Kevin Myatt's Weather Journal at roanoke.com

January 30, 2004

WATCHING EARLY NEXT WEEK

The blustery cold that you're feeling today, perhaps with a flurry or two of snow but nothing significant, is the arrival of a new reinforcement of Arctic air. It's this deep cold that will help set the stage for what could be another winter weather threat on Monday.

For early next week, we again expect the setup of Gulf of Mexico moisture overriding cold air damming at the surface. Forecast models have been trending colder and that's a big concern considering that they were all much warmer than the last round actually turned out to be.

One different twist, at least in the early consideration of what may happens, is that this system could be more of an ice-to-snow scenario rather than the more familiar snow-to-ice setup. There are indications that as a surface low deepens somewhere along the East Coast on Monday, it will pull colder air through all levels of the atmosphere behind it, changing precipitation to all snow. But it's way too far out to even be sure we have the right large-scale pattern, much less those kind of details.

While some precipitation could arrive as early as Sunday night, it looks more like a Monday storm, maybe into Tuesday. We'll take some time off for the weekend and check back in Sunday evening with the latest on the developing storm.

Jan. 29.

WHY SNOWFALL FORECASTS OFTEN MISS BY A MILE

Let's leave on the table for the moment the not-great chances of some snow with an Alberta clipper Friday and the prospects of storm early next week very similar to what we just experienced.

Instead, let's discuss the broader question posed by one reader, and echoed by many others, of why snow accumulation forecasts are often wrong.

Actually, the National Weather Service was pretty much right on the money with its snowfall forecasts in this past weekend's storm. But we've all seen times when the official forecast is for 1-2 inches and we end up with 4 or 6. This has happened twice since early December here in Roanoke. We also remember those 4-8 inch forecasts that produce only an inch or two, or sometimes, nary a flake.

The obvious problems in snowfall forecasts are predicting the amount of moisture available and the depth of cold air, each of which are affected by the precise speed, strength and track of particular storm systems. That's difficult enough on a broad scale, and pinpointing the specifics, like exactly when snow changes to sleet, is often little more than technology-aided guesswork.

Secondary problems result from local geographic details like mountains, valleys and bodies of water. Only the experienced knowledge of a local forecaster can compensate for these.

But beyond the obvious, there are many difficulties in the fine details of a snowfal forecast.

I would argue some apparent inaccuracy in snow forecasts is a matter of perception. Rainfall amount expectations are not listed in local forecasts issued by our local weather service office. Even if it were listed, differences are not as easily noticed. If you awake one morning and see wet pavement, it may not be readily apparent whether .60 inch or .20 inch of rain fell. But the difference in 6 inches of snow and 2 inches of snow is obvious and immediate. So we perceive the latter as a forecast discrepancy, but don't notice the first, even though the involve the same amount of precipitation.

One factor unique to snowfall forecasts is the snow-to-liquid ratio, bang for the buck, so to speak. You've probably all heard that 10 inches of snow is equal to an inch of rain. I used that as a presumption in the paragraph above, but really it's only a general rule. The colder it is at the atmospheric level in which snow develops, the more snow you can squeeze out of less moisture. Sometimes you can get 15 or 20 inches of snow out of what would be an inch of rain. Late last week, a very cold clipper system out of Canada produced 30:1 snow ratios in Maryland.

But perhaps the greatest bugaboos with snowfall forecast involve locally heavy snow bands and dry slots. Within almost every system, the dynamics are just right over a relatively narrow area to produce heavier precipitation over an extended period of time. Getting in one of these zones can double or triple snowfall quickly. It's next to impossible to determine where these bands will be until they develop on a radar screen.

While heavy bands give, dry slots take away. When storm systems get revved up, they tend to suck all the air around into them, and this usually means at least one band of relatively dry air gets pulled in. This can be kryponite for a super snowfall.

Snowfall forecasts are like crossing a field of potholes. If you're careful and keep an eye out, you may bump along and take some unexpected dips, but you can usually get pretty close to where you need to be. Sometimes, though, you just get stuck.

