Monday, July 30, 2012

It's not the heat ...

You know how the saying in the headline ends.

We got a little lesson in the impact of humidity on Sunday across much of the Charlotte region, when a push of drier air made outdoor life a little more tolerable -- at least for a day.

Now the humid air has returned, and it's probably here for the rest of the week.  What else would you expect in the dog days of summer?

The easiest way to measure humidity is with the dew point temperature.  Basically, anything in the upper 60s or higher during the summer is very humid air. When it falls into the lower 60s or below, and it's in May through early October, that's much more comfortable.

On Sunday, Charlotte's dew point readings were in the upper 50s and low 60s. Even though the air temperature climbed into the 90s, it was a more tolerable day outdoors than Friday and Saturday, which were two of the worst days of the summer, with dew point readings in the low 70s.

But the humidity returned. The dew point temperature climbed from 59 degrees at 7 p.m. Sunday to 70 degrees at 11 a.m. Monday. That has set the stage for the rest of the week.

The atmosphere is expected to remain generally stable Monday afternoon, and that will limit thunderstorm development to a few spots -- mostly southeast of Charlotte. But the arrival of a weak low pressure system Tuesday, combined with the heat and humidity, will bring more widespread thunderstorm activity. The only good news is that temperatures Tuesday are not expected to climb much above 90, due to ample cloud cover.

The rest of the week appears to be straight from the Carolinas Summar Playbook -- hot, humid, with a few thunderstorms around every afternoon. Highs will be in the low 90s. There's a chance that we might catch some storm-free weather Thursday or Friday, if a bit of dry air can wedge into the region again briefly.

Tuesday, July 24, 2012

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.

Friday, July 20, 2012

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.

Monday, July 9, 2012

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.

Sunday, July 8, 2012

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.

Friday, July 6, 2012

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.