Friday, February 22, 2019

Overnight storms, then severe storms -- but the end is in sight!

The drumbeat of raindrops falling is incessant... and we're not quite done. In fact, the next 24 hours or so pose more risk than we've seen in a few months, both in terms of the flash flooding potential and the severe weather risk. Let's start with the water, then move onto the storms.

Rest of today and tonight

Continuous rain today, though perhaps not as heavy as what we've seen recently, is keeping things saturated. Metro areas in west TN and east AR have seen 1/3 - 1/2" of rain since sunrise this morning (as of 2pm) while northwest MS is likely pushing an inch. The heaviest rain since yesterday has been southeast of the metro where Flash Flood Warnings are in effect. Today's rain is on top of 5-7" the metro has seen in the past 2 weeks, as shown below.

14-day precipitation totals (since Feb. 8) show 5-7" of rain has fallen through early this morning. (NOAA via WeatherBell)

Steady rain is likely to continue this afternoon and perhaps into the evening, though high-res models disagree a bit on a potential break in the rain for the early evening hours. However, once we get into the nighttime, after about 10pm, a warm front to our south will lift north and cross the metro overnight, pushed by an increasingly strong southerly low-level jet stream a couple thousand feet up. That will result in periods of thunderstorms overnight, perhaps lasting much of the night.

The high-res HRRR model future radar product through 7am Saturday shows the overnight storms lifting north through the metro ahead of a warm front. (WeatherModels.com)

Rainfall will be heavy enough in these storms to be classified as "frog-stranglers" and "gully-washers," dropping up to 2-3" overnight, perhaps higher in spots. Flash flooding becomes a much more concerning trend as the night goes on, so if you will be out overnight, be extremely cautious! Creeks and streams could overflow, rivers are already high, and low-lying or poor drainage areas in the urban jungle could see accumulating water. If you live near a creek or stream or other area that floods in very heavy rain, be aware that could happen overnight. The NWS Weather Prediction Center has the metro straddling the line between a moderate and high risk of rainfall that exceeds levels required for flooding, or about a 50% chance within 25 miles of you. North MS has already seen some water rescues in the past 24 hours - flooding is a threat to be taken seriously, particularly south of the state line!

The Memphis metro is in a Moderate (level 3/4) threat area for flooding through tonight, while north MS is in a High risk (level 4/4). This means there is about a 50/50 chance of having  rainfall exceed flash flood criteria within 25 miles of you. (NOAA/WPC) 

As far as storm threats, we're not necessarily expecting them to be severe. The Storm Prediction Center has our area on the edge of a Marginal Risk (level 1/5) for severe weather, with the possibility that a few storms could have some hail. Damaging wind and tornadoes are not currently expected overnight, but that doesn't mean the storms won't make a racket! If you had issues with storm noise Tuesday night, expect it again tonight (and it may last longer). Also know that due to the frequent lightning, power outages will also be possible.

SPC has the metro outlooked in a Marginal Risk (level 1/5) for severe storms overnight. The main threat is a few hail storms, but even that threat is very low. Storms will produce plenty of lightning and thunder though! (NOAA/SPC)

Saturday

Once the warm front moves to our north, we should catch a bit of a break for a while Saturday morning. By about sunrise, the storms will be to our north and a humid, very warm, and increasingly unstable airmass will overtake the area. Temperatures in the morning will rise to near 70° with gusty south wind and hit and miss showers or a thunderstorm. The stage will be set for the potential for severe weather in the afternoon as a potent cold front cuts through an airmass characterized by strong, turning winds aloft (bulk shear over 50 kts and SRH near 200 for you weather nerds), sufficient instability (CAPE of 1000-1500), and plenty of available moisture at all levels (PWAT near 1.6").

Storms will likely form ahead of and along the front by late morning in AR and move our way by early afternoon. We are expecting a line of storms, with perhaps additional storms ahead of the line, during the afternoon hours Saturday, or roughly between 1-5pm. These storms will tap into the springtime atmosphere and have the potential to produce damaging wind gusts and a low threat for a couple tornadoes embedded within the line.

