Friday, March 29, 2013

Ozone season starting ... technically

Monday marks the start of ozone season in North Carolina, but it's more a technicality than reality this year.

Regardless, it's a reminder of the potential health problems we'll all face in coming months across the region.

The ozone season stretches from April through October, and the N.C. Division of Air Quality will issue daily forecasts for the Charlotte, Hickory, Asheville, Fayetteville, Greensboro/Winston-Salem, Raleigh-Durham and Rocky Mount areas.

Ozone is the state's most widespread air quality program, state officials say, and it surfaces in the summer. When ozone levels are high, people with chronic respiratory problems can experience breathing troubles. High ozone levels are most likely on sunny, calm, hot days in metropolitan areas.

Ozone also can cause damage to trees and crops.

The daily air quality forecasts offer a prediction on the level of ozone and particle pollution for the following day.

The levels range from Green (good), to Yellow (moderate), and then to Orange or Red for unhealthy levels. On Code Orange and Code Red days, people are advised to limit their outdoor activities.

When I mentioned that the start of the ozone season was a "technicality" this year, it's because temperatures have been too chilly for ozone problems. And it doesn't look as if things will change anytime soon.  More colder-than-average weather is forecast for next week.

Eventually, however, the blocking pattern over the northern latitudes will break down, summer weather will assert itself, and we'll be worrying about ozone and particulate pollution again.

Thursday, March 28, 2013

A different way of measuring March's misery

We're nearing the end of what, by National Weather Service standards, will be the coldest March in Charlotte in more than 40 years.

The Weather Service measures monthly temperatures by compiling daily averages. In other words, you take the daily high and low temperatures, and get an average for the day. And for the month, you get an average of the daily averages.

By that measurement, we're on pace for the fifth or sixth coldest March in Charlotte history, somewhere around 5 degrees below average.

But here's another way of measuring just how miserable it has been in recent weeks -- by looking at afternoon high temperatures.

Most people don't have to deal with morning low temperatures. They're snug and warm, inside their homes or possibly already at work.  Afternoon highs are a different story.  We're walking home from school, getting in our cars to come home from work, trying to play scholastic or amateur sports, or doing any of a number of other things.

The point is ... we're out there, in it.

I've taken a look at how our afternoon high temperatures in Charlotte have compared to the daily averages, and the pattern is obvious.

We were mild for the beginning of winter -- through December and January, and into the middle of February. That's when a high-latitude blocking pattern became established, creating a kink in the jet stream that sent repeated bursts of polar air into the eastern United States.

Here's how our daily highs have compared to average:

DECEMBER ... 24 days above average; 7 days below.

JANUARY ... 18 days above average; 3 days average; 10 days below.

FEBRUARY ... 11 days above average; 17 days below.

MARCH ... 8 days above average; 20 days below.

So since Dec. 1, we've had above-average daytime temperatures about 55 percent of the time.

Now here's the interesting statistic:

SINCE FEB. 15 ... 10 days above average; 31 days below.

In other words, since the middle of last month, our daytime highs have been below average about 75 percent of the time. And on many of those days, highs were well below average.

It's a statistical look at the reason why many people are eagerly awaiting a warm-up.

Wednesday, March 27, 2013

Will it finally warm up in April?

Now that we've suffered through several weeks of unseasonably cold weather and are headed for one of the coldest months of March in Charlotte weather history, can we hope for something better in April?

The answer is "yes" -- sort of.

Climatology -- the increasing angle of the sun, the inevitable march toward summer -- will help insure that the pattern which has brought a winter-like chill to the Carolinas and the rest of the eastern United States this month will relent in April.

But don't expect the cold-weather switch to be turned off abruptly.

We'll begin to see better weather, as early as this weekend, but the progress into spring won't be smooth. And frequently in April, I think we'll be on the edge between warm weather and the chilly stuff.

First, a review.

The Greenland block -- high pressure at upper latitudes -- that some weather followers had been expecting all winter finally arrived in March.  Strong high pressure in the area near Greenland blocked the typical west-to-east jet stream pattern. Instead, it created a bend in the jet stream, which nose-dived from western Canada into the eastern United States.

That brought a persistent flow of cold air into our region.

