SR SSD 99-21
David Smith and Robert J. Ricks, Jr.
A large and complex low pressure system over the southwestern United States steadily approached the Gulf states on March 11-13, 1999. A stationary front over the Gulf of Mexico moved north as a warm front on March 12, providing a focus for locally heavy rain across southeast Louisiana north of New Orleans, and extreme southern Mississippi. A narrow swath of generally 4 to 6 in of rainfall occurred during the event, oriented west-southwest to east-northeast along the Louisiana-Mississippi border. Significant rises on rivers and streams occurred beneath the rain swath. While these rises produced swollen river flows, most locations remained below flood levels, with only minor flooding noted along the lower reaches of the channels draining into the Lakes Pontchartrain/Maurepas system. Several bridges across these streams were washed out, notably the State Highway 10 bridge spanning the Amite River near Darlington, Louisiana.
On the morning of March 12, 1999, a cold high pressure area was situated over the western Great Lakes and extended to the Gulf of Mexico coastal plains. A stationary front stretched from Cuba, northwestward to just off the Louisiana coast, continuing to a low center located in the vicinity of the Four Corners in the desert Southwest. The upper-level weather pattern indicated a deep low vortex over the Four Corners on March 12, which moved to the Texas Panhandle by the morning of March 13. Southwesterly flow aloft provided a source of Pacific-based moisture into the region, enhancing the return of moisture off the Gulf of Mexico.
A forecast problem during this episode was to accurately assess the amount of warm air advection in the mid-levels of the atmosphere that may provide a "capped" environment, thereby limiting the rain potential. The result was only a limited amount of warming to provide capping for only a short period of time. This feature diminished with the northward movement of the warm front from the Gulf, which placed the target region for rainfall under a deeper maritime flow. Nonetheless, the overall synoptic setup was typical of a Maddox Frontal Type heavy rain pattern (Maddox et al., 1979). This also resembled a 4B pattern published by Johnson, et al. (1987).
Recognition of a heavy rainfall pattern by the forecast staff at the NWSFO prompted the issuance of a Flash Flood Watch for southwest and south-central Louisiana at 950 PM CST, on March 11. The midnight shift of March 12 implemented the Precipitation Calculator (Ricks 1992) for determining forecast rain amounts. This approach utilizes equivalent potential temperature (theta-e) data obtained from a nearby upper-air sounding as input into a computer program. Output is generated for forecast rainfall amounts, potential rainfall maximum, forecast mean areal precipitation (FMAP), and severe weather parameterization. For this particular episode, a theta-e value of 332K was used, as determined by conventional meteorological model prognoses. A precipitation forecast of 100 percent probability resulted, with an amount of 1.68 in likely, a potential rainfall amount of 5.88 in assuming the surface temperature remained below 69F during the rainfall process. An FMAP amount of 2.10 in was obtained for a basin coverage of 1000 square miles. In addition, there was a high probability of severe weather in the area. (A few weak tornadoes, hail and wind damage also occurred with this event.) Output from this technique was used in subsequent flood watch statements issued by the NWSFO staff.
Given the temperature profile to the north of the warm front, it was deemed likely the potential rainfall amount close to six inches was achievable. Rainfall forecasts in the flood statements provided a range of generally 2 to 4 in expected, with locally higher amounts around 6 in possible. At 450 pm CST, March 12, the Flash Flood Watch was reissued to include the Louisiana Florida parishes (those parishes north and east of New Orleans) and the southwestern Mississippi counties.
As the weather situation unfolded, heavy rains were noted in the vicinity of the Louisiana-Mississippi state line. Due to the widespread rainfall ongoing at one point during the afternoon, the NWSFO issued an Urban and Small Stream Flood Advisory for the entire area to address minor flooding problems. A Flash Flood Warning was issued for Pike County, Mississippi, upon receipt of a report of a home being evacuated due to flood waters entering the structure.
