Precipitation moving into area; how much does it cool?Posted Jan31, 2008 at 09:59 PM
My best stab at it says we do end up as low as 31 for about 3 or 4 hours in Roanoke, enough for some ice on the trees. It could get quite slick in higher elevation and outlying areas, and on bridges, and I would not be surprised to see a few power outages as ice starts building on trees in many areas. This does have the potential to be a signficant ice storm across much of the area if the full extent of the evaporational cooling is realized and the cold air hangs in a few hours before the warmth modifies it. Once again, we're skating on the razor's edge. Latest Roanoke hourly conditions Many degrees of separation fuel ice threatPosted Jan31, 2008 at 04:07 PMCheck out the list of observations across Virginia and North Carolina. Note the numbers under "DP," or the dew point. Very, very low numbers in places away from the coast, single digits in many places. Roanoke's was 4 degrees at 3 p.m. A few are even below zero. The difference in the temperature and the dew point reflects how dry the atmosphere is. With temperatures in the 30s and 40s and dew points scraping zero, precpitation falling into that dry air near the surface will evaporate, taking heat out of the atmosphere. This evaporational cooling is where forecasters expect to get much of the cooling that would be necessary to trigger an ice storm. That cooling will eventually be overcome by warmth moving in from the south, but that may be hours later. It's another situation where a degree or two could make all the difference. Most of you will probably have some ice in the morning, even if it's just a little on car metal and trees, while a few of you may escape without any ice. I expect there to be about a 4-6 hour windown from 3 a.m. to about 7-9 a.m. where Roanoke may drop to 32 or even 30 or 31. I don't expect huge glaze ice issues on the roads in the immediate Roanoke Valley, but please do be careful driving, as even a small patch of ice can be treacherous. As you fan out from the Roanoke Valley in distance and elevation, conditions will be considerably worse. Probably just a few showers of rain/freezing rain and maybe some sleet before midnight, with the heavier stuff moving in after midnight.You can follow its arrival on the national Doppler radar loop. Latest advisories and warnings from the National Weather Service in Blacksburg. Ice storm warning is up -- will it verify?Posted Jan31, 2008 at 08:38 AMA big chunk of Southwest Virginia is now under an ice storm warning late tonight through midday Friday, including Roanoke County and every adjacent county, plus the New River Valley and everything north and west. Click here for the latest on warnings and advisories from the National Weather Service in Blacksburg. Everything about this storm system is more powerful than similar borderline ice situations we've had in the past year, particularly Feb. 13 and Dec. 16 of last year. The damming of cold, dry air ahead of the storm is stronger, which gives more opportunity for ice to develop, but so is the flow of warm, moist air aloft. That flow of warm air will eventually overcome the cold, but the longer it can keep from doing so, the more ice threat we will have. The weather service is thinking a quarter to a half inch of ice is quite possible over most of the area, hence the ice storm warning. That much ice would result in scattered power outages and some tree damage, locally very troublesome in areas that get the worst ice, but not an areawide paralyzing ice storm like many of you remember from 1994. The Roanoke Valley is always the big question. I will reiterate my thinking that the potential for the floor of the Roanoke Valley, all the way to downtown, to get some ice out of this situation is greater than in the 33-degree escapes Roanoke city had twice last year. The opportunity for evaporational cooling on the front end -- initial precipitation evaporating in cold, dry air at the surface, thereby lowering the temperature -- is much greater. I wouldn't be surprised to see an hour or two of sleet mixed in at the front end, either, which could lay down a cold surface layer. Higher elevations and the fringe areas of the Roanoke Valley are highly likely to see ice, and quite possibly the quarter-inch required to verify the ice storm warning. The trouble will come if the cold air holds in more stubbornly than expected on Friday. That is how this potentially significant ice storm could become a major one. For now, it looks like the falling rain itself will pull some warmer air toward the surface Friday morning, and an advancing warm front will get us above freezing before this can get extremely bad. Ice threat leads to winter storm watchPosted Jan30, 2008 at 03:28 PMUPDATED AT 5:30 PM The battle between the irresistible force -- warm, moist Gulf of Mexico air being pulled northward by a strong low pressure system to our west -- and the immovable object -- cold, dry air pushed southward by a high pressure system in southeast Canada -- will put Southwest Virginia in an atmospheric tussle that could lead to widespread freezing rain Thursday night and early Friday. As a result, the National Weather Service has issued a winter storm watch that includes the Roanoke and New River valleys and most counties along the Blue Ridge and points northwest. The irresistible force will win, and temperatures will rise above freezing Friday to melt the ice. It all depends on how much rain falls before temperatures can get above freezing how big a deal this will be. 
