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

Record cold, then record heat?

Is mowing the lawn part of your plans for Tuesday?

If not, you're like me, and we're probably going to regret that decision. We're looking at blast furnace weather across the Carolinas later this week -- on the heels of some unseasonably cool weather.

In fact, we could go from record cold to record warmth in Charlotte over a period of 60 hours.

Almost unbelievably mild high pressure has taken hold of Carolinas weather on Tuesday, bringing much cooler temperatures and very low humidity levels. Skies would be blue, if not for a few areas of cirrus clouds being spun into the region around the circulation of Tropical Storm Debby.

Wednesday morning's low temperature forecast in Charlotte is 55 degrees, and we seem almost certain to break the record for the date -- 59 degrees, set in 1879.

Temperatures will soar quickly Wednesday, climbing into the upper 80s. But with very low humidity, it won't seem intolerable.  So if you didn't get the lawn mowed today, you probably still have Wednesday.

Then it gets ugly. The cool high pressure area will modify by Thursday, and temperatures will keep soaring. We're looking for highs in the middle 90s, but once again with low humidity levels. Then again, 95 degrees is hot -- no matter what the humidity is.

The chance for record highs will come Friday afternoon, when temperatures will climb near the 100-degree mark in Charlotte. The same is likely Saturday. Record highs each day are 102 degrees, set in 1959 (for June 29) and 1945 (for June 30).

High pressure is forecast to weaken slightly Sunday and Monday, but all that means is the high temperature will be in the middle 90s.

There are signs that a slightly cooler air mass might arrive around or slightly before the Fourth of July, but there almost certainly won't be a return to what we experienced Tuesday.

Debby update: Weak tropical system are a real pain to forecast, and Debby was such a system. Originally, meteorologists expected Debby to move westward across the Gulf of Mexico and make landfall in south Texas as a Category 1 hurricane.

Instead, with very little in the way of steering currents and wind shear that disrupted the strengthening process, Debby remained a weak tropical storm and moved northeast. It likely will become a tropical depression later Tuesday, cross the northern Florida peninsula, and then swing out to sea while regaining tropical storm status.

But there's a lesson from Debby. Even as a weak tropical storm, it produced some amazing rainfall totals -- more than 20 inches in parts of Florida. Street flooding and power outages have been common across the Sunshine State.

It's the rainfall and spin-off tornadoes that can make a weak or dying tropical storm so dangerous, and the western Carolinas sometimes are the target of such a system.

Big cool down likely to miss Carolinas

A big cool down is coming to parts of the eastern United States next week, but it looks as if the Charlotte region will be a bit too far south to experience a major change in the summer conditions.

Many of the places that were hit with temperatures in the upper 90s this week -- New York City, Philadelphia, Washington -- will have highs only in the mid 70s for much of next week.

A trough will establish itself over the East, dragging down cooler air from Canada. The bottom of the trough is expected to be somewhere near the Virginia-North Carolina border. Areas to the north of that will see a big change in temperatures -- a change that will be heralded by strong thunderstorms this weekend.

Farther to the south, in the Carolinas, there is expected to be a slight cool down next week. We'll have highs in the low 90s Monday, dropping back to the mid and upper 80s Tuesday through Thursday. While that isn't anything like the cooler weather in the Mid-Atlantic and New England, it will be a break from the heat and humidity of this week.

By the end of next week, temperatures will be back on the rise, as we head into July.  At this early point, it looks like July 4 week will be (no surprise) hot.

In the tropics ... that mass of showers and thunderstorms we talked about Thursday is getting better organized and could become a tropical depression later Friday. Most forecasters believe it is destined to become Tropical Storm Debby and hit somewhere along the Texas coast.

Emergency management officials in Texas are getting prepared. Debby likely won't be anything more than a tropical storm, but even that can mean big trouble. Tropical Storm Allison had only 50 mph winds when it struck the east Texas coast in 1991, but it dropped 40 inches of rain in some areas near Houston. That caused devastating floods that killed 23 people.

Tropical activity possible in Gulf

The area between Cuba and Mexico's Yucatan Peninsula that bred two early tropical storms this season might be primed to create another.

A large area of thunderstorms between the Yucatan and western Cuba is showing signs of slow development, and the National Hurricane Center is giving it a 30 percent chance of turning into a tropical system in the next few days.

The chances of such a system having an impact on the Carolinas are low, but definitely not zero.

We'll look at the facts first, and then get into the conjecture.

The facts are that strong upper-level winds in the Gulf of Mexico are preventing the thunderstorms from organizing. However, the National Hurricane Center's Lt. Cmdr. Dave Roberts (he's a Navy meteorologist working with the Hurricane Center) says the winds are expected to relax in the next day or two. That opens the possibility of the system organizing.

Meteorologists expect the area of storms to drift slowly northward into the Gulf by later in the week.

That's where the guessing begins.

Some computer models predict the system will organize and move northeast across Florida and then somewhere along or near the Carolinas coast. That would be similar to the path of Tropical Storm Beryl in late May. And that could mean some lousy weather for people headed to the beach next week.

Other computer models predict the system would meander in the Gulf, searching for some type of steering current to move it along. Very little in the way of steering currents is expected by late in the week across the Gulf.

