Heavy rain fell in north central Kansas over Labor Day weekend. High rainfall amounts over a very short time period caused flash flooding, particular in Manhattan, on Monday, September 3. Many businesses, homes, and roads were inundated with high water from this event.
Meteorological factors influencing the heavy rain
An outflow boundary from thunderstorms in Nebraska pushed southward Sunday evening. This boundary forces air upward, making it condense into showers and thunderstorms along and north of it. As it moved southeast, it encountered southeast flow with very moist, warm air. This air originated from the Gulf, enhanced by a tropical disturbance in southern Texas, and was two to three standard deviations higher than normal, a near record for Topeka.
Figure 1. Surface pattern at 1:00 a.m. Monday, September 3, 2018. Background graphic via NOAA.
Air was pushed northward due to a high pressure east of the Central Plains. This air mass was enhanced by surface convergence due to a jet stream spreading out/diverging over the Central Plains. When this happens, thunderstorm activity is enhanced underneath the jet stream. Also, the stream was oriented from southwest to northeast. This provided shear to help thunderstorms sustain themselves, as well as venting the debris clouds (often called anvil clouds) to the north and east. This allowed thunderstorms to continue to develop to the south and east without any containment from previous rainfall, outflows, etc.
Figure 2. Upper level enhancements leading to the heavy rain event on September 3, 2018. Background graphic via NOAA.
In summary, it takes four ingredients to develop thunderstorms:
- Moisture - near record amounts from the Gulf
- Instability - provided by the moist air mass and weak cold air advection in mid-levels
- Lift - an outflow boundary and upper level divergence/surface convergence
- Shear - changing of wind direction with height not cutting off surface Gulf air inflow.
This indeed was a perfect storm of ingredients with all four ingredients present. What made this event stand out was the incredible amount of moisture present and the duration of the event stationary over one location.
Rainfall totals and rates
North central Kansas, and Manhattan in particular, saw the heaviest rains over the Labor Day weekend. The largest amounts occurred overnight from September 2-3. Rainfall amounts, as reported by 9:00 a.m. on the September 3rd, can be seen in the table below (Figure 3).
Figure 3. Rainfall reports in Riley County, Kansas. Data compiled by the Weather Data Library.
Flooding is influenced by more than simply total amount of rainfall. The rainfall accumulation rate also contributes to the intensity of flooding. The highest 1 hour (Figure 4) and 6 hour (Figure 5) rainfall totals from the Kansas Mesonet stations are shown below.
Figure 4. Highest 1-hour rainfall totals (Kansas Mesonet).
Figure 5. Highest 6-hour rainfall totals (Kansas Mesonet).
How often do these heavy rainfall events occur?
The Hydrometeorological Design Studies Center, a branch of the National Oceanic and Atmospheric Administration (NOAA), has produced a document that answers that question. NOAA Atlas 14 addresses the frequency of rainfall events on time-scales ranging from 5 minutes to 60 days. Other parameters, such as the probable maximum precipitation, are also available.
The 24-hour total of 11.00 inches reported from the Manhattan 9.8NW station falls in the 1000-year return rates, while the 1-hour total of 2.17 inches has a 10-year return frequency.
Figure 6. Rainfall return rates for Manhattan, KS (NOAA).
The full precipitation atlas, with access to other locations, can be found online at http://www.nws.noaa.gov/oh/hdsc/index.html
Rainfall data from the Kansas Mesonet is available at: http://mesonet.k-state.edu
Additional precipitation maps for Kansas can be found on the Kansas Climate website at: http://climate.k-state.edu/
Mary Knapp, Weather Data Library/Mesonet
mknapp@ksu.edu
Chip Redmond, Weather Data Library/Mesonet
christopherredmond@ksu.edu
Tags: weather