Latest HPC map puts moderate ice risk threat, in green, farther west than earlier maps The obvious question that arises for the immediate Roanoke area is whether the city can again pull out a 33-degree escape as we did on Dec. 16 and last Feb. 13. It is certainly possible that will end up happening, but three factors argue against it: (1) a greater degree of evaporational cooling, with moisture moving into cold, dry air at the onset, (2) more well-established cold air damming against the eastern slopes of the mountains than in those previous episodes, and (3) the overnight arrival of the precipitation. We could well see some sleet or even very brief snow on the front edge of the precipitation, which would help cool things that much more. It seems likely to me from here that we will see some ice even on the floor of the Roanoke Valley on Friday morning, but the low to the west will be so strong it will easily pull warm air northward to push things above freezing by mid-morning Friday. In an unusal twist ... the Roanoke Valley could actually end up being one of the last to warm significantly as milder air is pulled in at a higher elevation above the valley first. Would not be surprised if we struggled through the 30s during the day on Friday while some higher elevation areas to our south soared into the 50s. Rain on its way tonight, and more moisture where that came fromPosted Jan29, 2008 at 06:58 PM
The long-term outlook is still warmer than normal. At least we might get a little drought help the next 72 hours. The next wintry threat down the roadPosted Jan29, 2008 at 01:34 PMWe will get a fair amount of rain and wind tonight as a new cold front pushes through, and maybe some snow showers in the morning. Wind advisories and a few high wind warnings are out for tonight. The next big thing on the horizon is the ice storm potential on Thursday night and Friday. The Hydrometeorological Prediction Center already has us in slight to moderate risk of significant icing, defined as .25 inch or greater. This will be caused by a new low pressure system, quite a strong one, sweeping a lot of Gulf moisture up and over a cold air mass at the surface. Check out this map for forecasted precipitation totals -- yep, that's about an inch of liquid. Hopefully that won't all be ice for us. If the linked maps above look a little different than I described, it could be that they have been updated by HPC. I'll get some fresh information up later this evening. Not really looking that warm for next weekPosted Jan26, 2008 at 05:04 PMThe earlier arrival of a cold front and rain on Tuesday may stymie the potential warmup this week ... so maybe not the 60s like I hinted at in today's Weather Journal column. Still, temperatures will be well above freezing so there will be no significant threat of wintry weather with the next system. The late week system is another matter ... it is possible that some moisture may make it into cold air, at least to start. No use getting too specific about that now. The picture will change many times before then. One goes north, one goes south, both miss usPosted Jan25, 2008 at 05:10 PM
Typical mid-winter snow flurry-a-thonPosted Jan24, 2008 at 03:31 PMWe're in the typical snow flurry pattern this afternoon, as a new punch of Arctic air is bringing northwest winds that are carrying moisture up the western slopes of the Appalachians, where the lifted moisture condenses into low and mid level clouds and spits out snowflakes. While the western upslope regions near the Virginia-West Virginia border and points west get the most out of a setup like this -- a few inches may accumulate there -- some of the snow bleeds eastward over the mountains. In the Roanoke Valley, we have been seeing off-and-on snow flurries today. While you can never rule out a briefly whitened ground in a localized heavier snow shower, these usually don't amount to much for us on this side of the Appalachian spine, and that looks to be the case again today. UPDATE 9PM: Just drove home to south Roanoke County for a while to find a dusting of snow from about the Tanglewood Mall area southward, including some on parking lots and even streets. A heavier snow squall must have blown through that area earlier this evening. If you were under one, you had quite a show for a few minutes. Elsewhere, such as back here in downtown Roanoke, just some flurries. Peering down the road: A month-ending warmup?Posted Jan23, 2008 at 06:52 PM
