Saturday, August 13, 2016

Louisiana low produces record rain, but what about our precip chances?

You've no doubt seen the media reports of flooding that is rising to historic levels along the central Gulf Coast. It started earlier this week with a low pressure system that formed in the northeastern Gulf and brought heavy rain to western Florida and now has been sitting over Louisiana the past 24 hours. The situation in some areas of southern Louisiana, mainly west of New Orleans, is becoming dire as water rescues have been necessary and portions of towns and cities are flooding from 10-20" of rain that has fallen so far. Baton Rouge received over 11" of rain in the past 24 hours, which forced Louisiana State University to close today on what would have been move-in day for students and the governor has declared a State of Emergency for the entire state.

Unfortunately, today brings another day of heavy rain in some of the same areas. The low is centered over west-central Louisiana this morning and is combining with abundant atmospheric moisture fed by anomalously warm waters in the norther Gulf of Mexico to produce very high rainfall rates of multiple inches per hour in the heaviest storms.

With respect to how how this system affects Mid-South weather, the effect is indirect, as the low moves very little, into northwest Louisiana by Sunday morning. However, it will weaken a bit and merge with a frontal system that is loitering to our northwest. The front will move little, but act as a conveyor belt for the energy and rainfall from the low to move along the front northeast into the norther Mid-South and then the Ohio Valley.

NWS surface map valid 7pm Sunday evening shows weak low pressure over eastern TX as it gets ready to start it's journey northeast along the front that is draped out just to our north and into the Ohio Valley. That front will be the focus for the heaviest rainfall into next week.

With the metro sitting firmly in the "warm sector" south of the front, the proximity of front to the northwest, the low to the southwest, and upper level energy moving over the area will mean scattered showers and a few thunderstorms mainly during the warmer parts of the day this weekend with most people seeing a good chance of rain, especially Sunday and Monday. Abundant moisture is in place overhead, which means that any showers or storms that form will be capable of heavy downpours. Fortunately, showers and thunderstorm won't be sitting in one place very long, which should reduce the flash flooding threat this weekend in the metro.

Precipitable water values, which measure total atmospheric moisture in a column of air, will be very high this weekend, meaning any showers and thunderstorms that form will be capable of heavy rain in a short period of time. Values of 2" are considered very high and 2.5" excessive. Graphic courtesy PivotalWeather.

Closer to the front though, from northern AR into southern MO, rain and storms could "train" over the same areas repeatedly, thus setting the stage for higher risk of flash flooding.

NOAA/NWS forecast rainfall amounts through Monday evening indicate where the plume of heaviest rain to our west, although we could see more than an inch of rain the next couple days. Graphic courtesy WxBell.

Heading into next week, the weather pattern remains stagnant as the front sits just to our north. Rain chances remain at least 50/50 each day through at least Thursday. This wet pattern will mean a couple of things. First, periods of heavy rainfall will be possible each day, which will make up for our recent dry weather that for some is now going on two weeks.

Heading into next week, NOAA/NWS forecast rainfall amounts continue to build in the Mid-South and northward through the Ohio Valley into the northeast. Total amounts through Friday morning are shown, indicating 3-5" possible in the metro in the coming week. Graphic courtesy WxBell.

Also, temperatures will remain warm and humidity high, but we're not expecting to see 90° again for much of the next week. That may mean an end to the second-longest streak of daily average temperatures above 80° in the next few days. It currently stands at 64 days at Memphis International Airport. We also are in the midst of a stretch of 77 days with high temperatures at or above 85°. There's a good chance that will be broken in the next week as well with highs forecast in the mid 80s for much of next week. Finally, our streak with no temperatures in the 60s also stands at 64 days as of today. The fact that dewpoints will remain above 70 during this wet week ahead will likely keep that second-longest such streak intact.

Stay up to date with our latest forecast information and Mid-South radar via the MWN mobile apps or our website and social media streams. Links to all are provided below.

Erik Proseus
MWN Meteorologist

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