Historic rainfall amounts fell Wednesday evening in the Fort Lauderdale area and flooding could persist Thursday as more storms are expected.
Relentless heavy rain Wednesday evening resulted in widespread flooding in portions of eastern Broward County, with a number of sites reporting more than 10 inches of rain. Maximum rainfall totals of over 2 feet were observed, resulting in the continued closure of Fort Lauderdale-Hollywood International Airport through at least early Friday morning. Most of the 25.91 inches of rain that fell at Fort Lauderdale-Hollywood International Airport fell in a span of six hours, catching many travelers off guard.
The historic rainfall was made possible by several atmospheric ingredients coming together simultaneously. In the lower levels of the atmosphere, relative humidity values a few thousand feet off the ground were near 90 percent. This degree of saturation in the low-levels is a classic signal that forecasters use to determine likelihood of heavy rain and flash flood potential. Another measure to determine saturation levels of the atmosphere is precipitable water, or PWAT. Precipitable water refers to the amount of water vapor in a column of air if it were to be condensed. This value, expressed in inches, was between 1.8 and 1.9 Wednesday evening across South Florida and signaled an incredibly high moisture content. The depth of cloud cover without ice, often called the warm cloud layer, was incredibly deep across South Florida Wednesday. Research has shown a significant correlation between warm cloud depth and the potential for flash flooding, with many environmental case studies appearing similar to the atmospheric setup over South Florida Wednesday. Storm motion was another significant factor in Wednesday's flash flood event across Broward and Miami-Dade Counties, as storms were nearly stationary at times. Combine these factors with heavy urbanization in eastern Broward and Miami-Dade Counties and the flash flood potential is compounded significantly.
Thursday's forecast continues the risk of flash flooding across the entire state, with a higher probability of flash flooding across hard-hit South Florida. High-resolution models indicate storm development Thursday afternoon will occur along and east of I-75 during the early afternoon. As the afternoon progresses, storms are expected to push east along the I-95 corridor. As storms push along the I-95 corridor, the potential for flooding will peak, especially in South Florida where moisture-loaded storms will be slow-moving. While forecast guidance Thursday afternoon does suggest storm coverage will be more isolated in nature compared to Wednesday, an additional 2 to 4 inches of rain cannot be ruled out in Broward and Miami-Dade Counties. Outside of South Florida, rainfall amounts will generally be under 1 inch, however locally higher amounts will be possible where heavier storms develop or train over the same area. While most of the state can withstand an inch or two of rainfall, heavy rainfall rates could overwhelm drought-stricken soil across North and Central Florida through Thursday evening.
The environment will not only be prime for flooding across the state, but it will also be supportive of strong and severe storms. A Tornado Watch was issued early Thursday morning for portions of the Panhandle, where several tornado warnings were issued by the National Weather Service Office in Tallahassee. While the Tornado Watch is no longer a concern for Florida, a Severe Thunderstorm Watch is in effect until 10 PM Thursday from Jacksonville to Orlando.
In the watch area, damaging winds and large hail are most likely, though an isolated tornado cannot be ruled out. Outside of the watch area, the risk of strong and severe storms still exists through Thursday evening. Damaging winds and a quick spin-up tornado are main concerns as storms push east through the evening hours.
Models suggest the majority of convection will push offshore by midnight Friday. A few showers cannot be ruled out in coastal areas Friday and Saturday, though the coverage and intensity should be significantly lower.
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