Storm cluster breeds 'derecho' talk

Update at 2:10 p.m.: The storms are now covering a swath from the N.C.-Virginia border back up through West Virginia. A severe thunderstorm watch has been issued for Virginia and the N.C. northwest mountain counties.

The Storm Prediction Center in Oklahoma says the Piedmont could be placed in a severe thunderstorm watch before long.

The damage reports continue to come in, from southeast Ohio and now from western West Virginia.

Original post: Meteorologists and so-called weather weenies are watching the development today of a thunderstorm cluster that is ripping across the Midwest and barreling toward the Southeast.

The word "derecho" is being used in some quarters.

I've seen a number of different definitions of the word, but the most common use is that a derecho is a long-lived thunderstorm or line of thunderstorms that carries damaging wind gusts.  Typically, a derecho must last for several hundred miles.

I lived through one of those on a July 4 night in the late 1960s near Cleveland.  The storms formed upstream, over Wisconsin, and roared across Michigan, over Lake Erie, and onshore in northern Ohio. They arrived just about the time of the Independence Day fireworks displays, blowing down trees and causing several deaths.

More recently, earlier this month, a derecho ripped across Ohio, West Virginia, Virginia and Maryland, knocking out power for millions of customers. Some people were without power for more than a week in the baking heat that followed.

Today's area of thunderstorms formed in Wisconsin and Minnesota, then grew severe as it pushed across Chicago during the morning commute. As of late Tuesday morning, I've seen dozens of damage reports from Indiana and Ohio. The storms are forecast to push into Kentucky and West Virginia, and then into North Carolina by afternoon or evening.

Brutal heat's gone, but for how long?

We've caught a break in the Carolinas for nearly two weeks, as the searing heat wave that brought a steady string of 100-degree days has retreated to the west.

But the strong high pressure system responsible for the very hot weather from late June through the first week of July isn't gone.  It's off to the west, baking the Mississippi Valley and Great Plains. It could return, and some meteorologists predict it will.

Actually, it makes sense to expect the high pressure to build back over the eastern United States eventually.

In the meantime, we're enjoying a break. A weak trough of low pressure has persisted for the past two weeks in the Southeast, buffering us between the Midwest heat pump and another strong high in the Atlantic.

Temperatures frequently get into the 90s, but that's typical of summer in the Southeast. On occasion, a weak cold front has pushed into the Carolinas and died, but the front has brought more frequent thunderstorm activity and a few days when high temperatures stayed in the 80s. It looks like Saturday will be such a day.

The pattern appears to hold next week. High pressure will try to build inland from the Atlantic, but the weak cold front that moves into the Carolinas this weekend and then dissipates will still leave a presence in the region. That weakness will allow for thunderstorms to develop every afternoon, keeping temperatures in check.

If this were the height of hurricane season, we'd be in trouble.

The weak trough of low pressure across the Carolinas would provide a very attractive path for any tropical storm or hurricane steered toward the eastern United States, but the tropics have been quiet since early June, fortunately.

Paul Pastelok, a long-range meteorologist with Accu-Weather, said earlier this week that he expects a few more surges of heat in the East later this summer. Pastelok even advanced the idea that some of those heat waves could come in September or even early October.

Incidentally, I saw a report this week from Accu-Weather, in which one of its meteorological partners says 2012 could wind up as the second-hottest summer on record. Weatherbank Inc. measures heat by the number of cooling degree days. A cooling degree is every degree above 65 for an average. In other words, if the high is 75 and the low is 57, giving us an average for the day of 66, we had 1 heating degree days.

Weatherbank measures the total number of heating degree days at 59 major U.S. cities, from May 15 to Sept. 15. Meteorologists say we got off to a slow start, because June was rather cool. But we're making up for lost time. Weatherbank's Steve Root predicts we'll have the second-highest number of heating degree days this year.

The all-time mark of 60,402 was set last summer. Second-most was 60,078, set in 1951. The company keeps records back to 1950. The average for a summer is 51,923, but the average over the past 10 summers is 56,134 (there's some ammo for you climate change believers).

In Charlotte, the average temperature so far this month is 82.2 degrees, which ranks seventh overall. But with a few more hot days, we easily could reach No. 3 all-time.

The heat wave ... some amazing numbers

Some long-time Carolinas residents have responded to the soon-to-be-finished heat wave by saying, "It's summer. This is the South. Deal with it."

Well, we all dealt with it, because we didn't really have a choice.  And it is summer, and this is the South.

But it has been an unusually strong heat wave.  The record book shows that.

The National Weather Service says thousands of high temperature records have fallen in the last two weeks, over an area stretching from the Midwest to the East Coast, and from Canada to the Gulf of Mexico. The Carolinas have seen their share of it.

Here are several record-setting facts and some trends from the heat wave, which will come to an end early this week in the Carolinas with the arrival of a cold front:

-- It has reached 100 degrees or more on five of the last 10 days in Charlotte.  And it reached 96 degrees or hotter on 10 of the last 11 days.