Cold to return, but not much prospect of moisturePosted Jan23, 2008 at 09:48 AMThe rest of the week and the weekend doesn't present anything overly interesting in terms of precipitation-makers. It will be cold many mornings with lows slipping back into the teens and maybe some single digits. Weak disturbances moving through the next couple of days from the northwest could trigger a few snow showers, and another weak system approaching from the west could bring another borderline rain/ice/snow situation over the weekend, but not a very heavy one, it would appear now. It's well into next week when the next significant chance of widespread precipitation occurs, and temperatures may begin moderating above their present cold levels by then. Still, we've always got to keep an eye on each weak disturbance because it doesn't take much of a change to make something a little bigger than it looks now. But cold and mostly dry appears to be the mode for the rest of the week. Not much to this one for Roanoke, New River valleysPosted Jan22, 2008 at 02:52 PMWarm air advection at relatively low levels is overcoming any evaporational cooling effects, and very little precipitation is falling. So we've mostly been getting some very light rain with a few pings of sleet as temperatures have warmed above freezing. The sun has even come out briefly in the Roanoke Valley. This winter weather threat, if you can call it that, looks to be about over locally. If the precipitation had been harder, it might have cooled things enough aloft for more sleet and snow to make it to the surface. But this was not a really solid setup for wintry precipitation, so this marginal event not really coming through with anything significant is not a big shock. Dry atmosphere has mixed effects on today's precipitationPosted Jan22, 2008 at 11:07 AMI'm writing extensively about evaporational cooling for Wednesday's Weather Journal column in The Roanoke Times, but today we're seeing a good example of the push and pull dry air has on precipitation events. On the one hand, it tends it eat up moisture moving in and stop precipitation from reaching the ground. But it also can cool the atmosphere a lot more, so that once the precipitation does reach the ground, if it does, it begins as something frozen. Precipitation is struggling to move eastward into our area today, but our dew points are very low in the single digits. I'm still not terribly impressed with this setup, but I think it's quite likely that if we ever do see precipitation today, it will begin as a round of snow and/or sleet and then change to freezing rain or rain depending on how cold it is at the surface by that time. Can the precipitation get here before it warms up?Posted Jan22, 2008 at 01:13 AMVery early this morning, clouds are starting to stream in from the southwest ahead of the next weather maker that may give our area another borderline rake with wintry precipitation today. Normally, this kind of atmospheric setup -- with a low-pressure system crossing the Great Lakes dragging Gulf moisture ahead of a cold front -- wouldn't even be close to a wintry threat in our area. But the depth of the cold air mass that has been place, some leftover snow cover in many places locally, very low dew points to start wtih and just the fact that it is the point on the calendar which is historically coldest all mean that the cold air has a better chance to hang on for a while against the raging tide of warmer, moist air from the Gulf of Mexico. The amount of moisture that will fall to the ground doesn't look like much, and the key will be how much of it arrives early in the day before the warmer air can take hold and get us above freezing. If the system can lay down any kind of ice and snow layer early in the day, warming above freezing will be harder and slower. With the precipitation barely crossing the Mississippi River as I type this, it'll take some new development out ahead of the main band to make that happen. We might see a quick burst of snow on the back side of the rain shield with the cold front tonight. The next few days look cold as the Arctic air is renewed over our area. An Alberta clipper might brush us with some snow from the northwest Wednesday night and Thursday, and then there could be another moist system from the west by the weekend. Follow the latest National Weather Service-Blacksburg radar here Click here for the latest advisories from the weather service Current hourly reports from Roanoke and Blacksburg Tuesday looks kinda crazy, but probably not much to itPosted Jan21, 2008 at 12:35 PMAs southwest winds try to bring in warmer air on Tuesday, they