Late-night confusionPosted Jan31, 2007 at 10:54 PMHere we are within 12-18 hours of a long-forecasted winter precipitation event and the major American computer forecast models are showing extremely different scenarios, neither of which seem to match the current state of what's happening on radar. Welcome to the wonderful world of weather forecasting. One forecast model takes the entire storm south of us, while the other brings very light snow all day and much heavier mixed precipitation during the evening. By evening, the low pressure systems depicted by these models are several hundred miles apart. Something isn't right. The national radar has not looked very impressive to me tonight over the middle Mississippi River Valley. The precipitation shield has been very patchy, but of late, it seems to be filling in and advancing to the northeast. So I would still expect some snow getting in here during Thursday morning, just based on radar trends alone. This storm is coming at us in two pieces, one during the day and one at night, and that only adds to the confusion. One piece could end up stronger that the other. The computer models are probably having a hard time resolving this. Wintry precipitation still appears likely for Thursday and Thursday night when you consider the sum total of the evidence. I'll be back in the morning to see if it makes any more sense than it does now. It could be entirely different by then. Still some risk of a bigger snowPosted Jan31, 2007 at 05:47 PM
The developing system is producing scattered precipitation across the middle United States tonight, including snow streaking across Missouri and Arkansas into Tennessee (national radar linked here). It's a little patchy now, and this system may end up being a series of blobs of precipitation rather than one solid area. Don't be surprised to see a letup during the afternoon on Thursday, only to have precipitation restart later in the day. This does not look like what you would call a "big storm" but I wouldn't be surprised to see some local area get 6 inches if they get in a heavier band of snow for a while. Predicting where these heavier bands will set up is next to impossible before they develop on radar. We're at least 12 hours away from that. I still like a widespread 2-4-inch snow forecast for the area on Thursday, just a bit more than the weather service's 1-3-inch call.
Winter weather advisory for ThursdayPosted Jan31, 2007 at 04:44 PMIn expectation of 1-3 inches of snow on Thursday, with some sleet and ice on top of it, the National Weather Service has issued a winter weather advisory for areas generally along and south of Interstate 81 on Thursday, including the Roanoke Valley. The advisory doesn't include the New River Valley, even though the forecast is similar, because the bar is a little higher for an advisory there since you NRV folks often get more snow than we do over here. Arrival time looks earlier and earlier. I'm thinking 9 a.m. now. More thoughts a little later. Various opinions on ThursdayPosted Jan31, 2007 at 10:05 AMLet's see. The National Weather Service is, for now, holding off on winter storm watches and sticking to an inch of snow plus mix on top. A Weather Channel local forecast I saw this morning was calling for 2-4 inches. The Hydrometeorological Prediction Center has us in a slight risk of 4-plus inches of snow AND a slight risk of a quarter-inch of ice, with the moderate risk zone not far to the southeast. One private meteorologist's map I saw this morning, an early guess by Leesburg-based Dave Tolleris, suggests at least a possibility of 3-8 inches of snow. And that's not counting what any local television meteorologists are saying. It's the Tower of Babel typical the day before a winter storm in our area. And for good reason: It is confusing. So many different layers of the atmosphere than can vary a degree or two, storm tracks that can drift a few miles here and there, any slight changes in which can change everything. Suffice it to say: This looks a solid hit of winter precipitation, and probably a little wetter and colder than last time. Probably more snow than Jan. 21, though how much is still up in the air. I guessed 2-4 inches last night, and have no reason to change yet. Arrival time is equally as confusing, but mid-late morning for the serious precipitation (more than flurries) seems possible. I'm a little leery, though, because these overrunning precipitation events, with moisture overriding cold air at the surface, seem to arrive faster and heavier than the forecast models show them. I hope to break things down a little better for you this evening. A trend toward snowPosted Jan30, 2007 at 06:34 PMComputer model runs through the day have trended toward a somewhat weaker, more southerly track for Thursday's storm. If this verifies, it would mean more snow in our area and less sleet and ice, though that snow probably won't be extremely heavy. An early guess would be a 2-4-inch