1st major severe storm threat begins this weekend

The same slow-moving low pressure system that is expected to bring several rounds of heavy rain and thunderstorms to the Carolinas next week is forecast to produce the nation's first major outbreak of severe weather this weekend.

One round of showers and storms is predicted to affect the South on Thursday evening. That area of precipitation also might trigger some severe storms Friday afternoon and evening in eastern North Carolina.

But the big event will take shape Saturday and Sunday.

A deep low pressure system is forecast to move very slowly across the Midwest both days. The center of that low is predicted to move -- at a snail's pace -- along a stationary front that will be struck in an east-west line across Maryland, Virginia, West Virginia, Kentucky, Missouri and Kansas.

Areas to the north of that front are in store for a lot of rain and chilly temperatures through the middle of next week. Areas to the south, like the Charlotte area, figure to get milder temperatures but several clusters of showers and potentially heavy thunderstorms.

Evan Myers, an Accu-Weather meteorologist, said Thursday that the first major tornado outbreak of the year is expected to develop Saturday in northern Texas, Oklahoma, Arkansas, Missouri, Kansas and southwest Illinois. By Tuesday and Wednesday, that area will reach the Southeast.

"Indications are it will move slowly," Myers said.

NOAA's Storm Prediction Center in Oklahoma also is warning about severe weather caused by the slow-moving low. It is mentioning the chance of strong tornadoes this weekend in the Deep South and the Mississippi Valley.

By the time that precipitation area reaches the Southeast later next week (Tuesday and Wednesday, most likely), the nature of the threat is expected to change from tornadoes to a pattern of damaging wind gusts, hail and flash flooding.

Next week figures to be very interesting for the Carolinas.

Incidentally, meteorologists agree that once the low pressure system eventually wobbles off the East Coast late next week, a pattern of summer-like weather will take hold in the South. The week of May 5-11 could produce a lot of 80s in the Carolinas.

What to wear at the Wells Fargo: How about a rain coat?

The feature story on the Charlotte Observer's website Wednesday afternoon is a piece by Cristina Bolling on what fans should wear at next week's Wells Fargo Championship golf tournament.

Cristina's story focuses on what a well-dressed golf fan will wear to Charlotte's PGA tournament.

My suggestion: Rain gear.

By nearly every prediction, it looks as if we're in for a very wet week -- with the emphasis on "very."

The computer models insist on creating a blocking pattern over Greenland and northeast Canada. That will stall the west-to-east movement of weather systems across the United States. And considering that a slow-moving low pressure system is expected to be in the eastern half of the country next week, it's a recipe for days of rainfall.

The good news is that it apparently won't be a cold rain -- at least not in the Carolinas. Chilly temperatures and the rain could be confined to areas north of the Mason-Dixon line. High temperatures each day look to be from the upper 60s to the mid 70s.

But the upper-level low pressure system will barely move during the week, becoming a cut-off low pressure system that is left outside the jet stream and becomes stuck over the eastern United States. In those kinds of situations, we tend to get frequent periods of showers and thunderstorms. Severe thunderstorms usually don't develop in those patterns, but storms with heavy rainfall are more likely.

The exact timing is a bit uncertain, but it appears as if showers will begin to move into the Carolinas on Monday, with the amount of rainfall increasing Tuesday and Wednesday. It could be the end of the week before the cut-off low finally meanders off the East Coast.

Charlotte's rainfall since early December is several inches above average, and it appears as if we'll add to the surplus next week.

An update on this Easter weekend washout

It's late Saturday afternoon, and an update on what to expect for Easter might be in order -- especially many people will be attending sunrise services in the morning.

Obviously, the weekend so far has been a miserable washout. In fact, the spring break week for children in the area has been largely a bust.  That also goes for the original forecast for this weekend. If you'll remember, we were talking as late as Wednesday about 70 degrees (with clouds) for Saturday and sunshine and 70s for Sunday.

A low pressure system that was stronger than expected and slower than expected has ruined all that -- just like a cold high pressure system over New England ruined most of the outdoor plans for students last week.

We're approaching 2 inches of rain from this weekend's storm, and we've passed 4.5 inches for the month. That means April will make five of the last six months with above-average rainfall for Charlotte. Let's remember that the first time we have five straight dry, warm days, and someone starts talking about "drought."

So what about the rest of the weekend?

Easter Sunday will not start great.

Since the left-over moisture from the very slow-moving low pressure system will still be around in the morning, and we'll still have a cool northeast flow, I think daybreak will be cloudy, misty and cool (upper 40s).

