Check out this Statesman story, if you missed it, about Austin-area weather spotters:

http://www.mystatesman.com/news/weather/forecasters-harness-enthusiasm-of-amateur-spotters/nqfNZ/

Check out this video of Austin storm spotter Tricia Clarke explaining the difference between what she does and what the people who chase after tornadoes do:

http://www.statesman.com/videos/news/a-storm-spotter-is-not-a-storm-chaser/vDkjDX/

We’re starting to hear more about the threat of severe weather, including the storm that blew in last night and the bigger one that is probably coming this evening. (We’ll post something more specific when the National Weather Service posts its mid-afternoon forecast.) But what does it mean when we hear “severe weather”?

This handy graphic from the National Weather Service helps sum up what you hear about the possibility of severe weather:

As you might expect, not everyone thinks this is the ideal way to notify the public. Joe Lauria, a meteorologist with Fox-4 in Kansas City, writes in a blog post/critique that the severe warning is overused, and that it might not mean what you think it means.

“My feeling is that the vast majority of ‘regular’ people (our audience in Kansas City) has become numb to the numbers of severe thunderstorm warnings issued throughout the course of a spring and summer especially,” Lauria writes.  His worry, he writes, is that while “I want you to be warned, and have confidence, that what we say may happen will happen when it comes to severe thunderstorms especially,” the public could become inured to the point of dismissing severe weather warnings as “blah … blah … blah.”

This could become a bigger deal in the coming months. Some forecasters say that, with the El Niño weather pattern quite possibly making a comeback, we’re in for a wetter-and-colder spring than usual. And spring is usually a severe-weather season for Central Texas.

1:55 a.m. update: A severe thunderstorm watch for several Central Texas counties, including Travis, Williamson and Hays has been extended until 4 a.m., the National Weather Service said.

1:45 a.m. update: The National Weather Service has issued a Flash Flood Warning for Travis, Williamson and Hays counties until 5:45 a.m. Wednesday.

The service is warning residents of possible flooding as strong thunderstorms and heavy rain continue Wednesday morning.

More than 2 inches of rain have fallen in Jollyville, according to rain gauges monitored by the Lower Colorado River Authority.

Authorities are urging drivers to avoid flooded roadways Wednesday. Several low-water crossings along Spicewood Springs Road in Northwest Austin have been closed. Click here for a complete map of road closures.

1:20 a.m. update: Austin and several Central Texas cities along the Interstate 35 corridor are seeing lightning, heavy rain and in some cases pea-sized hail early Wednesday morning.

Small hail has been reported along the Interstate 35 corridor Wednesday morning

Over 750 strikes of cloud to ground lightning were recorded in the last five minutes along the I-35 corridor.

In San Marcos and Leander, residents have reported heavy rainfall followed by pea-sized hail, officials said.

The number of Austin Energy customers without power doubled to 800 in the past half hour, according to the agency’s outage map.

12:40 a.m. update: A flood advisory has been issued until 5 a.m. for several Central Texas counties, including Travis, Williamson, Hays and Bastrop, according to the National Weather Service.

The service is warning of possible flooding as heavy rain and thunderstorms continue moving over Central Texas early Wednesday morning.

About an inch of rain was recorded by 12:33 a.m. and 1 to 3 inches of additional rainfall are expected in the next hours, the service said.

Central Texas is also under a severe thunderstorm watch until 2 a.m. on Wednesday, the National Weather Service said.

Forecasters said a cell of strong thunderstorms possibly carrying dime size hail and causing strong winds was moving north at about 35 mph.

7:15 p.m. update: A severe thunderstorm watch has been issued until 2 a.m. on Wednesday for several counties, including Hays and Burnet, according to the National Weather Service.

The service expects possible thunderstorms, high winds and large hail starting Tuesday night into early Wednesday morning.

The alert covers 23 counties in South and Central Texas such as Hays, Burnet, Blanco, Caldwell, and Gillespie, the service said.

Earlier: Austin, and particularly the area stretching for several counties east of Interstate 35, are probably in for thunderstorms tonight, with high winds and large hail possible, according to the National Weather Service.