The lower end of that buckled jet stream will retreat northward, beginning this weekend. But it won't disappear.

Most meteorologists expect the blocking pattern to remain intact for much of April. The only difference is it won't be as persistent in the southern United States.  Draw a line from Richmond, to southern West Virginia, across southern Kentucky, and into Missouri. Area north of that line can expect to remain in the colder air for the next few weeks, still under the influence of the blocking pattern.

Down here in the Carolinas, we'll see periods of warm weather.

But every now and then -- maybe once a week, for a day or two -- the jet stream will dive southward into the Carolinas briefly.  That will bring a short period of much cooler weather, before the jet stream pushes back to the north.

It means we'll have several days in the 60s and 70s, but a back-door cold front will push into the region from the north, and we'll have two days of cold-air damming -- with temperatures in the 50s and a chilly breeze.

The best guess among meteorologists is that the pattern will come to an end in later April.

Basically, this means our weather is about three to four weeks behind schedule.  These inconsistent stretches -- a few mild days, then a day or two of chill -- are more common in March.

This also means severe weather is less likely in April than later this spring. We'll talk more about this later in the week, but springtime severe thunderstorms and tornadoes are more likely to the south and west of the buckled jet stream -- in places like Arkansas, Louisiana, western Tennessee, Mississippi, Alabama and southern Georgia.

Monday, March 25, 2013

End of the cold in sight ... but in time for Easter?

The blocking pattern and unseasonably cold weather across the eastern United States is holding tough Monday, with a snowstorm pounding parts of the Middle Atlantic after burying the Midwest and Ohio Valley.

And it appears as if temperatures that are 10 to 20 degrees below seasonal averages will continue for the rest of the week.

But there also are signs that the end is coming for this miserable start of the spring season.

The changing pattern will begin this weekend and be really noticeable next week, if you believe what many of the computer models and long-range forecasters are dishing up.

Charlotte's temperatures this month have been well below average. We've reached average daily highs about a half-dozen days, and our overall temperature for the month is more than 4 degrees below average. To give you some perspective, we're a staggering 14 to 15 degrees colder each day this month than last March (which, granted, was the second-warmest on record).

More below-average weather is likely through Friday, with highs around 50 degrees Monday and Tuesday, then into the middle 50s Wednesday through Friday.

The first landmark -- 60 degrees -- appears reachable Saturday. That should be accompanied by plenty of sunshine, after after the morning chill wears off, the day could be quite nice.

Then on Easter, we'll have a shot at reaching our average high for the date, 67 degrees.

Easter sunrise services should be OK, with dry weather and temperatures in the lower 40s. By late Sunday, an approaching cold front could bring showers, but the day figures to be OK, on the whole.

It's next week when the computer-generated forecasts point to a warmup.  Students enjoying spring break from the area's public schools could be playing in 70-degree weather several days next week. And it appears as if the trend will continue beyond that -- with an occasional cooler day mixed in.

Thursday, March 21, 2013

Early Easter forecast: Not great

The combination of an early Easter and a late-season outbreak of cold weather could add up to a nasty holiday weekend.

Easter is about 10 days away, and 10-day forecasts aren't the most reliable thing in the world. But odds are that the current pattern won't end by then.

The eastern half of the United States is locked in a mid-winter pattern, with high pressure near Greenland blocking the typical west-to-east flow of systems. Instead, the jet stream has buckled, allowing very cold air to funnel from the Arctic into the continental United States.

A storm system that will cross the country this weekend will bring snow -- some of it heavy -- to the Ohio Valley and parts of the Mid-Atlantic region. For the Charlotte region, the weekend will be a chilly washout. Rain will fall Friday night and again Saturday night, and temperatures will remain about 10 degrees below average.

Often, a pattern like this lasts a few days and then breaks. But there will be no let-up next week. Colder-than-average temperatures will continue to dominate the East, and it appears as if that pattern will continue through Easter weekend. Some computer models indicate a rain-producing system will cross the area on the day before Easter, but I wouldn't worry too much about that yet.  It's too early to pinpoint the development of low pressure systems.

But we likely can count on high temperatures only in the 50s.

It's starting to look as if the pattern might break a couple days after Easter.  When that happens, we'll probably experience a rather rapid warm-up.