Jan. 27, 2004 12 p.m.

SLIPPING NORTH OF US

Most of the precipitation band appears to be slipping north of Roanoke-- so it looks like we'll get out of this event with very little icing. Most people will be happy about that.

Still watch for some showers of freezing rain or sleet this afternoon, and some snow showers tonight. And lots of wind!

Jan. 27, 2004 7:30 a.m.

THUNDER?

While the threat of a widespread ice storm appears to abated, we do have one last band of precipitation to get through this morning, It's moving across eastern Kentucky and Tennessee this morning.

There have been reports of thunder and lightning with this band this morning, and it does contain some heavy cells. Aloft, a cold front is pushing into warm air, and this is creating instability. As this drags through, we could see some vigorous bursts of freezing rain, sleet and snow late this morning, but it probably won't last too long. It's something to watch.

Don't rule out a peal of thunder. This is a very energetic upper level system that will eventually spark a blizzard on the New England coast. When this comes together tonight, we'll get rattled by 50-plus mph wind gusts. Then, finally, a few days without precipitation to take an inventory of what might be coming down the pike this weekend.

Overnight freezing drizzle has left some icy patches. Do be careful.

Jan. 26, 2004, 4:30 p.m.

IT GOES ON AND ON

We have at least two disturbances to deal with in the next 24 hours, and the concern remains how much moisture they will pull up over the Arctic air at the surface. The National Weather Service feels good enough about the first wave to pull down its winter storm warning in favor of a winter weather advisory tonight.

That said, a flare-up of precipitation in southern Alabama and Mississippi has me a bit concerned. This could represent the first wave of something developing that might be pulled over us later tonight or early Tuesday. The band from Georgia into the Carolinas looks to stay mostly to the southeast, but this new development looks like its headed more our way.

On Tuesday, as yet a second piece of upper level energy approaches, more freezing rain will break out. There's a chance we might get above freezing on Tuesday, but I'm doubtful with snowpack and potentially icy trees. It's like a big ice chest out there, and the mercury might get stuck at 32 degrees for hours instead of warming above it.

More trouble Tuesday night: High winds may develop as a low pressure system tightens to our northeast. Hopefully, we do get above freezing on Tuesday, because icy trees and high winds are a very bad combination.

And we're still going to have to keep an eye on this weekend.

Let me catch my breath.

Jan. 26, 2004

MORE TROUBLE WHERE THAT CAME FROM

There's a second act to yesterday's winter storm, and it could be big trouble.

All eyes will be on the Gulf coast today watching to see how a new plume of moisture develops northward. Yet another upper level disturbance is sliding eastward and this one is expected to tap the Gulf later today and begin streaming moisture our way.

The problem is that Arctic air is still trapped at the surface against the mountains, enhanced by the widespread 4-8 inches of snow/sleet that fell on the area Sunday. Temperatures will begrudgingly climb to maybe 20 or 25 today. Warmer air will deepen aloft, and this is mouth-wrenching prescription for an ice storm.

The biggest worry is that some of the heavier 1/2-inch plus rainfall amounts expected over the eastern Carolinas and Virginia will develop more westward instead. Were this to happen, it would create a major ice storm with widespread tree and powerline damage across our area. The expectations now are for more like 1/4-inch of ice, still a problem.

Best-case scenarios would be: (1) less moisture than anticipated being pulled up and (2) cold air staying deep enough for a lot of sleet -- which is gritty on roads and bounces off objects instead of weighing them down.

Another problem: As a strong low pressure system winds up northeast of us that will cause a blizzard in parts of the Northeast, and a powerful upper low tracks across on the very strong jet stream, high winds could develop by Tuesday night. Hopefully, we warm just enough on Tuesday to melt any glaze off the trees, because high winds on icy tree limbs mean mayhem.

The active weather is likely to continue past this system, as we watch another possible event for the weekend, and continue to pay attention to developments over Canada for a possible major push of extremely cold air. The next two weeks will likely be the apex of winter for us.