will bring in some added moisture, too. This moisture moving into cold air will bring the chance of precipitation ... snow, sleet and freezing rain in the morning cold air, gradually mixing with more rain as temperatures rise above freezing. Then, as a new cold front arrives early Tuesday evening, the whole thing could go back to snow for a while. It looks like precipitation amounts will be light, though it doesn't take a lot of freezing rain to cause problems. Don't expect a lot of accumulation out of this one, just some nuisance precipitation. UPDATE 5:30 PM: The National Weather Service has issued a winter weather advisory for muc of the area on Tuesday. Again, it doesn't look like a whole lot of preciptiation, but it could be cold enough for some ice or snow for a while, enough to possibly be a nuisance for a short while. Click here for the latest advisories and warnings from the weather service and click here for the latest National Weather Service radar. Very cold tonight ... but it's been even colder on this datePosted Jan20, 2008 at 06:34 PMWith widespread lows in the single digits to low teens -- and a few snow-covered valleys and higher mountaintops possibly dropping below zero -- Monday morning may be one of the coldest if not the coldest in the last decade. But there is virtually no chance of any daily records being set for January 21. That's because it is the 23rd anniversary of the massive Arctic plunge of 1985, when temperatures across the eastern two-thirds of the U.S. dipped to levels rarely or never seen. Roanoke's all-time record low of -11 and Blacksburg' all-time record of -18 were set on Jan. 21, 1985, as was the state's all-time official record low of -30 at Mountain Lake. Tuesday is our next precipitation chance. A surge of warmer air will bring in Gulf moisture, but how strongly the cold air holds its ground will determine how much of that moisture falls as something frozen. We'll get back to that question on Monday. Split jet-stream flow might bring wintry precipitation chancesPosted Jan19, 2008 at 09:53 PMYou've got to love it when the Deep South gets a taste of snow as parts of Mississippi, Alabama and Georgia did today. The snow tonight has been raking central and eastern North Carolina up into the Tidewater area of Virginia. The week ahead will bring a sharp shot of Arctic cold the next couple of days, with lows in the single digits to low and mid teens likely across the area by Monday morning. After that, there will be a couple of shots at precipitation -- the first around Tuesday or Wednesday, the second very late in the week or during the weekend -- as disturbances move eastward. From here, there doesn't look to be any blockbuster storms on the horizon, but the split-flow jet stream pattern that is expected to set up later this week -- dividing the jet stream into a cold-supplying northern branch and potentially moisture-supplying southern branch -- will offer a few shots at wintry precipitation. Each one will have to be monitored for its specifics, the most critical being the storm tracks and depth of cold air available. The early outlook is for warmer-than-normal temperatures to return near the start of February, but that is subject to change, and how February turns out overall is still very much in question. Let's see just how close this storm getsPosted Jan19, 2008 at 12:29 AMI like the high-resolution national radar available by clicking here, but our ancient home computer can't handle it (works well here at the Roanoke Times, though). If your computer has the juice, you can follow today's storm system to see if it can scrape closer to us than expected or even surprise us with more snow than we think now. Just because you see echoes over us doesn't mean it'll be snowing ... there is dry air to overcome by any falling precipitation. But if there are many shades of green streaming in, we would have to re-think things. Accumulating snow looking less likely for SaturdayPosted Jan18, 2008 at 03:49 PMThere is more and more indication that the low on Saturday will be too weak and track too far to the southeast to do more than throw a few snow showers into our region, and even that is doubtful. The trend on the forecast models is for the back edge of the precipitation to be farther east. The Hydrometeorological Prediction Center is now highlighting an area farther southeast, mostly in North Carolina, for a slight chance of 4-plus inches of snow. All the winter storm watches and snow advisories are going up along that general path, with the closest snow