Issues Shortly after, water reached the top of the barriers and the

highway became impassable

6 people died by not abandoning their cars as the water surge carried 7 vehicles downstream

Hydrological Issues

Overall, there is a great sensitivity of drainage basin to repeated bursts of intense rainfall.

The combination of saturated soil, heavy rain, and unnatural structures in the floodplain can be disastrous when an LEC hits.

Compare 2 LEC Storms

Fort Collins, Colorado:

-Flash Flood July 28,1997

-LEC Storm produced over 10 in of rain in less than 6 hrs

-5 casualties

LEC Characteristics:

-IR imagery showed warm cloud tops

-No hail

-Weak mid-tropospheric winds

-Warm rain process---Mechanism for precipitation growth

Compare 2 LEC Storms

Comparing Ft. Collins event with Kansas Turnpike event:

We can see that they similar characteristics

Fort Collins Kansas Turnpike

CAPE 868 J/kg 316 J/kg

Freezing Level 3.6 km AGL 4.2 km AGL

LI -2.8 deg. C -2.0 deg. C

LCL 764 hpa (700 m AGL) 911 hpa (365 m AGL)

PW 3.4 cm (179% of normal)

4.8 cm (120% of normal)

Compare 2 LEC Storms

There are many differences:

-Fort Collins: Region was under the influence of weak S to SW flow aloft in association with a negative tilted 500 hpa ridge

-Kansas Turnpike: Associated with a cut off low to the west of the area

Overall, both cases the flood event was not associated with a surface frontal boundary

Compare 2 LEC Storms

Major hydrological response differences:

• Fort Collins:

-Topography, there was heavy precipitation run off flowing down from the mountains the day before LEC hit

-Water flowed into Spring Creek and overflowed several retention areas upstream

-The storm followed the route of the main drainage area of Spring Creek

• Kansas Turnpike:

-The culvert under the highway was unable to accommodate the amount of water flowing through the basin

-This resulted in the powerful flow of water over the interstate

Factors that promote LEC development:

Overall:

• Highest reflectivity's are in warm portion of the cloud

• Very high influx of moisture into the storm allowing heavy rainfall characteristics to develop

• Weak instability, weak cloud layer flow, a deeply saturated sounding

• The natural flow of water is altered

Summary

These LEC storms can become difficult to recognize in a real time event.

These factors can be useful to operational meteorologists in better anticipating LEC storms because of their subtly heavy rain signatures

Not all LEC storms are the same. Many meteorological and hydrological characteristics can be different but overall will have the same results; devastating flash floods, property damage, and deaths.