By the evening of the 13th, a cold front moved eastward across the affected area, bringing an abrupt end to the rainfall. Much of the watch area received the expected 2 to 4 in of rain, with the most pronounced rainfall occurring in a 40 mi wide swath in excess of 4 in along the Louisiana-Mississippi border. Maximum reports were 7.38 in at Liberty, 7.65 in at Centreville, and 7.09 in at Gloster, all in Amite County, Mississippi (Fig. 1). In addition, 7.33 in fell at Kentwood, Louisiana, in northern Tangipahoa Parish.
Automated tipping bucket rain gauge traces showed nearly 3.5 in of rain fell over the Amite/Comite River watershed in a four-hour period (Fig. 2a, b). Seven-inch rains created excessive discharge on headwaters of rivers and on creeks and bayous which scoured out river banks and carried damaging debris downstream into bridges and low water crossings in east central Louisiana.
Five bridges were damaged or destroyed in East Feliciana and West Feliciana Parishes in Louisiana. The most significant damage was to the Louisiana Highway 10 bridge across the Amite River near Darlington (Fig. 3). According to a news report in the Baton Rouge Advocate, the sheriff of East Feliciana Parish had stopped on the bridge Sunday afternoon and noticed a "raft" of trees and limbs lodged against almost all of the 10-span structure. The Louisiana Department of Transportation and Development (DOTD) had already requested equipment to remove the debris. Before the work crew could arrive two of the bridge spans collapsed when the supporting concrete pilings failed. Fortunately, sheriff's deputies and DOTD personnel had stopped traffic over the bridge and no injuries occurred.
LA 10 is a major commercial route for east-to-west commerce from Mississippi to St. Francisville on the Mississippi River above Baton Rouge. Repairs will take up to four months at an estimated cost of $750,000. The Thompson Creek bridge was already in poor condition and scheduled for structural overhaul; nevertheless, the flood damage created on that ungauged stream was significant (Fig. 4). As in the case of the LA 10 bridge failure, there were no injuries, but transportation inconvenience for local residents will be a problem during the lengthy reconstruction period.
The Polly Creek bridge near Plettenberg collapsed when pilings were scoured out in the high volume flow. That structure will cost approximately $47,000 to replace. Little Bayou Sara washed out the road at one end of the Highland Road Bridge in West Feliciana Parish. Officials indicated the washout can be repaired by extending the bridge and refinishing the approach.
Four of the five bridge failures were in remote areas with low population density. The subsequent damage was not sudden, but caused by gradual scouring out of bridge supports. While more of the creeks and streams actually went into flood (exceeding bankfull) during this event, their channels became swollen rather quickly. Nevertheless, the possibility of injury to the public was present. In the case of the Amite River bridge on LA 10, public danger was a greater possibility given the volume of traffic on a major roadway.
The absence of injury during this intense rain event can be directly attributed to the successful public outreach and severe weather awareness programs of the New Orleans/Baton Rouge Area NWS Forecast Office. As in most severe weather cases structural damage was unavoidable, but public safety was paramount. An excellent communication system and cooperative effort among the NWS staff, USGS officials, state and local law enforcement personnel and emergency managers has evolved over time that facilitated local action to timely weather watches and warnings, all of which saved lives and prevented injuries.
Johnson, G. Alan, E. Mortimer and H. W. N. Lau, 1987: Synoptic Climatology of Heavy Rainfall Events in Louisiana. Preprints, 7th Conf. On Hydrometeorology, Amer. Meteor. Soc., 169-173.
Maddox, R.A.,C.F. Chappell, and L.R. Hoxit, 1979: Synoptic and Meso-Alpha Aspects of Flash Flood Events. Bull. Amer. Meteor. Soc., 60, 115-123.
Ricks, R.J., Jr., 1992: An Experimental Program for Determining Precipitation Potential Based on Equivalent Potential Temperature. NOAA Technical Memo NWS 87-ER, 195-201.