MWN Lightning Round: On heat, haze, and high-latitude auroras

It's once again time for the MWN Lightning Round, in which we hit a few different topics in rapid succession and let you get on about your day, armed with insightful new information!

Heat Builds as High Pressure Expands

A hot day was once again tempered just a bit by thunderstorm-induced cloud cover today, although high humidity values still pushed the heat index over the century mark this afternoon. With a large upper-level ridge continuing to expand the next two days, fewer storms are expected, thus we expect highs to reach their full potential in the mid to upper 90s. Combined with dewpoints in the 70s, we'll see heat index values near or exceed the danger level of 105° Tuesday and Wednesday. Practice the heat safety tips below and check out our complete guide to staying safe on a Memphis summer day!

Remember also that successive hot days have a cumulative effect and heat illness can occur more quickly in the same heat when there is less opportunity to recover from previous days. Overnight lows in the upper 70s can provide little relief.

Walk Like an Egyptian - in the Saharan Dust

If our previous article on a lone thunderstorm casting a shadow into Alabama wasn't enough to convince you, we have another example of how weather hundreds (or thousands!) of miles away can affect an area.

You may have noticed on your way home from work today that the sun seemed filtered and the sky hazy when looking off in the distance. Although there was a veil of high cirrus clouds from earlier thunderstorms, there also was a noticeable haze. It wasn't just summertime urban air pollution you were seeing. In fact, it was a thin veil of dust from the Saharan desert!

The dust was transported across the Atlantic Ocean into the southern U.S. on trade winds. This is the same dust that is sometimes mentioned in Atlantic tropical discussions as a deterrent to tropical organization. The dust was even mentioned in today's air quality forecast discussion issued by the Shelby County Health Department:

A combination of local emission sources and transported Saharan dust are creating very hazy skies as the particles are easily remaining suspended in the very humid conditions over the Mid-South. 

We've written one other time about dust that actually accumulated on vehicles and outdoor objects - that was April 2014 from dust storms in the Central Plains. This dust has a much more exotic origin and won't be enough to create a coating, but is reducing visibility just a bit. The image below shows the concentration of Saharan dust over the Atlantic Ocean and into Central America and the southern U.S.

Saharan dust thickness at 7pm Monday is shown in this model graphic from WxBell Analytics.

Storms Originating on the Sun Reach Earth

Another long-distance connection - this one farther than the Sahara to the Mid-South - is occurring today as well. A geomagnetic storm on the sun hurled multiple coronal mass ejections (CMEs) at the Earth over the past couple of days. Today, the strongest of these reached our magnetic field and prompted space weather warnings indicating a blackout of low-frequency radio waves over North America. The "Planetary K-Index" is a scale used to measure the strength of these waves, which are then tied to expected impacts. The index spiked this afternoon at a magnitude 8. This is strong enough to have some effect on power grids, radio waves, and communication with aircraft, as well as on orbiting spacecraft.

The Planetary K-Index spiked at a level 8 this afternoon as a strong CME bounced off the earth's magnetic field.
Image courtesy SWPC.

In addition, strong CMEs tend to produce more vivid aurora borealis, which can be seen further south from the poles as they get stronger. Some northern U.S. locations could have a decent chance of seeing the aurora tonight if skies are clear. While there is some chance of aurora dipping farther south, it is highly unlikely we'll see the northern lights in the Mid-South.

An aurora map produced by KATV meteorologist Ryan Vaughan (@ryanvaughan on Twitter) indicates a low probability of  aurora viewing into the central U.S.

More really cool graphics, animations, and information can be found on the Space Weather Prediction Center's homepage.

Erik Proseus
MWN Meteorologist

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This rain won't clean your car off!

We've received several reports from social media this morning of sprinkles on cars and other outdoor objects that leave behind a residue, or dust, after evaporating. In fact, I had noticed the same thing on my windshield yesterday and assumed pollen, but didn't look closely. The pictures received though we definitely NOT pollen. It was dirt.

Morning sprinkles evaporated, leaving these dust spots on Carissa H.'s car (@rissa424)

A couple of thoughts immediately came to mind. First (and this one seemed less plausible than the second) was dust/dirt from Arkansas that was sucked up by tornadoes on Sunday and deposited downstream. I had heard of such a thing happening, but the timing (2 days later) and wind patterns didn't quite make sense. The second option was dust from the plains carried east on upper level winds. Knowing there was a massive trough over the middle of the country and a strong jet stream blowing from west to east across our area, I knew that option was more plausible.

Sure enough, after a little digging and checking in with the NWS to get their opinion, I'm fairly confident that it is indeed dust from the plains. For the past few days, strong and gusty north wind has prompted dust storms across the central and southern plains. In fact, the NWS watch/warning map below shows Wind Advisories and High Wind/Dust Storm Warnings in effect right now across the plains.

Then, I got a text from a family member who happens to be on the interstate in the TX panhandle describing vicious wind and "raining mud!" Sure enough, dust storms were in progress and light rain was dropping the dirt back to earth! Dust storms are known to eject fine dust particles thousands of feet into the air where they are carried by the currents downstream.  Below is the current upper level (about 35,000') jet stream courtesy of an awesome animated web tool called the "Wind Map."  The large L is the low pressure trough that the jetstream wind is flowing around. Key points are labelled.

So you can see that as the dust is picked up and ejected into the upper levels of the atmosphere, it gets carried downstream and deposited or rained out of the atmosphere! The light rain falling this afternoon is not going to be good at washing pollen off your car, but instead could result in a trip to the car wash thanks to strong wind and plains dust that is transported all the way to Tennessee!

Erik Proseus
MWN Meteorologist

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Follow MWN on Facebook, Twitter, and Google+
Visit MemphisWeather.net on the web or m.memphisweather.net on your mobile phone.
Download our iPhone or Android apps, featuring StormWatch+ severe weather alerts!

Dust storm hits Riyadh, Saudi Arabia

Now here's a type of weather we don't see around these parts! Embedded below is a clip taken from a cell-phone video camera by a person in a high-rise office building as a major dust storm blows into the capital city of Riyadh, Saudi Arabia. Pretty amazing! It is not a time-lapse, so the dust actually envelopes the city that fast. Visibility was reduced to about 500 meters (about 1650 feet, or 1/3 mile) and the city was said to have taken on an orange hue, which is evident in the second video below. For reference, the skyscraper in the first video is the Al Faisaliyah Center, which according to Wikipedia is 267 meters tall (875 feet, 44 stories).