“High Impact” is wording that we don’t throw around too lightly around here, but the flooding situation that will unfold today and tonight across portions of north Alabama and adjacent areas is expected to be just that. The National Weather Service has placed a large part of north and central Alabama in a rare HIGH RISK of excessive rainfall and flooding/flash flooding for today and tonight, with the remainder of north Alabama, northeast Mississippi, and southern Tennessee still in a Significant Risk. This level of flood risk is rarely used, maybe a few times a year across the entire country. At least for portions of north and central Alabama, this may be comparable to the flooding events we had in February of last year and in December of 2015. Because of the rare and serious nature of the flooding threat and the increased threat for flash flooding (rapid water rises, as compared to gradual water rises over a large area),the National Weather Service offices in Huntsville and Birmingham have converted the Flood Watch over to a Flash Flood Watch, valid until 6:00pm tomorrow. A few different rounds of heavy rain and thunderstorms will bring a widespread 2 to 5 inches of rain across the area through tonight, and some localized areas under the heavier storms could very well get more than that. This is a serious flooding and flash flooding risk that could become potentially life-threatening. We seriously ask that you do not try to drive through flooded roadways and that you are prepared to move to higher ground if necessary, especially if you are in an area that frequently floods.
In addition to the flooding situation, the Storm Prediction Center has our southwestern and southern coverage counties in a Level 1 of 5 risk of severe weather for today and tonight. We will say up front that, for our specific coverage counties, the risk of severe weather for today and tonight is very very low. However, just like we saw in Itawamba and Tishomingo Counties earlier this morning, “low” does NOT mean “zero”. We can’t rule out a storm or two producing gusty winds of 40-60 mph or quarter size hail. The better surface-based instability is going to be on the south side of all the heavy rain today, down in central Mississippi into west central Alabama, off to our south. Because of that, the tornado risk will be off to our south as well. We can never 100% rule out a tornado if there’s a severe storm, but the risk in our coverage counties today is exceedingly low, about as close to zero as possible without being zero. The main impacts in our coverage counties today and tonight will be from flooding and flash flooding.
The second heavy rain complex that moves through tonight and into tomorrow morning will help to push the frontal boundary south of our area and into central Alabama on Tuesday, and we temporarily get into cooler and much more stable air. Highs on Tuesday will stay in the 50s with northerly winds. There will still be periods of rain at times, but not nearly as heavy as today and tonight. For this reason, the flooding threat for Tuesday may become more limited, especially in terms of flash flooding, but ANY rainfall that happens on Tuesday could cause problems to at least some degree.
Things rapidly change on Wednesday as the front races back north across the area as a warm front during the morning and warm, humid, and unstable air rapidly moves northward from the Gulf of Mexico. There may even be breaks of sun in the clouds, and that will warm us into the low 70s. We can’t even rule out some afternoon temperatures getting close to the mid 70s across parts of the area. As this happens, the main storm system finally ejects out and approaches and spins up a surface low on the front over Louisiana and Arkansas that tracks into Tennessee and Kentucky. By afternoon and evening, ingredients will rapidly be coming together for the threat of severe weather across the area. You may have seen on Sunday that the Storm Prediction Center already put out a Level 3 of 5 “Enhanced Risk” of severe weather for portions of Mississippi and Alabama for Wednesday. They maintain that risk on their update this morning. It is a little trimmed back in Alabama because of some uncertainties with just how likely it will be to get supercells ahead of the front during the afternoon and evening, and we currently agree with this assessment. Outside of the Level 3 risk, a Level 2 of 5 risk covers almost the entire remainder of the coverage area, and we have already seen several events just this winter where strong and significant tornadoes have happened within Level 2 risks AND within squall lines instead of fully discrete supercells. The threat for tornadoes and damaging winds of 60-70 mph will be elevated with the line ahead of the cold front that moves through, but the actual magnitude of the tornado risk will be determined by whether individual supercells can develop ahead of the line. There is a bit of warm air in the mid-levels of the atmosphere that is keeping instability a bit on the lower side, and this may make it harder for supercells to form. However, we have seen several times already that the models have been too warm with mid-level temperatures, and if that is the case again, instability may be higher, and it may be easier for supercells to form. We will continue to assess the data as it comes in and provide updates to the severe weather threat as we get closer.
With the storms late Wednesday bringing another round of heavy rain, even if the storms are faster movers, the flooding threat will again be elevated. We can’t rule out another 2 or 3 inches of rain during this time period as well, especially in our northwestern counties that may stay north of the warm front in stratiform rain for longer on Wednesday. The flooding threat already for Wednesday is Elevated, but we may have to bump that up as we get closer and are able to assess where the heaviest rain Wednesday is likely to occur. ANY rainfall during this time will cause problems. We are already saturated or near saturated going into the rain event we have on our hands TODAY.
Rain ends going into Thursday morning as the cold front brings drier and cooler air into the area. The models are currently in serious disagreement for the weekend, with the GFS showing another rain and storm event (likely not severe though) Saturday night and Sunday, but the Euro remains dry. For now, we will split the difference and introduce a low chance of showers during this period, and then we will adjust the forecast one way or the other as the situation becomes more clear.