Wednesday, April 30, 2014

Record Rain in Alabama and Florida

Source: NWS Advanced Hydrologic Prediction Service
For the second time this month heavy rain has produced widespread flash flooding over southern Alabama and the western Florida panhandle. On April 14 three to five inches of rain fell from Mobile County, AL into the western Florida panhandle, with 6 to 8 inch amounts from the eastern side of Mobile Bay along I-10 into Florida.  In the past 48 hours the area was hit again, but with twice the amount of rain as on April 14.


Rainfall totals of up to 26 inches were estimated by radar in the westernmost portion of the Florida panhandle, though no actual amounts that high have been reported. Measured 24-hour rain totals in excess of 12 inches were common. The rain came in two waves, the first of which occurred Monday night into early Tuesday morning. This round of storms dropped from 3 to 8 inches of rain along the coasts of Alabama and Florida. The second wave of storms produced 10 to 15 inches of rain in roughly a period of nine to ten hours. The National Weather Service reported that 5.68 inches of rain fell between 10:00 p.m. to 11:00 p.m. CDT in Pensacola, FL on Tuesday night.

2-day radar estimated precipitation ending at 8:00 a.m. CDT April 30. The Mobile, AL radar was knocked out by a lightning strike during the storms and was not available. This image combines the radars from New Orleans (KLIX) and Eglin AFB (KEVX).  There is a slight difference between the start of the accumulation periods for the two radar sites.

Previous daily rainfall records for Mobile, AL and Pensacola, FL were annihilated Tuesday. Mobile measured 11.24 inches of rain on April 29, breaking the old record of 3.67 inches in 1996. In Pensacola, 11.13 inches of rain broke the old record of 3.06 set in 1918. The new record may in fact be a little low because a power outage resulted in the loss of an hour of data. The NWS radar estimate of rain at the Pensacola observation site was 15.55 inches.

24 hour radar estimated rainfall. KPNS is the Pensacola Regional Airport where the weather observations are made.
Credit: NWS Mobile/Pensacola

The CoCoRaHS reports coming in this morning were amazing. (The high amounts, compared to what was happening in the rest of the country, are what caused the national map to "skew to the blue", so to speak).  Six observers in southeastern Alabama and western Florida reported 24-hour totals of more than 17 inches, and 50 other observers reported more than 6 inches of rain.  Here is a list of the CoCoRaHS observers in the affected area reporting 10 inches or more of rain in the past two days. Stations highlighted in green reported that the gauge overflowed, so actual amount is likely higher.