A low-pressure system moving in from the west will probably collide tonight with a stew of other meteorological phenomena in the Austin area, including moist air, with those storms resulting, Weather Service meteorologist Steve Smart said. Some of the storms could be severe — winds of 58 miles-per-hour or more; hail the diameter of a quarter; and the possibility, if slight, of tornadoes.

Expect one to three inches of rain as well, with as much of five inches possibly falling over pockets of the area. That means there is a slight chance of flooding, according to the Weather Service.

A front that rolled through the Austin area yesterday dropped a little rain last night and this morning, but gave way to sunny mid afternoon weather that should give way some time in the evening to the incoming storms. The thunderstorms could last through the end of the week, according to the Weather Service.

Smart said people in the Austin area should “continue to monitor the situation closely … everyone has a responsibility to keep themselves safe, and staying informed is a big part of that.”

The Weather Service web site and Twitter feed provide updates, as does the Statesman weather app, which can be downloaded for Android or iPhone. Those who want to submit weather reports by tweeting directly to the Weather Service or use the hashtag #eWXspotter.

No, seriously — it’s time to talk about flooding.

The fall rains saturated the ground. In the past 48 hours Austin has seen more than two inches of rainfall in places such as the Onion Creek area, with nearly three-and-a-half inches in portions of the Hill Country where runoff feeds the Highland Lakes, which are nearly full. Rains are expected to continue through the week and, though the heaviest is probably behind us, even a relatively modest downpour in the right place could cause flooding.

El Niño is still spinning the Pacific, as well, and will likely continue into summer,  producing a rainy spring, most forecasters say. And — March is traditionally a month that brings severe storms.

“When I talk about severe storms, I’m referring to storms that can produce damaging winds, large hail, tornadoes and even flooding,” Bob Rose, a meteorologist with the Lower Colorado River Authority, said in a recent blog post. Add El Nino to the typical pattern and you get a strong possibility of a spring “with more severe storms than we’ve seen” in recent years.

The LCRA manages the Highland Lakes, a series of connected bodies of water that are dammed to limit flooding. There is no imminent flood threat, according to the agency. But lakes Travis and Buchanan, which rise and fall and are often used to absorb flooding, are 95 percent and 83 percent full, respectively. A good rainfall in the right place can cause a rapid rise “with very little warning,” said John Hofmann, who oversees river operators for the agency.

That can put homes and businesses along the lakes at some risk. The agency must strike a balance during a flood between releasing water down the lakes and ensuring it does not release so much it imperils downstream communities such as Austin, which the dams were built partly to protect. The agency recently advised property owners to repair docks and generally prepare for the possibility of flooding.

Other areas are also at risk in “flash flood alley.” The soil is still plenty moist, diminishing its absorptiveness and putting places such as Southeast Austin’s Onion Creek neighborhood, which was hit with devastating floods in 2013 and 2015, at greater risk. Last Memorial Day weekend, San Marcos was hit with floods so devastating that the city still has not fully recovered. That led the U.S. Department of Housing and Urban Affairs to recently take the unusual step of earmarking $25 million relief to the city, specifically, along with Houston ($67 million) and the rest of Texas ($51 million).

Though floods are generally the most devastating creation of Central Texas storms, they can bring other damaging phenomenon. One day to watch this month: March 25. That’s a day to park the cars in the garage, if possible, because the hail that tends to come in spring has a nasty habit of hitting hard that day. Three of the six costliest hail storms in Austin history, Rose said, struck on March 25.

Paul Yura is an Austin-born, Texas-educated forecaster who, naturally, wants the Lone Star state to win an annual contest near and dear to his heart.

Yura is the second-in-command of the National Weather Service office that serves the Austin-San Antonio region. The office has powerful weather-observation technology, but in weeks like this one in particular, it also relies “ground truth”: what people see, hear, feel and report — observations that will either square with the tech or show where it’s still imprecise. A big piece of ground truth comes from rain gauges in peoples’ backyards. A great way to supply that particular kind of truth comes from CoCoRaHS, which is somewhat short for Community Collaborative Rain, Hail and Snow Network.

Those are large rain gauges distributed by the CoCoRaHS (“sounds like cocoa puffs”). The nationwide network of weather enthusiasts — which you can join — is now having its own “March Madness.” It’s this contest that Yura wants to win. He wants Texas to add the most CoCoRaHS of any state this year. They look like this:

And they’re used like this:

Why does that matter? Precipitation measurements help with everything from large-scale agriculture to knowing whether you should fix your gutters. In Colorado, where CoCoRaHS began, snowfall influences a great many things. In Central Texas, it is particularly useful in understanding the flood patterns of “flash flood alley.”