advisory to us in the Danville area. There is still a chance that the system could make a jog farther to the northwest, or throw back a larger shield of precipitation -- I've seen each of those happen before -- but this will probably not be our snow, but rather Greensboro's, Raleigh's, and maybe Richmond's and Norfolk's. Be cautious if traveling in that direction. Saturday's snow potentialPosted Jan18, 2008 at 01:21 PMIn some ways Saturday is a more tricky forecast than even Thursday was. There came a point Wednesday when it looked almost certain we would see precipitation and most of it would be frozen. The frozen part is even more certain for Saturday, owing to very deep Arctic air moving into many layers of the atmosphere, but the precipitation part is not certain. A fast-moving low to the northeast from the Gulf toward the Carolinas could (1) spread precipitation into our area, (2) spread moisture but not enough to reach the ground through drying air or (3) skirt by us to the southeast. And even within possiibility 1, still a slight favorite of the three, there is the question of how much moisture. As of now, it looks like light amounts, translating to an inch or two of snow mostly from Roanoke south and east. But if the low takes a slight northward jaunt, it could spit out more snow. Also, with the atmosphere as cold as it will be aloft, we are likely higher snow-to-liquid ratios than in Thusday's storm. In other words, the snow will likely be drier and fluffier, smaller flakes with longer arms, and we could get more snow out of less moisture. The same amount of moisure an inch of snow on Thursday might be enough for 2 on Saturday. As of early afternoon, the Hydrometeorological Prediction Center still has our region in a slight risk of 4-plus inches of snow for Saturday. (If this linked map now longer shows us in the circle for 10 percent chance of 4-plus inches, you'll know HPC has changed its mind. A new update is due around 4:30 p.m.). The winter storm watches, however, are going up in central and northeast North Carolina. So if I am breaking down percent chances for Roanoke and Blacksburg right now, it would look like this: 1-3 inches of snow: 30 percent In other words, still pretty even odds of any of those things. Hopefully, things will be clearer by evening. Thursday's forecast only really came into focus by Wednesday night. Where we go from herePosted Jan17, 2008 at 10:43 PMFriday will be a "warm" day, though not as warm as it would be if there wasn't snow cover. We will probably see sunshine and highs in the 40s. There'll be lots of slush around. Saturday through Monday will not be "warm" in any sense of the word. On Saturday, we're going to have to keep an eye on this low moving northeast out of the Gulf. Best early call is that we get some light snow on Saturday while heavier stuff hits North Carolina and eastern Virginia, but it wouldn't take much of a shift to change that whole idea entirely. Then, the bitter cold sets in Sunday and Monday. It's possible some locations in Southwest Virginia won't make it to 20 on Sunday. Wrapping up the little winter storm that couldPosted Jan17, 2008 at 04:56 PMThis little winter storm really made the most out of a not-so-good setup. The cold air was departing, warm air was encroaching aloft, there was nothing to hold in the cold air, the low pressure system was not that impressive, it took hours to saturate the dry air over our area, and yet somehow enough moisture caught enough cold air for just long enough for a solid snowfall. It's the opposite of so many other cases when a classic winter storm setup for our area goes awry somehow. Yeah, it wasn't perfect for all the snow lovers, ending as sleet and freezing drizzle, but two days ago it looked like it would be a lot of sleet and ice with just a little snow to start, instead of the other way around. In football talk, this was a busted play that went for a big gain. Not quite a touchdown, but enough of a gain to placate snow-starved winter lovers for a little while ... at least until Saturday. The warmth-lovers didn't like it, and will despise the next 5 days or so, but your time is probably coming back for February. In terms of recent years, you have to go back to Feb. 11, 2006, to find a similar snowfall over such a widespread area of Southwest Virginia. That one left 5 inches in Roanoke and 3-8 inches across the area. Last year's Feb. 6 Alberta clipper dumped some heavy snow over part of the area, especially Alleghany and Rockbridge counties, but left folks south of the Roanoke and New River valleys with little to nothing. Feb. 28, 2005, was the last time we had a snow that was clearly heavier than this one ... 