Station No. Station Name County    4/29     4/30     TOTAL
AL-BW-45  Silverhill 5.0 SW  Baldwin 1.80 20.00 21.80
AL-BW-9  Foley 0.5 ESE  Baldwin 1.83 18.93 20.76
AL-BW-27  Orange Beach 2.1 NE  Baldwin 2.01 18.52 20.53
FL-SR-9  Milton 10.9 SSW  Santa Rosa 1.48 18.91 20.39
FL-ES-21  Pensacola 9.2 NW  Escambia 1.86 17.70 19.56
AL-MB-50  Mobile 5.1 S  Mobile 4.21 12.99 17.20
AL-BW-3  Daphne 1.2 NNW  Baldwin 3.48 12.99 16.47
AL-BW-32  Fairhope 3.1 NNW  Baldwin 3.43 12.70 16.13
AL-BW-40  Fairhope 1.5 WSW  Baldwin 2.56 12.95 15.51
AL-BW-65  Silverhill 0.9 SSE  Baldwin 2.45 13.02 15.47
AL-BW-68  Spanish Fort 1.6 W  Baldwin 3.12 12.33 15.45
FL-ES-4  Gonzalez 2.5 NNW  Escambia 1.50 13.93 15.43
AL-BW-13  Fairhope 3.7 NNW  Baldwin 3.41 10.88 14.29
AL-BW-4  Daphne 0.4 SW  Baldwin 3.33 10.90 14.23
FL-OK-29  Mary Esther 0.6 E  Okaloosa 1.72 12.43 14.15
AL-BW-1  Fairhope 2.3 N  Baldwin 2.63 11.49 14.12
AL-MB-49 Mobile 1.2 WSW Mobile 4.31 9.73 14.04
AL-MB-26 Mobile 4.4 WNW Mobile 4.82 9.18 14.00
FL-SR-12  Navarre 2.1 WNW  Santa Rosa 2.98 11.00 13.98
AL-BW-58  Spanish Fort 1.2 NE  Baldwin 2.87 10.96 13.83
AL-BW-36  Daphne 4.2 NE  Baldwin 2.61 11.20 13.81
AL-BW-26  Loxley 0.4 SSW  Baldwin 2.42 11.36 13.78
AL-MB-35 Mobile 7.4 W Mobile 4.63 8.91 13.54
AL-BW-8  Foley 7.4 SW  Baldwin 1.92 11.55 13.47
AL-BW-41  Fairhope 2.3 E  Baldwin 2.43 11.01 13.44
AL-MB-1 Tillmans Corner 4.3 WNW Mobile 4.56 8.73 13.29
FL-OK-33  Destin 1.3 NW  Okaloosa 1.02 12.20 13.22
FL-ES-15  Gonzalez 2.1 E  Escambia 1.69 11.53 13.22
AL-BW-31  Foley 2.0 SSW  Baldwin 1.87 11.30 13.17
FL-SR-2  Milton 1.4 NNE  Santa Rosa 2.06 11.09 13.15
AL-BW-20  Summerdale 4.3 WSW  Baldwin 1.78 11.25 13.03
AL-BW-60  Daphne 1.5 SSW  Baldwin 1.98 11.00 12.98
AL-MB-44  Mobile 1.8 W Mobile 3.96 8.92 12.88
AL-MB-10  Mobile 11.4 WSW Mobile 4.42 8.43 12.85
FL-SR-12  Navarre 2.1 WNW Santa Rosa 1.78 11.00 12.78
AL-BW-53  Elberta 3.1 SSW Baldwin 2.81 9.86 12.67
FL-OK-17  Ocean City 1.3 NNW  Okaloosa 1.56 11.04 12.60
AL-MB-4 Mobile 2.6 WNW Mobile 4.09 8.21 12.30
AL-MB-63 Tillmans Corner 8.7 WNW Mobile 4.43 7.66 12.09
FL-SR-17  Milton 6.0 SW  Santa Rosa 0.00 12.00 12.00
FL-WT-14  Inlet Beach 0.7 E  Walton 1.86 10.00 11.86
FL-OK-16  Shalimar 1.0 N  Okaloosa 1.25 10.38 11.63
AL-BW-30  Fairhope 3.5 E Baldwin 2.50 9.04 11.54
AL-MB-41  Theodore 8.0 SSE Mobile 2.97 8.25 11.22
AL-MB-32  Grand Bay 3.1 NNE Mobile 4.70 6.13 10.83
FL-OK-15  Niceville 4.5 SE Okaloosa 1.50 8.55 10.05
AL-BW-35  Orange Beach 1.3 E  Baldwin       M 19.00 INC
FL-ES-10  Pensacola 3.8 N  Escambia M 12.00 INC

The thunderstorms that produced the rain resulted from a "perfect storm" of factors.  There was strong upward motion in the atmosphere generated by small waves of energy rotating around the massive low centered over the central U.S.  That upward motion was aided by difluence in the upper atmosphere as well. In simple terms, difluence is the horizontal "spreading" of air at a level. As air is removed from a given level air rises from lower levels to replace it. That rising motion can aid in thunderstorm development. A third ingredient to this event was the surface cold front to the west, which helped provided lifting of the air from the surface. Lastly, warm, moist air from the Gulf flowed into the area ahead of the cold front. All of these came together to produce the conditions that gave rise to the torrential rain and resulting flash flooding.

Surface weather map at 7:00 p.m. CDT April 29.
This same area experienced a similar heavy rain and flash flooding event less than two years ago on June 9-10, 2012. Eight to 12 inches of rain fell from around Mobile Bay into the Florida panhandle. The CoCoRaHS observer at FL-ES-2 (West Pensacola) reported almost 22 inches of rain with that storm. You can read more about this in the July 2012 edition of the Florida CoCoRaHS newsletter.


More information on this event can be found at the NWS Mobile/Pensacola web site. 

1 comment:

Scott said...

Steve:
Is there any way I can get that listing of gauges from you in a CSV or an excel file, but with the lat/lons of each site included in separate columns?

Contact me directly (scott.lincoln@noaa.gov) if you want to discuss.