Oct20-21_2014Flood

 Palm Beach Flood Event of October 21, 2014

Photo:  NBC

During the afternoon hours of October 21, 2014, portions of coastal Palm Beach county were inundated with flooding rains for the second time this year.  Although this event was not near to the magnitude of the flood event that overwhelmed the area back in January, it did produce copious amounts of rainfall over a short period of time.  Many roads were blocked due to the flooding which left many motorists stranded.  Portions of metro Palm Beach county received anywhere from 1-3 inches of rainfall while some coastal locales received nearly 10 inches (Figure 1).  The worst impacted areas were between downtown West Palm Beach and Riviera Beach where many roads became flooded and impassable. Extreme southeastern Miami-Dade county was also impacted by torrential rains a day earlier.  During the evening of October 20, rainfall amounts ranging from 3 to nearly 7 inches of rain inundated a small section of coastal Miami-Dade county at Biscayne Bay.  Mesonet sites at Palmetto Bay and Cutler Ridge recorded 6+ inches of rainfall in only a 5 to 6 hour time period during the afternoon and early evening hours (Figure 2).  Street flooding was reported in these areas and in South Miami Heights.

                                                        Figure 1:  Rainfall Totals Across Palm Beach County

                                               Figure 2:  Rainfall Totals Across Coastal Miami-Dade County

Antecedent Meteorological Factors

Two major synoptic features played critical roles in the severity of the flooding.  A cold front pushed southward down the Florida peninsula and stalled near Lake Okeechobee on the evening of October 21st.  At the same time, tropical disturbance 93L was located in the Bay of Campeche.  This disturbance funneled ample amounts of deep tropical moisture across the Gulf of Mexico and into the south Florida peninsula.  This allowed precipitable water values to surge to 2+ inches across much of south Florida (Figure 3).  In fact, the 12z MFL sounding for October 21st registered a precipitable water value of 2.13 inches, which is very close to +2 standard deviations above the normal value for that date (Figure 4).  Moderate to heavy showers associated with this deep moisture plume began to train along the southern periphery of the stationary front in place.  These factors combined with weak coastal convergence facilitated the large rainfall amounts and flooding along the Palm Beach county coast.

                                                      Figure 3:  Precipitable Water Amounts

                                                     Figure 4:  South Florida Precipitable Water Climatology

 Hydrologic Products Issuance

The potential for flooding was first mentioned in the Days 2 through 7 portion of the Hazardous Weather Outlook at 5:10 PM on October 20 and remained in subsequent HWO products through the duration of the event.

An Urban Flood Advisory was first issued at 5:46 PM for on October 20th for eastern Miami-Dade county as Doppler Radar precipitation estimates were near 2.5 inches with showers continuing to develop over Biscayne Bay.  A follow-up statement was issued at 6:07 PM as report of 5 inches of rain was received from a rain gage in Cutler Bay.  A second Urban Flood Advisory was issued at 6:22PM for the same area as additional reports of flooding rains were received.  This advisory was then canceled at 7:41 PM as the heavy rain ended and flood water began to recede. 

On October 21st, an Urban Flood Advisory was issued at 1:45 PM for east-central Palm Beach county.  Rainfall amounts of 2-3 inches had already fallen across the area over the last hour, and training of cells was occurring.  It was also mentioned in the body of the advisory that a Flash Flood Warning may be necessary.  A follow-up statement was issued at 2:33 PM as reports of flooded and impassable roads came in across portions of Riveria Beach and the Port of Palm Beach.  An additional Urban Flood Advisory was issued at 3:03 PM for northeastern Palm Beach county and was then upgraded to a Flash Flood Warning at 3:55 PM as reports of 6+ inches of rain were received.  A follow-up Flash Flood Statement was issued at 4:32 PM when the heaviest rainfall reports of around 9 inches were relayed to the office along with road closures.  A final Flash Flood Statement was issued at 5:24 PM to relay the latest public and media reports.  The Flash Flood Warning was then canceled at 5:50 PM as high water began to recede and no longer posed a major threat.