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Question: Who decides what constitutes a 10 or 100-year rainfall event? We seem to be getting a lot of 100-year rainstorms lately, leading me to suspect that our stormwater runoff regulations need to be updated to reflect a new post-global warming reality. — Oak Rapp
Answer: The values of "average recurrence interval" or "return period" for rainfall over a given duration are statistical calculations based on historical data, and are compiled and published by the National Oceanic and Atmospheric Administration's National Weather Service in a series of documents called NOAA Atlas 14. The results can also be accessed and viewed online via a web page called the Precipitation Frequency Data Server (easily found with a web search), where you can drill down to any particular location to find the interval associated with any given rainfall amount and duration, along with a confidence interval of what the true return interval is likely to be based on the available period of record and the uncertainties associated with such analyses. Of course, as conditions change over time, those numbers will be recalculated to reflect new data. It is worth keeping in mind that while the terms like "return interval" imply that a 100-year rain rate should only happen every 100th year, it really means that in a given year, there is a one percent chance of that rate occurring. That doesn't preclude a similar event happening just a few years later. Of course, if they continue to happen at rather frequent intervals, that will eventually cause the calculated recurrence intervals to become smaller. Climatologists have to balance the desire to have such calculations reflect current reality against the possibility of changing the numbers too much based on what may turn out to be a short-term anomaly that isn't representative of longer term trends.
Dec. 9, 2016 | Tags: cool sites, normals, rain
Question: We live near Wake Crossroads in NE Raleigh. Like today, when storms come through Raleigh, it will be raining all around us, but we will not get any rain. Is there something unique about this area that it misses a lot of rain storms? — Jan
Answer: We haven't seen anything to indicate that should be the case, as average monthly and yearly rainfall totals for that general area appear similar to most surrounding locations. Of course, in any given system passing across the region, there can be considerable variability in rainfall amounts due to the typical fluctuations over time and distance associated with many weather features. On the Sunday afternoon that you wrote in, for example, dry air in the lower atmosphere evaporated a lot of the precipitation that fell during much of the daylight hours, especially from around Raleigh northeastward, but as the lower atmosphere became more humid, rain became steadier and heavier heading into the evening and overnight. Observations from a pair of personal weather stations in your immediate vicinity, in agreement with radar rain estimates for that period, indicate you should have ended up with around 3-6 tenths of an inch of rain, similar to much of our area with that disturbance. We've found that the variability of rainfall amounts, and the availability of radar displays to track systems passing through the area (often affecting part but not all of the region) has led people from pretty much every part of our viewing area to have the same perception - that something routinely and uniquely blocks precipitation, storms, etc from their particular location.
Dec. 8, 2016 | Tags: general meteorology, rain
Question: Is it going to snow Friday? — Daniel
Answer: We usually leave short or mid-term questions about the local forecast to the published information on our main weather page, or that you would see in our on-air forecasts, but thought we'd go ahead and note that as we answer this question, it appears there is very little apparent chance of precipitation, and probably little in the way of cloud cover, likely for this Friday. Current projections indicate a strong cold front will pass through on Thursday, followed by the arrival of cold, dry air and a high pressure system to our west as we end the week. if this holds up, we'll see a sunny, breezy Friday with below-normal temperatures.
Dec. 7, 2016 | Tags: snow, wral.com
Question: How come people can survive being struck by lightning? I read somewhere lightning bolts are hotter than the surface of the sun, and I know they can turn sand to glass. Also the Joules contained in one would wreak havoc on the nervous system, so how do people survive? — Alexander Roa
Answer: A big reason people can survive some strikes is that not all strikes are equal. Some strikes involve current that passes mainly over the outside of the body, some pass through the body in a narrow channel that only involves an extremity or two but does not pass through vital organs, some strikes involve ground currents or side flashes that have been dissipated somewhat in energy, and the impact on a person depends heavily on how the person is oriented with respect to the direction that current is flowing, what body parts are in contact wit the ground or other conductor, etc. The result is that strikes involve a wide range of physical impacts to people, from just minor discomfort to fatal injuries. As to the temperature for a lightning channel, the resistant air through which lightning passes can heat to as much as 50,000 degrees F, about 5 times the temperature of the sun's photosphere (the surface you referred to). However, the amount of mass heated to that temperature is very limited, and the duration of a strike is often very short (a small fraction of a second), so that the heating effect of a strike is typically limited, since only a small patch or two of skin would be briefly exposed to the heated air. Of course, burns do occur with some strikes to people, but due to our bodies being reasonably good conductors, the temperatures are generally not extreme over a large area or for an extended time.