“Having real rainfall reports from actual locations can really help us with our watches and warnings,” Yura said.

The gauges are available online (one site has them for $30.50) and there is a link through the web site of the National Weather Service’s Austin-San Antonio office. One you have a suitable rain gauge, you can go to the CoCoRaHS web site and register. Then, just enter rainfall totals in the database. Both you and the experts can track rainfall totals. Or, email Bill Runyon, a Texas CoCoRaHS observer and retired Weather Service employee, at Texas.CoCoRaHS@austin.rr.com.

But hurry. March Madness is already underway. As of March 6, the most recent day available, North Carolina led the way with 14 new CoCoRaHS added. Texas was in fifth place, with 11 added.

Roughly 60 low-water crossings in the Austin metro area have been closed, mostly in rural parts of Williamson, Hays, Caldwell and Bastrop counties.

Public-safety officials are urging drivers to be cautious around the crossings. The number of closures has fluctuated a bit during the day but stood at 61 at 10:30 a.m. The atxfloods.com web site lists the closures. The number has been dropping through the morning, but with more rain on the way and the soil already saturated, the number could rise again.

Driving around barricades at low-water crossings is illegal.

The National Weather Service is forecasting a 50 percent chance of rain tonight in Austin, with a chance of thunderstorms and a low around 55 degrees. But then Austin will probably do what it usually does for the SXSW visitors — turn gorgeous. Saturday will have a slight chance of showers and should be mostly sunny, with a high around 75. Sunday: sunny skies and a forecast high around 84 degrees.

Hopefully the life-size TIE fighter will stick around for the lovely weekend weather.

National Weather Service

What are the chances?

National Weather Service forecasters say there is a 40 percent chance of precipitation in Austin on Thursday night. But what does that mean, exactly? And how does that relate to the more general terms you hear forecasters use a lot, like “scattered showers”?

Troy Kimmel and the National Weather Service can explain. Kimmel, who teaches meteorology at the University of Texas, said you’ve probably got a decent sense of the basics, even if you don’t know exactly what the terminology means. But we should briefly cover this.

For instance, let’s say the forecasters call for a 50 percent chance of rain — that does not mean it’s simply a flip of a coin. Nor does it mean that the forecast calls for rain to be falling 50 percent of the time, or for rain to be falling across 50 percent of the region.

It means that if you stand in a particular spot, there is a 50 percent chance you’ll get rained on at some point over the forecast period (usually 12 hours). Within the five-county Austin metro area, a 50 percent chance of rain means that, if I stand in my yard for the entire night, I have a 50 percent chance of getting wet.

Calculating . . .

So where does that 50 percent come from? This guide from the National Weather Service explains. Basically, a forecaster decides the likelihood rain will fall, then determines how much of the forecast area will probably be rained on.

Let’s say a forecaster thinks that the odds of rain falling are 100 percent, and that rain will drop on 50 percent of the forecast area. From there, it’s multiplication: 100 percent times 50 percent (or 1.0*0.5).

Voila. A 50 percent chance of rain.

Sometimes forecasters substitute descriptions for percentages. But for those of you who like forecasts expressed as percentages can match them up. The Weather Service’s terms go as follows:

0 percent = no chance of rain, anywhere in the forecast area
10 percent = slight chance of isolated showers
20 percent = slight chance of rain
30 percent to 50 percent = a chance of scattered showers
60 percent to 70 percent = showers are “likely” and/or will be “numerous”
80 percent or above = stay inside, otherwise your pant cuffs are gonna be damp

Okay, I made that last one up. The term is actually “categorical.” But that means your pant cuffs are gonna be damp.

National Weather Service

Wildflowers along the Round Rock West greenbelt. Henry Huey for Round Rock Leader.

The plentiful rainfall of last year should mean a spectacular wildflower season in Central Texas.

Um — probably?

That is the basic assessment of the University of Texas’ Lady Bird Johnson Wildflower Center. The soil in Central Texas is still moist from the fall rains, which is good for wildflowers. Some of which have popped up early this year to and are waiting for a little more warmth and moisture to bloom, according to the Wildflower Center.