Figure on the clouds hanging around all morning, then slowly breaking in the afternoon. It won't be a warm day, either. Those morning clouds and the northeast breeze will limit highs to the mid 60s, and that might be a stretch.

Temperatures today were more than 20 degrees below average, and they'll be 10 degrees below average Sunday, too.

In short, it'll be a great indoor Easter.

It looks like conditions turn warmer next week, and since the students will be back in school and most of us back at work, that figures.

Back in March, we said spring would be slow to arrive this year in the eastern United States. And it has played out that way. Cold air has been persistently lodged in the northeastern part of the country since January. Here in the Southeast, we've had brief warm spells over the past few weeks, but cold high pressure repeatedly has wedged its way into the Carolinas, and we've been visited by these cold rainy systems.

Until the pattern breaks, expect more of the same.

The good news is that the pattern has kept storm systems from moving up the Tennessee and Ohio valleys, for the most part, and that has saved us from severe weather outbreaks.

Are we safe from frost? What about Easter weather?

The two questions I've been hearing a lot in recent days concern frost and the advance outlook for Easter weekend.

As you well know, there are dangers in giving definitive answers in either case.  The date of the last frost is an average, with wide variations. And predicting the weather for Easter, which is 10 days away, taxes the limits of meteorology.

But let's talk about it.

The average date of the last frost in Charlotte is April 2, according to numbers I found, although I also discovered a listing that said "somewhere between April 1 and 10." By either of those averages, we are safe.

But you'll be taking a chance, as you know, by planting anything tender vegetation now.  It's a small chance, but we've had freezing temperatures as late as April 25 in recent years (31 degrees on April 25, 2005). And it doesn't take freezing temperatures at Charlotte Douglas International Airport for frost to develop. If it's in the mid 30s at the airport, there easily could be frost nearby.

Looking at the last several years, here are the last dates of chilly temperatures:

2013 ... April 20 (39 degrees)

2012 ... April 24 (39 degrees); also April 13 (34 degrees)

2011 ... May 5 (36 degrees)

2010 ... April 29 (38 degrees)

Computer models show some chilly air infiltrating the Carolinas next week after the current warm spell is broken. If skies are clear at night, we easily could see some mid and upper 30s in the region. However, there are signs that the period from Wednesday through Good Friday, the three-day period of the coolest weather, could be cloudy. That would keep overnight readings warmer than they otherwise might be.

About Easter ...

Several long-range forecasts show the same thing next week -- cloudy and rather cool temperatures from Wednesday through Saturday, the day before Easter.  Predicted highs are in the mid and upper 60s, and there could be some rain around Good Friday.

But the good news is that most of those forecasts show clearing skies and much warmer temperatures for Easter Sunday, with highs around 80 degrees.

The amazing power of the cold air wedge

We talked for several days about the revved-up weather that we were expecting today. It was to be a one-two punch of heavy rain in the morning and then severe storms in the afternoon and evening.

The first part developed pretty much as expected.  But the severe storms apparently will be a no-show for the Charlotte region.

And it's all because of the amazing power of the cold air wedge.

A quick review ... the cold air wedge is the name given a condition in which cool air becomes trapped in the Piedmont and foothills of the Carolinas and Virginia.  Typically, an onshore wind brings the cool air into the Carolinas. Since cold air is heavier than warm air, it sinks -- and becomes trapped against the mountains.

Sometimes we get what's called an in-situational cold air wedge.  The air is relatively cool and quite dry. Then precipitation from a low pressure system arrives. As it falls into the dry air, it cools temperatures even more.

Often in these situations, a warm air mass is lurking somewhere to the south or southeast. Occasionally, a warm front plows into the cold air wedge and erodes it.  Usually, the cold air wedge holds on. When that happens, Charlotte is stuck in the soup, with chilly, damp conditions.

Weather computer models frequently err in predicting the erosion of the wedge. They tend to forecast the wedge will break down, but that often doesn't happen. Today is the latest case in point. The warm air that several of the models predicted would reach the I-85 corridor by Monday afternoon hasn't even pushed west of I-95.

The cold air wedge is a real pain for meteorologists, whose forecasts are busted when the warmer air never arrives.  Charlotte was supposed to have reached the middle 60s by late Monday afternoon. Barring something unexpected, temperatures will remain stuck in the lower 50s.

At 3 p.m. Monday, it was in the upper 70s along the coast and about 40 miles inland. Temperatures were 30 degrees colder along the I-85 corridor. And to the west of the mountains, temperatures were in the low and mid 60s, on the other side of the trapped air.

Monday could be a wild, wacky weather day

Rainy days and Mondays are not a big hit with most people, especially when they're one and the same.