type snow, mixing with sleet or freezing rain during Thursday evening. The Hydrometeorological Prediction Center has now put all of Southwest Virginia in a slight risk for heavy snow (4-plus inches) on Day 2, which would be 7 p.m. Wednesday through 7 p.m. Thursday. (If the linked map looks different than what I've described, there may have been an update; click here for the entire HPC Winter Weather page for the latest information.) The National Weather Service in Blacksburg has also trended its forecast toward snow; here is the latest forecast for the Roanoke Valley. There's still many miles before we can sleep on this one, but a moderate winter storm looks likely for Thursday. So ... what about Thursday?Posted Jan30, 2007 at 09:49 AM
It all depends on the exact track and strength of the storm. A more southerly track keeps more cold air in aloft, leaning the precipitation toward snow with some sleet mixed in, while a more northerly track would allow warm air to surge in aloft, creating more freezing rain. A stronger storm would also wrap more warm air in, producing more ice and sleet, while a somewhat weaker storm would be less capable of scouring out the cold air aloft and allow more snow to fall. At this point ... it does look like this will be a stronger and wetter system than the sleety slush of Jan. 21, and we've had more time to get things cold. So travel troubles appear likely Thursday and Thursday night. Brrrrrrrrr ....Posted Jan29, 2007 at 11:11 AMThe low was 15 in Roanoke and 10 in Blacksburg ... not record stuff by any means, but the coldest weather since last February, and feeling all the more frigid since it was in the 50s as recently as Saturday and since December and early January were so warm. There are three or four more shots of cold air where this came from over the next couple of weeks. Snow to our west and to our eastPosted Jan29, 2007 at 01:14 AMThe Outer Banks of North Carolina are expecting 1-3 inches of snow by mid-morning Monday as a low pressure system tightens up off the coast ... a little late for most East Coast areas to see a lot of snow. Meanwhile, parts of eastern West Virginia already have up to 4 inches Sunday evening with maybe another 4 expected in some areas as familiar upslope snow showers coat their usual areas. Roanoke, again, has had flurries. This will be the coldest morning of the winter so far, though, with widespread teens and likely a few single digits in protected valleys. The late week continues to look interesting and may kick off a 1-2 week period of several precipitation threats. The overal Arctic air regime will continue, but will the individual storm systems be timed right for the individual cold shots? Stay tuned. Snow showers possible today and tonightPosted Jan27, 2007 at 11:20 PMDon't be surprised to see a few snowflakes on Sunday morning, possibly mixed in with the rain. When the windy Arctic front passes later in the day, the upslope snow squalls will get going again in West Virginia and the higher mountains of Western and Southwest Virginia. By Sunday evening, a few of those snow showers will spill into the New River and Roanoke valleys, but little accumulation will occur. It will probably be a lot like Thursday was, with snowflakes in the air but not much to show for them. This is going to be a cold week, with most days struggling to top freezing and nighttime lows ranging from the low teens to the low 20s. Precipitation is still a fairly good bet toward the end of week, but what type of precipitation is a huge question mark. The track of the system will be all-important, as any jaunt to the west of us would actually pull milder air into the region with the precipitation before a new round of Arctic air comes in during the weekend. It might hit 60 today, but don't get used to itPosted Jan27, 2007 at 12:21 PMWe're having a strange little one-day interlude in the overall cold pattern that has established itself. With the help of sunshine and west winds downsloping off the mountains, warming and drying as they do so, we might get close to 60 degrees today. It's already 52 at noon in Roanoke. The Arctic surge returns by Sunday afternoon, though. Other than the typical snow showers/flurries action we get when winds turn out of the northwest, the weather systems just aren't in sync for any major wintry precipitation through at least midweek. Could be some very cold weather with lows in the teens and even some single digits by Monday and Tuesday. UPDATE 7:25 PM: Well, close ... it reached 59 in Roanoke. The cold front is coming with a few overnight showers, maybe snow showers toward morning. Where it is snowing a lot this winter ...Posted Jan27, 2007 at 12:16 PMAnchorage, Alaska, is having an incredible amount of snowfall this winter, already having topped its annual average with at least four typically snowy months to go. Click here for article. Two systems won't pull togetherPosted Jan26, 2007 at 09:49 PMDisturbances moving