But when the great spigot in the sky turned off late last year, the lack of rain — ahem — dampened the otherwise optimistic outlook about wildflowers. That February of dry, often spectacular weather spurred by the Godzilla El Niño’s siesta might have come with a downside, after all. The rain that finally fell in late February and early March should help. But it did not fall evenly across the region, so wildflowers in places that got significant rainfall should do better than those in places that didn’t.

“The trick is always the weather,” said Andrea DeLong-Amaya, director of horticulture at the Wildflower Center.

The spring weather to come could help. Winter is over — both in the physical sense, and according to one official definition — but most forecasters say Texas is probably headed for one more damper-and-colder-than-normal spell before the Godzilla El Niño is spent and returns to the ocean from whence it came. The best thing for spring wildflowers is intermittent rains as they’re growing and prepping to bloom, according to Wildflower Center horticulturists.

And don’t worry about a cold snap. If one does blow in before El Niño bows out, DeLong-Amaya said, it probably would not damage native wildflowers already putting out buds.

Austin styles itself the allergy capital of the world, and with cedar fever, something to be allergic to nearly year-round and the city’s frequent appearances near the top of top-10 lists, it’s an understandable assumption.

But this spring Austin has ceded the title, according to the Asthma and Allergy Association of America. The association recently released its “Spring Allergy Capitals” ranking. And Austin (for once) is pretty far down the list, finishing 58th, between New Haven, Conn. and Scranton, Pa.

The spring allergy capital, according to the association, Jackson, Miss., with Memphis just behind. Opinions may differ about whether San Antonio is winning the taco war against Austin — the mayors have declared a cease-fire but hostilities are still simmering — but allergy association list suggests the Alamo City is winning the allergy rankings, finishing 21st.

Most lists rank cities’ allergy problems for spring or fall. But eossinus.com takes Austin’s cedar season into account and consistently places Austin in the top 10. Because cedar season sucks.

Congratulations to Scott Fisher, Mark Murray, Jim Spencer, Chikage Windler and the rest of Central Texas’ meteorologists. You are apparently the least-creepy people out there.

Researchers at Knox College, in Illinois, recently conducted what might be the first-ever empirical study of creepiness. Their work included ranking the public’s perception of how creepy various professions are, from least to most. Meteorologists finished as the least creepy, ahead even of teachers, farmers, college professors and doctors. (Writers finished in the middle of the pack, right alongside actors, so, um … yay us?)

That’s the good news. The most-creepy professions, according to the study, were taxi drivers, funeral directors, sex-shop owners, taxidermists and — wait for it; wait for it — clowns.

Because of course clowns are the creepiest. Of course they are. How could clowns not be the creepiest? What could be creepier than clowns?

The researchers write that creepiness is more than just a response our brains developed to the threat of violence: “It is our belief that creepiness is anxiety aroused by the ambiguity of whether there is something to fear or not and/or by the ambiguity of the precise nature of the threat (e.g., sexual, physical violence, contamination, etc) that might be present.”

Also — if a profession inspires a horror novel and has terrified whole generations, it’s probably creepy.

Nothing to fear with meteorologists, though. The study does not differentiate between television forecasters and the roughly 90 percent of meteorologists who are not on air, but we’ll assume our friends at the National Weather Service are on the not-creepy end of things. (I can personally confirm that Paul Yura and his crew are among the nicest people out there, and most definitely not creepy.)

Still, it’s safe to suppose the study respondents had television personalities in mind. And they scored exceptionally well. That is probably in part because television forecasters tend to be well groomed (greasy hair and odd dress are among the characteristics of a creep, according to the study), they generally don’t make a habit of staring at people before interacting with them (another characteristic of a creep) and they don’t have a peculiar smile (they’re on television and so tend to be telegenic).

The creepiness rankings are based on 1,341 peoples’ responses, most of which came from American women. A Washington Post blog focused on respondents’ answers about what hobbies are creepy. Bird watching was listed among the creepiest, a correlation the blog attributes in part to the use of binoculars. Another blogger suggests dealing with this perception by turning the binoculars away from windows.

The creepiest hobby, according to the study, is “collecting things,” particularly insects and reptiles.