But Monday could be a very interesting day for weather geeks, especially if you live in the southern portion of the Charlotte region.

A big storm system that covers hundreds of miles and much of the South will affect the Carolinas on Monday, bringing plenty of rain and a chance of severe weather in some places.

If you live to the northwest of Interstate 85, chances are Monday will be a chilly, wet day -- and not a whole lot more. Places like Hickory and Statesville probably will stay in the lower 50s for much of the day, with a steady, soaking rain falling.

To the southeast of I-85 ... that's another story.

First, for the basics.  The center of low pressure will move northeastward Monday, crossing Tennessee and into Kentucky. A cold front that pushed south of Charlotte on Friday evening is moving back to the north as a warm front. Forecasters say any severe weather that develops Monday will be in the warmer, more unstable air.

The question is how far north that front will get. Cool high pressure to our north and falling rain have created a cold air wedge over the Carolinas, and severe weather won't develop in that cool pocket. But the northward-moving warm front on Monday will trim back the cold air pocket. Computer-generated forecasts show the warm front pushing as far north as I-85.

Typically in these situations, the cold air wedge is very difficult to dislodge. If Charlotte, Monroe, Rock Hill and Lancaster get into the warmer air, it probably won't happen until mid or late afternoon.

So here are some things to look for Monday:

1. A lot of rain across the entire area. The National Weather Service is predicting about 1.6 inches in Charlotte, Concord, Monroe, Rock Hill and Salisbury. The forecast is closer to 1.8 or 1.9 inches in Gastonia, Lincolnton, Hickory, Statesville and Shelby. Heavier amounts are possible, however.

2. Most of South Carolina will get into the warm, unstable air.

3. As the warm front moves into North Carolina, it will push inland toward the foothills. Areas east of U.S. 1 are likely to get into the warm, unstable air. How much of the area between U.S. 1 and I-85 gets into the warmer air? That's the question mark.

4. Winds in the atmosphere -- south of the warm front, at least -- will be set up to create rotating thunderstorms.

5. There will be remarkable contrasts in temperatures Monday. Don't be surprised to see some afternoon temperatures in the foothills in the upper 40s, while it's 75 degrees in Columbia or Florence, S.C.  And especially in the early afternoon, don't be surprised to see readings around 70 degrees near Wadesboro or Pageland, S.C., while it's 15 to 20 degrees colder in Gastonia or Lincolnton.

6. If fog develops, it could be a sign that you're near the warm front. Warmer air will pile into the area from upper levels of the atmosphere, squashing the cooler air toward the surface. That's when fog develops. If it's foggy in your area, and the ceiling suddenly lifts (and the temperature climbs), it's an almost certain sign that you're south of the warm front.

Monday could be a stormy and very wet day

We've been spared the typical springtime weather woes of heavy rain events and severe thunderstorms, largely because of that persistent block of cold air to our north that pushed its way into the Carolinas frequently in February and March.

But things are shaping up for a potentially wild day on Monday.

Our string of 80-degree days will come to an end when a cold front pushes across the region later Friday night. Behind the front on Saturday, we'll have a very nice day, with partly sunny skies and temperatures somewhere around 70 degrees.

But then things go downhill.

Low pressure is expected to form in the lower Mississippi Valley on Sunday and push northeast. The track of the system is important, because the center of the low is predicted to push across Louisiana and Mississippi through Tennessee and then Kentucky.

Clouds will increase in the Carolinas on Sunday, and it probably will be a chilly day, with temperatures not getting much above the lower 60s. And rain from the low pressure system is expected to arrive by Sunday evening.

Andrew Kimball of the National Weather Service's office in Greer, S.C., said computer models indicate "copious amounts" of moisture will be spread northward from the Gulf of Mexico into the Carolinas.

How much is "copious"?  Some of the computer-generated forecasts show up to 3 inches of rain falling from the system.  That would be enough to cause some flooding problems in areas where the heaviest rain falls.

But that's not all.  Remember the cold front that passes through our area tonight. It will stall somewhere to the south of us this weekend, then be dragged northward by the low pressure system pushing up through Tennessee and Kentucky. All the ingredients will be there for severe weather sometime Monday.

Areas to the south of that front will be at risk of severe thunderstorms with damaging winds and hail. And tornadoes will be possible, too.

It's nearly a sure thing that much of South Carolina will be in the warm sector south of the front. Kimball said computer models predict the front will move far enough north to put the I-77 / I-85 area in the warm sector, too. And there even is a possibility of the front pushing up to the foothills.

Rain, some of it heavy, is a very strong possibility from Sunday evening through late Monday. And severe weather will be in the mix, too.

We'll watch how this develops over the next few days.