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The surprising, tropical location of the worst drought east of the Rockies

This week’s updated drought map, with severity color coded and whether the drought is short-term (S) and/or long-term (L). (National Drought Mitigation Center)
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The historic, widespread drought in California and other parts of the western United States is justifiably getting a lot of attention. But the worst drought east of the Rocky Mountains is digging in its heels in a place that might surprise you: southeast Florida.

This week, the worsening drought in southeast Florida was upgraded from “severe” to “extreme” by the National Drought Mitigation Center. The area in the “extreme” category just so happens to be heavily populated, including Miami, Fort Lauderdale, and scores of smaller cities. It’s currently affecting roughly 6 million people in this region.

Along with not enough precipitation and too much evaporation, fresh water usage has been a big factor in southeast Florida drought. Agricultural irrigation, commercial and residential needs place demands on the region’s fresh water supply, both in lakes and in the subterranean aquifers.

In south Florida’s tropical savanna climate, there are two seasons — dry and rainy. During the dry season (there is not an exact definition, but I’ll use November-April), about 25 percent of the annual rainfall occurs, so the rainy season (May-October) contributes the remaining 75 percent, with a normal annual total around 62 inches in Miami.

But since May 1, this year has been the driest on record for Miami since 1945, having only recorded seven inches of rain — more than 10 inches below average. For year-to-date totals, Miami is at 16.1 inches compared to the average of 27.3 inches.

Unfortunately the outlook for the next three months does not offer much reason for optimism, nor does the likely onset of a strong El Niño, as there is typically not much of a precipitation signal in this region.

The prolonged precipitation deficit is leaving its mark on the region’s significant source of fresh water, Lake Okeechobee. The large shallow lake has an obvious seasonal cycle, lagging the precipitation cycle by a few months, but by this time of year, it should be increasing and it is still decreasing. The lake level is managed through a network of canals, but without sufficient rainfall to replenish it, human demands and evaporation will bring it down.

As mentioned earlier, aquifers play a role in the region’s freshwater supply. However, the aquifers also contain saltwater closer to the ocean. The saltwater is more dense, and forms a wedge under the freshwater as it sinks to the bottom. That wedge advances and retreats naturally with the seasons, but when there is insufficient freshwater input, the saltwater wedge advances further inland than normal, and can result in “saltwater intrusion”. Saltwater intrusion has and does affect municipal and residential wells, rendering them useless, so hopefully the current drought does not extend long enough to make this an additional problem.

Looking into why it has been so persistently dry along the southeast coast of Florida, most atmospheric fields look close to normal from May 1 through today, with the exception of wind. Through the depth of the troposphere, there have been abnormally strong easterly winds over south Florida (an example from 500 millibars, around 20,000 feet is shown below). This is important because during the rainy season, the daily seabreeze is a big player in generating thunderstorms. With stronger easterly winds, those thunderstorms rarely drift eastward. Also, the abnormally strong trade winds can overwhelm the subtle sea breeze cycle, resulting in fewer thunderstorms being generated in the first place.

To pull the area out the drought, NOAA’s Climate Prediction Center estimates that 20-22 inches of rain is needed — the highest values anywhere in the nation. The map below shows the additional precipitation needed to bring the Palmer Drought Index up to near normal levels (-0.5). This is has not been updated since July 4, but the color scale saturates at 15+ inches, while the actual values in the two south Florida zones are 20.8 inches and 21.8 inches. The update next week will show even more rain is needed to bust the Florida drought.

Of course, one or two tropical cyclones or disturbances lingering over the area could make up a huge portion of this deficit during the summer, but they are not a sure thing to rely on. And they can also deliver rainfall at incredible rates over a day or two, resulting in flash flooding and excessive runoff. Ask Texas and Oklahoma about this! Routine afternoon thunderstorms can dump a lot of rain in a short amount of time (if you’ve never experienced a bona fide tropical downpour, you are missing out!), but they are normal during the summer, so in general, one would expect them to at least keep the deficit constant, not necessarily improve it.

All rainfall statistics are from KMIA, and all averages are for the 1981-2010 climate normal.