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Question: Outflow boundaries from storms...are these phenomena only recently detectable by our technology? — Tom Noffsinger
Answer: They have been well-known as a feature of thunderstorm structure and effects for quite a long time, but have become increasingly detectable through the decades. Once reasonably high-resolution visible satellite imagery became available in the late 1970s and early 1980s, outflow boundaries could clearly be seen on that imagery in the vicinity of thunderstorms, but the imagery itself was updated infrequently and had a considerable time lag. Most weather radar at the time, and into the late 1980s and early 1990s, was in the form of WSR-57 and WSR-74 National Weather Service instruments that produced about half the output power of the WSR-88D ("NEXRAD") Doppler radars that replaced them. The added power, and upgraded receiver sensitivity, of those radars (along with the power, sensitivity and shorter wavelength of privately owned radars like our DualDoppler 5000) have made the boundaries more routinely visible in the last 10-20 years. Because the outflow airmasses tend to be rather shallow, their boundaries are still seen mainly when they occur fairly close to a radar site, since the upwardly tilted radar beam can pass over top of them at greater distances from the transmitter.
Jul. 18, 2017 | Tags: general meteorology, thunderstorms, weather radar
Question: Hi! Is there a calendar online that shows when we last had rain; specific daily rainfall amounts for the area (instead of aggregate monthly or yearly data for the area (or less helpfully - the State))? Getting the right amount of water to my summer garden is tricky and being able to look back and determine how much water it needs would be helpful - relying on my memory alone is a faulty system. — Anonymous
Answer: There are a number of resources we can suggest, keeping in mind that rainfall can vary a good bit over a short distance. In terms of looking at specific dates and how much was received in rain gauges at individual stations, you can use the "Almanac" function of our web site to check a selected date - when you do that, the default is to show a single day's info for RDU. However, you can then change the view to "monthly," which will allow you to quickly scan a table of observations near the bottom of the page that includes daily precipitation observations for the entire month - you can also use the "Search for Another Location" box to change the observation site to another town. Another place to look for a fairly dense network of daily precipitation observations is the Community Collaborative Rain, Hail & Snow network, at www.cocorahs.org. Once you learn your way around the site a bit, you can get good localized history of recent precipitation for the region there. Another option is a National Weather Service Precipitation Analysis page that uses radar estimates of precipitation, adjusted to fit with rain gauge reports, to plot daily contours of precipitation that you can step through. The advantage here is that radar fills in some of the gaps between fixed observation sites. The page also allows you to view precipitation amounts summed over the past week, past month, past 90 days, etc, and to view how those amounts compare to normal. The address is water.weather.gov/precip/, and you might like to go to the "location" button, select "WFO," choose "Raleigh" and click on the county borders for a good view of our area.
Jul. 17, 2017 | Tags: cool sites, past weather, rain
Question: Why is it that when I'm looking at the live online radar, I'm seeing areas of blue in the Triangle area? Is that where the Doppler radar is reflecting off water or lakes? — James
Answer: When the Dual Doppler
5000 radar includes a display of some of the lowest reflectivity values (often shaded blue), which are good to have in picking up drizzle or light sprinkles, outflow boundaries from storms, and light snow in the winter, they can become rather enhanced when we have temperature and moisture profiles that are favorable for "anomalous propagation" that bends the radar beam down a bit and increases returns from the ground and from low altitude non-meteorological targets like dust, insects and birds. This is most noticeable when there is both a temperature inversion near the surface and a rapid decrease in humidity with increasing height. When you lapse the radar in these situations, you can often notice the ground returns staying in the same place or jumping around somewhat randomly, while precipitation echoes move along with steering winds. This time of year, you may also notice some movement with the low-level winds in the light, non-meteorological returns as they pick up on the concentrations of insects or dust.
Jul. 16, 2017 | Tags: weather radar, wral.com
Question: Where is the dew point forecast on this website? — Jimmy Pines
Answer: Dew point is included in the hourly forecast on our site. The one thing to note is that it isn't shown by default when you first access the hourly forecast section. Instead, once you do that, look for the "More Details" link just below the initial list of hourly information. Once you click that, you will see dew point, relative humidity and wind speed/direction added to the previous information for each forecast time.