The HRRR model forecast radar simulation from noon to 6pm Saturday shows a broken line of storms moving through the metro, intensifying as they near the Mississippi River about 4pm. A few storms are also possible ahead of the line. (WeatherModels.com)

A second possible solution for Saturday, from the high-res NAM model looping from 10am-8pm Saturday, shows an earlier arrival of the storms, and perhaps a couple of lines between 1pm-4pm. It also depicts more organization of the storms as they move towards the metro. (WeatherModels.com)

Severe weather threats

While the threat of damaging wind will be much higher (right now pegged at about 30% within 25 miles of any point), a Tornado Watch is likely during the afternoon as tornado probabilities are currently forecast at about 10% within 25 miles of you. Consider now what your plans are for Saturday afternoon and be prepared to take shelter wherever you are, if necessary. In addition, with the excessively wet ground, storms that produce sub-severe wind gusts (40-50 mph) may be sufficient to uproot trees and cause power outages. A Wind Advisory has also been issued for non-convective wind gusts to 30-40 mph Saturday would could pose an additional threat to trees with shallow roots in saturated soil.


After the storms

By 6pm Saturday evening, the storms will likely be gone and evening plans should continue with little concern. Moving into Sunday and early next week, I am pleased to report that it will be DRY with plentiful sunshine and seasonal temperatures. A few mid to late week showers are possible, but there is currently no threat of heavy rain that would cause additional flooding concerns as high temperatures remain in the mid 50s to near 60.

Stay tuned to our social media channels for the latest updates and be sure you have the MWN app downloaded with StormWatch+ Alerts activated for your locations of interest. Links are presented below.

Erik Proseus
MWN Meteorologist

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Follow MWN on Facebook and Twitter for routine updates and the latest info!
Complete MWN Forecast: MemphisWeather.net on the mobile web or via the MWN mobile app
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MWN is a NOAA Weather Ready Nation Ambassador Meteorologist Erik Proseus is an NWA Digital Seal Holder

Wednesday, February 20, 2019

Weather 102: Why is nighttime thunder so loud?

MWN friend Kristen Boyden asked a great question: "Why were Tuesday night's storms so loud?"

The overnight storms (and this is pretty common for nighttime storms and some during the day as well) were what we call "elevated thunderstorms." Basically, we had a very strong temperature inversion in place, in which the temperature rises with height rather than cooling. It was about 45° F at the surface during last night's storms, but 57° F at just 4,000 feet up! That inversion creates a stable layer in the atmosphere at the low levels and basically acts like a lid at 4,000 feet above us.

An atmospheric sounding (temperature/humidity profile) of the airmass over Memphis Tuesday night shows temperature (red line) increasing as you go up into the atmosphere, an inversion. That inversion "caps" the atmosphere below, trapping the sound of the thunder in the lowest few thousand feet. (NOAA/AMDAR)

When lightning discharges under that lid, the sound of the thunder is trapped close to the ground. Since the sound can't escape UP ↑, it bounces around between the ground and the stable layer above (the lid), amplifying the noise. It's like setting off firecrackers in grandma's pressure cooker!

Elevated thunderstorms occur when warm air overrides cooler air near the surface, creating a temperature inversion, and thus a stable layer of air in the low levels, above which convection occurs. (Graphic courtesy @wxbrad)
In addition, general daytime noise is reduced at night, so the thunder seems louder because the ambient noise level is quieter. (Like when your baby lets out a cry in the doctor's waiting room vs. in a house full of other kids!) I frequently say that thunder at night is often louder than during the day, but in fact, ELEVATED thunderstorms are truly the cause for the noise level. They just happen to occur more often at night! It doesn't mean they are "more severe" or even more damaging. They're just louder.

Erik Proseus
MWN Meteorologist

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Follow MWN on Facebook and Twitter for routine updates and the latest info!
Complete MWN Forecast: MemphisWeather.net on the mobile web or via the MWN mobile app
Download our iPhone or Android apps, featuring StormWatch+ severe weather alerts!
MWN is a NOAA Weather Ready Nation Ambassador Meteorologist Erik Proseus is an NWA Digital Seal Holder