If you think the last five years of weather were extreme in Texas — with the pendulum has been swinging between drought and deluge without much in between — you may want to rethink what extreme means.

State climatologist John Nielsen-Gammon says there is a decent chance those swings will be the norm over the next few years. A inkling of what that might be like: in 2010-11, Texas had the driest 12-month stretch on record, while 2015 was the wettest year on record. And even within that wet 2015, the state swung from drought to downpour to drought to downpour. We had what many meteorologists called a “mini-drought.”

Even this El Niño season, which was supposed to bring cooler and wetter weather than usual, has been a sort of pendulum. It started with an extraordinarily wet fall and early winter that brought major flooding on back-to-back weekends in the Austin area. Then the skies dried up, even as El Niño persisted. Austin was in the midst of its fourth-driest start to the year in history before the overnight rain late February rain ushered in another wet period and a spring that has, with the exception of a few hot spells, been relatively wet and cool, as El Niños tend to be. That is likely to continue into summer, said Nielsen-Gammon, a professor of atmospheric sciences at Texas A&M University.

But wait: there’s yet another reversal of the pendulum threatening in the near future.

The El Niño will lose strength as summer approaches, Nielsen-Gammon said, with the unusually warm surface temperatures in the Pacific that characterize an El Niño returning to normal. Some time around mid-summer, the Godzilla El Niño, as it’s been called, should finally succumb to the fatigue it must be feeling after so much activity.

There is a 50-50 chance that the Godzilla El Niño will give way this fall to his evil twin, La Niña. That is a weather pattern that tends to make Texas hotter and drier — as it was at the height of the drought in 2010 and 2011.

Some Central Texans have acclimated to the general pattern of wild swings. But Nielsen-Gammon has a reminder for those who’ve become inured to what we’ve experienced: “Many other parts of the country are not so heavily influenced by El Niño and La Niña as it is in Texas, so their climate tends to be more regular from year to year.”

Early release

Lockhard ISD will release students three hours early on Monday, April 18.

School closures

Del Valle ISD has canceled classes for Monday April 18.

Hays Consolidated Independent School District has canceled classes for Monday, April 18.

Smithville Independent School District has canceled classes for Monday, April 18 due to growing concerns with weather and road conditions.

Thrall ISD has canceled classes for Monday, April 18.

Wimberley Independent School District has canceled classes for Monday, April 18.

School delays

The Austin Independent School District class day will start two hours late on Monday.

Eanes ISD schools will start two hours late on Monday.

Leander ISD schools will start two hours late on Monday.

Manor ISD schools will start two hours late on Monday.

Pflugerville ISD schools will start two hours late on Monday.

KIPP Austin classes will start two hours late on Monday.

Wayside Schools will also have a two-hour delay.

Austin Community College will also delay start times until 10 a.m., according to an ACC press release.

Round Rock and Lake Travis schools will start on time.

The Georgetown school district does not anticipate a delay. But Hutto will have no school, owing to an unused bad weather day in the winter; as always, trust the Hippos to plan ahead.

Consider this a TV news counterpoint to the cringe-worthy footage of a Houston reporter attempting to help a man out of his car.

On Monday, Houston Fox 26 meteorologist Mike Iscovitz wanted viewers to know they should not drive through flooded areas. He had seen an awful lot of them that morning. But, he noted on air, a woman on Facebook told him it was not that people had been lulled into a false sense of security. They were worried they would be fired.

“And I gotta tell you man, if that happened, you better take it to (reporter Emily) Aken or something, because we are going to kick somebody’s behind if they fire you because you didn’t head out on a deadly flooded street,” Iscovitz said.

“I know a lot of people have that fear,” he continued.  “It’s a tough decision, I know there’s a lot of moms and dads out there trying to pay the bills … but if anybody got fired because they didn’t go out to work in this, call us … and we will expose that person on the air in front of millions of people and embarrass them.”

The segment is attracting attention around the country. The clip posted by Fox 26 Houston has been shared nearly 20,000 times and garnered more than 1,800 comments.

Green skies are gonna clear up, put on a happy face …

Strong to severe storms are expected in the Austin metro area late Tuesday afternoon and evening. Large hail and damaging winds main threats but forecasters aren’t ruling out the possibility of a weak tornado.