Jan. 26 9:55 a.m.

ORDER UP: A ROANOKE SPECIAL

I've seen so many of these kinds of winter storms in my four years here, I just call it the "Roanoke Special."

You know the routine: We get a few inches of snow, then a crusty layer of sleet on top, then a little icing on the cake. It seems like it happens at least once every winter. It's happened three times in four winter storms this season.

Our geography makes it just right for this to happen. We're located just right to have mountains hold in the cold air from the north but also have warm, moist air riding in from the south on top of it. When the warm air gets deep enough, the fluffy snow turns into sleet and freezing rain.

As I write this, I do not know what the outcome of Monday night's potential ice storm will be, and what postscript it will add to this winter storm.

Snow amounts on Sunday were pretty close to what almost all forecasters ended up calling for, with 4-8 inches common across the area, though some private forecasters' expectations of locally 10-12 inch amounts didn't come to fruition. The moisture, while plentiful, was not quite as rich in our region as it could have been for a couple of reasons. One was that thunderstorms firing up along the Gulf coast actually siphoned off some of the moisture stream, shooting it miles up the in the atmosphere in thunderstorm cloud tops rather than allowing it to stream northward into our Arctic air.

Secondly, a "dry slot," or an area of sparse or no precipitation amid a storm, worked into our area from Tennessee and Kentucky late Sunday afternoon. The dry slot was just enough that the heavy snow that had been falling around 3 p.m. Sunday quickly tapered a little after 5 p.m. Another hour or two of snow at the intensity that had been falling, and we would had more like 6-10 inches across the area.

Our weather pattern ahead promises to be active and confusing. The moist southern jet stream is now fully in the game, bringing moisture-laden systems out of the Pacific, and there are signs that high pressure blocking over the northern Atlantic will buckle the cold northern jet stream deep into the United States.

Late last week, two sets of computer forecast models, one American the other European, depicted a historic Arctic air mass diving south into the United States. They showed sub-zero readings in Atlanta and even snow on Bourbon Street by this weekend.

The next day, the models promptly lost the whole thing. What does it mean? Sometimes the models latch onto something that has the potential to happen, but don't get the timing quite right. Sometimes they just burp. But to have two respected models do the same thing at the same time ... hmmmm. The National Weather Service has backed off the super duper Arctic surge idea for the moment, but some private forecasters are still gung-ho about it. We shall see.

Another note: There are hints of a possible low pressure forming in the northeast Gulf of Mexico this weekend. What has been missing in all of these Roanoke Special winter storms this year is deep surface low formation in the Gulf. Such a development could make one of these storms more than just "special."

Jan. 25 3:03 p.m.

BRUNT OF SNOWSTORM

The next 3-6 hours or so will be the brunt of the snowstorm in Roanoke. After steady snow for several hours that has left about 2 inches, we are just now getting into the meat of moderate to heavy snow. We could start collecting up to an inch an hour as the heavier bands come through.

I think the sleet holds off until after dark. Weather balloon soundings are showing temperatures freezing or below throughout the atmosphere, though in one place, it's creeping close to the freezing mark. A warm nose of air will eventually push into the atmosphere and we'll probably starting mixing with and changing to sleet, but it may well be that the heaviest precipitation will be over. There's also a "dry slot" in Tennessee that may move in and cut off the precipitation tonight.

Widespread 5-8 inch amounts look reasonable, with a few locally heavier totals up to 12 inches.

Monday evening is going to be a big concern. A new system is looking stronger on models and this one would almost certainly be ice. Ice storms are big issues. We'll watch it. Let's get through this one first.

Jan. 25 6:29 a.m.

HERE IT COMES

This winter storm is a bit slower than was expected yesterday. Probably a mid- to late-morning start in Roanoke, between 9-11 a.m., which makes it lousy for church services.