through the northern and southern branches of the jet stream will move fairly close to us Saturday night and Sunday. If they were more in sync, it could blow up a larger storm. But the northern stream disturbance will be a little faster and farther north, while the southern stream system will be too far south and a little slower. We may well get moisture into the area, but not a lot, and it won't be very cold, catching the gap between Arctic air masses. So expect to see scattered rain showers along and east of I-81 on Saturday night, with snow showers to the west, changing to snow showers with whatever moisture's left at all locations by Sunday morning. The chance of a measurable snow in the Roanoke Valley is low, certainly no better than 20 percent. The main show for the eastern half of the U.S. will come later this week when it seems there is a much greater chance to get the moist southern branch and the cold northern branch of the jet stream involved together. Where that will come together and where it will move will be critical for who gets rain and who gets snow. The trend this year has been for big storms to travel more through the middle of the country. This one will probably be farther east, but far enough east for us? We'll have to keep an eye on it. A mini-warmup for SaturdayPosted Jan26, 2007 at 12:42 PMBetween the Arctic blasts, and with the help of some downsloping winds from the southwest, we may see some highs in the 50s on Saturday. It won't last long, as a new round of Arctic air begins pouring in by late Sunday. The bit of moisture coming up late Saturday or early Sunday may actually catch the in-between time and be as much rain as it is snow. So probably not another slippery Sunday. Another slippery Sunday?Posted Jan25, 2007 at 09:41 PM
This is far less certain than last Sunday's ice/snow threat was at this time. The system throwing this moisture northward may simply be too far south and moving too fast to the east. The National Weather Service is not buying into it at this time. But I pay attention to these southern stream systems scraping the Gulf of Mexico. It wouldn't take a vast change to give us another slippery Sunday. Flurries, showers, squallsPosted Jan25, 2007 at 10:37 AMSome pretty intense snow showers are moving through the Roanoke Valley. Don't be surprised if it snows pretty hard for a few mintues. What I'm seeing out my window now is certainly the hardest snow I've seen at this south Roanoke County location since last February. We'll see lots of flurries today, when you see a flake flutter by every few seconds. We may get a few more showers, when it snow noitceably hard for a few minutes, possibly even briefly dusting things. Some areas of West Virginia and far western Virginia may get snow squalls, where the heavier snow continues for quite a while, accumulating in inches. We have half-mile visibility or less here now. It can still snow hard here. It's even starting to accumulate a little. Maybe this shower is becoming a squall. UPDATE 10:52: The grass is white here now. My wife tells me it's bright and sunny downtown ... and yep, there comes the sun, even as it continues to snow moderately. It's gonna be that kind of weird day. A few snow possibilitiesPosted Jan24, 2007 at 10:14 PMMany of you are wondering about our chances of snow in the ongoing weather pattern being dominated by Arctic air sinking southward. You'll feel the bite of this by Thursday evening, with temperatures falling into the teens across much of the area. A few things to watch: * In the next couple of days, a series of weak disturbances will be accompanying new arrivals of Arctic air. The upslope snow machine should get going pretty robustly, with a few inches in those places in West Virginia that get snow on every northwest wind (Salt Pond Mountain and Potts Mountain in Virginia often get some out of this, too, among other higher places). Unless one of the disturbances is stronger than expected, or digs farther south than expected, we usually only get a few flurries in the Roanoke Valley, maybe a dusting in parts of the New River Valley. * There is some indication on a few model runs of moisture trying to build northward from the Gulf of Mexico late in the weekend. There's definitely nothing worth jumping on at this time, but it bears watching. Probably, this will be more of an issue for Georgia and the Carolinas. * The weather pattern may shift slightly late next week or the week after to allow a strong subtropical branch of the jet stream to undercut the cold air mass sinking southward relentlessly from Canada and ultimate the north pole. If this develops fully, there could be the potential for winter storms across the southern and eastern U.S. A long way out, and not a sure bet. Cold and dryPosted Jan23, 2007 at 05:10 PMThis winter is about to add insult to injury for snow lovers. After snow fans suffered through all that unseasonable warmth that sent warmth lovers into ecstasy, the