Concerning reports that 25 inches of rain fell in one day on Fort Lauderdale, some historical context is provided by the Florida Climate Center article Anticipating Heavy Rain in Florida. Excerpts in italics with my bolds.
Florida is situated within a part of the United States where torrential rain is a common occurrence. Torrential rain is here defined as at least 3 inches in a single day. Rain of at least this magnitude is far more frequent along Florida’s coasts than in its interior. The Panhandle and the Gold Coast experience such weather events more than elsewhere in the state. Except for North Florida, rainstorms are heavily concentrated in the warm months. Mid- latitude low pressure systems, that pass over or near North Florida in the winter, often produce heavy rain. As a consequence that part of the state has no seasonal concentration.
The distribution of torrential rain throughout the state is much more uneven during years when they are most frequent than when few such storms occur. During the 51 years of daily observations for 48 weather stations no cyclical pattern of torrential rain was discerned. However, when data were organized by Enso phase it was shown that during the La Niña phase torrential rainfall, especially of 5 inches or more in a day, was more frequent than during the phase named El Niño. This was most true of South Florida stations.
Florida lies within a broad region along the Gulf and Atlantic coastal plains of the southeastern U.S. that experiences frequent episodes of torrential rain (Map 1). Torrential rain in Florida is here defined as three inches or more in one calendar day. Rain of this magnitude contributes approximately ten percent of the total precipitation that falls on the state, more in some parts of it, less in others. Along the coasts of both northwestern and southeastern Florida torrential rain makes the heaviest contribution, while in the interior of the peninsula it contributes the least. For several decades a Florida weather station held the nations record for the most rain to fall in a 24-hour period. Yankeetown, a small fishing port on the northwestern side of the peninsula, during September 5th, 1950 was swamped by 38.7 inches of rain. The village retained the national record until July 25th-26th, 1979 when 43 inches fell on Alvin, Texas, situated between Houston and Galveston. This record still stands.
Torrential rain, since it is usually accompanied by intense atmospheric turbulence, has the potential of causing much property damage, as well as the destruction of agricultural crops and livestock. Florida is especially vulnerable to flooding because it is both low and flat. Although the sandy soils of the Peninsula are capable of absorbing water rapidly, their ability to absorb large amounts is limited because the water table is normally very close to the surface. Most of the state’s densely populated areas are situated on the shore of either the Atlantic Ocean or the Gulf of Mexico, and are especially vulnerable to rainstorms. Not only are such storms more frequent than in the interior of the state, but a large share of the urban area is covered by pavement and roofs, which concentrate runoff into low areas. Most of Florida’s cities today have adequate storm drainage systems to meet the demands of a sudden intense downpour, but flooding, sometimes on a major scale, does occur.
The reader should be aware that the amount of daily precipitation, especially from the cooperative weather stations, which constitute the majority, is not necessarily that which fell between one midnight and another. Cooperative weather stations usually depend heavily on non-professional volunteers who read the gauges when it is convenient, hopefully each day at a time agreed upon. Few are read at or near midnight. Only a small number of Florida stations record hourly precipitation. Consequently, a rainstorm may begin during one calendar day and end in another. The total rainfall of the storm would then be shared by two days, and although it may be higher than three inches, if no calendar day had a total of three inches it would not be counted.
It should be noted that the monthly frequencies of torrential rain, if graphed, do not conform to a bell shaped curve, increasing to a peak in the hottest month of the year. Instead, the curve is bi-modal, there being two peaks, one in June and the other in September. It is presumed that the June peak is the result of the state then coming strongly under the influence of the intertropical convergence zone, and the September peak is due to the greater frequency in that month of tropical low pressure systems such as tropical storms reaching the state.
The ENSO (El Niño Southern Oscillation) phenomenon, which has been given much justified attention in recent years, is now generally regarded as being able to influence climate over a huge area of the world. It would be irresponsible to ignore the possibility that it could influence the frequency of Florida rainstorms. To ascertain if there is a relationship the three phases of Enso (El Niño, La Niña, and the neutral phase) the frequency of rainstorms were calculated by ENSO phase (Table 4). There does appear to be a relationship, and it doesn’t seem to be spurious. The share of the 48 Florida weather stations that reported no torrential rainstorms during a year is somewhat higher during the El Niño phase than the other two. The share of stations that reported only one storm during the years of the El Niño phase also was higher than the share of those that were reported in the La Niña phase. Thereafter, except for the shares of the ‘four storm’ category, the La Niña phase produced more torrential storms than the El Niño phase. In South Florida, It has become generally accepted that in La Niña years precipitation is generally wetter than during the El Niño phase. From the data we might conclude that weather controls that become important during this phase also promote a higher frequency of torrential rain. The neutral phase of Enso has little to no effect upon the frequency of Florida’s rainstorms, some neutral years producing many more episodes of torrential rain than others.
In conclusion it would be derelict not to address the issue of the relationship between the frequency of torrential rain in Florida and global warming. In studies of the consequence of global warming on climate the possibility of greater climatic extremes has been predicted, including storms that could produce large amounts of precipitation. This is usually based on the assumption among other factors, that the temperature of the water of the oceans would rise, heating the air above them, increasing evaporation and the air’s ability to hold water vapor and consequently its ability to produce more powerful rainstorms.
Table 5: Number of times 48 Florida weather stations
experienced at least 3″ of rainfall for
five decades between 1950 and 1999.
To ascertain if Florida has been experiencing an increase in the number of storms that produce torrential rain the torrential storm data for the 48 stations within the state which became the primary data source for this study were divided into the five decades between 1950 and 1999 (Table 5). When the average for the 50-year period is compared to the frequency by decade no trend is discovered. When the frequency in earlier decades is compared with the later ones there also appears to be no trend. For example, between 1950 and 1969, during that 20-year period there were 684 episodes at the 48 Florida weather stations in which 3 inches or more rain fell in one day, while in the 20-year period between 1980 and 1999 there were 695 episodes, a difference of only eleven episodes. The frequency of episodes during the year was also examined. An examination reveals, for example, that there were 23 stations between 1950 and 1969 that reported five episodes during the 20-year period, and between 1980 and 1999 the number fell to 20. Such a small drop does not suggest an increase in torrential rain over time.
via Science Matters
April 15, 2023 at 11:36AM