-- Charlotte's all-time record high of 104 degrees was equalled three straight days -- June 29, June 30 and July 1.

-- The June 29 and 30 readings were the hottest ever in June for Charlotte. And the July 1 104-degree high set a record for July.

-- Through Sunday, Raleigh had six straight 100-degree days. And nine of the last 10 were 100 degrees or hotter.

-- Typically, Fayetteville and Florence are hotter than Charlotte and Raleigh, but not this time. Florence had only two 100-degree days in July, through Sunday. Fayetteville had just one. The core of the heat was centered a bit west of the Sandhills. Columbia, Charlotte and Raleigh got the hottest weather.

-- Columbia equalled its all-time high, 109 degrees, on June 29 and 30. It hit 106 degrees on July 1. And the morning low Monday was only 81 degrees.

-- Greenville-Spartanburg's high of 107 degrees on July 1 was the all-time hottest for the city. It was 100 degrees at 11 a.m. that day.

Still ahead ... some frightening power bills. And the high pressure system responsible for the heat could build back into the East again, in coming weeks.

For now, however, enjoy the change to more tolerable conditions.

Keep the faith ... heat wave will break

We've been promising an end to the heat wave for several days, and it appears to be on schedule ... pretty much.

As we've been saying, today (Sunday) will be the last full day of the stifling heat. And Sunday's conditions might turn out to be among the worst. At noon, Charlotte's temperature was 96 degrees, and humidity levels were higher than in recent days. The heat index was 103 degrees.

It appears likely that we'll see heat indices near 110 degrees in places near Charlotte later Sunday afternoon. That's about as bad as last Sunday, which was perhaps the worst day of the current streak of miserably hot days.

But the cold front that will bring a change to the weather is being pushed southward by high pressure in Canada.

That cooler, dry high pressure system won't take control of our weather.  Those nice conditions will be reserved for areas north of the Mason-Dixon Line. But the front will stall somewhere near the North Carolina-Virginia border, and that will be enough to bring a change.

Monday will be the day of change. It will start hot once again, and temperatures probably will soar into the middle 90s by early afternoon. But the front is expected to be pushing ever closer Monday, and thunderstorms likely will develop shortly after noon in the mountains and push into the foothills and Piedmont. The Monday evening commute could be a stormy one.

And that will set the stage for what could be five straight days of humid, wet, stormy weather.

Tuesday through Saturday certainly won't be wash-outs.  There will be sunshine each day, but the sun will only serve to help stir the atmosphere. And with the stalled cold front nearby, a stirred-up atmosphere will trigger the development of showers and thunderstorms in the afternoon and evening.

Additional showers and storms will be set off by the passage of weak low pressure systems each day. Those could bring precipitation in the overnight or morning hours.

Daytime high temperatures will be a lot cooler, probably only reaching the mid 80s most days. Don't look for cool nights, though. We'll still be in the humid air on the south side of the cold front, so morning lows won't fall much below the low 70s. And humidity levels will be very high.

National Weather Service forecasters still think most of the area will get an inch or more of rain this week, and some places could get quite a bit more.

The strong high pressure system responsible for our scorching, mostly rain-free weather the past 10 days will retreat westward, bringing hot weather to the Rockies and High Plains. That system could push back eastward again later in July, so we're certainly not out of the woods, when it comes to the heat.

But this week will be a nice break.

Major change forecast next week

As temperatures have soared near or past the 100-degree mark, and lawns and fields have turned brown with only spotty rainfall in the last week to 10 days, I've heard a lot of people praying for a change in the weather pattern.

It appears as if those prayers might be answered next week.

The computer models are in agreement that a significantly different pattern will establish itself, starting about Monday in the Carolinas. It will mean a considerable drop in daytime temperatures (although nighttime readings won't change much) and soaking rains for just about everyone.

A strong dome of high pressure has governed our weather for about two weeks. Initially, that high pressure brought record cool temperatures to the Carolinas, but once it warmed up, the end result was record highs and mostly dry weather.

The high was centered over the Mississippi Valley, and being on the eastern edge of the circulation, parts of the Carolinas received occasional and scattered thunderstorms from little pockets of low pressure that spun around the clockwise flow of the big high. That explains the severe weather we got Sunday and Thursday nights.

But the big high is predicted to push back toward the west, starting this weekend. At first, a little area of high pressure will break off, bringing more triple-digit heat and dry weather to the Charlotte region through Sunday. Eventually, though, the Carolinas will join the rest of the eastern United States and come under the impact of a large trough of low pressure.

Moisture from the Atlantic and the Gulf of Mexico will stream into the Southeast, starting Monday and lasting through most of the week.With low pressure nearby, we'll have high temperatures gradually fading to the mid and upper 80s. There'll be frequent outbreaks of showers and thunderstorms.

Don't look for chilly mornings. Under that pattern, our morning lows will be in the 70-degree neighborhood.

Humidity levels will be sky-high.  Even those of you with straight hair will develop curls next week. But it isn't beyond the realm of imagination that most areas will get several inches of rain next week.

