Heavy rain and strong wind on track for the Mid-South

A lot of rain to be sure, but Biblical in nature? Don’t hunt for gopher wood or pair up the animals just yet…

Another strong transition-season cold front will move across the country this week. Similar to the past two events of last week and the week before, there will be plenty of wind at all levels of the atmosphere with this system, but little instability. The lack of unstable air will greatly reduce the threat of severe weather, but it won’t make much difference in precipitation rates as the heart of the system moves through. Let’s take the next few days a piece at a time.

Tonight / early Monday

Clouds thicken overnight tonight as upper level moisture increases on west-southwest wind aloft. Atmospheric energy originating from an upper-level low over the Desert Southwest will move over the area late tonight and early tomorrow. These “upper level disturbances” as they are sometimes called are areas where the air is lifted in the vertical. Where moisture is also present, the lifted air cools and condenses, resulting in precipitation. Thus scattered light showers that result will help to moisten the lower levels of the atmosphere and begin priming the pump for heavier rain to come. Rainfall will generally be 0.10″ or less and should mainly affect areas north of I-40 with measurable precipitation.

North American Model (NAM) forecast depicting a few upper-level disturbances, including a couple strong ones in red/purple, moving through southwest flow aloft early Monday morning. They will trigger scattered showers, mainly for northern areas of the Mid-South. Graphic courtesy Pivotal Weather.

Monday afternoon / night

By Monday late afternoon and evening, steadier light rain is expected to begin, lasting through the overnight hours. This additional shower activity will be in response to increasing moisture content in the atmosphere, additional upper level disturbances moving through southwest wind aloft and providing lift necessary for precipitation, and a rapidly-strengthening low level jet stream Monday night. As low pressure over the Front Range strengthens and moves slowly east into the Central Plains, wind increases ahead (or downstream) of it. At the low levels (a couple thousand feet up), south wind will increase to nearly 50 knots (almost 60 mph) by early Tuesday morning! This strong wind above the surface but in the low levels of the atmosphere is termed a low-level jet (LLJ).

North American Model (NAM) forecast showing low-level wind increasing to over 50 mph straight off the Gulf of Mexico by Tuesday morning, bringing abundant moisture into the region. Graphic courtesy Pivotal Weather

With the low-level wind straight out of the south, a flood of atmospheric moisture from the Gulf of Mexico will inundate the lower Mississippi Valley and Mid-South. Despite this, overnight rainfall amounts Monday night should still be less than 0.50″ as the best upper level dynamics remain well to the west of the region. Surface wind will also increase in response to the low level jet with steady south wind likely reaching or exceeding 20 mph.

The NWS Weather Prediction Center forecast of total rainfall from Sunday night through Tuesday at 6pm indicates only about 1/2″ of rain is expected in the metro. The band of heaviest rainfall totals shows up to our west in OK, AR, and MO.

Tuesday daytime

A relative lull in precipitation will occur Tuesday, though the metro will continue to see the pump primed for heavy precipitation later as very strong southerly flow begins to increase moisture content in the mid levels as well. With no significant upper-level disturbances traversing the area, significant precipitation is not expected before dusk, though scattered showers certainly are possible given the amount of moisture in the air above us. Plan for plenty of leaves to fall as wind will remain strong and gusty at the surface Tuesday with south wind frequently gusting to 25-35 mph. A Wind Advisory may be needed on Tuesday. With a break from steady rainfall and south wind pushing warm air into the area, expect highs to eclipse 70°, even with overcast skies.

North American Model (NAM) forecast showing surface wind increasing to 20-25 mph with higher gusts Tuesday afternoon. Parallel lines from south to north indicate a flow of air from the Gulf of Mexico. Graphic courtesy Pivotal Weather.

