August 30, 2003
TROPICS COME ALIVE
As we speculated on Friday, we now have two tropical systems to watch in the Atlantic basin.
Fabian has roared from a modest tropical storm to a Category 3 hurricane in the past 24 hours. He's about 850 miles east of the Lesser Antilles, with 100 mph winds.
Long term, Fabian could be a major story. Conditions seem favorable now to carry Fabian for a rendezvous with the southeast U.S. coast about next weekend. That is a long way off in meterological terms and there is plenty of time for Fabian to fade wide right into the open Atlantic as other weather features come into play. Still, Fabian will be front and center next week for anyone who is interested in eastern U.S. weather.
Also, a tropical depression has formed in the western Gulf of Mexico about 300 miles southeast of Corpus Christi, Texas. This is the system computer forecast models were showing on Friday -- it is no longer a digital fantasy, but a real storm. With winds of 35 mph, it will take only a slight increase to 40 to become Tropical Storm Grace.
Tropical storm warnings are already up for the Texas coast. Following in the footsteps of Hurricane Claudette and almost-hurricane Erika earlier this summer, would-be Tropical Storm Grace will likely do the Texas two-step and not directly impact Southwest Virginia weather.
And if that isn't enough tropical excitement for you, Hurricane Jimena -- which at least one prominent meteorologist has suggested may in effect be the Pacific regeneration of Gulf-born Erika -- appears headed for a Monday brush with the big island of Hawaii.
It's that time of year, folks.
August 29, 2003
LOTS TO WATCH
We appear to be on the fringe of an active weather pattern that will stem from two distinctively different sources: the Canadian tundra and the tropical Atlantic. Really, that's much less unusual than it sounds. It's what we call "autumn."
The festivities begin today with the potential for some heavy downpours (surprise, surprise) in scattered afternoon and evening thunderstorms. This threat is the result of southerly winds feeding Gulf moisture and turbulence from "outflow boundaries" left behind by last night's thunderstorms in other areas around us -- mini-cold fronts that flow out from storms as they die.
But the real action will be coming next week as Canadian cold fronts try to slap down the hot/humid dome over us and one or two tropical systems threaten to take aim on the U.S. coast.
Tropical Storm Fabian is chugging westward across the Atlantic this morning, some 1,200 miles east of the Lesser Antilles. Its winds are already 60 mph and there seems little that will stop it from becoming a hurricane. The long-term questions ares (1) how strong will Fabian become and (2) will he be turned northward into open water by passing low pressure troughs, or will he proceed more northwestward and threaten the East Coast by late next week.
Many computer forecast models are also indicating that another tropical system will develop in the Gulf of Mexico early next week. Because this one hasn't even formed, it's hard to really discuss it in detail yet. But the possibility is there.
Whether or not we're affected directly by any tropical systems next week, it would seem we have lots of potential for more of those tropical downpours we have come to know so well. It's quite simply the start of the annual war that pits winter against summer, the one that winter will inevitably win in about three months.
The cold fronts will come out of Canada and crash against the steamy warmth, and that clash will touch off thunderstorms. The first will probably not quite make it to us on Saturday, but each one will probably come a little farther south. And before you know it, there'll be crisp mornings and turning leaves, then snow flurries. However, we run ahead of ourselves.
I will go out on an easily cut-off limb on one subject. Thursday's high at Roanoke Regional Airport was 90 degrees. It might make 90 today or Saturday. After that, I will stick myself out there and say there will be no more 90-degree weather for Roanoke in 2003. I think the clouds and rain and, eventually, the Canadian air will put the kibosh on the 90s for the next couple of weeks; after that, it becomes very difficult to reach 90. If that verifies, the 93 on July 27 and again on Aug. 26 and 27 will stick as our hottest temperatures of the summer.
The late August mini-heat wave has been enough to push our monthly temperatures about a degree above normal -- an oddity in this cool, wet year.
