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Heavy rainfall deluged coastal Palm Beach County on Jan. 9, 2014

NWS radiosonde sounding for Miami, FL early on Jan. 9, 2014.  Note high precipitable water value and vertical wind shear pattern.
NWS radiosonde sounding for Miami, FL early on Jan. 9, 2014. Note high precipitable water value and vertical wind shear pattern.
Plymouth State University

Following two brief, but significant, cool spells across south Florida (Jan. 3 – 4 and Jan. 7 – 8, 2014), temperature and dew point (a measure of absolute atmospheric moisture) readings soared across south Florida on Jan. 9. However, high dew points near the ground (where we humans experience humid discomfort) were only part of the story. Humidity values throughout the entire atmospheric column (from the ground to altitudes about 6 miles above the ground) rose to near record levels. At Miami, FL, precipitable water values (a measure total column moisture) reached 1.82 inches early on Jan. 9, 2014 (Fig. 1). The record for January, spanning the period 1948 to 2013, was around 1.98 inches (Fig. 2). The observed value on Jan. 9, 2014 exceeded 99 percent of all observations (roughly 4,000) taken in January during the most recent 65-year period.

In short, the atmosphere was primed for a significant rainfall event. It did not disappoint.

Starting around midday, heavy showers developed near Vero Beach on Florida’s south-central East Coast. The showers redeveloped southward, hugging the coast. By early afternoon, heavy rain started to fall in the Fort Pierce area. During the next six hours, more than six inches of rain fell at the Fort Pierce Airport.

The rainfall area continued to move and redevelop southward, eventually locking itself in place along the coast of Palm Beach County (Fig. 3). From about sunset though the overnight hours, torrential rains fell over coastal Palm Beach County, FL. During a roughly 12-hour period, widespread ten to fifteen inch rains fell in the eastern parts of the county (Fig. 4 and Fig. 5). Locally, amounts topped 22 inches. Most of the rain fell within a six- to ten-hour period.

The heavy rains did not make it southward to Broward County (including Fort Lauderdale).

There were reports of significant flooding (Fig. 6); some highways (including Interstate 95) were closed due to deep water.

The event can be likened to numerous other excessive rainfall events that have occurred in recent years along and near south Florida’s East Coast. Many of these involved a combination of most of the following: local convergence zones (where winds at low levels blow together), interactions with a sea breeze front, favorable vertical wind shear (easterly winds near the ground and westerlies aloft) and an unseasonably moist atmosphere.

In this case, the main convergence zone seemed to have been driven by the larger scale wind flow. Offshore, strong easterly winds (30 to 35 miles per hour) were blowing into an onshore region where easterly winds were much weaker (10 to 15 miles per hour). Coupled with this larger scale convergence zone (Fig. 7), the rain area moving southward from Fort Pierce brought its own low level convergence zone. Torrential rains fell where the two zones merged and locked into place.

© 2013 H. Michael Mogil

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