It will be a scenario where the afternoons bring thunderstorms with drenching downpours. There probably will be one or two episodes where weak lobes of low pressure circulate across the region during the overnight hours, adding to the rainfall totals.

Meanwhile, the searing heat will push into the West. That sounds ominous for those Rocky Mountain areas trying to deal with wild fires, but it'll be a big break for the East and Midwest.

St. Louis has hit or exceeded 100 degrees on nine straight days, as of Thursday. The National Weather Service says more than 3,000 heat records were established in the seven-day period leading up to July 4. Charlotte tied its all-time record of 104 degrees three times.

Eventually, that high pressure system probably will push eastward again -- probably later in July. But it looks as if we'll get at least a one-week break. All we need do is get through the next three days, with more 100-degree readings.

Thunderstorm relief? Careful what you wish for

I'm sure you've heard the phrase "cooling thunderstorm."

That sounded pretty good Friday afternoon, when the temperature was 104 degrees in Charlotte and it seemed nearly impossible to breathe outdoors.

I was talking to John Tomko, meteorologist at the National Weather Service office in Greer, S.C., around 4:45 p.m. Friday about high temperature records and other meteorological trivia when he asked if I had seen the radar in Indiana. I told him I hadn't been looking at any radar, because the high pressure system covering the Southeast was squashing all chances of precipitation.

"You need to look," he said.

I saw a fairly large area of strong thunderstorms moving southeast out of Indiana, crossing into Ohio.

Tomko said the system, known to meteorologists as an MCS (mesoscale convective complex), could be headed for the Southeast, and possibly the Carolinas, although such systems are not easy to predict.

I thought it might be nice to get a thunderstorm overnight, because it would cool things down.

As it turned out, we were better off without that MCS.  Instead, the system stayed to our north, crossing West Virginia, Virginia, Maryland, and the District of Columbia. It caused incredible damage.

There were 886 reports of wind damage, according to the Storm Prediction Center in Norman, Okla. In addition, there were two reports of tornadoes and 57 of hail.

More than 2 million customers lost electric power late Friday and early Saturday. That meant millions of people spent part or all of Saturday without electricity and air conditioning on a day when temperatures approached 100 degrees. And it meant crews responsible for cleaning the damage were outdoors, doing heavy work in that heat.

Friday night's MCS stayed to our north because of the position and strength of the high pressure system responsible for our heat.  That high pressure dome is expected to weaken slightly and move a bit west on Sunday, possibly putting the Charlotte region in the path of MCS systems Sunday and Monday nights.

A little rain would be nice, but we don't need what happened Friday night in the Mid-Atlantic.

How hot will it get?

The words "100 degrees" are scary to some people, but anyone who has spent a bit of time in the Charlotte region has lived through those temperatures before and knows it comes with the territory.

We also know that 100 degrees can mean several things, depending on the humidity.

The National Weather Service is forecasting high temperatures in Charlotte of 100 degrees Friday, 103 Saturday, and 100 Sunday. It's possible Monday could hit the 100-degree mark, too.

But it also appears as if the humidity will be rather low through at least Friday. That doesn't mean the inside of your car won't feel like a blast furnace when you unlock the door at 3 p.m., but it also means the atmosphere won't feel like a wet rag.

That was the problem last year. We had a pair of 100-degree days in 2011, but what made the summer somewhat miserable was the persistent high humidity. Want to know when the humidity is up?  Check the morning low temperatures.  If they're in the mid 70s, it's humid. That happened a lot last summer. That's forecast for Saturday through Monday, so be prepared.

Here are a few things I've picked up while looking at the forecast for this weekend and early next week:

Hot 2007 ... It appears as if the upcoming heat wave is the hottest since August 2007. We hit 100 degrees six times that month -- Aug. 8, 9, 10, 16, 21 and 22. It hit 104 degrees on Aug. 9 and 10, 102 on Aug. 8, and 101 degrees the other days.

"Mild" Atlanta ... In many people's minds, Atlanta symbolizes the heat of summer in the South. But it hasn't reached 100 degrees in Atlanta since Aug. 22, 2007 -- nearly five years.  We've had four 100-degree days in Charlotte since then.

The Heat Belt ... Long-time Carolinas residents know that the hottest summer temperatures always take place in a corridor stretching from Augusta, to Columbia, to Florence, and then up through the Fayetteville area. Forecast highs this weekend in that area are about the same as Charlotte, but the humidity is almost always higher there.  Watch for some really high heat index numbers, measuring the combined impact of heat and humidity. It's possible to see 110-degree heat indices in that area this weekend.

Charlotte's 100-degree history ... Here's a look at the last 10 years:

2011: 2 days (July 29, 100; July 30, 101)

2010: 2 days (July 8, 101; July 25, 101)

2009: none

2008: none

2007: 6 days (Aug. 8, 102; Aug. 9, 104; Aug. 10, 104; Aug. 16, 101; Aug. 21, 101; Aug. 22, 101)

2006: none

2005: 2 days (July 26, 100; July 27, 100)

2004: none

2003: none

2002: none