Aug. 26, 2003
HOTTEST DAYS OF THE YEAR
Sometimes it's hottest right before the cool.
The big high that's been sizzling parts of the West and Midwest most of the summer will finally have a major impact on our weather over the next 48 hours, as it forces dry west and southwest winds across us. The hot dry air is aloft and this heat will be pushed down toward the surface -- "advected" in National Weather Service parlance. Cold air advection is often a bugaboo in the winter when it can quickly turn a 45-degree rain into a 32-degree snow (a la March 30, 2003). Hot air advection, obviously, will have a different flavor.
Adding to the heat some will be a downslope effect off the mountains, compressing the air and thus heating it. Plus, hot air often compacts in front of an approaching cold front, which we expect Thursday. The result will be temperatures in the mid to upper 90s. Farther east in the Piedmont, one or two stations could scrape the 100-degree mark; I expect more like 93 or 94 in the Roanoke Valley.
With our guess the hottest day contest closing on Labor Day, it could give someone with a late-August guess victory.
The overall pattern suggests that this heat will get beaten back over the next couple of weeks by a series of cold fronts, beginning on Thursday, possibly making it quite cool by early to mid-September. Summer's late appearance will likely be too little, too late to sustain a heat wave. The high, while spitting a little heat our way, will not move eastward over us, preventing us from getting under a late summer/early fall "heat dome."
The best news is all the dry weather we're having right now. We need a break from the rain, as it may be back all too soon.
OLD FARMER'S ALMANAC
The new version of the Old Farmer's Almanac is calling for another snowy, cold winter over the eastern U.S.
The almanac, which claims to use a variety of climatic and astronomical factors in forecasting weather long-term, has long been trusted by a wide variety of people. Arguments break out about the accuracy of the almanac, with its backers claiming an 80 percent accuracy rate, and "real scientists" disputing that figure and dismissing the almanac as pseudo-science.
I think there's a bit of a trick with the accuracy claims. The Farmer's Almanac will pick off four days at a time and say something like: Jan 13-16, rain, cold. So what happens if Jan. 13 is rainy and cold but Jan. 14-16 are sunny and mild? I'm guessing the almanac people call it an accurate forecast, but I would call it 75 percent wrong since three of the four days were sunny and mild. In any four-day period you sift out through the winter months, one day is typically going to have some kind of precipitation.
But let's give credit where credit is due: The almanac scored big last winter with its overall forecast of a cold, stormy pattern, and did particularly well calling for a mid-February major East Coast snowstorm (that's the one that gave us a 5-inch-thick coating of sleet instead of 2 feet of snow like D.C. got).
The government's Climate Prediction Center, fresh off falling on its face with its fall 2002 advisory that there was "no end in sight" for the drought just days before the months-long rainy cycle began for us, chased a flagging El Nino all the way to a horribly inaccurate prediction of a warm winter. So however the almanac formulates its forecast, it put treadmarks on Uncle Sam's best last winter.
Looking through the long lens right now, there is still no sign of a clear signal in the Pacific that would denote a La Nina or an El Nino, and that would seem to show the other factors in the Arctic, northern Pacific and Atlantic will dominate the scene again. All of those have consistently favored cooler, wetter patterns for nine months now, and the current mini-heat spell notwithstanding, there's no real sign that will change dramatically. Besides, cold, snowy winters seem to run in groups of three or four years, like the early 1960s and late 1970s.
So it might pay to start thinking about investing in tire chains and sleds this winter. Global warming hasn't turned Roanoke into Tallahassee-on-the-Blue Ridge just yet.
August 22, 2003
NOT THAT HOT
A year ago Saturday was our hottest day of 2002, when it hit 97 degrees in Roanoke.
Roanoke has yet to have a triple-digit high temperature in the 21st century and I think it's a safe bet to say that it will not happen this year, either. It's hit 90 only once at Roanoke Regional Airport in August; today might be the second day, but there just are no indications of heat waves on the horizon.