Tuesday night

Tuesday night is the time period of most concern, with the main threat being flooding and flash flooding and, secondarily, wind. As the north-south aligned cold front moves through the area, it will provide the necessary lift promote heavy precipitation and will open the faucet wide. For 24 hours prior to the front arriving, strong southerly wind carrying Gulf moisture-laden air will provide abundant moisture for the front to tap into. Upper level dynamics increase as wind increases to around 30 mph at the surface and 60-75 knots (70-85 mph) from 2,000 feet to almost 15,000 feet. In addition, jetstream wind about 35,000 feet up will increase to over 100 knots (115 mph). Wind Advisories will likely continue into the night.

Atmospheric profile from the NAM model at 9pm Tuesday showing very strong southerly to southwesterly wind in the low levels of the atmosphere and a strong southwesterly upper level jetstream above. In addition, the red line (temperature) and green line (dewpoint) are very close together from the surface to about 25,000′, indicating high relative humidity through most of the column of air above Memphis.

With all of this wind coming from the south, it is originating from the Mid-South’s biggest moisture source – the Gulf of Mexico. Precipitable water (PW) values, which are a measure of total atmospheric water content, will approach 2.00″, which would be record territory for this time of year according to climatology provided by the Storm Prediction Center. With a slow-moving cold front and record atmospheric moisture levels, the stage is set for possible flooding and flash flooding Tuesday night. Various computer model solutions and Weather Prediction Center forecasts paint a picture of rainfall totals between 2-3″ in a roughly 6-hour window overnight.

North American Model (NAM) forecast showing precipitable water values reaching near-record levels by Tuesday evening with abundant moisture in all levels of the atmosphere. Notice also behind the cold front how quickly moisture falls off. Graphic courtesy Pivotal Weather.

There are 3 big concerns related to the flooding threat on Tuesday night:

1) Precipitation rates. Widespread 2-3″ amounts in a roughly 6-hour window, on top of precipitation that has already fallen from Monday morning through Tuesday, will lead to high water in the usual places. Drainage ditches, streams, and creeks will fill rapidly.
2) Leaves. It’s fall, y’all! I drove around some today and saw piles of leaves sitting on curbs and in gutters, not to mention all over yards. We have a lot of trees in the Mid-South and the leaves are falling (and if they aren’t yet, wait until the wind has been blowing 20-30 mph for 24 hours!). The leaves will block natural areas that water flows, namely along the sides of streets in the gutters. With drainage grates potentially blocked by leaves, street flooding becomes a bigger hazard as the water can’t run off as quickly.
3) Darkness. All of the heavy rain will occur when it is dark out. Pair darkness with torrential rain and leaves blocking gutters and we have a recipe for big problems for motorists. Streets that haven’t flooded in a long time will. We have one suggestion: if you don’t HAVE to get out Tuesday night, DON’T! Get your errands done on the way home from work then stay put overnight.

Moving on, the threat of severe weather will be low Tuesday night as the lack of instability (as with the previous systems the past couple of weeks) and overall saturation of the atmospheric column will limit the formation of strong storms. Convective precipitation (that which occurs in an area of rising air) and thunder is certainly possible and would serve to increase rainfall rates, but the bulk of the severe weather should remain to our southwest. If there is a severe weather threat locally, it will be strong(er) wind from a possible squall line of storms. Again ,we expect that threat to remain to our south, but it’s worth mentioning.

The severe weather threat on Tuesday will be southwest of the Mid-South where instability is greater and a squall line could develop, according to the Storm Prediction Center convective outlook.

As the cold front passes sometime in the wee hours Wednesday, the faucet will shut off as wind shifts westerly, cutting off the feed from the Gulf and ushering in drier air from the west (as seen in the precipitable water graphic above). By dawn, the rain will probably be over (and your rain gauge could be overflowing). Your school’s car line should be dry.

Wednesday – Friday

The second half of the week will be a time to dry out as high pressure of Pacific origin moves overhead. Temperatures will remain seasonal with highs in the mid 60s and lows in the 40s. Clouds will depart early Wednesday, leaving the rest of the week mostly sunny.

Follow us on Facebook and Twitter and on our mobile apps (all links below) for the latest information on this week’s wind and heavy rain threats, as well as any changes to the forecast.

Stay safe and (relatively) dry!

Erik Proseus
MWN Meteorologist

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