Memorial Day is when I’ll be headed out with the Hokie Storm Chasers. The first group returned safely today after an intense trip. Looks like it’s going to get active in the central U.S. again in the coming week.

Wednesday’s Weather Journal column on this past week’s sudden burst of tornadoes linked here.

Tornado chances in our region will be very low, not quite zero, but very minimal, as the same storm system that has affected the central U.S. with violent storms the past several days approaches. Severe storm chances, primarily strong winds in some storms, will be somewhat higher, but also not extraordinary. As is so often the case in Southwest Virginia, the greatest amount of atmospheric shear aloft will be traveling northeast toward the Great Lakes, and we will be left primarily with a cold front moving west to east. It will be colliding into thick humidity, so some degree of strong to locally severe storminess is possible just from that.  The front is not expected to arrive until late Thursday — until then, spotty showers and storms may break out in today’s warmth and humidity, and more organized frontal-enhanced storms overnight and during the day Thursday. Locally heavy rain will be the greatest threat, as it has been for the past several days, with high winds (brown in inset Storm Prediction Center map shows 5% risk  of 58+ mph winds through Thursday morning within 30 miles of any given location.)  (UPDATED MAP THROUGH REMAINDER OF THURSDAY LINKED HERE)

Behind the front, we could have a pretty chilly morning on Saturday — lots of 40s, possibly some 30s in higher elevations and outlying rural valleys west of I-81 — but the weekend looks mostly dry with highs in the 70s. There may be some chance of showers and storms on Memorial Day.

Hokie Storm Chasers were just north of Houston last night after chasing storms in central Texas on Tuesday. They begin the long trip home today, with the second trip I’m aboard leaving Sunday or Monday.

UPDATE 11 PM: While it doesn’t appear a well-organized cluster of storms will plow through Southwest Virginia tonight, some storms with heavy rain and gusty winds have been occurring primarily west of I-77 and north along the Virginia-West Virginia border. These storms may linger a while overnight, even drift north or east a little bit. We’ll have chances of storms Wednesday through Friday as a cold front approaches from the west and daily warmth and humidity continue. END UPDATE

Some counties in Southwest Virginia west of Interstate 77 are under a severe thunderstorm watch until 10 p.m. because of cluster of thunderstorms that has actually propagated eastward out of Monday’s tornado outbreak in Oklahoma. The line of storms now in central Tennessee and Kentucky is encountering more unstable, sun-heated air as it moves eastward and poses mainly a large hail/strong winds threat eastward. Though there are some reasons this line may diminish later crossing the Appalachians or running out of daylight, I would advise Roanoke/New River valley folks to keep at least an eye and an ear out for this (RADAR LINKED HERE) as it approaches this evening. Sometimes these “mesoscale convective systems” — MCS’s, in weather geek talk — sort of get a mind of their own about where and how fast they want to travel, as their updrafts and downdrafts actually change the weather conditions around them, defying forecast models. The cluster is moving east-northeast, so it’s pretty much on a beeline for all of Southwest Virginia, IF it were to maintain its strength and heading. Other scattered showers and storms may pop up this afternoon simply because of the heat and humidity.

Courtesy Hokie Storm Chasers Facebook page

At left is an image of a possible tornado photographed by the Hokie Storm Chasers on Monday somewhere near the Red River (southern Oklahoma or northern Texas). We say “possible” because it is unclear if the funnel is in contact with the ground behind the cluster of trees. As you know, Monday was a vicious severe weather event in the southern Plains, particularly because of the deadly tornado at Moore, Okla., in the Oklahoma City suburbs. That storm has recently been rated EF-5, the highest on the scale, based on some new findings by the National Weather Service office in Norman, Okla. The Hokie Storm Chasers are in Texas today on what is likely to be their last day of tagging severe storms before the first team returns home. A second team is headed out on Sunday or Monday for 10+ days — I will be on that trip as a co-leader, which I will be posting about on the Weather Journal blog. In the Weather Journal column also posted at top, I discuss the current upsurge in tornadoes as I prepare for this trip. 

* Destructive and deadly tornado in Moore, Oklahoma — same place leveled in 1999 F-5 tornado.

* Flooding  from a localized but very heavy downpour in Roanoke.

The Hokie Storm Chasers were on storms south of Oklahoma City and were not near the Moore, Okla., tornado.

In Southwest Virginia: Expect a warm, sticky day on Tuesday (after some locally thick morning fog) with highs in the 80s and some scattered afternoon showers and thunderstorms — fewer than on Monday, but a few spots could some locally heavy rain, as occurred in downtown Roanoke on Monday.