The fact is, a winter-like pattern is setting up. A low pressure system over Greenland is buckling the jet stream into the "negative-NAO" pattern that, were it about 6 months later, would mean cold days and probably some ice and snow. It disrupts a pure west-to-east upper air flow and bends the jet stream over the eastern U.S. more from the northwest.
As a result, a series of cold fronts moves across from Canada. In winter, these things would just plunge across us unchecked and chill us out; in summer, some of the dynamics remain that would wash them out or get them hung up at or just below us. In other words, don't get the coats out next week because the days might be downright toasty. But especially early in the week and again late, we could have really pleasant mornings. Wouldn't be surprised to see some 50s even in Roanoke on Monday morning, and maybe even a 40-number in Hot Springs or Bluefield.
Plus, with this kind of upper level flow, the Gulf of Mexico is cut off. Other than whatever turbulence might be kicked off with the fronts, we're not likely to have the tropical rains we came to know so well. Next week might, just might, be a totally dry week. Such dry air allows for wide temperature ranges morning to afternoon, where a morning low of 55 might jump to an afternoon high of 88. This is why arid areas of the West can swing from the 40s to the 90s in a day ... wet air is like a heavy blanket holding the heat or chill, dry air is more quickly heated and cooled.
As summer continues fading, the fronts will get bolder and colder as they dive southward. Will this bring an early September chill as we discussed Tuesday? And will this be the pattern that predominates through the fall and winter?
The first front comes through this evening and may find enough moisture and heating to trigger some pretty serious thunderstorm action.
FOGS IN AUGUST
A reader sent me an e-mail regarding the old saying that the number of fogs in August will equal the number of snows in winter.
Many old sayings have a grain of truth; some are pure bunk. I have struggled to find out what may be the grain of truth in this saying, and all I can figure out is that foggy Augusts are usually indicative of wetter, cooler than normal weather that, if it should hang around until winter, would lead to more snow.
A problem this saying runs into a mountainous area such as ours is that deep valleys have a lot more fog than higher elevations ... and as you know, higher elevations end up getting more snows than valley areas. It's possible that there might be 10 or 12 more foggy mornings in a wet August like we've had in, say, the James River valley near Buchanan than there would be at the Peaks of Otter lodge. But count the snows this winter and I will guarantee you that Peaks of Otter lodge will come out on top.
August 20, 2003
DID SOMEONE SAY FROST?
With cool weather slated to move into the fire-charred northwestern United States, and a brief shot of cooler weather for us this weekend (60 at night, 80 by day), there are a few signs that the seasonal shift is waiting in the wings.
A British forecast model brings an even more substantial cool wave to our area late next week. This is not a consensus opinion, but certainly an interesting one.
Private long-range forecaster David Tolleris on his WxRisk Web site (www.wxrisk.com) even notes a "risk" of an early-season Eastern U.S. frost for Sept. 9-16.
Just for the record, Roanoke's earliest sub-40-degree day on record was Sept. 19, 1984, when the low hit 38. Its earliest day to fall to freezing (32 degrees) was Oct. 1, 1993. It's latest 100-degree day was Sept. 5, 1954, when it hit 101.
What begins to happen as the days shorten is that the atmosphere tends to become more "progressive" — that is, systems actually move across the country instead of lingering in one place for days on end. We typically start getting cold fronts in quick succession.
The jet streams become more vigorous as more and more cold air leaks southward against the waning Northern Hemisphere sunshine, breaking down the domes of stagnant hot air that choke the summers. Jet streams that typically spend the summer in Canada begin diving southward again ... those we've actually already seen much more of that than is normal this summer. Later in the season, we see more and more collisions between truly wintry air masses and some sultry summer leftovers, and these collisions can fire up some really potent storm systems. And all this is not even counting the influence of the Atlantic tropical season, which hits its stride in late September.