Dry air at the front end will eat away the first bands of precipitation appearing on radar as they move over, but it will also allow for more evaporational cooling throughout the atmosphere as the precipitation evaporates, dropping temperatures to near the very low dew points. This will increase the likelihood that we stay snow longer, perhaps throughout the daylight hours today.

The National Weather Service is calling for 4-7 inches of snow here in the Roanoke Valley, and I'm sticking at 5-8 inches, though I think somewhere will crack 10 inches. Models continue to show between one-half and one inch of liquid, which translates well to 5-10 inches of snow, cutting a little down because of sleet mixing tonight.

There are new concerns about a potential second act to this winter storm late Monday, which in all likelihood would be ice or sleet here. It's doubtful now that temperatures will get above freezing Monday as the cold air wedge hangs in on top of snow and ice at the surface.

The big X factor today continues to be when or even if the changeover to sleet occurs. If we go over early, say by mid-afternoon, the amounts probably won't top 4 inches. If it doesn't change at all and the moisture is any tiny bit heavier than expected, this could be more of an 8-12 inch storm. NWS is favoring a late afternoon switch and I'm guessing after dark, so at that, the 4-8 inch expectations look solid.

Jan. 24, 9:30 a.m.

PIECES ARE ON THE BOARD ... BUT IT'S STILL A PUZZLE

Deep in the heart of Texas this morning, one piece of our winter storm is coming together. The other piece is sliding southeastward in southern Canada.

If you check out a national radar, either via the Weather Channel or National Weather Service links on this page, you will see lots of colors in Texas, and probably farther eastward and northward if you're looking at this several hours after I'm writing this.

The high pressure system in Canada is what will bring the cold air. By later today, it sets up just right to pump cold air southwestward down the east side of the Applachian Mountains. Don't let today's 40s and sunshine fool you ... the atmosphere is chilling out today. It won't get close to freezing tomorrow. Forecasters may be generous in calling for mid-20s with the depth of cold air damming that is occurring, not to mention the evaporative cooling of the falling precipitation.

The cold and moisture are clearly there. The remaining questions that are still puzzling for forecasters, and frankly, will remain so even for some time even after the precipitation begins, are how much moisture will fall and how quickly the warm air moves in aloft to change snow to sleet or freezing rain.

On the first point, most models are spitting out .5 to 1 inch of liquid on most of our area. As all snow, that would be 5 to 10 inches; a mix would cut it down some. With a prolific stream of combined Pacific and Gulf moisture roaring at us, that could be a tad on the light side.

The warm air aloft, which is what's bringing the moisture, presents a thornier issue. Computer models are a hung jury; some are showing cold air sticking with us throughout the storm to cause nearly all snow, while others are warming the upper layers much more quickly and deeply to change us to sleet by early afternoon and then to freezing rain.

Let's face it, folks: this is a crap shoot. You could throw darts or draw straws and have as good a chance as any weather-obsessed people at calling when the snow changes over.

In some cold air damming events, I've seen the warm air move in rapidly aloft and snow go to sleet and ice really quickly. At that end of the measure, we'd get 2 or 3 inches of snow and maybe an inch or 2 of sleet, then a tree-coating, perhaps tree-sagging though probably not tree-breaking, glaze of about 1/4 inch. The National Weather Service in Blacksburg, while fully admitting its quandary in its forecast discussion today, is buying a ticket on this forecast train.

In early December, though, the cold air aloft stuck with us more than 8 hours after it was forecast to warm up, and we ended up getting 6 inches of snow when most forecasts were 1-2 inches. The precipitation with this should be much heavier on the front end than in the December storm, which piddled around with glorified flurries all day before an evening rush hour burst of heavy snow.

My educated guess is that the cold air hangs thick enough to keep us almost all snow through the daylight hours on Sunday, and that we get some rounds of heavy snow during the day. My call is 5-8 inches of snow and some sleet in Roanoke and over most of the area north of a Martinsville-Wytheville-Bluefield line. South of there, more like 2-5 inches with a thicker glaze on top.

Someone in central or western Virginia will get a foot in this. Big Meadows in Shenandoah National Park would be a good guess.