8 inches in Roanoke, 6-12 across the area. I probably don't need to remind you again that it has been 12 years since we had a 12-inch-plus snow in Roanoke ... and we had THREE such storms that year, including the Jan. 6-7, 1996, two-footer. (I say "we" even though I was living 730 miles to the west back then.) Below on the extended entry is the National Weather Service's last list of snow totals across the area from today. Peruse the comments on previous blog entries and you can find some other snow totals reported by Weather Journal readers. A big thanks to everyone today who visited this blog and who commented! You are turning this into a great online community. Continue reading "Wrapping up the little winter storm that could" » Anybody for a sequel?Posted Jan17, 2008 at 04:02 PM
For the remainder of tonight -- a winter weather advisory is in effect as freezing drizzle may cause some icy spots. Temperatures are going to hold steady or maybe creep upward a tad. Unlike many of our winter storms, there is no cold air ready to sweep in behind this one ... in fact this winter storm was caused by warm air making a move against the cold air. Will be back to recap today's storm later. The end is in sightPosted Jan17, 2008 at 12:35 PMNational Weather Service radar shows the back edge of the precipitation not that far west of us now, probably only another couple hours worth. While there may be some light freezing drizzle or some of those tiny ice bits I call "snizzle" into the evening, it looks like the bulk of this winter storm is over, and will end with just a little more sleet. Accumulations of 4 to 6 inches are common in Roanoke and New River valleys, so this did meet the low-end criteria of a "winter storm." Certainly it was a pleasant event for many snow-starved winter lovers, but it caused its share of problems, too. Let me catch a deep breath and take a look at Saturday's potential. I'll also post final accumulation reports later today. Winter storm warning goes upPosted Jan17, 2008 at 09:21 AMThe National Weather Service has come around to issuing a winter storm warning for the Roanoke and New River valleys, bascially everybody from Bedford and Franklin counties westward. Really, in some parts of the valleys, we have already hit the winter storm criteria threshhold with some reports of up to 4 inches in the Roanoke Valley and up to 6 inches in New River Valley. Here is the latest on advisories and warnings from the weather service. How much snow you end up with the rest of the day depends on two major factors: (1) how many times, and for how long, you get under one of the heavier snow bands (which you can follow on radar by clicking here) and (2) how long it can stay snow before the sleet and freezing rain get mixed in. Freezing rain has worked into Southside Virginia around Martinsville and Danville, which is what was expected. Hourly observations at sites in Virginia, North Carolina and West Virginia can be found by clicking here. On the extended entry below I have added the most recent list of snow totals reported to the National Weather Service in Blacksburg. Feel free to add comments reporting on the snow at your location. Winter storm arrives right on schedulePosted Jan17, 2008 at 06:16 AMI saw the first flakes at my south Roanoke County residence at 4:45 a.m. Yes, I got a few hours of sleep. This winter storm arrived pretty much right on time. What will be interesting to see now is if and when we get upgraded from a winter weather advisory to a winter storm warning in the New River and Roanoke valleys. Based on the intense way the snow has started, radar trends, snow continuing to fall far south of us, model run data on moisture, I think we're headed for at least 4 inches today. The National Weather Service in Blacksburg recently upgraded Grayson County to our south to a winter storm warning, but is holding the advisory for 2-5 inches elsewhere. Sleet and freezing rain will come, eventually, but it may not be a factor until the precipitation is winding down in the afternoon. Yes, Southwest Virginia, it looks like our biggest, most widespread snowfall since mid-February 2006, maybe since late February 2005. Thanks everyone who commented overnight. Please note that there is often a delay between posting a comment and it appearing on the blog. I have to approve each comment, and that involves sifting through spam to find your jewels. |