We're starting to see a little of the inevitable rush of fall developing. Give it another month and the trees on the tops of our ridges will start looking colorful. About 90 days from now, we'll be speculating on our first snow. If it weren't for seasons, weather would be really boring.
August 15, 2003
FOILED AGAIN
It looks like we're going to be slopping the trough again within a few days.
We've had a few real summerlike days -- mostly sunny, warm to hot, some humidity, puffy thunderheads over the mountains that sometimes squeeze out a few showers or maybe even a clap or two of thunder. The Bermuda high has indeed ballooned far enough west to restore some normalcy to mid-August. Today will be the peak of that, with highs maybe creeping into the low 90s.
But lo and behold, forecast models are now showing that pesky low pressure trough reasserting itself by the middle of next week, and that will mean a return to the tropical soupy stuff we've been experiencing. If it verifies, we'll be getting persistent south to southwest winds aloft that will open up that seemingly endless supply of Gulf of Mexico moisture, fronts and disturbances will be sliding across us regularly, and we'll be back in the day-after-day rains again.
Meteorologists have a phrase they sometimes use called "persistence forecasting," which simply put is the principle that whatever has been occurring a lot over the past several days, weeks or months is what will likely occur again. Last year at this time, during the drought, persistence forecasting led meteorologists to put the kabosh on every promising rain system that came along. The opposite is the case in swampy 2003. If it looks a little like rain, it probably means a lot.
The Bermuda high tried. It really did. But it just looks like the pattern is too overall well-established. When something tries to shift, there's just too many things that won't budge much and it snaps back where we were. With autumn rapidly closing in on us and our first 40-degree mornings probably not much more than a month away, it is really doubtful we'll get any hotter than we already have. In many ways, we'll remember 2003 as our year without a summer, or kind of an extended wet spring.
ERIKA
Tropical Storm Erika is rapidly intensifying in the Gulf of Mexico and seems to be headed for a date with the Texas coast by Saturday morning. It may very well be a hurricane by then.
It looks like Erika might strike some of the same that suffered in Hurricane Claudette earlier in the season. It seems some years like hurricane paths get locked into patterns, too, and some areas suffer multiple strikes. Pensacola, Florida, got nailed by Erin and Opal within two months in 1995. The North Carolina coast got pounded by Fran in 1996, Bonnie in 1998, and then by Dennis and Floyd in 1999.
Like Claudette, Erika appears to be headed for the streets of Laredo once it makes landfall and won't be a direct factor in our weather.
August 12, 2003
GET OUT THOSE BERMUDA SHORTS
Lots of things are supposed to have disappeared into the Bermuda Triangle. Something will emerge from the Bermuda Triangle this week that will likely make our day-after-day rainy pattern disappear for a while.
A "Bermuda high" is a common feature of summer, and in many years, becomes the dominant force in our weather through July and August, bringing week after week of stangnant hazy heat. In extreme circumstances, it can balloon so large that it controls the weather all the way to the Rockies, creating a massive heat wave over the central and eastern United States.
More commonly, it drifts or grows far enough westward two or three times a summer to become sort of a forcefield over us, deflecting cold fronts and keeping Gulf moisture flowing up its side several hundred miles west of us.
The dominant feature of this summer has been a low pressure trough. The usual jet stream pattern in midsummer is across southern Canada, keeping storm systems up there and allowing hot air to rise to the Great Lakes. But this year, the jet stream has dipped southward over the eastern U.S., enough to bring storm systems over us repeatedly and pull up copious Gulf of Mexico moisture.
And over the last two weeks, an upper level low pressure system has become trapped in this jet dip. The low has been sitting a little to the west, perpetually spinning moisture-laden disturbances over us.
But as the computer models began picking up on last week, it looks like the Bermuda high will finally quit being such a wimp and will expand westward, pushing the upper level low westward back to Texas late this week.