It should begin in Roanoke between midnight and sunrise Sunday and be in full swing no later than 9 a.m. It's going to be a messy day for travel, and church services will be probably be postponed or cancelled. I'll pray that we're all safe.

Jan. 23, 2004

TWO QUICK NOTES

(1) Lenticular clouds are again being seen in the area today. They're not as numerous or large as last Friday, and they're mixed with some more typical stratocumulus clouds, but you might be seeing some above mountains this afternoon. Look for the smoothly rounded look, maybe some in layers like pancakes.

(2) The National Weather Service is not yet issuing a winter storm watch for our area. The system looks more like a Sunday event now and NWS rarely issues winter storm watches past 36 hours. But I would expect to see this flag raised on Saturday.

It still looks like irresistible force of moisture meeting an immovable object of cold air -- the perfect prescription for a winter storm. Stay tuned.

January 23, 2004

MAJOR WINTER STORM

I don't throw the word "major" around lightly, but it looks like that is what is on tap Saturday night through Sunday when an enormous plume of moisture, supplied both by the Pacific Ocean and the Gulf of Mexico via a strong upper level low riding a fast southern jet stream, overrides a solid cold air dome being built by Canadian high pressure.

As time goes along, the cold air looks deeper and deeper, which would lend itself more to a snow/sleet situation rather than a whole lot of freezing rain, at least in Roanoke. Southside Virginia may be in for a tree-busting ice storm, while places to the north may get out of the storm with almost all snow. Everybody will probably see at least some sleet and ice; this is a "warm air advection" situation where the upper levels will be warming as the storm progresses. But as it looks now from Roanoke north, it may only be a thin film of crunch on top of the snow as opposed to a thick layer.

A good area-wide guess on accumulations would be 3-7 inches with a few higher amounts where it snows particularly hard or the sleet and freezing rain hold off the longest. I am a bit concerned that some of the earlier model forecasts of 1-1.5 inches of moisture will come true. That would up the ante quite a bit on snowfall/ice expectations.

Expect it to start fast and furious Saturday night or early Sunday morning. It should all be over by sunrise Monday, and it may even be a tad above freezing by then. With the warm air advection continuing into early next week, we'll get a warmup/meltdown for a few days, but winter will be building again over Canada and prepared to dive southward. Our coldest weather of the season probably hasn't happened yet. But let's just get through this weekend first before looking too far ahead.

January 22, 2004

DAM THE COLD AIR ... WINTER STORM AHEAD?

Cue the music. Let the dance begin.

Our shy friend, the upper level low pressure area that's been hanging out in the southwest corner of the country all by its lonesome, is getting pushed out in our direction.

Unlike last weekend, when the cold air went running away, this weekend's cold is running right into the low's arms. As of Thursday evening, it looks like the cold air will build southward Saturday, timed almost perfectly to interact with the wads of moisture the southern jet stream storm will be bringing with it by late Saturday.

It's that refrain we play over and over in a winter season. Can you sing along with me?

Cold.

Air.

Damming.

Weather geeks often call it "CAD" for short.

Computer models are coming together in showing a high pressure system position over the Northeast that will push cold air down the Eastern Seaboard, and these lovely clumps of rock and dirt we live among will keep it pooled up over us.

As is usually the case, some models have been having a hard time simulating this effect, so they've been showing ridiculous things like 50-degree weather swarming over us with the approach of the upper level low. That's not going to happen this time, at least not here at ground level. As time gets closer, the computer screens are showing thicker and thicker cold air as they catch onto the trick that old-school meteorologists in these parts learned long, long ago.

There's a slight chance we could squeak out with 35-degree rain, but even that would probably be mixed with sleet or snow for a while. Most likely, this is going to be one of those snow-sleet crunchy nuisance storms that Roanoke seems so adept to get. We may be a bit too far north to get into the heavy glaze ice and bit too far south for all snow.

