Spring begins this Sunday, March 20, at exactly 6:33 a.m. Why is spring said to begin at such a precise time, regardless of day or night, snow or warmth? Because at that moment, the Sun passes over Earth's equator heading north, an event called the vernal (spring) equinox. The Sun appears to roam north or south in our sky, depending on the time of year, because of what some might consider an awkward misalignment of our planet. Earth's axis is tilted about 23.5 degrees with respect to our orbit around the Sun.

So when we're on one side of our orbit, the Northern Hemisphere is tipped sunward and gets heated by more direct solar rays, making summer. Six months later, when we're on the opposite side, the Northern Hemisphere is tipped away from the Sun, the slanting solar rays heat the ground less, and we get winter. For a skywatcher in Alabama, the effect is to make the Sun appear to creep higher in the sky each day from late December to late June, and back down again from late June to late December. An equinox comes when the Sun is halfway through each journey. This celestial arrangement makes several other noteworthy things happen on the equinox date. For example:

*Day and night are almost exactly the same length; the word "equinox" comes from the Latin for "equal night." Day and night are not exactly 12 hours long at the equinox, for two reasons: First, sunrise and sunset are defined as when the Sun's top edge -- not its center -- crosses the horizon. Second, Earth's atmosphere distorts the Sun's apparent position slightly when the Sun is very low.

*The Sun rises due east and sets due west (as seen from any location on Earth). The fall and spring equinoxes are the only times of the year when this happens.

* In the Southern Hemisphere, March's equinox marks the start of fall, and the September equinox marks the start of spring. (Summer for kangaroos begins in December, their winter in June). Not even the most diligent jet setter could manage to live in an endless summer, but by traveling between hemispheres at just the right time, you could live in an endless spring AND summer.

Thanks to skyandtelescope.com for input to this article.

Posted the Tuesday afternoon video update a little early today:

http://beta.abc3340.com/weather/video.hrb

Have to run to Tuscaloosa early this afternoon to see the kids at Woodland Forrest Elementary, and then I have a promo shoot after that, which means no time for this if I wait.

Still working on that March Madness bracket... tough choices.

COMING WEATHER HIGHLIGHTS:

*Cold and wet tonight and tomorrow thanks to a surface low moving from the Louisiana coast to Dothan and on to Savannah. Just think what would happen if we had some Arctic air around here. But, cold and wet is the story with temperatures tomorrow holding in the 40s in most places all day. The colder readings will be on the eastern side of the state. Rainfall amounts of 1/2 to one are likely here, with the heaviest rain near the coast where thunderstorms will be involved.

*The upper trough swings through here on Thursday with a chance of more showers during the day.

*The cold front Friday night/Saturday morning looks drier and drier with each model run. I do still expect showers, but hopefully the drier look will be correct and amounts will be light. Sunday looks nice for the first day of spring with ample sunshine and a good chance of seeing 70 degrees.

*The 12Z GFS continues to advertise a potent severe weather setup across Alabama and the deep south early next week on Monday, March 21. We will have to keep out eyes on this.

Have to hit the road... be back in the office later today as we get set for the ABC 33/40 weathercasts at 5:00 and 6:00.

The Tuesday morning web video is on the server:

http://beta.abc3340.com/weather/video.hrb

As you expect in March, a very busy map this morning with lots of issues ahead:

*Rain should increase over the state during the day today, becoming widespread tonight as a surface low forms on the Gulf coast. Tomorrow morning, the low should be near Panama City with the most widespread rain east of Alabama by midday. A low level eastery flow around the top of the surface low could pull in some pretty chilly air into east Alabama tomorrow. Might be a day where Anniston, Heflin, and Centre will hold in the 40s all day, while the western counties reach the 50s. Cool and damp statewide.

*THURSDAY: An upper trough swings through, and it should squeeze out some light rain or a few showers. I sure have to think the NAM is too cold (it shows a high of only 44 on Thursday), but it will be a cloudy and cool day. Snow fans get excited about the thickness scheme on the NAM and the GFS, and you can bet we will have to watch for dynamic cooling, but for now we won't even go in that direction.

*FRIDAY NIGHT/SATURDAY: We have highlighted a good chance of rain and storms in this period, but the latest GFS looks drier with only limited moisture with a northern branch system and the associated cold front. We don't change the forecast at this point; we will take a look at the 12Z run and see what it looks like.

*EARLY NEXT WEEK: If the 06Z run is correct, a potent severe weather threat could be setting up here in about a week. A deep surface low over southern Missouri along with a strong negative tilt trough, and the Gulf of Mexico wide open. We will sure keep an eye on it.

*END OF THE MONTH: The GFS suggesting we might have the warmest weather so far this year around the end of March, possibly low 80s here. Summer must not be too far away.

*SPRING BREAK WEEK: Like this week, if you are headed to the beach or the Great Smoky mountains (or hanging around here), you will have to deal with occasional storm systems. That system early in the week could bring some active storms late Monday or Tuesday, with another system toward the end of the week. Of course, there should be one or two nice days between those systems....

Headed to Tuscaloosa today to see the kids at Woodland Forrest Elementary school...