Hokie Storm Chasers look at a wall cloud on a supercell near Hays, Kansas, on Saturday. — Courtesy, Hokie Storm Chasers Facebook page

The Hokie Storm Chasers — who have had two days of  tagging big supercell thunderstorms but have yet to spot a tornado amid the Plains outbreak — will have another opportunity today with yet another moderate risk of severe storms. On Sunday, the team was on a storm that spawned a tornado in Wichita, but had to bail out for safety reasons just before it entered the metro area — we generally do not chase storms in metro areas. This first trip — also being covered by WDBJ Channel 7 in the field for the first time — will be wrapping up late this week, and I’ll be Plains-bound with the second group starting near Memorial Day, my ninth year to be on board, eight as a co-leader.

Speaking of Memorial Day — that weekend is looking really nice for Southwest Virginia at this point, with warm temperatures and likely dry weather.

The Hokie Storm Chasers are positioning for what could be an intense severe weather day in the central/southern Plains, likely somewhere in Kansas. This new tracking map shows their position, with the ability to zoom in (so the chase van isn’t 100 miles long) and add radar, warning boxes, and weather station observations. In a couple of weeks, you can follow my position on this map. Also, eventually, a webcam sending back pictures every several seconds from the front windshield of one of the vans will be operational. Here is a look back at their Wednesday chase in north Texas.

Southwest Virginia’s weather will also be unsettled this weekend, though not in the same way as the Plains states. The same upper-level low that spawned Wednesday’s deadly tornadoes in Texas is moving across the South, and will most directly influence our weather later today and Sunday. This will mean lots of Gulf of Mexico moisture (and eventually some draw off the Atlantic, too) being pulled around against our higher terrain. The coverage of rain and thunderstorms will be greater this afternoon and evening than it was Friday, with just about everyone in the region getting wet, and some getting heavy rain. The Weather Prediction Center is showing the potential for an inch or more over much of the area through Sunday morning … again, this map may be a little more smoothed out than what actually occurs, with more patchy/streaky amounts based on terrain and where individual bands of rain and heavy storms set up. The best bet for outdoor activities today is the earlier, the better, but keep an eye on radar and be prepared to go inside. Sunday is also likely to be a wet day, maybe not constantly, but frequently.

A RATHER OBNOXIOUS PATTERN SETTING UP THROUGH THE FORECAST PERIOD. — National Weather Service-Blacksburg forecast discussion

What makes it obnoxious? For the next 4 or 5 days, we’ll be dealing with a diffuse frontal boundary drifting around somewhere near us. Meanwhile, the same upper-level-low that sparked the tornadoes in Texas will be wobbling our direction, spreading out and circulating Gulf of Mexico moisture and periodic disturbances across us. The combination of a difficult-to-define front and troublesome-to-time disturbances means that there will be a middling to low likely (40-70 percent) chance of showers and storms throughout the weekend. Timing exactly when it may rain or storm in any given location is an exercise in futility — obnoxiously frustrating with graduations and sports and other spring events on tap across Southwest Virginia –  though afternoons and early evenings, when daytime heating (70s-low 80s) will be in play to destabilize the atmosphere, will tend to have higher chances of showers and storms. Late Saturday may be the peak of rain potential in our region, as the upper-level low comes into play most directly by streaming moisture and some upper-level wind energy. The Weather Prediction Center is highlighting our region for 1.5 inch or more of rain through Sunday evening, but I really doubt the pattern of heaviest rain will be the neat little smoothed out purple egg shape shown on the map. It will probably be more splotchy and patchy, with every location seeing some rain, but some seeing multiple inches while others get tenths. Some strong to locally severe storms will be possible, especially in the afternoons, but the tendency for more pervasive showers/clouds in the day and the much weaker upper-level winds than what Texas experienced will ensure no repeat of what happened there.

Meanwhile, deepening low pressure and an associated cold front in the Plains states will be setting that region up for what may be its longest and most extensive severe storms outbreak so far in 2013 — that’s where the Hokie Storm Chasers will be figuring out where to place themselves on the ongoing first of two trips. Eventually that system will help clear out the obnoxious pattern, but when and how exactly that will happen next week is a little fuzzy.

For Southwest Virginia, if you have outdoor plans Friday through Sunday, pay attention to the radar. It won’t rain anywhere constantly , but everywhere it will rain sometimes.