The amount of moisture that will be thrown into this cold air is one of the biggest issues to be resolved. Some of the forecast models have painted 1-1.5 inch amounts of liquid. That would be 10-15 inches of snow at one extreme or a paralyzing ice storm at the other. That's probably a high amount. Most estimates are more in the .5-.75 inch range, which still could be 5-8 inches of snow, a damaging ice storm, or a few inches of snow/sleet/ice mish-mesh.

The other factor is just how much warmth, while rebuffed at the surface by the dense layer of dammed-up cold air, will stream overhead. The quicker and deeper this happens, the more sleet and freezing rain will fall instead of snow. If happens early, we could be looking at a significant ice storm. It happens later, then there would probably several inches of snow before any ice occurs, if at all.

Saturday night into Sunday is the time frame for this. The late night-early morning arrival will also bring it through in the coldest part of the 24-hour cycle, aiding and abetting the CAD caper.

There's still time for the situation to change, but once the cold air damming train gets running, it's hard to knock it off the tracks.

Thanks for the photos

My thanks to everyone who sent in photos of last Friday's lenticular clouds. Click here for two of the best submissions, taken by Harper Vaughan of Roanoke from the parking lot at Tanglewood Mall.

If you ever have photos of anything interesting weatherwise in our area (or extremely interesting in another area), you're welcome to email them to me.`

January 19, 2004

ANOTHER RAIN DANCE, OR IS THIS A SNOW JOB?

If you're a snow fan, this past weekend was a huge missed opportunity. If you're a snow detractor, the weekend's turn of events was a huge relief. Though the overall pattern has been and continues to be favorable for winter storm threats, an important detail kept this past weekend's system wet for us.

When it was time to do-si-do, one partner didn't want to dance.

The high pressure supplying the cold air slipped off to the east, out of the position that would ensure it damming the cold air against the mountains. The incoming moisture-rich system on the southern jet stream swept away the freezing air in favor of milder, moist air. That's why it was almost all rain around here, with minimal ice or sleet and no snow.

But here's the catch: We might have the same song and dance to contend with this coming weekend as the "split flow" continues, with a moist southern jet stream and cold northern jet stream in place.

We have a wallflower in this dance that will, reluctantly, become the key player.

A strong upper-level low is expected to become closed, or cut off from the jet stream, in northern Mexico, and it will sit there twirling aimlessly for a few days.

Later in the week, a similar system will dive southeastward down the West Coast, bumping the wallflower eastward, like someone pushing his friend onto the dance floor, saying "Go ahead, get out there."

What is expected to happen then is that the low newly set in motion will sweep in a lot of both Pacific and Gulf of Mexico moisture over the cold air mass to the north, creating a whole lot of rain in the South and much precipitation in the eastern U.S.

The issues on which our forecast will hinge will be the strength and depth of the cold air mass: whether there will be a cold high pressure system in place to honor its partner, or if it will get shy and run out the door again.

It's too early and too complicated to go through all the possibilities, but they run the gamut. Early guesses are for some kind of snow changing to ice changing to rain episode, but that's really a default "play to the middle" kind of guess. There appears to be a growing amount of evidence that the whole weather pattern after the weekend storm will change from the cold pattern of air descending out of Canada to a milder west-to-east jet stream, or "zonal flow," from the Pacific. This gives me pause for concern because our most significant storms often occur near or during weather pattern changes.

Lenticular clouds

If you looked up into the mostly clear blue skies on Friday, you may have noticed roundish clouds here and there that might have reminded you of white, fluffly flying saucers.

These were lenticular clouds, so called because they're often lens-shaped.

These oddly rounded clouds sometimes develop when when wind moves perpendicular to a mountain range, rising and falling as the air moves up and down the successive ridges, creating clouds where the wavy air flows causes moisture to bubble upward. They're much more common in the much taller mountains of the Western U.S., but under the right conditions, our mountains are quite enough to cause them.

I asked for lenticular cloud photos from last Friday, and several readers responded. Here are two of the best photos from the selection. My thanks to all for their submissions.

January 16, 2004

RETREAT!