Dec. 6, 2016 | Tags: lightning, weather & health
Question: Just for curiosity since we are getting cold at night, do you think we have a good chance of having SNOW, not ice but Snow, this winter? And when in the forecast is the first time Lee County will see SNOW this year? — Heather Thompson
Answer: You may have seen several recent discussions about the reasons predicting snow amounts, or even whether snow will likely be above, near or below normal for the winter season is generally unreliable. For those reasons and more, we really can't give you a meaningful answer about whether Lee County will have some significant snow, or whether your area will likewise escape having any significant freezing rain or sleet, as we head through the winter. Instead, we have to wait and take each potential wintry weather event as it develops, and try to predict, starting a few days in advance, whether it will feature mostly rain, mostly snow or a mix of rain, snow and ice, and how much will occur. Even a few days or a day ahead, those can be very hard to pin down with high confidence!
Dec. 5, 2016 | Tags: snow, winter weather
Question: Last night (11/28) at about 7:15 PM EST there was a glowing bright orange what appeared to be a small piece of a contrail near the apex of the sky, in a roughly SW to NE orientation. This is in the Brier Creek area. I could see three additional ones-in a different direction, more W to E, but they were mostly obscured by clouds. The sun had set by this time, so the sky was completely dark. I am curious as to what this may have been. — Brian K. Fletcher
Answer: We checked into the possibility of sounding rocket launches from Wallops Island or other NASA facilities that might have left a very high altitude trail to be illuminated by the long-set sun that evening, or a tracer launch that might have released very high-altitude materials that can glow as you noted against an otherwise dark sky. So far as we could determine, there was no such activity at that time. Otherwise, your notation of the orientation of the streaks you saw matches reasonably well with directions often flown by airliners following high-altitude routes overhead. We verified that radiosonde data showed the higher altitudes at which commercial aircraft often cruise (35-40,000 especially) had temperature and moisture conditions that could have supported contrail formation. While contrails often glow orange in reddened sunlight well past sunset due to their altitude, the time you noted was over two hours past sunset for that day, which seems awfully late for the contrails to still be directly lit by the sun, but we wonder if there might have been some favorably oriented clouds to the west that scattered enough light toward some contrails that you were able to make them out, or perhaps if the relative darkness of the surrounding sky allowed even some weakly illuminated contrails to stand out to your dark-adjusted eyes. Without further information, it remains a bit of a mystery, but that's the best we can speculate at this point.
Dec. 3, 2016 | Tags: atmospheric optics, past weather
Question: What is percentage of snow looking like this winter? — Amy
Answer: There really isn't a reliable way to assign a meaningful number for that. In a broad, general sense it is possible to make seasonal forecasts that provide a percentage chance that the overall season ends up with precipitation amounts or temperatures that are in the above normal range (meaning within the upper third of observed conditions over a 30-year period from 1981-2010), the below normal range (lowest third of average seasonal values over that period) or "near normal," that being the range in between the highest and lowest thirds. For the upcoming winter, for instance, the Climate Prediction Center indicates that for the interior Carolinas, the chance of above normal temperatures is 36.9%, the chance of near normal temps is 35.7% and the chance of below normal temps is 27.4%. For precipitation, the chances are 29%, 33.9% and 37% respectively. As you can see, the odds are a little tilted toward warmer and drier than normal weather, but only by a little, and with significant chances that we could fall into the other two categories. You'll notice that no similar probabilities are available for snowfall amounts, because those are so dependent on individual storm behavior and very short-term trends. It's very easy, for example, for a couple of storms systems to pass through during warm periods of the winter and leave us with little overall snow despite an overall cooler and wetter than normal winter, or conversely for us to have a notably warmer and drier than average winter, yet we have a single really significant snow storm pass through during a short but intense cold spell, giving us a big snowfall that significantly exceeds our seasonal average right by itself (part of the reason this can happen is that the depth of snowfall can easily range from about 5-15 times the melted water equivalent, so in especially cold conditions a relatively small amount of water falling from the sky, had it occurred as rain, can amount to a sizable snowfall).
