From Drought To Flood (H. Michael Mogil, CCM, CBM, NWA-DS*)
About 10 days ago, a major brush fire (linked to a growing drought) erupted to the east of Naples, along I75. The fire eventually grew to more than 8,000 acres. Today, the same area is under a Flood Watch (Fig. 1) meaning that some two to four inches rainfall could occur during the next 36 hours. Local amounts, according to the National Weather Service in Miami, could reach six inches, and localized flooding could occur.
Due to the topography of south Florida, the greatest flooding risk is ponding of water in both urban and rural settings. With the ground already saturated from nearly a week of heavy rainfall (parts of northwest Collier County have already received two to four inches of rain), and with standing water in some swales, roadway flooding is a real risk.
Already, early this Sunday morning, a large area of moderate to occasionally heavy rainfall (with embedded thunderstorms) is spreading northwestward across south Florida. Shortly after 7:30am E.D.T. this Sunday, the leading edge of the heaviest rain area was along an Everglades City-Fort Lauderdale line (Fig. 2).
The culprit is a slow-moving upper-level weather pattern. The counter-clockwise circulation around an upper-level low-pressure system over the central Gulf states is allowing copious amounts of moisture to stream northeastward across the so-called” Sunshine State. This mid-level (roughly 20,000 feet above ground level) water vapor satellite image (Fig. 3) clearly shows the moisture source and the associated wind pattern aloft.
Further testimony to the risk for heavy rainfall is the “precipitable water” values across south Florida. Precipitable water is the value obtained when the total amount of moisture is presumed to be squeezed out of the atmosphere above a point. It is similar to squeezing the water out of a sponge.
Compared to long-term (1948-2019) data, precipitable water values measured last evening over south Florida (2.08 inches at Miami) were just shy of the moving maximum average of 2.16 inches for the date. The maximum observed value for the 71-year period on May 23 at 8:00 pm E.D.T. was 2.36 inches.
Precipitable water only considers the moisture present at the time. It now appears that advection (horizontal movement) of additional moisture can be expected for at least the next 36 hours.
Looking ahead, forecasters have indicated that the persistent pattern could lead to heavy rainfall at times throughout the rest of the week.
© 2020 H. Michael Mogil
Originally posted 5/24/20
* 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.