That refers both to my expectations on this weekend and to the Arctic high pressure system.

The high that supplies our cold air isn't inviting itself to the party this weekend. It's slipping out the door to the east, and so it won't be where it should be to pump the cold air down and trap it against the mountains. This will allow the advancing warmer, moist air to simply sweep it out like so much leftover confetti.

That means the mild, wet jet stream will win round one and we'll probably go the way of cold but above-freezing rain, and not a lot of it at that, this weekend. In some of the deeper valleys and areas just to the north there is a threat of freezing rain Saturday evening.

Behind the storm there are reinforcements of Arctic air, and the southern branch of the jet stream may well bring additional storm systems through during the next week or two. Each one will present its own problems and possible surprises. The pattern remains favorable for some kind of winter weather mayhem -- eventually -- but that's never a guarantee.

I'll keep an eye on this weekend's system in case there are any surprises yet to come, but it look likes kind of a damp, chilly weekend rather than anything white or icy.

January 15, 2004

THE SUBTROPICAL-POLAR TANGO

Riding out of the dry, dusty deserts of the Southwest is ... moisture?

Well, that may seem counterintuitive, but it's true. The national radar today shows moisture building across New Mexico and Texas, and this portends an important and complicating development in our weather picture.

This horse has a name, and it's called the "subtropical jet stream."

When this branch of the jet stream becomes active, moisture-laden low pressure systems ride in from the Pacific across the southern part of the country. The storm systems that float along this powerful river of wind miles into the atmosphere also sweep close to the Gulf of Mexico, which is the ultimate treasure trove of moist air.

The trouble that is forthcoming for us is that the subtropical jet stream will be interacting with a persistent polar jet stream across Canada that keeps resupplying Arctic air for much of the eastern United States. This is sometimes called a "split flow" and when this really gets rocking in mid-winter, it almost always means ice and snow.

Our first touch-and-go round of subtropical-polar interaction occurs this weekend, and it's going to be a booger of a forecast that will almost certainly trigger a blizzard of acid reflux for meteorologists charged with tracking and forecasting the storm.

One extreme scenario with any disturbance on the subtropcial jet is that a large low pressure area could form in the Gulf of Mexico, additional upper level energy could drop into it from the polar jet, and a huge car- and house-burying nor'easter could roll up the East Coast.

There has been some cyber-speculation about the potential for a blockbuster storm over the past couple of weeks, but as of now it looks unlikely, for this weekend, at least. Forecast models are leaning more toward a series of smaller pieces coming through rather than one big storm.

The other extreme is that the high pressure areas to the north are so strong and the jet stream so fast that the systems zip by quickly and harmlessly to the south. It's called "suppression." While the highs have been strong, they haven't been dropping so far south to push things way out of our way, so a highly suppressed system that would brush by or miss us entirely is not probable.

The middle scenario seems more likely. That scenario involves these low pressure areas sweeping moisture up and over the cold air with each passage, or what's called "overriding precipitation." Depending on the depth and intensity of the cold, you get snow, sleet, freezing rain or just a cold, dreary rain.

The National Weather Service has had all of the above in our weekend forecast, really all the way from Saturday to Tuesday, but that's more out of uncertainty than any solid commitment to exact precipitation at certain times. The question with each passing system will be the cold air supply, whether it will be dammed up against the mountains or slipping out to sea a bit.

Usually, there is deeper cold air damming than computer forecast models show a few days out. My lean is to sporadic snow and sleet from Saturday night through Monday. Even though it's coming through in pieces there is concern that one of the pieces could intensify to a big storm in its own right.

January 14, 2004

WEEKEND THREAT?

Arctic air has returned from Canada, the latest in an ongoing series of chilly chunks of air from our northern neighbor.

A low pressure area tonight and Thursday will move just north of us. Most measurable snow occurs north of a low's path, so this will be a storm for Pennyslvania, Maryland, northern West Virginia, and places north of there. In our neck of the woods, expect some scattered light snow or rain this evening, with a bit more of it (maybe an inch or two) in those favored upslope areas on the west side of the mountains in West Virginia and far southwestern Virginia. The winds will crank up pretty hard tonight as the low pressure area tightens.

