Late-season Mississippi River flooding a concern for many

Typically springtime is when farmers, tug boat captains, the Corps of Engineers, and countless others whose livelihood depends on the Mississippi River anticipate high water. This year, however, the spring flood season was marked by minimal, if any, flooding. The Mid-South appeared to have made it through high-water season without a hitch. Unfortunately, the excess water was just late.

As we move into early July, recent heavy rain in the Mid-Mississippi River Valley and the Ohio River Basin has resulted rising water levels downstream. As shown in the graphic below, rainfall over the past 30 days has been double normal amounts in those areas, which all drain south into the river that flows past Memphis.

Accumulated precipitation for the past 30 days shows nearly double the normal precipitation upstream from the Mid-South.

As far as local precipitation, the past 30 days has featured accumulated precipitation that is above normal, however there is a major caveat in that data. Nearly 2" of rain fell right at the beginning on that 30-day window (May 31) and the amount above normal has decreased pretty much ever since. In fact, for the month of June to date, the total precipitation at Memphis International Airport is under 2.5", which is well below normal. Thus, the river can continue to rise towards flood levels even with below average precipitation for a 4-week period locally. Rain that falls locally, while having a significant bearing on the levels of tributaries and streams that flow into the Mississippi River, actually have little to no effect on the main channel itself here in Memphis.

The graphic below shows the current Flood Warnings (lime green with green outline) and Flood Advisories (green with no outline) in effect along the Mississippi River in the Mid-South. Flood Warnings are issued with flood stage is expected to be exceeded, while an advisory is issued when the river level will result in some impacts locally, but flood stage is not expected to be exceeded.

Flood Warnings (outlined) and Advisories (no outlines) line the Mississippi River throughout the Mid-South.

An article in this morning's edition of the Commercial Appeal details the impacts to local farmers, particularly those outside the levees in east Arkansas, of the late flood. In particular, thousands of acres of maturing soybeans are flooding with little chance of being salvaged this season. In addition, by the time the river recedes and fields dry, it will likely be too late to re-plant and get a full season of growth in before harvest time. Typically, these crops are planted after the "usual" spring flood season is over.

Flood stage for the Mississippi River at Memphis is 34.0 feet. This morning's stage was 28.5 feet and the river is expected to crest at 30.5 feet on Wednesday, July 8. Obviously, it doesn't take a stage of 34 feet to begin having an impact on local agricultural, and other, interests.

Today's Mississippi River stage and this week's forecast at Memphis is shown in the hydrograph above.

Just upstream, the river level at Osceola, AR, which also affects Tipton County, was 28.8 feet. Flood stage is 28.0 feet, thus the river is in flood at that location. It is expected to reach 30.5 feet by next Tuesday with possible additional rises thereafter. South of Memphis, the water level at Tunica is also expected to rise above the flood stage of 41.0 feet early next week. The river levels can be monitored anytime via the Mississippi River Forecast page on MWN.

Unfortunately for those who are hoping for a quick decrease in river levels, forecast precipitation in the Ohio/Tennessee River basins and Mid-Mississippi Valley shows a lot more water could be headed into the river here in the Mid-South (graphic below).

Forecast precipitation for the coming 7 days would indicate the likelihood of water levels remaining high on the river for the forseeable future.

Erik Proseus
MWN Meteorologist

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Pleasant weekend in the books, unsettled holiday week ahead

An unseasonably strong cold front dove through the Mid-South early Saturday morning, bringing with it drier and cooler air just in time for the weekend. With highs in the 80s and dewpoints in the low to mid 60s, the sunshine has resulted in pleasant days, rather than nature's sauna. Surface high pressure that is overhead this weekend will shift east though, meaning we're back in southerly flow by Monday and humidity levels will begin to increase.

Sunday mid-day surface analysis shows our recent cold front down near the Gulf coast while high pressure dominates the eastern U.S., bringing pleasant conditions.

Well above the surface, the eastern U.S. is dominated by a large trough of low pressure, which means the potential for unsettled weather. That pattern includes the Mid-South as cold fronts move by to our north, with the potential for thunderstorm complexes generated by these fronts diving south. These complexes are called MCS's (mesoscale convective systems) and we've talked about them before. They can be unpredictable both in the long term and the short term and they can bring damaging wind and torrential rain, as well as a lot of lightning.

In the Mid-South, they are most common in "northwest flow," which means the wind at the mid levels that drives storm complexes is blowing from the northwest, and that's the pattern we'll be in to some degree most of this week and right into next weekend.

The pattern at the mid levels this week will be dominated by northwest flow over the Mississippi Valley, as shown in this 500mb (18,000') forecast for Wednesday evening from the GFS model. A huge dome of high pressure over the western U.S. and general trough of low pressure over the eastern U.S. results in northwest wind over the Mid-South, which is a conducing pattern to unsettled weather and potential MCS activity from the Mid-South into the Ohio Valley.

As far as the forecast is concerned, though there is a very slight chance of a thunderstorm tomorrow, chances increase a bit Tuesday, then scattered thunderstorms are in the forecast for the remainder of the week. However, the main factor in our daily rain chances will be any MCS's that form and move into the area. Rain chances will be likely when one moves through, then probably dry for the rest of the day or night behind one as the atmosphere "re-energizes."

At this point we don't have our eyes on any one particular time period when one could affect the metro, so rain chances are fairly broad-brush. That's not good if you're planning to attend (or organize) a holiday fireworks display, so the best advice is check the MWN Forecast multiple times throughout the week as the forecast could change drastically from one day to the next as we get a better handle on timing of individual storm systems. For now, here's what we're expecting this week:

Erik Proseus
MWN Meteorologist

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