We don’t have the kind of ingredients the Dallas area did, but there will be a stationary front draped just north of us for a few days. This will be close enough that some waves moving along the front could trigger some scattered showers and storms in or near our area in the warmth and some humidity that has built. A repeat of Wednesday’s not-quite-record high temperatures (91 at Roanoke, 85 at Blacksburg) probably will not occur with more clouds and a bit thicker humidity, but highs in the 80s are likely, and that will be enough to help cook up some showers and storms. It appears we have several days of the spotty/chancy kind of showers and storms ahead, with periodic weak impulses and this front drifting around. By next week, we’ll finally get a push from a cold front to the west to clear this out a bit, with possibly a stronger, more organized round of showers and storms as it pushes through. There are some indications of another cut-off or at least slow-moving low developing over the eastern U.S. next week that could spin in a few days of cooler-than-normal temperatures. We haven’t entirely shaken the “blocky” weather pattern of slow-moving features and south-displaced cool air masses just because it got suddenly hot on Wednesday.

And that’s what we’re about to experience, again. After Tuesday morning’s scattered frost, temperatures are likely to be 15-20 degrees warmer to start Wednesday … and then shoot up into the 80s by Wednesday afternoon. It wouldn’t be totally out of the question for Roanoke to record its second 90-degree high of 2013 on Wednesday, though the forecast high is just a tad cooler. After a record-breaking low of 35 at Roanoke and record-tying low of 31 in Blacksburg on Tuesday morning, May 15 record highs of 92 at Roanoke (1962) and 88 at Blacksburg (1985) will probably be just a wee bit out of reach — but not inconceivable. Westerly winds blowing down the slopes of the Appalachians will enhance the effects of the strong warm air advection rolling in from the west to produce the high temperatures 10-15 degrees above normal for mid-May. We’re not the only part of the country experiencing temperature whiplash, or even the part experiencing it most severely — some parts of Nebraska, South Dakota and Iowa soared past 100 on Tuesday, just a couple of days after sub-freezing lows and less than 2 weeks after abnormally late and in some cases record-large snow for May. Omaha, Neb., for instance, has recorded its earliest 100-degree temperature and its heaviest May snow just 11 days apart.

Some hard-to-time disturbances and ill-defined fronts will begin to affect our weather by Thursday continuing well into next week. That’s why you see forecasts with chances of showers and thunderstorms stretching out for days. In time, those forecasts may become a little more focused on particular periods with higher chances of showers and storms.

The Hokie Storm Chasers, who left chilly Blacksburg on Tuesday morning, may see some severe storm action Wednesday afternoon/evening in north-central Texas, or somewhere reasonably nearby. Targets sometimes easily shift 100+ miles as new data develops on these trips. We’ll see if they come up with any memorable storm photos on the second day of this first trip.

The first group of Hokie Storm Chasers head out Tuesday morning for 8-12 days or so in the central U.S. Looks like they’ll get a rather potent set-up for the weekend in a season that hasn’t had much severe weather. While preparing for my own journey with them on the 2nd trip starting near Memorial Day, I’ll be following this first trip closely from home base. We have a few new tracking features that I’ll share on here as they become operational.

The storm chasers may have to scrape a little frost off the windshield leaving home base.  However, you may have noticed a somewhat odd configuration of frost and freeze alerts for Tuesday morning from the National Weather Servie in Blacksburg, with the freeze warning primarily along and north of I-64, and the frost advisory primarily east of the Blue Ridge, leaving most of the Roanoke and New River valleys west and southwestward out of them. This configuration is a result of the expectation that light southwesterly winds and possibly mid to high level clouds will stymie the radiational cooling slightly in the areas from the Roanoke and New River valleys south and west. Still, with many lows in the 30s, there is likely to be at least patchy frost, especially in any valleys protected from any developing breezes. The record May 14 low of 37 at Roanoke could be challenged tonight, nonetheless, and the record of 31 at Blacksburg (both records date were set in 1996) definitely isn’t entirely safe if the weather service is even just a little wrong on the timing of the southwest winds and possible clouds. Roanoke’s latest freeze of May 11, 1966, will very likely stand, but I wouldn’t rule out a few 32-degree lows in the some of the areas rimming the Roanoke Valley. In any event, it does not appear a fruit harvest disaster is afoot for the region (per Lindsey Nair’s report on roanoke.com today), but it would be wise to protect any tender outdoor plants especially in areas away from Roanoke’s urban valley floor.

The southwest winds expected to start overnight west of Roanoke are the start of a warm surge that will overtake our region through Thursday, with highs returning to the upper 70s to mid 80s in most areas on both Wednesday and Thursday afternoons.