Historic Palm Beach Flooding January 9-10 2014

Photo:  Kenneth Kraus

During the night of Thursday January 9, 2014, several mesoscale meteorological factors combined to produce torrential rainfall across portions of coastal Palm Beach county over a rather short period of time.  From roughly 8pm until midnight, several locations received over 12 inches of rain in just those few hours Thursday evening with one mesonet site just west-southwest of Hypoluxo receiving an astonishing 22.21 inches during the same time frame (Table 1).  Steady rain rates were on the order of 3-5 inches per hour.        

      Table 1:  Storm Total Rainfall Palm Beach County                      Figure 1:  24 hour rainfall map ending 7am January 10th

These staggering rain rates led to severe flooding overnight and through Friday January 10th well after the rain had stopped.  This prompted the South Florida Water Management District to hold an impromptu weather briefing Friday afternoon to discuss the safest and most efficient ways of moving water out of low lying areas and into the regional flood control system.  The most severely affected communities were Delray Beach, Boynton beach, and Lantana.  Severe flooding of neighborhoods led to the closing of numerous roadways, including all lanes of Interstate 95 in Boynton Beach.  Several lakes and canals overflowed their banks.  Flood waters entered a number of homes and businesses, but no major structural flooding was noted.  All Palm Beach county schools were also closed due to the flooding.  Tragically, two people perished as a result of this historic event.  A 56 year old woman drowned in Delray Beach after she drove her car off of a flooded roadway and into a lake.  Also in Delray Beach, a 90 year old man drowned after slipping into a canal while walking down a street.  Rainfall totals of 8 to 10 inches were also experienced as far west as the Florida Turnpike in Palm Beach county (Figure 1).

                Figure 2:  12z MFL Sounding January 9th                                     Figure 3:  Historic January Precipitable Water Amounts

              Image:  Plymouth State University/Mike Mogil                                                     Image: NOAA/NWS/Mike Mogil

Several mesoscale meteorological factors came together to produce this rare and historic flood event.  A stalled frontal boundary across south Florida edged to the north through the evening of January 9th.  As it did, it placed south Florida in a moisture rich environment as was indicated by the 12z MFL sounding indicating a precipitable water amount of 1.82 inches (Figure 2).  This amount was near the January record of 1.98 inches spanning the last 65 years, placing it in the 99th percentile of roughly 4000 observations taken in January since 1948 (Figure 3).  At the same time, a mesoscale low was slowly tracking to the south across the St. Lucie and Martin county coasts producing heavy rainfall in these areas.  Fort Pierce recorded over 7 inches of rainfall on January 9th in nearly a 6-hour time span.  As this low moved to the south and into Palm Beach county, storm motion slowed and eventually became nearly stationary across extreme southeastern Palm Beach county.  Heavy rainfall then became enhanced as it encountered convergent flow along the coastline (Figure 4).  The main area of convergence was driven by the low level wind flow.  An area of speed convergence was also seen as strong easterly winds offshore were blowing into an onshore region of much weaker winds.  Favorable vertical wind shear was also present as low level onshore flow opposed mid level southwest flow.  This also allowed for the training of storm cells over the same areas.  According to the Office of Hydrologic Development plotted observations from the Boynton Beach rain gauge show that for durations between roughly 1.5 and 18 hours, the rainfall was more rare than a 1000 year Average Recurrence Interval when plotted against depth-duration frequency curves from NOAA Atlas 14 (Figure 5).

    Figure 4:  Convergence along Palm Beach coast                         Figure 5:  Average Recurrence Interval for Boynton Beach

                 Image:  NOAA/NWS Mike Mogil                                      Image:  Office of Hydrologic Development (OHD)/NOAA Atlas 14

NWS Miami issued the following hydrologic products during this flood event:

An Urban Flood Advisory was issued at 8:25 PM for NE Palm Beach County as reports of flooded roads were being received. This was upgraded to a Flash Flood Warning at 9:29 PM as more and more reports of more serious flooding were being received across NE Palm Beach County from the media. Three Flash Flood Statements were issued, serving to update the public on the ongoing radar trends along with the reports being received.

As the heavy rain spread further down the coast, an Urban Flood Advisory was issued for SE Palm Beach County at 11:08 PM.  This was upgraded to a Flash Flood Warning at 12:46 AM as radar and observations showed excessive rainfall totals along with reports of flash flooding being received. One Flash Flood Statement was issued to update the information on the flooding.

Since flooding remained ongoing during the morning of Jan. 10th but the rain had ended, Flood Warnings were necesistated for NE Palm Beach County (issued at 3:29 AM) and SE Palm Beach County (issued at 6:27 AM). The Flood Warning for NE Palm Beach County was allowed to expire during the morning as the flood waters subsided there. However, since SE Palm Beach County had the brunt of the excessive rainfall, flooding continued not only through the day but even during the overnight hours of Jan. 11th, so the Flood Warning remained in effect through mid-morning of Jan. 11th.