Tuesday, March 19, 2013

Spring? Not for 2 weeks, computers say

It's getting more obvious by the day that the atmospheric blocking pattern I wrote about Monday will have a profound effect on weather in the Charlotte region, the Carolinas, and the eastern United States for the next few weeks.

If you take the computer models at face value, it appears as if we won't see any prolonged spring weather until at least the period around April 7-10.

That includes prospects of a chilly Easter weekend, late-season frost and freeze, and even a chance of snow this weekend in the northern part of North Carolina (especially the mountains).

First, a review.  The Atlantic Oscillation (AO) has gone strongly negative, and with high pressure in place near Greenland, we have a classic "blocking pattern" across the eastern United States. That means the jet stream has a big kink, allowing very cold air from Canada and points north to race southward, into the eastern United States.

At the same time, any storm system that glides from the Pacific into the continental United States will encounter this cold air, and we face the possibility of wintry weather -- even though the calendar says "spring."

Such an event is taking place today in New England, with heavy snow forecast for places like Hartford and Boston, and up into Quebec.  Air travel to places like Hartford, Boston, Montreal and Portland (Maine) could be impacted.

As a series of cold fronts moves across the Carolinas over the next few days, there could be a bit of mountain snow.

Certainly, ski slope operators and ski enthusiasts will love all this. The ski season, which ended very early in 2011 and 2012, apparently will continue into April this year in the North Carolina mountains.

By this weekend, the computer models continue to advertise the development of two storm systems. It's the second system, expected to move through the area later Sunday and Monday, that bears watching. Cold air will be locked firmly in place across the Carolinas, in a "wedge" pattern as was the case Monday.

In Charlotte, for example, I could see Sunday's temperatures stuck in the upper 30s and low 40s again.

The models can't seem to agree -- as usual -- on the path of the Sunday storm.  The trend seems to be pointing toward a snowfall for the Ohio Valley and Middle Atlantic. But there could be wintry precipitation north of Interstate 40 at lower altitudes.

And there doesn't seem to be any sign of a temperature recovery to average levels (middle and upper 60s) next week -- at least, not for anything more than one day at a time.  The trend seems to be for below-average temperatures through Easter, and for that matter, through the first week of April.

We'll continue to watch this pattern in coming days.

Monday, March 18, 2013

Cold air floodgates opened

Snow-lovers waited impatiently throughout the winter for a strong negative Atlantic Oscillation (AO) to develop, and it never really happened.

Now, on the week when the calendar switches from winter to spring, the AO has gone strongly negative.

That's why the Carolinas -- and, for that matter, most of the eastern United States -- are in for a week of chilly weather. Someone to our north could be facing a big snowstorm later in the week, before the pattern changes.

The Atlantic Oscillation is one of several atmospheric steering patterns, and when the AO goes negative, it means steering currents are carrying cold air from Canada or even the Arctic into the eastern United States. In the winter, a negative AO puts the Carolinas in position for snow, sleet or freezing rain, because temperatures are cold enough for frozen precipitation if a storm system comes through.

The AO went negative a few times during the recent winter, but its impact was blunted by another pattern -- the Pacific Oscillation. Persistent low pressure off the West Coast often blocked cold air from pushing into the United States.

Now the Pacific pattern is not blocking the cold air, so temperatures that are well below average are likely during the next week in the Carolinas and elsewhere in the East.

And by the weekend, a couple of storm systems are expected to move across the United States.

If this were January or February, we'd be on "snow watch."  But it's March.  Average temperatures are almost 15 degrees warmer now in Charlotte than in early January, so an outbreak of cold air and a storm system probably means chilly rain, rather than snow.

Indications are that the cold pattern will persist for about 7 to 10 days before relenting. If this pans out, we'll have milder weather again by Easter.

But it will be chilly, especially Friday through Sunday, thanks to the long-overdue negative AO.

Wednesday, March 13, 2013

(Official) winter ends with question marks

The last few days of "official" winter will end with an uncertain forecast in the Charlotte region, and that includes the weekend, meteorologists say.

Meteorological winter ended Feb. 28, but according to the calendar, spring doesn't start until later next week.

