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August climatology in the Tennessee Valley, and an update on rainfall so far.


As we always do, before going forward and looking at the Tennessee Valley's climatology stats for the new month, we will take a peak back at the previous month. Rainfall was a little below average in July at the Northwest Regional Airport in Muscle Shoals, Alabama, the official climatology reporting station for our local area. Rainfall for the month of July officially came in at 3.96 inches, 0.82 inches shy of what is considered normal for the month. That puts the reporting station now 0.66 inches below normal for the year, after being off to a wet start and having a rainfall surplus through spring.


Elsewhere across the Tennessee Valley, things are a bit mixed. Much of southern middle Tennessee has actually gotten back into the plus column in terms of average rainfall for the last 30 days, and a lot of that recovery happened over the weekend! However, a lot of the heaviest rain missed northwest Alabama to the north, and these areas are still running near or a little bit below average for the month, despite averaging 4 to 5 inches of rainfall for many (but not all) areas for July. The bigger rainfall deficits for the past 365 days are from Hardin and McNairy Counties in Tennessee down through north Mississippi though. Some of these areas are running with double digit rainfall deficits, and it's no surprise that these areas have the more intense drought conditions.


As mentioned, the good news is that a lot of these drier areas have been able to catch up the past few days. Most of southern middle Tennessee, down into the areas of Lauderdale and Limestone Counties of Alabama north of Florence to Athens have gotten anywhere from 2 to 4 inches of rain the past five days, with the majority of that coming over the weekend. A few areas in here have gotten as much as 4 to 6 inches of rain. An additional half inch to one inch of rain earlier this morning fell across some portions of northwest Alabama that had missed much of the weekend rain, especially across the immediate Shoals metro.


All of this rain is definitely welcome, with most of the area in drought conditions of some kind or another. However, by the new update to the Drought Monitor that comes out Thursday, the recent rainfall may have helped remove or drastically recover some of these areas from the drought conditions, especially across our Tennessee viewing area counties. Further good news is that the latest outlook update for August from NOAA's Climate Prediction Center office of the National Weather Service shows that they expect the drought to either drastically improve, or be removed, for a large part of the Tennessee Valley and Mid South! Ridging aloft looks to stay centered a bit to our west over the Plains through at least mid month, and while we may still be close enough to it to still have hot weather, having that weakness in the ridge centered over our area will allow a much better potential for multiple rain chances compared to what we saw the past month or two. Definitely welcome news!


So, now we look ahead to the month of August and what our climatological records suggest the weather is normally like here in the Tennessee Valley. One thing to note is that we are in the peak of the summer heat, statistically speaking, but temperatures do start to trend down toward the end of the month. Our monthly average high for August as a whole is 91 degrees, and we keep that as a daily average through the middle of the month, but by the time we reach August 31st, our average daily high temperatures has slipped back into the upper 80s. Our average monthly morning low also comes in right around 69 degrees, a signal of the humid atmosphere we often have in place this time of year. We are certainly no stranger to big heat and triple digits this time of year though! The monthly record high is 108 degrees, set in 1930. We can also surprisingly get a little cool at night, especially if a front sneaks in late month. Our record monthly low is 47 degrees! That was set back in 1917. However, overnight temperatures cooler than the 60s are certainly not frequent in August in the Tennessee Valley! We tend to average a bit drier in August than June and July, with our normal monthly rainfall coming in at 3.15 inches. We tend to be even drier in September and October. We can occasionally see heavy rainfall events in August though, either from thunderstorm clusters producing heavy rain and flooding or from landfalling tropical systems.


August starts the rapid ramp up toward the peak of the Atlantic hurricane season, and it also begins the period of time when major hurricanes (Category 3 and higher) are a good bit more likely. Anywhere in the Atlantic basin is fair game, but the highest frequency of systems tends to be from waves coming off Africa and tracking near the East Coast or just offshore, or sometimes from systems that make it into the Gulf of Mexico. So far this season, we have already had three named storms. The next name up on the Atlantic list is Danielle. For right now, the tropics look mostly quiet for at least the next 7 to 10 days, with sinking air aloft and dry Saharan air and dust providing unfavorable conditions over much of the Atlantic basin.


In terms of severe storms, if that kind of thing is going to happen in the Tennessee Valley in August, it happens one of three ways:


  1. Pop-up afternoon and evening thunderstorms that pulse up and briefly become severe with gusty winds or hail. These also produce intense cloud-to-ground lightning and isolated areas of heavy rainfall and sometimes localized flooding.

  2. We sometimes see complexes of thunderstorms with widespread wind damage potential. These don't happen as often in August in our area as they do May through July, but they can sometimes still happen. Because of internal processes within these lines or complexes of storms, this is also one of the ways you can rarely but occasionally get a short-lived, spin-up, lower-end type tornado.

  3. Landfalling tropical systems often carry the potential for spin-up tornadoes in their outer rainbands on the right side of the system relative to its track and forward motion. Tornadoes are very infrequent in the Tennessee Valley in August, but if they ARE going to happen, this is the most likely way. Statistically speaking, the majority of tornadoes from tropical systems are of the short-lived "weak" (EF0-EF1) variety. However, EF2 intensity tornadoes are not uncommon, and tornadoes as strong as EF3 occasionally happen in association with tropical systems and their landfalling circulations as well. Anything stronger than that, there have only been two recorded "violent" tornadoes (EF4 or greater) from a tropical system in the United States. Both were F4 intensity, happening in the 1960s (Texas and Louisiana). There has never been a documented F5/EF5 tornado from a tropical system or its remnant circulation in our nation's weather records, dating all the way back well into the 1800s.

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