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Heavy rainfall gradients and rainfall rates (H. Michael Mogil, CCM, CBM, NWA-DS*)

Across Louisiana, Mississippi and parts of far East Texas, torrential rainfall occurred Sunday and Sunday night. Widespread one to four inch rainfall amounts were reported, with some locations noting amounts of six, eight and even ten inches (Fig. 1). Alexandria, LA, in the west-central part of the state, reported more than nine inches over a nearly 12-hour period, with nearly continuous thunderstorm activity for a roughly 8-hour period (Fig. 2).

By early Tuesday, the large-scale storm system responsible for the storminess has moved well to the northeast and the heaviest rainfall had moved well to the east of The Pelican State. In the wake of the Sunday rainfall activity, flood warnings remain in effect for many areas in Louisiana that received excessive rainfall or are downstream from these areas (Fig. 3).

Two things are striking in Fig. 2. First, there are numerous bands of heavy rainfall aligned southwest-to-northeast. These mark the locations in which thunderstorm lines became at least somewhat stationary. Then thunderstorms, moving along the line, dumped successive bursts of heavy rainfall. This scenario, in which numerous storms move across the same area, is known as “training.”

The second thing of note is the large precipitation gradients in areas near the heaviest rainfall. One only needed to have traveled a relatively short distance to go from eight to ten inches of rainfall to amounts of around an inch.

A similar, albeit greater, rainfall gradient occurred in my backyard, Collier County, in southwest Florida, on Sunday. A small convective weather system (without thunder and lightning), developed along the sea breeze front and moved slowly northwestward across parts of north-central Collier County. Heavy rainfall (between 1.47 and 1.78 inches according to two nearly co-located reporting stations) fell just two miles to the northeast of my rain gauge. According to a high school student interested in meteorology (and one

who attended a Naples weather camp last summer), most of the rainfall that he measured fell in just under 20 minutes (Fig. 4). That translates to an hourly rainfall rate of 4.5 inches to 5.5 inches. I measured a scant 0.07 inches of rainfall, although I did briefly experience gusty outflow winds from the convective weather system. Fig. 5 shows radar-based rainfall estimates for southwest Florida on Sunday.

While these situations were somewhat similar in their excessive rainfall and localized rainfall gradients, one was linked to a large-scale low-pressure system and the other to a small-scale sea breeze zone. This is testimony that different meteorological scenarios can yield similar meteorological outcomes.

© 2017 H. Michael Mogil

Originally posted 4/4/17

* The National Weather Association Digital Seal (NWA-DS) is awarded to individuals who pass stringent meteorological testing and evaluation of written weather content. H. Michael Mogil was awarded the second such seal and is a strong advocate for its use by weather bloggers.