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Weather Questions tagged “tornadoes”(remove tag filter)
Question: Can the severity of a tornado (EF scale) be estimated using the Doppler radar? — Byrd F.
Answer: Not in any consistent, reliable manner, because radar resolution and the height of the lowest usable radar beam elevation both limit the ability to resolve detailed velocity estimates for the tornado funnel itself, and even in rare cases when the tornado passes unusually near the radar site, the lowest usable wind estimates will be well above ground level. There have been some good measurements of wind speeds in tornadoes made with mobile research Doppler radars, but in those cases as well, the measured wind speed were generally at least 1-200 feet above the surface. There are some research efforts underway to develop usable correction factors to estimate surface speeds based on radar measurements at higher altitudes, but this would still leave the problem of most tornadoes being too far from the radar to resolve the funnel wind speeds (when you see the rotation couplets often shown to note the potential location of a possible tornado, the inbound and outbound wind maxima generally correspond to the much larger mesocyclone within the storm, rather than the rotation of an individual tornado funnel, and the speeds or shear measured in this case would not give a good indication of the tornado intensity).
For the time being, the great majority of tornado intensity ratings will continue to be based on post-storm assessments using the survey techniques and damage indicators that resulted from the development of the Enhanced Fujita scale.
Aug. 14, 2015 | Tags: tornadoes, weather radar
Question: On the mobile app for WRAL weather alerts, I am seeing alerts like "dangerous storm approaching 0.9 BTI". What is BTI and what are the thresholds? How does it differ from a severe thunderstorm or tornado warning (which I know means those are imminent)? — Diane Wade
Answer: We partner with a company called Baron Services for some of the data and features in the new app. BTI = “Baron Tornado Index”. It’s a scale of 0-10, with higher values indicative of environments progressively more favorable for tornadoes. They’re processing observational, radar, and model data in real time to assess the tornado favorability of an environment that a storm is in. Numbers higher than 6-7 or so should warrant attention, as well as numbers that increase progressively from radar scan to radar scan. In the case you noted, the value of .9 would be quite low and indicate poor conditions for a tornado to exist or develop. That doesn't mean that the storm couldn't produce intense straight-line winds, lots of lightning, large hail or heavy rain.
The BTI, and the alert for a potentially dangerous storm, are automatically generated and meant to highlight approaching weather that could be important to know about, but they differ from a severe thunderstorm or tornado warning in that those are issued by humans (at the nearest National Weather Service office) who have determined that based on all available information, an existing storm is either producing or is about to produce severe weather (winds gusting to 58 mph or more and/or hail reaching 1" in diameter or greater), or that a tornado has formed or is likely to develop very soon.
Jul. 30, 2015 | Tags: preparedness, severe weather, tornadoes, wral.com
Question: Was there any evidence of a tornado with the storm that came through near Holly Springs 7/5? I saw what looked like a tornado moving in the sky before the fireworks and took a picture. — Sherry Holloway
Answer: We didn't see anything in archived radar for that time frame that indicated a typical tornadic storm cell, and the overall structure of the atmosphere, while conducive to gusty storms and brief heavy downpours, was not favorable for tornadoes to form. There can be spinups and transient rotations that don't show up on radar, however. While these aren't classic tornadoes, they can look similar and are sometimes referred to as gustnadoes or landspouts, depending on the mechanism that forms them.
In the case of the photo you sent along, we're pretty sure what you're looking at there is an edge-on shot of a "shelf cloud" that often precedes gusty winds ahead of an approaching thunderstorm. They are often perceived to be possible tornadoes when a part of the cloud is a good distance away so that it disappears behind the horizon. There are a number of resources online with good explanations and images of these kinds of clouds. Just do some searches for "shelf clouds" and "arcus clouds" and you'll get some good background on what was happening. Anyone else interested in seeing the photo Sherry sent in can find it posted at scontent.xx.fbcdn.net/hphotos-xfp1/v/t1.0-9/11666302_851173198263073_7747299980481654533_n.jpg?oh=0c6d88c2bbb829796b2419a8f4d626dd&oe=561A5477
Jul. 27, 2015 | Tags: clouds, past weather, thunderstorms, tornadoes
Question: What do you think the percentage odds were for us in Lillington to see a Tornado on July 14, 2015? — Steven Schlink
Answer: On that date, our part of NC was under an "enhanced risk" as assessed by the NOAA Storm Prediction Center (SPC). "Enhanced" is a new category that represents the higher end of the older "slight risk" category. In this case, the atmosphere appeared to favor the potential for straight-line wind gusts more so than tornadoes, but in early morning projections for the day ahead, SPC estimated about a 5% chance of a tornado occurring within 25 miles of any point in our area. This probability was lowered to 2% in an updated projection issued around daybreak.