And possibility No. 3 calls for the system to drift far enough north to be caught in a trough that is predicted to develop across the central United States by the weekend. In that scenario, the tropical system would weaken but would be carried northeast -- somewhere over the Southeast (could be Mississippi into Tennessee; could be Alabama and Georgia, into the Carolinas) -- next week.

The likely scenario at this time of year would be for the system to drift across the Gulf.

But with so many people vacationing along the Gulf and up the Southeast coast at this time, of year, it'll be worth watching.

UPDATE AT 2:45 P.M. ...

It looks more like a Texas-Louisiana problem. The Hurricane Center now gives the system a 50 percent chance of developing, and more of the computer models are coming in line with a slow northwest drift. That could put a tropical storm along the Texas Gulf Coast by the weekend.

Does summer really start tonight?

The answer to that question is "Yes" -- sort of.

The summer solstice in the Northern Hemisphere is reached at 7:09 p.m. Eastern time Wednesday evening, and that's the official start of summer.

Today has the longest time of the year between sunrise and sunset -- 14 hours 31 minutes and 16 seconds in Charlotte. So in that way, June 20, 2012, really fits the description of summer's first day.

But in three ways, it won't seem like summer:

-- The earliest sunrise actually arrived earlier this month.

-- Meteorologists consider June 1 to be the start of summer.

-- Our average temperatures haven't reached their highest levels and won't until next month.

Here are the reasons for those seeming oddities:

SUNRISE

Our earliest sunrise in Charlotte was 6:09 a.m., and it happened from June 4 through Wednesday. The earliest sunrise time arrived 16 days before the start of summer because of a phenomenon called "solar noon."

"Solar noon" is the time of day when the sun actually reaches the highest point in the sky, and it's not always at 12 noon. That's because of the earth's elliptical orbit and its 23-degree tilt. The "solar noon" is actually after noon at the start of summer. It'll be about 1 p.m. Wednesday in Charlotte.

That's why sunrise times start getting later shortly before summer begins, and sunset times don't get earlier until a few weeks after June 20. For example, sunset Wednesday in Charlotte is 8:41 p.m., but it'll be 8:42 p.m. from June 26-30 before it starts getting earlier again.

That time difference is even greater at latitudes farther north.

By the way, the same thing happens at the start of winter. Our earliest sunset, 5:11 p.m., happens Dec. 1-8. By the time winter starts Dec. 20, sunset is three minutes later. Sunrise is 7:27 a.m. on the first day of winter but slides back to 7:32 a.m. in early January before it starts getting earlier.

METEOROLOGICAL SUMMER

I've discussed this before. Meteorologists consider December-February to be winter, March-May as spring; June-August as summer, and September-November as fall. That is because, according to meteorologists, those timetables seem to coincide better with what we consider "winter," "spring," "summer" or "fall" weather.

So from a meteorological standpoint, summer started June 1. And autumn begins Sept. 1, although anyone who's spent even one year in Charlotte knows real autumn weather usually doesn't arrive until a lot later.

AVERAGE TEMPERATURES

There is a lag in seasonal temperatures, because it takes a while for the atmosphere to heat up and cool down. Our coldest winter temperatures usually come in January. Our hottest temperatures typically are in July and August.

The average high and low temperatures for the first day of summer in Charlotte are 87 and 66 degrees. But our hottest average high is 89 degrees, from July 1-Aug. 5. And if you take it down to fractions, the hottest readings are usually in late July.  Our warmest average morning low, 68 degrees, is from July 2-Aug. 16.

There's no trend in our 90-degree history

Without wading into the climate change debate, let's just say that people on either side of the argument won't find ammunition from Charlotte's history of 90-degree days.

The Queen City has experienced only one 90-degree day so far this year, which is below the average for this time of year.

But if you've lived around here for more than a few summers, you know how quickly that can change. And that's exactly what could happen over the next week. While temperatures might not quite get to 90 today (Tuesday), they're expected to reach or surpass the 90-degree mark daily for the next week.

Yesterday, I wrote that computer models are pointing to a cool down next week. Now some of those models have flip-flopped and indicate the heat will hang around.

If you believe that our weather is getting warmer, the number of 90-degree days over the past two years will support your cause. Charlotte had 73 of those days in 2011 and 87 in 2010. That total in 2010 was the second-most on record, falling a bit short of the record 88 set in 1954.

There also were 75 days of 90 or hotter in 2007.

But you don't have to go back very far to find much smaller numbers. Three years ago, in 2009, Charlotte had only 28 days of 90 degrees or more. We had 36 of those days in 2006.

And if you go back to 2003, we had only nine 90-degree days. That was the second-smallest number on record. Only the summer of 1967, with eight 90-degree days, was cooler.

More than afternoon high temperatures go into the overall picture of summer heat.

Morning lows are another issue. If you remember last year, we experienced a larger-than-average number of mornings when lows fell only into the middle and upper 70s. That was a sign of the high humidity levels that covered the region like a wet blanket for much of June, July and August.

A third factor is rainfall.  No matter how hot it gets, summer seems a bit easier to take when a late-afternoon or evening thunderstorm rolls along.

Long-range forecasts call for average summer conditions in Charlotte.

Over the past 30 years, the Queen City has averaged 45.6 days of 90 degrees or more.  Let's see what unfolds.