new and still intensifying cold pattern is pushing the moisture out of the way and offers little hope for widespread snow at least through early next week. If we are to get snow in this pattern, it will be on the little "Alberta clipper" systems sliding southeastward; this usually means a good dose of snow showers in the mountains but only flurries on this side of the Appalachian spine. Occasionally, though, one of these systems has more gusto and dumps a decent snow, rarely more than 3 inches. The best shot at this happening would be Wednesday night and Thursday morning, but it looks pretty small. The overall cold pattern may relax a little over the weekend before coming in with even more ferocity next week. The next 2-3 weeks could be an extremely bone-chilling time in Southwest Virginia, peaking about Groundhog Day. As for snow ... some things will have to change to get the southern branch of the jet stream involved to provide moisture. If the cold hangs around long enough, that will probably eventually happen. Until then ... the warmth lovers can despise the cold, and the snow lovers can despise the dry weather. Late-snow record avertedPosted Jan22, 2007 at 04:39 PMSince sleet accumulation counts as snowfall in climatological statistics, Roanoke officially received half an inch of "snow" in Sunday's winter storm. That was the city's first measurable "snow" of the season. My research of weather records determined that, since records began being kept at what is now Roanoke Regional Airport in 1948, the latest first measurable snowfall in a winter season was January 28 in 1995. So we have missed that record by a week. The record lowest snowfall for a season in Roanoke history was 2.3 inches in 1975-76. So that's still in play, if the upcoming bouts of Arctic air don't meet up with enough moisture for significan snowfalls. But I really don't believe the low-snow record will still be on the table by mid-February. So, what's next?Posted Jan22, 2007 at 10:54 AMThere is no clear-cut "next big thing" winter storm on the horizon. There is some chance of snow in the Wednesday-Thursday time frame but that has looked less likely has time has passed. What is in evidence is a weather pattern ripe with potential. We have several more shots of Arctic air coming the next 2-3 weeks, a rather potent one late this week. It will be a matter of getting the wet subtropical jet stream and the cold polar jet stream to work together. I don't think we're done with wintry precipitation this winter. In fact, we've only just begun. But we'll probably get a breather for a few days, with temperatures likely entering the 40s during the day today and Tuesday. Sharp contrastPosted Jan22, 2007 at 10:37 AMSunday's ice is fresh on your mind. Do you remember the Sunday before, Jan. 14? Perhaps this will jog your memory. Winter weather advisory for rest of tonightPosted Jan21, 2007 at 09:31 PMThe ice storm warning has been dropped by the National Weather Service in favor of a winter weather advisory. Light freezing drizzle has begun again in parts of the Roanoke Valley and other areas of Southwest Virginia, and will continue off and on through the night. The threat of heavy icing has ended but freezing drizzle can cause a dangerous coating on roadways, many of which are still slick from today's ice, sleet and snow. Winter storm appears to be winding downPosted Jan21, 2007 at 07:38 PMRadar trends indicate that the main band of precipitation is working its way east of the area. There will probably be some light freezing rain or freezing drizzle overnight, but it appears that we have gotten out of this storm without the major ice storm we feared earlier in the day. The roads are treacherous, but the trees and power lines, at least in the Roanoke area, are not sagging to the ground. Instead, we are left with up to an inch of a white wintry concoction covering the ground. In the end, speaking for the Roanoke Valley, we got the precipitation types we were expecting, but in the reverse order. First came the freezing rain, then hours of sleet, then even a little snow mixed in toward the end. It was supposed to be a snow to sleet to freezing rain day. It appears that we had so much cold, dry air in place that the daylong precipitation was able to gradually chill the atmosphere, allowing for the freezing rain to change to sleet for much of the afternoon. Wherever you may be reading this from, the storm may have varied for you. Perhaps you got a lot more freezing rain and have more glazing; perhaps you got more snow than we did and therefore a thicker mantle of white. Please leave your comments or email me about what this storm was like for you. Roads deteriorating in the Roanoke ValleyPosted Jan21, 2007 at 03:33 PMHaving been out and about the last couple of hours, there has been a quick and noticeable deterioration to the conditions of many main roads that were just wet before. We've had some sleet that has done just what I