Note that this kind of value is used because the probability a tornado will strike at a specific location is extremely small, even on days when there is a significant chance that a tornado could form somewhere within a large region. This is because tornadoes, compared to many other weather phenomena, are very small and tend to affect very limited areas (of course, where they DO strike, the impact can be severe, a good example of a low probability, but high consequence phenomenon). The earlier, higher value indicated by SPC in this case (5%) represented, then, about a 1 in 20 chance that a tornado would occur somewhere within a 1963 square-mile area (25-mile radius) surrounding your location, and the later update reduced this chance to about 1 in 50.
In the end, most severe weather associated with the pattern in place on that day was concentrated west of the Appalachians, and in NC, all reports of wind damage and large hail were confined to the mountains. No tornadoes were reported in our state during that event.
You can see a description of the new severe storm outlook risk categories used by SPC, which includes five levels compared to the previous three, at www.spc.noaa.gov/new/images/Outlook-category-descriptions.png
Jul. 23, 2015 | Tags: cool sites, past weather, preparedness, tornadoes
Question: Haven't been able to catch the news since last Sunday because I have been clearing extensive storm debris from 2 cells that formed into one right at my rural house in Pittsboro/Moncure Sunday, July 5th around 8:00pm. Have you had any reports or confirmed cells from that area? Pretty sure we had a small tornado but because of being rural, and no direct neighbors, other than our home, believe it mainly hit in unoccupied areas. Very interested in finding out more. Thank You! — Susan Turner Neff
Answer: The storms you noted formed in an overall environment that was not favorable for developing tornadoes, but did favor some cells producing strong downburst winds. The NWS issued a severe thunderstorm warning for Chatham County a little after 7:30 PM Sunday evening, and extended that over eastern Chatham and parts of Lee and Wake just after 8 PM. There were several reports of trees down in the area associated with gusts from the storm. Radar imagery from the event appears to confirm that winds would have been associated with thunderstorm outflows rather than a tornado, as reflectivity patterns did not include hook echoes and inflow notches usually associated with tornadoes, and Doppler velocity displays didn't appear to show organized rotation indicative of a mesocyclone. It's worth remembering that "straight-line" winds are quite capable of damage similar to lower end tornadoes, and the damage can be confined to rather short and/or narrow swaths in what are known as "microbursts."
Jul. 14, 2015 | Tags: past weather, thunderstorms, tornadoes, weather radar
Question: Not much flooding here but winds and lightning took down many large branches and destroyed a new Japanese Maple I purchased just yesterday. Could there have been a tornado or something along Chatham/Orange county line? — Greg
Answer: The organization of the atmosphere and storms that were underway at that time were both much more favorable for "straight-line" wind gusts than for tornadoes, and it is very likely the damage you noted was due to these intense thunderstorm outflows. It's worth noting that thunderstorm downburst winds of this sort can reach wind speeds near 150 mph in rare instances, though less than 80 mph is much more common. Also, the winds can sometimes occur in a highly channeled fashion, leading to narrow swaths of damage surrounded by lower wind speeds, and the shear between the strongest wind and weaker winds to either side can lead to swirls and vortices that can mimic to some extent the kinds of damage patterns seen in tornadoes.
