Monday, June 22, 2015

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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