I would be very surprised to see even an inch of snow in Roanoke ... but recall how I was surprised to see 4 inches last week. The key difference in this week's storm and last week's is the path: much farther north this time around. Last week, the storm was moving perfectly, but there were questions about strength and moisture availability. For this week's storm to be much more than a few snow showers, it would have to take a serious deviation to the south -- not totally unheard of, but certainly not likely.

I'm increasingly concerned about a Sunday-Monday winter storm threat. We'll have a low pressure area riding the southern branch of the jet stream out of California and across the southern states, which is the path most of our big winter storms take. Its path, intensity and moisture it pulls up and the cold air available at various levels of the atmosphere will be the critical factors.

Some models are showing just enough of a warmup to bring up a rain-snow quandary. Were it November or March I might buy that, but with lingering cold air at the mid-January apex of the winter season, held in place partly by what should be a sizeable snowpack north of us, it seems unlikely that such a storm would be all rain for us. It could be ice, it could be snow, it could be a mix with or without some above-freezing rain, but I doubt highly that it will be all rain. I think you'll see the models trend colder and the National Weather Service forecasts trend upward from the conservative 20-30 percent probabilities now.

This is something to watch. I'll keep an eye on it for you.

Jan. 12, 2004

WINTER RESPITE ... THEN MORE TO COME

This week begins with a "warmup," two days of temperatures in the 40s and maybe even 50s, which should be enough to melt almost all of last week's snowfall.

But by Wednesday, the winter pattern will be back upon us as reinforcements of Arctic air arrive from Canada, and it looks like this cold could be around for at least another 7-10 days.

A midweek system that once looked like a clone of last week's snow for us now appears as if it's more likely to clip just north of us, bringing significant snow to areas north of Interstate 64 and D.C. I wouldn't be surprised to see some snowflakes in the air for us on Wednesday or Thursday -- maybe even a drop of rain or a pellet of sleet because its northern path might be enough to draw up a tad of slightly-above freezing air. But don't expect a big snow here in the Roanoke Valley. That would take a considerable dip to the south for this system that doesn't seem likely as of now.

What could in time be a bigger concern is a system that will come onto the West Coast later this week on the southern subtropical jet stream. The question out there is whether a system in the cold northern jet stream will hook up or "phase" with this southern system and create a major snowstorm in the Eastern U.S. over the upcoming weekend or early next week.

Computer forecast models, not surprisingly, are handling it differently from run to run, and many private and public forecasters alike are in a quandary. Later this week we'll have a less clouded picture.

Here's what is known: There will be lots of Arctic air over the next couple of weeks, and occasional upper level disturbances riding through. It's a recipe for winter weather mayhem sooner or later. Stay tuned.

FARMER'S ALMANAC UPDATE

The Old Farmer's Almanac wasn't doing too bad through the first 20 days of December. Then came a 17-day losing streak.

From Dec. 21-31, the almanac called for "heavy rain, then sunny, very cold." There was no heavy rain during any of the final 11 days of 2003, and there certainly wasn't any very cold weather. For Jan. 1-6, it called for "Sunny, cold." This was a period of record warmth. Oops.

Since then, it has recovered. Jan 7-10 was "Mild, then cold, rain turning to snow." Since it did in fact turn cold and there was at least flurries on three of those days, I gave it credit for three right and one wrong -- the first day was cold, so there was never any mild weather during that time. For Jan. 11-13, it's calling for "Sunny, cold." Jan. 11 and 12 fit the bill.

So, for December, the Old Farmer's Almanac got 15 days right and 16 days wrong. In January, through the 12th, it is 5 right and 5 wrong. For the season, since the start of November, it is 30 right and 41 wrong, or just a shade over 42 percent accurate. Doesn't look like a playoff season to me.

WEATHER JOURNAL