As it grows overhead, this dome of hot air will gradually shut off the hose. Oh, we might see some widely scattered afternoon thundershowers -- typical summer fare -- but by the weekend, it's finally going to look and feel like a summer should: Hot, hazy days, the kind I hate in abundance, but would appreciate a smattering of right now.
How long will this last? If this were early July, I might say three or four weeks. My best guess now is that we'll squeeze out five to eight days of real summer before the pattern readjusts. In summer weather, old patterns are hard to break, and I figure that rainy low pressure trough will be back before school starts.
BLOODY HOT, OL' CHAP
100 degrees in London.
Of all the weather events of 2003 so far, that one amazes me the most.
London's average high in early August is 71. So that's 29 degrees above normal.
If Roanoke were as far above normal as London was, we'd have a high of 116.
Roanoke's all-time record high, set in August 1983, is 105.
CONTEST TIME, ALMOST
So far, the hottest day this summer was 93 on July 27. Of the scores who entered our pick-the-hottest-day contest lst month, a few of you are real close to that temperature and day.
Early next week could be a time to watch. If the Bermuda high gets real carried away, we could go into the mid or maybe upper 90s.
We'll pick our winner on Labor Day.
August 8, 2003
HOT AND DRY? COULD IT BE?
The West has been sizzling. Europe has been sizzling. But the Eastern U.S. has had a cool, wet summer that is bordering on the ridiculous.
In the short term, there's no change in store. A whirling dervish of an upper level low is spinning wickedly just to our west, trapped in a downward dip in the jet stream from Canada that keeps us the same soupy goop we've been in since spring. As it spins, we'll have tons of Gulf moisture and periodic disturbances to deal with. It's now rained 11 of the last 12 days in Roanoke ... we may well add to that streak.
But signals are meshing that this pattern could, finally, break down. Some long-term forecast models are indicating that a high pressure system over the Atlantic -- commonly called a Bermuda high -- will build westward and may link up with the vicious heat-dome in the West. If all this transpires, it would mean real summer weather with temperatures in the 90s and much, much less rainfall.
The cautionary note here is that these are fickle bundles of microchips and wires that sometimes flip-flop their forecasts entirely as new data becomes available. Also, because the ground is so incredibly moist now, you can pretty much take it to the bank that evaporation and topography alone (differential heating -- hot air in valleys clashing with cooler air on mountaintops) will trigger some scattered showers and thundershowers on many days even in a hotter, drier regime. But, at least, we might deep-six this daily downpour beat we've been marching to.
When could this pattern shift happen? Mid- to late next week is a decent guess.
My guess is it will be short-lived, a few days to a week or two, and a turbulent pattern will re-establish itself for fall and winter.
August 6, 2003
STORMY WEATHER
There were indeed many reports of hail, up to an inch in diamater, in Southwest Virginia on Tuesday as severe storms moved through. Some places also had wind damage.
The weather highlight of the day, though, occurred in the early morning hours when an apparent tornado touched down briefly to the north of Roanoke, damaging two businesses. The tornado was rated F1 on the Fujita Scale, which means it had winds estimated between 73 and 110 mph.
Tornadoes are rare but not unheard of in our neck of the woods, and like this one, most of the time they are weak and brief. The reasons we don't have many really destructive tornadoes are the same as why we don't get large hail ... generally unfavorable atmospheric dynamics and topography for intense "supercell" thunderstorms. (See entry below.) But there can be exceptions to almost any rule.
The next week looks about the same -- a chance of thunderstorms every day, with occasional bursts of upper-level energy triggering bouts of heavier storms. The beat goes on.
August 5, 2003
VOLLEYBALL-SIZED HAIL
Just in case you missed The Associated Press story in The Roanoke Times on Sunday, a record-size hailstone has been confirmed by weather experts.
This iceberg of a hailstone fell on Aurora, Neb., on June 22. It was 7 inches wide and measured 18.75 inches around. Now, that could do some damage to your windshield, don't you think? (It did strike the gutter of the house, chipping off part of the hailstone. I wonder what it did to the gutter.)
