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Question: Nice coverage of the 3-body scatter spike on the 11pm news tonight (June 25). I hadn't heard of that, but it makes huge sense -- and I found the 1998 Leslie Lemon paper that labeled/described the phenomenon. We don't always understand what you're talking about, but we love your passion and enthusiasm. — Phil Earnhardt

Answer: For those who didn't catch the weathercast that night, a 3-body scatter spike on a weather radar image is a narrow extension of somewhat reduced reflectivity that stretches a ways past an intense thunderstorm echo on the opposite side of the storm from the radar location. This feature, also known as a "hail spike" or "flare echo," results when part of the radar beam strikes hailstones, is scattered downward toward the ground, reflects back up to the hailstones, and then is scattered back toward the radar. The extra time it takes for the signal to make the entire trip leads the radar to display it as if it is farther away than the location of the hailstones (in effect, adding the altitude of the hailstones to the distance of the echo shown). The 3 bodies in question here are (1) the hailstone, (2) the Earth below and (3) the hailstone a second time.

Fore a more in-depth discussion, with a diagram and example radar image, see the Weatherwise Magazine "Weather Queries" feature at www.weatherwise.org/Archives/Back%20Issues/2012/May-June%202012/queries_full.html. Also, simply searching for "three body scatter spike" should turn up many example images.

Finally, just an added note that those of us in the WeatherCenter were fortunate to have Les Lemon come to the station for two days a few years ago to present a series of classes for us on radar interpretation and severe weather. It was a really enjoyable learning experience!


Jul. 7, 2015 | Tags: hail, weather radar

Question: 8:16 pm Thursday, June 25th, .... saw one large long lighting strike south of Youngsville . . . 8 seconds later, one long thunder for at least 15 seconds building to a louder ground shaking boom around the middle of the 15 seconds .... did anyone else experience that ... how does thunder last so long? — Anonymous

Answer: The duration of a single rumble of thunder depends in large part on the length and orientation of the lightning channel relative to the position of the person hearing it, but also can be affected by atmospheric temperature and wind conditions that can bend and/or focus the sound waves involved. Also, there can be instances where multiple lightning strikes produce thunder that merges together into what sounds like one very long rumble.

Regarding the orientation, imagine a bolt of lightning that runs from the sky to the ground a few miles away from you that follows a path in which almost the entire channel is about the same distance from your location. That would produce a rather brief rumble of thunder as the principal group of sound waves pass your location at about the same time. If a long lightning channel is oriented so that one end is rather close to you and the other end is stretched away at a significant distance, then you may hear thunder from increasingly distant portions of the channel arrive over quite a lengthy time.

In your particular case, the 15-second duration suggests that, assuming there was only one lightning strike that produced the thunder you heard, the most distant part of the lightning channels was around 3 miles more distant from you than the closest segment, since the sound travels about 1 mile for every 5 seconds. Anything beyond that is more speculative, but regarding the boom in the middle, it seems possible that perhaps that part of the thunder came from a segment of the lightning channel that was more parallel to your location and focused a greater amount of energy, or that some temperature and moisture gradients in the air concentrated more acoustic energy from different parts of the channel at your location right at that moment.
Jul. 6, 2015 | Tags: lightning, thunderstorms

Question: What caused the eerie "golden glow" in the sky early this Thursday morning (June 25th)? — Sue Jennings

Answer: Early that morning there was a fairly intense cluster of thunderstorms drifting into and across the Triangle area from the west. We checked satellite imagery for that time frame and found that the storms were producing a large, thick shield of cirrus cloud spreading out as an "anvil" above the storm, with the bottom of those clouds around 25-30,000 feet above the ground. While there was significant cloud cover at other levels and locations toward the east and the south, there was a notable lack of clouds toward the northeast. This time of year, the sun is rising well north of east, so the pre-sunrise and near-sunrise rays of sunlight were able to shine onto the bottom of that extensive high cirrus anvil cloud, being reflected down to the ground from there. Since the sun was low in the sky, or even below the horizon as seen from ground level, it was passing through a long stretch of atmosphere that scattered away the shorter (blue and green) wavelengths of light) and what remained to strike the base of the clouds were longer wavelengths (yellow, orange and red mixing to a golden hue) that led to the glow you noticed. This can also happen around and after sunset if there is a clear path for the sun's rays to come in from the west. Depending on the particulars of how dusty or hazy the air is at the time, the resulting "eerie glow" can range from yellow to gold to orange or pink. The right conditions for this are not extremely rare, but do happen infrequently enough to make it really noticeable when it does!
Jul. 5, 2015 | Tags: atmospheric optics, past weather, thunderstorms

Question: Did you say on last night's forecast that it could be in the 60s (daytime) over the weekend due to "?" effect? Coworker swears she heard this. — Charlene

