...More Heavy Rain Across the Lower Missouri River Valley...
Yet another powerful upper level storm system pushed towards into the heart of the country this week. This was a rather anomalous storm system for October as it extended all the way down into the Great Basin region, bringing heavy snow to parts of Utah and Colorado (Figure 1). This storm system pushed a large volume of moisture northward into the lower Missouri River Valley out of the Gulf of Mexico on Wednesday, October 28. The first area of thunderstorms to impact the area developed before daybreak as an abnormally strong lower level wind fields combined with this increasing moisture. The end result was widespread "elevated" thunderstorm activity which developed in southeast Kansas and lifted northward into Iowa and Illinois during Thursday morning. Additional elevated convection also developed during the afternoon hours of Thursday. Further to the west, an additional line of showers and embedded thunderstorms slowly crawled across central and eastern Kansas during the day on Thursday, accelerating across Missouri late Thursday afternoon and evening as an extensive frontal system pushed through the region (figure 2).
|Figure 1: Upper Level Storm System||Figure 2: Surface and Radar Depiction|
Rain quickly came to an end in the area by midnight as drier air aloft punched into the region. However, most every location in the Kansas City/Pleasant Hill forecast area received upwards of an inch of rainfall. The heaviest 2-day rainfall totals occurred in areas from Richmond, to Chillicothe, Unionville, Kirksville, Macon, Fayette, and Boonville, where upwards of 2 to as much as 3 inches of rain were reported early Friday October 30th. In figure 3 below is a graphical map depicting some of the rainfall totals during the heaviest periods of rain on the 29th.
|Figure 3: Rainfall Totals From October 28-29, 2009|
So what is an elevated thunderstorm? A common occurrance in the heart of the United States at nighttime, an elevated thunderstorms is simply a thunderstorm which is based at and lifted at a level above the surface of the earth. These types of thunderstorms can be forecast from looking at vertical plots/diagrams of the atmosphere otherwise known as "skew-t" diagrams. Skew-T diagrams depict the three most important variables in the atmosphere that Meteorologists examine when forecasting weather conditions, mainly temperature, dew points, as well as wind speed and direction. In figure 4 below is the 12z (7am) atmospheric sounding from Topeka, KS on the morning of the 29th of October during the initial stages of thunderstorm development in the local area.
|Figure 4: Atmospheric Sounding From 7am October 29 at Topeka. KS||Figure 5: 850 MB Lower Level Atmospheric Plot|
At first glance on this vertical sounding, a key indication for the potential of elevated thunderstorms appears. Note the "v" looking signature on the sounding indicated at around 850 mb. This is an indication of a warm "nose of air aloft" which is present in the atmosphere at that level. Also note the speeds of the winds at that same level. At 12z on the 29th, the winds in the lowest level of the atmosphere (figure 5) were rather strong in nature, with indications on this sounding and the map view in figure 5 of wind speeds around 50 knots at the layer of the atmosphere where this warm nose was present. In fact, by 5 am in the morning of the 29th, these strong winds began to focus on and "converge" a large quantity of moisture over southeastern Kansas and western Missouri, lifting warm saturated air northward at the 850 mb level and upwards in the atmosphere (for more information see "warm air advection"). You can visualize this by looking at the clustering of green contours (moisture values), temperature values (red contours), and winds in figure 5 above. The strongest winds in Oklahoma and Texas are pointed at the Kansas City area, with lighter winds to the north of this area.
In addition the vertical sounding also indicates that air temperatures higher up in the atmosphere were colder than that of the air at 850 mb. This allowed the air which was being lifted upwards at the 850 mb level (or 3500 ft AGL) to accelerate upwards along the dashed brown line (indicating path of upward travel of a saturated air parcel) without any resistance from further up in the atmosphere. Once this air starts its upwards movement it can utilize the "convective available potential energy" or CAPE which allows it to remain buoyant and continue its upwards movement. The amount of CAPE can also be a determinate of storm intensity, hail potential, as well as numerous other thunderstorm components. Showers and thunderstorms raced very quickly northward through the area once they formed, due in part to the strong driving winds some 40-50 knots in the area where thunderstorms developed. These elevated thunderstorms, despite their rapid movement, used all the moisture that was available to them to produce areas of at least 1 inch of rainfall in a short amount of time and setting the stage for more widespread flooding later in the day.