Jul. 1, 2015 | Tags: past weather, thunderstorms, tornadoes, winds
Question: I've lived in NC for 30 years. I've noticed a trend in severe weather in NC as it relates geographically to the fall line. It seems there is a statistically significant amount of storms (particularly with tornadoes) following paths south and east of the fall line separating the coastal plain and the Piedmont. Is this pattern recognized as influenced by the geography at all? Other than the mountains shearing passing storms, what would make the coastal plain that much more conducive of an environment versus areas a few miles west of the fall line? — Stan
Answer: It's worth noting that tornadoes can occur anywhere in our state, but the distribution you've noticed does have statistics in its favor, especially for more intense tornadoes, and does have some physical reasoning behind it. Several factors play a role, including the location of the area relative to the mountains, which can induce a "lee trough" that can set up along or just west of the fall line and act as a storm trigger in some cases. In addition, a "Piedmont trough" that is influenced by the differential heating of soils over the Sandhills region of the state and areas farther west that sometimes sets up on rather hot days. Probably a more important factor than either of those, however, is the presence of the Atlantic Ocean just not far south and east, and the intrusion of moist, warm marine air in the lower atmosphere that often precedes the arrival of the kinds of sharp upper-level troughs and attendant frontal boundaries that are most often associated with our tornadic outbreaks. The coastal front that can push inland in those situations provides both enhanced instability (through a combination of low-level warmth and humidity) and can also serve as a boundary that enhances low level wind shear that can be translated into rotation in storms that form in the vicinity of that boundary.
Jun. 14, 2015 | Tags: severe weather, tornadoes
Question: Last year on 5/15/14, an EF1 tornado passed one block away from my home. There was considerable property damage with many trees down. The only notice was Greg Fishel telling me to head for the basement on my phone. Back when I lived in the Midwest, there were sirens in every neighborhood to warn of severe weather. My question is; why don't we have sirens in the Triangle? — Mike Bethelzs
Answer: Compared to many populated areas in the midwest and parts of the plains, central North Carolina has historically had considerably fewer tornadoes, in particular of the more violent variety (there has never been a documented F5 or EF5 tornado in our state, for example) and this probably led to resources being directed to other things besides procuring and maintaining siren systems that would be rather rarely used in many communities. A few other considerations include the difficulty of hearing them inside homes, vehicles and businesses that, through the years, became increasingly buttoned up and resistant to sound from outdoors, the potential for confusion between weather sirens, volunteer fire department sirens and nuclear power plant/civil defense sirens, and the density of the siren network that would be required to overcome rolling terrain and thick tree cover that limits the distance a given siren could cover. In more recent times, the ubiquity of weather information available by way of radio, television, internet, home phone and cell phone has further reduced the call for sirens related to weather. Despite this, debate on the issue continues and a few locations in our state have added sirens in recent years, for use in flash flooding or severe weather events, and several college campuses have done the same.
May. 22, 2015 | Tags: spotters/skywarn, tornadoes
Question: How bad are 200-mph winds? — Antonio
Answer: It all depends on perspective. A 200-mph wind at jet stream level may be very beneficial in terms of fuel economy for an aircraft traveling in the same direction, although shear in the vicinity of the strong jet core might also produce some clear air turbulence. On the other, at or near ground level in a populated or built-up area, 200-mph winds can be extremely destructive. Such a wind speed rates at the top of the Enhanced Fujita EF-4 scale (and anything higher at EF-5) for tornado damage potential, representing a violent tornado capable of producing extreme damage. Likewise, for hurricanes, 200-mph is far up into the Category 5 level on the Saffir-Simpson Wind Scale, and would be expected to result in "catastrophic" damage.
Apr. 7, 2015 | Tags: hurricanes, preparedness, tornadoes
Question: How many tornadoes have we had in North Carolina this year? — Josiah
Answer: All we can offer for now is a rough estimate that will likely be revised as more complete data is collected and posted by the Storm Prediction Center and National Climatic Data Center. Tornado reports initially come in the form of a product called local storm reports, for which a very up to date archive is readily available. However, in some cases these preliminary reports turn out to have been due to non-tornadic storms, or there may be several reports that all arise from a single tornado, so that the initial number of reports is usually an overestimate. Using that value, there were 40 tornado reports in NC for January through October 2014. The Storm Prediction Center applies a ratio of .85 as a rough correction factor to reduce potential duplicate reports, which would give an estimate of 34 tornadoes. A more accurate, final count for our state for 2014 will be available from SPC and NCDC by sometime in the spring of 2015, as by that time tornado damage survey reports from local NWS offices will have been collected and entered into databases that reflect, as near as is possible, the actual number of confirmed tornadoes.
Nov. 4, 2014 | Tags: past weather, tornadoes
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