Heavy Rainfall and Supercell Storm May 2-4th
Heavy Rainfall Event May 2-4, 2013
On May 2-4, 2013, a classic “Nor’easter” developed over Florida and Georgia, with a strong area of high pressure north of the area and low pressure off the east coast of Florida. The combination of the two set up a strong inflow of Atlantic moisture, lifted by an upper level weather system approaching Florida from The Gulf of Mexico. The end result was a nearly stationary northwest-to-southeast oriented swath of heavy rainfall that extended over extreme southeast Georgia and much of northeast Florida. Figure 1 below shows radar estimated rainfall amounts for the event through late in the day on May 4th. Red shades represent 4-8 inches of rainfall, with a few locations likely receiving at least 10 inches, which resulted in areas of flooding and river flooding along Black Creek and the St. Marys River. Figure 2 below shows ground truth rainfall amounts.
Figure 1 (below): Doppler Radar Storm Total Precipitation (STP) rainfall amounts from May 2nd through May 4th.
Figure 2 (below): Ground truth 3-day rainfall totals.
The excessive rainfall produced a number of flooded roads, causing stranded vehicles and road closures.
In addition to heavy rainfall, Nor’easters bring strong northeast winds to the area. The surface map below depicts the strong high pressure well north of the area, and low pressure to the south. A strong pressure gradient existed across southern Georgia and far northern Florida. Winds of 20 to 30 mph, with occasional gusts up to 40 mph were reported over coastal land area with gale conditions over the offshore waters.
Figure 3: Weather regime over the southeast region; high pressure to the north and low pressure to the south kept a persistent easterly flow over the area.
Wind gusts up to 42 mph were measured at Craig Airport, and 43 mph at St. Augustine Airport. These wind gusts were enough to bring down a few shallow rooted trees in St. Johns and Flagler counties.
May 4th Supercell & Flooding
As if that wasn't enough, an unusual supercell thunderstorm developed over northeast Clay county and northwest St. Johns county, as illustrated on the radar image below. Unlike surrounding showers that spread northward, the supercell motion was toward the south over St. Johns county. At one point, rotation within the storm became strong enough to warrant a tornado warning for St. Johns county. While some trees were blown down, no tornadoes were confirmed. The storm also produced penny size hail. Figures 4, 5, 6 and 7 detail the supercell's evolution.
Synopsis of Storm Envinronment
There had been an Omega Block in place with a large upper level ridge over Quebec and the Great Lakes and a large upper level low over the Mississippi Valley. At the surface, there was a strong (1030 MB) high over New England which was wedging south over the area and a weak surface low across east central Florida. The strong pressure gradient was producing gale force winds and convergent rain bands along the coast. A cold front was also moving east across the eastern Florida Panhandle and southwest Georgia. At around 21Z, a mid level impulse, rotating around the base of the upper low, moved in from the Gulf and produced a cluster of thunderstorms over the Okefenokee Swamp. By around 22Z, the storms split, and outflow from a storm over Charlton and Nassau counties started to track SE with a slow moving cluster of showers drifting with the outflow. Strong low leve easterly inflow combined with the SSE propagating outflow continued to produce rainfall...with increasing intensity when the cell drifted over the Jacksonville Westside. Hourly rainfall rates progressively increased from less than 1 inch over the Jacksonville NW side to near 3 inches near Orange Park. Flooding started to occur quickly due to already saturated ground between 7 pm and 9 pm, with innundation of homes in Orange Park and road 2-4 feet under water. The radar details of this flooding rainfall are addressed in Figure 4 below.
As the storm drifted ESE, it intensified further over northern St. Johns and southern Duval county around 8 pm, where a large hail core was indicated aloft. As the storm track southward over St. Johns county, low level shear (helicity) was very strong with windy ESE winds off of the Atlantic, and the cell began to take on supercell characteristics including a deep layer mesocylone, bounded weak echo region (BWER), overhang, and a tight low level hook echo signature, which is evident in the images of Figure 5 below. A tornado warning was issued at 8:25 pm when the storm was over north-central St. Johns county, with an extension of the warning issued around 9:16 pm as the cell continued to track southward over southern St. Johns county. The tornado warnings are detailed in Figure 6 below. While there were no confirmed tornado reports, widespread wind damage occurred, including strucutral and crop damage from the World Golf Village to Elkton. This is detailed in Figure 7.
Figure 4 (below): The storm started as a heavy rain maker over the Jacksonville Westside were a shower started to have strong ESE inflow.
Figure 5 (below): Dopper radar products focused on the May 4th supercell. The storm started to become severe over south-central Duval county, then tracked ESE over northern St. Johns, where it then started to take on supercellular characteristics including a hook echo and deep layer rotation (mesocyclone).
While the rainfall was excessive, causing flooding in some areas, in many areas it was much needed to alleviate dry conditions. The Drought Monitor image below, as of April 30th, indicated dry ground conditions over coastal southeast GA and much of northeast and northcentral Florida. The soaking rainfall will help to reduce the short-term Spring wildfire threat.
Figure 8: Drought monitor.
Write-up by: P. Wolf, P. Peterson, K. Guillet, A. Enyedi