Dec. 2, 2016 | Tags: general meteorology, winter weather
Question: Does a 30% chance of rain mean that if I walk outside my house at any time during the specified period that it will be raining, or does it mean that if I stand outside for the entire period that I will get wet? Does it imply that I would only get wet if I stood in 3 out of 10 locations in the area? — Steve
Answer: Those are all interesting interpretations of precipitation probability. A 30% chance of rain does not mean you would expect it to be raining anytime you walked out during the specified period. It just means that during that period, the weather pattern is one that should tend to produce measurable precipitation within the specified period about 3 out of 10 times. It is possible that this one time there would be precipitation for the entire period, some fraction of it, or none at all (in fact, for a 30% chance you would expect that 7 out of ten times with the pattern in place, measurable rain would not occur during the valid period of the forecast). Similarly, if you stood outside for the entire period on ten occasions with a 30% chance of rain, measurable rain would fall on you for some portion of that period on about 3 out of those ten periods - it is simply difficult to know which of the ten periods would be the ones in which rain would occur at your location. The last part of your question is also not what is generally meant in probabilistic forecasts, although there are situations (summertime scattered showers and storms that pop up in the afternoon and die off in the evening, for example) that can approximate that meaning. Again, if about 30% of the area will experience those showers, the difficulty lies in knowing which 30% will be affected and which will not. The bottom line is that the percent chance of measurable precipitation generally boils down to a statement of confidence, given the information available to a forecaster, that precipitation will fall during the forecast period, from zero for a near-guarantee of dry weather, to 100% for nearly certain occurrence of precipitation.
Dec. 1, 2016 | Tags: general meteorology, maps & codes
Question: How did rainfall totals compare between Hurricane Matthew and Hurricane Floyd? — Dustin R
Answer: There are some similarities and important differences, especially with regard to distribution. The areas that received over a foot of rain from Floyd were focused over the northern coastal plain and along to just inland of the southern coast. Those kinds of totals from Matthew were mainly focused over the southeastern Sandhills and southern coastal plain. In each case a large section of the eastern third or so of the state received over 8 inches, and there were sizable pockets with over 15 inches. You can compare similar contour maps of rain from each storm by going to www4.ncsu.edu/~nwsfo/storage/cases/20161008/precip.20161008.png for a Matthew map, and scrolling dow a bit at www4.ncsu.edu/~nwsfo/storage/cases/19990915/ for a Floyd map.
Nov. 30, 2016 | Tags: cool sites, hurricanes, past weather, rain
Question: What are your snow predictions for NC 2016-2017? — Ryne Barnes
Answer: We don't attempt to make a seasonal snow prediction, simply because amounts of snow are some highly variable and dependent on the specific track, intensity, moisture content and temperature structure of individual storms systems, with small variations in any of those characteristics leading to large variations in both precipitation amount and type. It is possible to assess, at a fairly low confidence level, certain large scale influences like El Nino/La Nina, the Quasi-biennial and Pacific Multidecadal oscillations and extent of autumn snow cover over Eurasia, for example, to estimate that seasonal precipitation and temperatures are most like to be above, near or below normal, but unfortunately they do not translate to meaningful, reliable assessments of whether snow will be above, near or below normal. This year may involve even more uncertainty than is typical, since a weak La Nina pattern in the Pacific would tend to tilt our odds a little toward a drier and warmer than normal winter period. Conversely, Eurasian snow extent through October was well above normal, which can correlate to colder than normal weather for the northeastern third or so of the U.S. - bottom line is we'll have to wait and see, and deal with individual storms that have wintry potential one by one, a few days to a week in advance, as they approach or develop.
Nov. 28, 2016 | Tags: el nino/la nina, preparedness, snow, winter weather
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Published: 2007-10-09 14:40:00
Updated: 2014-06-24 16:06:51