You should stay inside during tonight’s storms. But if you’re out as they approach, or get caught in it, there is a chance — albeit small because it will be nighttime — that you could see something strange: green skies.

It’s a phenomenon so old that sailors from ancient Greece wondered about it, but it’s one that has been reported so often that meteorologists are certain it exists. But as far as the science of meteorology has progressed since ancient Athens’ heyday, it’s still not clear why the sky turns green, according to weather experts at Texas A&M’s Department of Atmospheric Sciences.

“The most popular theory is that thunderstorms contain a lot of water – often in the form of hail – and this water or ice tends to scatter green light during the strong updrafts that occur in severe storms,” said Brent McRoberts, a postdoctoral research assistant in the atmospheric science department. “That’s why many people say the sky appears green right before a hailstorm.”

The sky can also turn green before a tornado, McRoberts said. Green skies do not necessarily mean a tornado, though, and they may or may not contain hail. But they are almost always a sign that bad weather is on the way, said McRoberts, adding that the main point to keep in mind with green skies is: “You should take cover immediately.”

The Statesman’s Amanda O’Donnell posted a blog earlier this month about how drone video taken near Wylie, where grapefruit-sized hail caused severe damage to many homes and vehicles earlier this week, made social media users curious about the strange color of the clouds. The Weather Channel reported that the green glow was from hail about to fall.

On Wednesday — as Lower Colorado Meteorologist Bob Rose gave his summer forecast — Rose also noted a few forecasters’ projections for the number of major storms that could hit the United States later this year.

Colorado State University tropical-storm forecaster Phil Klotzbach is anticipating “average activity” — 12 named tropical storms, five hurricanes and two major hurricanes.

Accuweather, Rose said, is projecting not only a higher number of named storms (24 named storms) but also more hurricanes: eight total, with four falling into the major category and three making landfall.

Joe Bastardi gives himself a little more wiggle room: he is projecting 11 to 14 named storms, six to eight hurricanes and two to five major hurricanes.

Rose said that national forecasts call for a “brutally hot summer” in most the country, as the Benevolent Godzilla El Niño that brought so much rain to Central Texas gives way to its bizarro twin, La Niña. (Do they have air conditioning in Oklahoma?)

Image from the Weather Channel

A phenomenon with a sci-fi-worthy name could keep Central Texas dry for the next few days.

The midweek “Omega Block,” as the Weather Channel is calling it, should deflect the storms that the Godzilla El Niño will probably continue to hurl toward Central Texas. Basically, a system of low pressure will settle over the East Coast, while a separate, roughly parallel system of low pressure will settle over the Left Coast. In between, according to the Weather Channel, “will be a ridge of high pressure that settles over much of the country.” Weather patterns that generally move from west to east — say, those thunderstorms that have been soaking Central Texas — will be slowed or stalled for several days, essentially being “blocked” by this low-pressure/high-pressure combo that’s also contorting the jet stream.

So if an El Niño-inspired storm comes spinning off the Pacific this week, it will run into the block before it hits Central Texas, according to forecasters. Thus, a break from the perhaps-heavy rainfall that could be dousing the region until summer.

Why do they call it an “omega block?” We’ve covered the “block” part, and as to the “omega”: Think of the shape of the jet stream being drawn from the Southwest, up the above the Plains, then back down the eastern third of the country. Now extend both ends out a bit and then curl them back up again in the ocean (to complete the full pattern). It’s sort of an upside-down U shape: or this Ω, which is an omega (a Greek letter).

No word if Michael Crichton is planning a sequel.

I hate to break it to you, Internet denizens, but if something goes horribly wrong this month, it probably is your fault, despite what the astrologers are telling you.

Yes, Mercury is in retrograde, and that is an interesting astronomical phenomenon, one that we’ll get into in a moment. But first: no, there is no scientific evidence linking that phenomenon to your flat tire, bad date or that lost Japanese satellite. Astrologyzone.com may claim understanding retrograde “can allow you to increase your productivity and avoid at least some of the frustration they can bring about,” as well as asserting that Mercury “rules all types of communication” including speaking, reading and negotiating contracts, but the web site cites no scientific source for why your May is going to be so messed up. You should, as they say, consider the evidence before blaming the Mercury retrograde for the next death of a prominent singer.