described previously, created slush on road surfaces that is starting to freeze. This is old news to more outlying areas where many roads became icy a long time ago. It looks like we have several more hours of precipitation to go. Sleet -- a mixed blessingPosted Jan21, 2007 at 01:15 PMIt has actually started sleeting more here in the last 30 minutes or so at my location in south Roanoke County. In fact, we had a burst of what I would call moderate sleet, with no freezing rain at all mixed in. That could indicate that evaporation from some of the morning precipitation falling through dry air may have cooled the atmosphere to allow for a thicker layer of surface cold, freezing droplets on the way to the surface instead of them reaching the ground as liquid and freezing on things. Sleet is a good thing in this regard: Anytime we are getting sleet, we are not getting freezing rain. Sleet bounces off things, doesn't coat trees and power lines in ice like the freezing rain does. Sleet can be bad in this regard: It can accumulate on the roads more easily than freezing rain. Even warm roads can develop a slushy coating when sleet falls; this can freeze over later in freezing rain. If we get some rounds of heavy sleet this afternoon, even some of these warm asphalt roads in Roanoke will get slick, the way bridges and roads in outlying areas already are. Drive cautiously this afternoon, please. Don't presume everything is "just wet." Sleet and freezing rain can change things quickly. Ice Storm Warning now in effectPosted Jan21, 2007 at 11:27 AMThe National Weather Service in Blacksburg has issued an ice storm warning through 6 a.m. Monday for the expectation of 1/4 to 1/3 inch ice accumulation. Those kind of ice accumulations are capable of producing some damage to trees and power lines. So the worst case scenario, the one I labeled as a 15 percent chance a couple days ago (primarily ice), is the one that appears to be coming to fruition. Ice is definitely freezing on trees and exposed objects here in south Roanoke County where I am at. Most roads don't look bad, and many may not freeze because of recent warmth. But there will be icy patches, especially on bridges and overpasses, and less-travelled roads away from the city may freeze solid. The heaviest precipitation is yet to arrive, and now it appears it will be mostly freezing rain. Follow the latest on the National Weather Service-Blacksburg's Current Event page. Ice storm under wayPosted Jan21, 2007 at 10:08 AMUPDATE 10:50 AM: Southwest Virginia considered moderate to high risk for quarter-inch or greater ice accumulations today. It looks like the worst of all the possibilities is under way in Roanoke, as the precipitation is primarily freezing rain. The road surfaces may not freeze too bad at first because of days of warmth they have absorbed, but bridges and overpasses will freeze quickly. Exposed objects are already icing over. Let's hope this doesn't get too carried away, or trees and power lines will be an issue. There have continued to be reports of snow in the New River Valley, so it may not be freezing rain everywhere, though it likely will be before too long. However, it's obvious to me now by how fast we went to freezing rain in the valley that the folks at the weather service were on top of it overnight and correct in downgrading our snow/sleet expectations. Warmer air has moved in aloft. A week after 70-degree warmth, it looks like the snow lovers and the warmth lovers will both lose out today. Precipitation developing in Southwest VirginiaPosted Jan21, 2007 at 07:52 AMSnow is already being reported this morning in the New River Valley. Radar shows precipitation overspreading the area, though as we've discussed previously, much of it is not yet reaching the ground through the dry atmosphere. It will take at least another hour or two to do so in the Roanoke Valley, as the surface dew point is still in the single digits. When it does, expect to see 1-3 hours of mostly snow, 3-6 hours of mostly sleet, then freezing rain into the evening. The weather service has actually backed off the snow/sleet accumulation in Roanoke to "little or no snow and sleet accumulation" with one inch expected most other areas. Not sure I quite get that with several hours of sleet likely on top of beginning snow. We'll know soon enough. In any event ... a wintry mess is likely to develop on roads by midday, so be careful. UPDATE 8:10 A.M. -- A very fine sleet is beginning to fall here in south Roanoke County. I still would expect some snow as the atmosphere moistens and cools with precipitation through the morning, then primarily sleet into the early-mid afternoon, then an icy glaze on top of that. Temperatures will likely hold in the upper 20s to low 30s during the day ... and may even edge upward overnight after the precipitation stops. Late-night thoughts on Sunday winter weatherPosted Jan20, 2007 at 11:25 PMNow near midnight, radar is showing precipitation