Answer: You wrote in about a forecast on Tuesday evening, June 23. At that time, we had forecast model projections indicating a frontal system would move into the state over the weekend. Some models (notably the European model, which on average is a little more accurate at longer time frames than a similar American model called the GFS) were pushing the front through central NC and developing a series of low pressure areas that would keep us in clouds and periods of rain in Sunday, holding the high temperature in the low to mid 60s. However, we also showed that evening that if we looked at ensemble averages, based on running the same model many times with slightly altered starting conditions, that the average results were much warmer than this. for that reason, we only lowered our forecast high for Sunday at that time to 77 degrees, from 80 the previous day. The very next day, most model projections pushed the front through more rapidly, but also concentrated low pressure well to our north and showed that we'd have sunshine and little or no rain on Sunday, allowing for notably warmer temperatures. We increased the forecast high back to 80 degrees with more sunshine, and as the week went on we showed highs for Sunday in the mid 80s with bright skies. As it turned out, we had a mostly sunny day on Sunday, with an observed high of 84. This was a good example of the benefits of using ensemble forecasts, and also illustrated that even though the European model scores as a little more accurate overall than the GFS, the results can vary a good bot from case to case. This time around, the GFS was consistent in forecasting sunshine and highs in the 80s for Sunday even while the European model was showing clouds, rain and 60s.
Jul. 4, 2015 | Tags: maps & codes, past weather

Question: My grandmother use to say that she'd seen snow in July in Vass, NC. Is that true? — Kathy

Answer: We can't really imagine any circumstance that would allow it to snow in Vass at that time of year. However, it seems more likely to us that she might be remembering a few hail storms, or perhaps a thunderstorm that produced significant amounts of graupel, sometimes called "soft hail" or "rice hail." In either case, there can be times when the hail or graupel is intense enough to cover the ground in a white coating of ice, enough that it looks a bit like snow cover, and she may be remembering some events of that nature. The atmospheric structure and processes leading to the occurrence of hail or graupel, however, is very different from that leading to snow.
Jul. 3, 2015 | Tags: hail, snow, thunderstorms

Question: What was the name of the storm that flooded Crabtree Mall? — Melissa

Answer: Crabtree Valley Mall had a history of flooding frequently in its early years, but later construction of Lake Crabtree and some other improvements reduced the frequency of serious flooding. Even after that, however, heavy rains associated with tropical cyclones have caused serious flooding in and around the mall in more recent years. The storm you are probably asking about was Tropical Depression Alberto, which contributed to flooding at the mall on June 14, 2006. In addition, there was significant flooding there from the passage of Hurricane Fran in September of 1996.
Jul. 2, 2015 | Tags: flooding, Fran, hurricanes, past weather

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: We've just had 12 consecutive days with highs of 95 or above, although it looks like today won't make that level. Saw your recent response about the longest stretch of consecutive days of 90 or above. Whats the record for 95 or above? — David Rubin

Answer: Rich M. asked about the same question a few days back. At that time, it appeared we had a somewhat uncertain chance to end up with as many as 14 days in a row reaching 95 or higher at RDU. As it turned out, the day you wrote we ended up with a high of 92 and broke that streak, ending it at 12 days in a row. This was, as it turned out, a new record for that statistic, breaking the old mark of 9 consecutive days that ended on July 21st in 1977.
Jun. 30, 2015 | Tags: heat, records/extremes

Question: How many days have been at least 90 this year--not necessarily in a row, but total? Is that way above normal? Seems like it to me! — M. Lee

Answer: That's a bit of a moving target since we put these answers in a database a few days before they post sometimes. At this writing, it appears we'll finish the month of June with 19 or 20 days having reached or exceeded 90 so far in 2015. Historically, this would be 5 or 6 days above the "normal" (1981-2010 average) of 14 days, but still 12 or 13 below the maximum number for the first 6 months of the year, which was 32 days in 1944.
Jun. 29, 2015 | Tags: heat, past weather, records/extremes

Question: How is the heat index determined? — June

Answer: The idea of heat index is that when humidity level are elevated and temperatures are high, the body is less able to cool itself by way of the evaporation of sweat, so that in terms of maintaining a normal body temperature, high humidity reduces our ability to do so to a rough equivalent of what it would be at a higher temperature, but lower humidity. The index used in the United States today is derived from work published by Robert G. Steadman in 1979 in a paper titled "The Assessment of Sultriness, Part I." His tables of apparent temperature, based on many factors and with a specific value assigned to each combination of temperature and relative humidity, can be seen, for example, in the graphic at www.nc-climate.ncsu.edu/climate/heat_index_climatology.php.

These values were later reduced to a regression equation that calculates heat index given inputs of temperature and humidity, and that can be adapted to calculate the values based on inputs of temperature and dew point. One version of that equation is shown next to the table at the address above, and it is worth noting that the equation is approximate, so its output will not exactly match all the values in the table.

You can find another, shortened and more approximate version of the equation relating heat index and relative humidity at www.srh.noaa.gov/fwd/?n=heat5. A handy calculator for computing approximate heat index based on knowing either temperature and relative humidity, or temperature and dew point, is available at www.srh.noaa.gov/ffc/html/metcalc.php.

Jun. 28, 2015 | Tags: apparent temperature, cool sites, humidity/dew point

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