There have been a few mild days this month, but for the most part, it has been chilly and rather wet. That trend could continue later this week.

A gradual warming trend is expected to start Friday, and originally, the weekend forecast was superb. Meteorologists were predicting highs near 70 degrees Saturday and in the middle to upper 60s Sunday. That could change -- in a downward direction.

A "back door" cold front is predicted to push southward Saturday. A "back door" front is one that moves south to north, rather than the typical west to east. When back door fronts move southward into South Carolina, a cold air wedge often becomes established in North Carolina -- with a northeast wind off the ocean, lots of clouds, chilly temperatures, and a bit of rain.

Computer models are wrestling with the southward movement of the back door front. If it stays north of Charlotte, we would have a very warm weekend.  If it pushes into South Carolina, our weekend weather could be chilly and damp.

The uncertainty extends into the beginning of next week.

The good thing about cool weather and rain is its impact on agricultural and horticultural interests. With temperatures remaining below average, the water evaporation rate from the soil has been a lot lower than during March 2012, which was a record-setter for warmth.

Friday, March 8, 2013

Winter is dead

Stick a fork in it.

Winter 2012-13 is finished.  You might want to remember this column, in case it snows some morning in late March, but I don't see that happening.

The storm system that swept from the Midwest across West Virginia, Virginia and Maryland before going offshore earlier this week was our last chance at a real winter storm. Computer models earlier had pointed to the Carolinas as a possible target, but high pressure over the Northeast wasn't as strong as first thought. That allowed the big storm to stay north of the Charlotte region.

We certainly won't be vaulting straight into 80-degree weather. There is quite a bit of consensus among the computer models and meteorologists that temperatures overall for the next few weeks will be generally in the vicinity of seasonal averages.

Our average high temperature at this time of year is 61 degrees.

It looks as if we'll have a lot of 60s between now and the end of the month -- with a few days in the 70s and a few in the 50s mixed in.

But there is no sign of real cold weather that would allow a passing storm system to drop snow in our region.

For the record, we got 2.7 inches of snow in Charlotte, which fell Feb. 16. A trace of snow fell several other times, and we had the one January encounter with sleet and freezing rain. But we managed to avoid a devastating ice storm again. Our snowfall total was about half the seasonal average for Charlotte.

I suspect statistics will show that temperatures this winter were around average, but it seems as if we had an awful lot of chilly days, especially since mid-January.  I'll look closer at the numbers in the next few days.

Friday, March 1, 2013

Wednesday -- next (last?) chance for snow

Winter is certainly not ended in North Carolina's mountains, with up to 5 inches expected this weekend in some of the higher elevations.

But time is running out for snowfall in the Piedmont and foothills.

Our Charlotte weather history is loaded with cases when snow fell in the latter two-thirds of March, but a lot of the long-range forecasts indicate warming will arrive in the Carolinas later next week, perhaps bringing an end to the chance of a sustained outbreak of cold, like we're in right now.

So what's left on the horizon for snow-lovers?

A weak system will cross the region Saturday, and it's possible a few areas could see a brief snow shower, perhaps heavy enough to coat the ground for a while. But meteorologists say the impulse doesn't have the clout of a system that moved through the Charlotte area two weeks ago, leaving a 2-inch snowfall.

Instead, Saturday's precipitation more likely will be in the form of a cold rain, or perhaps a rain-snow mix.

Perhaps snow-lovers should look to Wednesday.

A strong upper-level low pressure system is expected to cross the eastern United States on Tuesday and Wednesday. As has been the case all winter, temperatures will be at the critical point -- on the edge between rain and snow.

And has also been the case, the computer models are waffling all over the place on the track of the system. Some predictions take the storm far to the north of the Charlotte region. Others are south. Still others have the storm dumping several inches of snow in the area.

The consensus seems to be that precipitation would begin as rain, then change to snow as the storm pushes away.

It probably will be Monday before we have a better idea about this, but I'll keep an eye on things this weekend and post updates.

After Wednesday's storm, it appears as if a warm-up will arrive. I saw a discussion from government meteorologists early Friday that described Wednesday's storm as probably the last chance for measurable snow at lower elevations this season.

We could be looking at upper 60s for high temperatures next weekend, according to some forecasts.