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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Rain returns Thursday, severe weather possible Saturday

Our rain has finally gone away, but unfortunately it won't stay that way for very long. Multiple rounds of showers are expected to return starting tomorrow, with some severe thunderstorms being possible on Saturday. Over the past day, most of the Mid-South has seen between 2 to 3 inches of rain.

Observed precipitation across the Mid-South shows much of the Memphis area receiving 2 to 3 inches of rainfall over the past day with some areas receiving even more. (NOAA/NWS)

Unfortunately, a lot more rain is expected from tomorrow through Saturday. We will not begin to transition away from this perpetual wet period until next week, but even then the ground will still be pretty soggy.

The Weather Prediction Center's precipitation forecast from now through Saturday evening shows additional rainfall of 2-3" is expected across the Mid-South. (Pivotal Weather)


Rest of today

Skies are expected to continue clearing out for the remainder of the day. While we won't completely clear out during the daytime, we could get a glimpse or two of some sunshine by this afternoon.

Currently visible satellite imagery shows a blanket of clouds to our East, with some clearing to our West. We should continue to see clearing skies this afternoon. (College of DuPage)

As for this evening into the overnight hours, skies will continue to clear leaving us with a mostly clear sky overnight. Conditions will be favorable for some patchy fog to develop in outlying areas. Temps will dip back down to near 38 overnight so a bit of frost is possible on your car if you park outside overnight.

Thursday and Friday

The first half of tomorrow will actually be pretty nice. Clouds will remain, but we may get a peak or two of sunshine in the morning. By tomorrow afternoon, clouds will continue to increase ahead of our next rain maker with highs nearing 52. Scattered showers will arrive during the afternoon hours with even more showers expected in the evening and overnight hours. A thunderstorm or two is possible, but we are not expecting any severe thunderstorms with this round of precipitation. 

Friday appears to be a continuation of Thursday evening, with showers across the area. Once again, a thunderstorm or two is possible during the day, but we are not expecting any severe thunderstorms. Highs will reach near 58. Showers and a few thunderstorms will likely hang around into and through the evening hours on Friday as well. 

The GFS model shows rain remaining across the Mid-South through much of Thursday evening and Friday. (Tropical Tidbits)
*Note: a Flash Flood Watch has been issued for much of the Mid-South in anticipation of this additional rainfall. This watch is set to begin tomorrow (Thursday) at noon and continue through Saturday night. Stay safe over the next few days folks. If you come across a flooded roadway, turn around, don't drown.*

Latest hazards map outlines Flash Flood Watches in green with Flood Warnings in the hatched lime green.

Saturday

While it is still a few days away, we will need to keep an eye on things on Saturday. Showers and thunderstorms are expected through the day on Saturday, but severe thunderstorms appear to be possible for the afternoon hours (timing will become more refined as we get closer). A combination of highs reaching into the low 70s (yuck), along with plenty of moisture (dewpoints in the "muggy" category), strong wind fields aloft, and the arrival of a potent cold front leads to this severe weather potential. 

The Storm Prediction Center currently has the Mid-South at a 15% chance of seeing severe weather within 25 miles of you on Saturday, equivalent to a "Slight Risk" (or level 2/5) if this were within the first 3 days of the forecast. (NOAA/SPC)
The main concerns with this system are damaging winds, but there is a low-end tornado threat as well. Additionally, since we have seen so much rain already this week, any more rain we see (especially heavy downpours), could become problematic over some areas. Also, due to very saturated ground, wind gusts could uproot trees at a lower speed than would typically be required. Like I mentioned before, we are still a few days away with lots of details and logistics to be figured out. It is worth mentioning and definitely worth keeping an eye on. 

Sunday into next week

We will finally get a small break from all of this rainfall next week. Sunday and Monday look to be clear with the possibility of a shower or two by midweek. Temps will remain right around average, leaving next week looking pleasant to "normal". For those wondering when/if winter will come back, it looks like temperatures may return to below average by the following weekend, but that's getting a bit into la-la-land forecast.



Caroline MacDonald
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