There is some good astronomical science here, though. Basically, Mercury will appear to be moving the wrong way across our sky until May 22. It looks, to casual observers, as if it reversed its orbit and now goes around the sun in the other direction.

That’s not actually the case. It’s an illusion caused by the fact that Mercury, being much closer to the sun, rotates around it faster than the Earth does. Several times a year, the relative position of the Earth and Mercury make Mercury appear to be going backward. NASA explains it with the following example:

“Start out standing side by side with a friend. Have a friend walk forward slowly. Now you walk forward at a faster speed. Watch your friend and think about how they are moving relative to you. At first, they move away, then as you pass them, they APPEAR to be moving backward relative to you – even though they are still walking forward.”

(A few years ago NASA put together a graphic about how this phenomenon

Imagine courtesy NASA. This is actually a depiction of Mars retrograde, but the general explanation holds.

works with Mars.)

This particular Mercury retrograde — apparent retrograde, it should be emphasized — comes with another bit of interesting astronomical phenomena, said Keely Finkelstein, a research associate and lecturer in the University of Texas’ Department of Astronomy. Mercury will pass between Earth and the Sun on May 9, an event that occurs only 13 times every century.

“You’ll need decent equipment to view the transit, but this will be a real astrophysical event, where interesting science can be done,” Finkelstein said. Mercury will appear to be a small dot against a very bright object, so the naked eye won’t work, though NASA will be streaming a live program through its TV station and Facebook page.

This Mercury retrograde will last until May 22. Some people suggest waiting until the retrograde is over to move, apply for a new job or buy things. The advice is debatable. Until then, I guess it couldn’t hurt to follow one piece of #mercuryretrograde advice, if only because this is Austin:

Courtesy National Oceanic and Atmospheric Administration

The last year was a weird one for weather. Epic drought gave way to deadly floods a year ago, which gave way to a mini-drought and then again to floods, and then to one of the driest starts to a year and now an unusually wet period.

Now we’re in the month with the most severe weather. So it’s no coincidence that the latest forecast calls for severe storms in the Hill Country late Tuesday.

May has been the month that brings the most tornadoes, hail and severe thunderstorms, according to data provided by Paul Yura and his crew at the San Antonio-Austin branch of the National Weather Service. It’s not even close:

The threat posed by tornadoes and high winds pale in comparison in Central Texas to that of flooding. Still, tornadoes have occasionally struck with devastating effect, such as on May 27, 1997, when a tornado tore Jarrell apart, leaving 27 people dead.

And, contrary to conventional wisdom, they actually might hit the same place twice.

“The belief that a tornado will never hit the same place twice is totally wrong,” said Brent McRoberts, a postdoctoral research associate in the Department of Atmospheric Sciences at Texas A&M University  “There are numerous cases of a city or town being hit several times. Proof of this is the small town of Codell, Kansas.  It was hit by a tornado on the same day for 3 straight years – On May 20 of 1916, 1917 and 1918.  The 1916 tornado is believed to have been a fairly strong one, but the worst was the 1918 storm that we now think was probably an F-4 with winds of at least 150 miles per hour and it destroyed most of the town. Although some of the buildings were eventually rebuilt, the town never fully recovered from the 1918 tornado.”

This has been an unusually wet spring, partly because of the El Niño swirling in the Pacific. Each month has seemingly brought warnings of look out for severe weather, particularly thunderstorms.

Despite the Jarrell tornado, May actually does not produce the most severe tornadoes in Central Texas, said Troy Kimmel, a forecaster and instructor at the University of Texas. April tends to produce

Where do tornadoes come from? Science apparently doesn’t have a complete answer. Warm, moist low air from the Gulf sweeps west, hitting high dry air from the Rocky Mountains. Storms form. This video from Scientific American does a good job of explaining the leading hypothesis about what happens from there.

So why do certain cities, such as Oklahoma City, which has been hit more than 140 times since the 1890s, get nailed more often?

“There seems to be no explanation,” McRoberts said, “other than Oklahoma City is smack in the middle of Tornado Alley, and conditions in and around the town are perfect for tornado formation during the spring months. Dallas-Fort Worth, Wichita Falls, Kansas City, Tulsa, Lincoln and Omaha are just some of the cities that have been multiple times through the years by tornadoes.”