entering far southwest Virginia. This precipitation is almost certainly virga, or precipitation that evaporates in the very dry air aloft before it can reach the ground, but as the virga arrives it will be moistening and cooling the atmosphere, setting us up for whatever precipitation finally makes it to the ground. Seeing the robust nature of the precipitation shield and the lingering cold, dry air, I'm inclined to think we might come out with a bit more snow/sleet than current forecasts indicate. Perhaps more like 2 inches, locally 3, rather than the 1 in the forecast. In the end, I think we'll end up with more sleet than snow or freezing rain. Arrival time: Always tricky with these kind of overrunning precipitation shields, with the warm, moist air coming on top of the cold, but mid-morning, 10 a.m. or so, seems about right for the first flakes or pellets to plop on Roanoke. Earlier to the west, later to the east. You can add a plus or minus 3 hours factor to that. I know it could be rough on churches tomorrow morning trying to make decisions. I'm personally opting for an earlier service rather than a later one, just in case. Really, we're almost into a "nowcasting" phase. The best thing to do at this point, rather than forecast models or weather geeks like me speculating, is to look at radar. Two links to radar are below. I'll check back in after daybreak to see where we stand. Cold, hard facts about dry air and precipitationPosted Jan20, 2007 at 06:30 PMLook at the hourly Roanoke conditions today and note the dew points for each hour. It's been in the single digits a good part of the day, while the temperature climbed into the low 40s. The wider the difference in the temperature and the dew point, the drier the air is. Humidity readings have been running below 20 percent at times today. This is important how, you may ask. Precipitation moving in on Sunday will have to fall through this dry layer. That will mean two things: (1) A period of "virga," or precipitation evaporating before it hits the ground; you will see the radar light up but not see anything falling until it saturates the dry air aloft; (2) perhaps more importantly, evaporative cooling. As each snowflake, rain drop or sleet pellet enters that dry air and evaporates, heat is used up and the air cools. This will likely ensure that even if temperatures are a little above freezing when precipitation begins, the temperature will fall below freezing quickly and stay there shortly after the precipiation starts falling. If precipitation begins fast and furiously on Sunday, falling through this dry air, the cooling from cloud to ground could be so rapid that we would likely get more snow and sleet than freezing rain. But the current thinking is a more gradual start to the precipitation, which would start with snow and sleet before the upper levels become overcome by the warming aloft, changing it over to freezing rain. It's all very hair-trigger and borderline. We're within 12-18 hours now of seeing how it plays out in the real world, not on a silicon chip in a computer hard drive. Winter weather advisoryPosted Jan20, 2007 at 04:38 PMAll of Southwest Virginia is now under a winter weather advisory for Sunday, as depicted by this map from the National Weather Service in Blacksburg. Click here for the latest information on this storm from the weather service. No winter storm watches for this onePosted Jan20, 2007 at 10:56 AMYou may wonder why there aren't any winter storm watches out for Sunday. It takes an expectation of a quarter-inch of ice or at least 4 inches of snow to trigger a winter storm watch or warning in the Roanoke area; the bar goes up to 5 inches of snow a little west of here. This storm does not look extremely wet. It is actually weakening as it travels east, essentially just carrying a big slug of moisture on the jet stream. So it's likely we'll see only a quarter to a half-inch of precipitation in terms of liquid. That would translate to about 3-5 inches were it all snow, and would easily qualify as an ice storm worthy of a winter storm watch. But with precipitation expected to be mixed, it is unlikely that the snow will reach 4 inches or the ice will reach 1/4 inch. And everything that is sleet will reduce the totals on both, as sleet doesn't accumulate as deeply as snow for the same moisture content and doesn't accumulate on trees/power lines as does freezing rain. So most likely, we'll get an inch or two of snow/sleet with a tenth of an inch of ice on top. The precipitation looks as if it will begin by mid to late morning (in Roanoke; earlier farther west), probably just as snow flurries. Road conditions probably won't begin to deteriorate until midday to early afternoon (again, in Roanoke) as snow picks up then changes to sleet and freezing rain. This does not look to be a huge winter storm, but in a season that has seen so little cold or snow or ice, even a little wintry precipitation will be a big deal. |