Most the time when we talk about large hail in thunderstorms, we're referring to hailstones the size of golfballs, tennis balls or baseballs. One of the criteria for thunderstorms being considered severe is hail at least 3/4 of an inch in diameter.
Hail is formed when intense updraft winds in thunderstorms carry raindrops to the highest parts of a cumulonimbus cloud, or thunderhead, freezing them solid. The winds keep carrying the raindrops-turned-hailstones back into the top of the cloud, growing new layers of ice around them, until they finally become so heavy they fall to the earth. Most of the time, hail is only pea- or marble-sized, 1/2-inch or less in diameter. But in particularly violent thunderstorms, it can become much larger -- even deadly.
The Aurora hailstone beat out a 5.7-inch wide, 17.5-inch around chunk of ice that plummeted on Coffeyville, Kan., in 1970.
If you see a common theme here, it is that the High Plains are particularly susceptible to large hail. It is not uncommon in severe weather months to have reports of baseball- or softball-size hail out there, and it is not unusual to see pictures of hail accumulations that look like snowstorms. It is my suspicion that many hailstones as large or larger than the Aurora glacier probably fall every year, but go unnoticed as they fall in the middle of huge prairies and melt into anonymity.
One reason the High Plains has so much large hail is because the plains are so high. A hailstone melts in the warmer air below thunderstorms, but if the ground is higher, it has much less time to melt before hitting the ground. The High Plains -- the Oklahoma and Texas panhandles, western Kansas and Nebraska, eastern Colorado and Wyoming, the western Dakotas -- are about 3,000 to 5,000 feet in elevation, though almost totally flat. Only our highest Blue Ridge peaks touch that elevation.
A second reason is that the vast flat plains, where dry air blowing down off the Rockies is free to mingle with warm, moist Gulf air with little topographical interference, is a breeding ground for monstrous "supercell" thunderstorms that can rise 10 or even 12 miles into the sky. These storms have more vicious updrafts (and downdrafts) and also reach even colder altitudes.
The short summary is that, while never impossible, it would be extremely unlikely for volleyball-sized hail to pelt western Virginia. Our terrain is much lower than the High Plains -- excluding the mountain peaks, where a small boulder of a hailstone might go unnoticed anyway -- and we almost never have the kind of stratosphere-scraping thunderheads that are almost a daily fact of life in the land of wheat and steers.
It's a good thing too ... the National Weather Service in Blacksburg is noting a somewhat higher risk of large hail in this afternoon's thunderstorms due to a pocket of very cold air aloft.
Aug. 1, 2003
DOG DAYS
We now enter a period of time typically called the "Dog Days." Seems kind of a like a slur against man's best friend.
What it actually refers to is a time of August that is typically noted for dreadfully dull hot, humid, stagnant weather. The reference to "dog" days is because Sirius -- the brightest star in the sky, and the "Dog Star" in the constellation Canis Major or "Big Dog" -- is now rising at about the same time as the sun. That of course means you can't see it because it's up the same time as the sun.
Some ancient cultures believed that the combined energy of the sun and Sirius caused the year's hottest weather to occur. We know now that whatever energy is reaching Earth from Sirius 9 years out would be pretty small. And in the southern hemisphere, this is the peak of winter right now.
This is the hottest time of year in the northern hemisphere simply because we're in the period of time between the peak sunshine in June and July and before the time when the cool systems near the poles begin moving across us in September and October.
While we often associate "dog days" with hot weather, typically it's more about stagnant weather. So whatever we exit July with is likely to continue, as steering currents are weak. For us, that means more damp, dreary weather as a seemingly endless chain of weak disturbances float over us -- all of them strong enough to kick off showers, none of them potent enough to sweep the moisture out.
So this year, we'll have a wet dog, which is often the worst kind.
Copyright 2003