Jul. 15, 2017 | Tags: humidity/dew point, wral.com
Question: The Capital Weather Gang blog for the Washington Post had an excellent write-up of the impact of the June 29 2012 derecho that led to severe damage in the D. C. metro area. The blogpost was on June 29 this year. Do you have any memories of the effects of the event on the Triangle area or NE North Carolina. I recall a pretty major thunderstorm around midnight in Raleigh with a lot of wind and damage to trees. — Dave Crotts
Answer: A derecho is a long-lived convective storm system that produces widespread reports of wind damage over a very large area, and the event of that day that stretched from the Ohio Valley across to the mid-Atlantic and northern parts of our state was certainly an impressive example. The core of that system and its most extreme effects (which led to nearly two dozen fatalities) was confined to areas north of our state, but the southern fringes of the system did produce intense winds that in some cases produced damage well south of the storms themselves. In our state, there were some 52 reports of wind damage, mainly in the form of trees and power lines down, that did include a number of reports around the Triangle, northeastern parts of the state, and also toward the northern Piedmont to our northwest. It was notable that the way in which winds spread south of the heaviest storms meant that some parts of our state had wind damage, but actually had no lightning and little or no rainfall. Less than .01" of rain fell, in fact, from around Greensboro to Durham to Rocky Mount and points south. There is a very good overview/summary of the entire derecho event available at www.spc.noaa.gov/misc/AbtDerechos/casepages/jun292012page.htm. Also, you can retrieve a zoom-able map with clickable storm reports by setting the red search box to that date on the State Climate Office's Local Storm Reports database page, at climate.ncsu.edu/lsrdb/index.php.
Jul. 14, 2017 | Tags: cool sites, past weather, severe weather, winds
Question: I'm somewhat of a severe weather lover and I've been watching Greg for 20 plus years. My question is, is there some kind of geothermal dome of some sort preventing severe storms from developing or even moving into the inner Beltline or downtown area? It seems more often than not that approaching storms split up and go north and south, no matter the track or direction of a large storm. — Jimmy H
Answer: We don't think this is likely the case. Although there can certainly be localized microclimate effects from terrain and land use, they tend to be a little less impactful when it comes to intense thunderstorms that can produce severe weather. Instead, you are likely noticing the fact that severe weather tends to affect rather small areas, and to impact any given location quite infrequently. Because of that, and the wide reporting of severe weather and the ready availability of radar displays that can make storms appear to cover a bit more area than they really do, especially when they are at greater distances from the radar site, there is a tendency to to take great note of storms that affect nearby areas and to feel that your particular area of interest is being left out. In addition, even for intense storms well represented by radar displays, the portion of the storms where winds or hail reaches severe levels can be quite localized. The result is that we get much the same question/comment from almost every part of our viewing area. To get some sense of a rough quantification on this, we took a look at local storm reports collected by the NWS over the ten year period 2007-2016 for Wake County. There were 222 such reports for Wake, which covers an 857 square mile area. "Inside the Beltline" takes up about 60 square miles (the entire Raleigh city limits encompasses about 144), or about 7 percent of Wake's area. During that time, there were 22 of the storm reports that came from inside the Beltline, or about 10 percent of the Wake County reports, not much out of line with the percent of Wake real estate covered by that part of the city. Of course, "downtown" is an even smaller target for any intense storms passing through the area, but in spite of this it was struck by a tornado that passed through its southeast quadrant on April 16, 2011.
Jul. 13, 2017 | Tags: past weather, severe weather
Question: On the 7/6 5:30 PM broadcast, Mike showed a camera view of RDU and mentioned that the clouds shown were actually in southern Virginia. Google maps shows that distance as 46 miles. How far away can we see clouds? — Chuck
Answer: He was likely referring to either some high cirrus clouds, or perhaps to the upper portions of some tall cumulonimbus clouds, which can be see at a surprisingly great distance, assuming the air in between the viewing location and the distant clouds is sufficiently clear. You can get an idea as to the possibilities by considering a simplified formula for estimating distance to the horizon as seen on the spherical earth, including a small correction for refractive bending of light by the atmosphere. The equation gives distance to the horizon (in miles) as 1.32 times the square root of the height of the observation point (in feet). For the simple case of someone standing by the ocean looking out at clouds in the distance, clouds will appear above the horizon if they are any closer than the calculated distance from the observer's eyes (or camera) to the horizon, added to the distance from the height of the viewed object (in this case clouds) from the horizon. Suppose the person's eyes were 5 feet above the ocean surface - the distance to the horizon would be about 3 miles. If the clouds in the distance are cirrus clouds 25,000 feet off the ground, then the distance from the clouds to the horizon is about 209 miles, so in theory they would be right at the horizon level if seen from 212 miles away. Of course, the closer the same clouds are, the higher above the horizon they would be seen. When it comes to seeing distant thunderstorms, it's worth noting that cumulonimbus clouds can occasionally reach heights around 40-60,000 feet tall, so they can be visible from a great distance away. Of course this simple approach ignores complications associated with more complex topography and with obstructions to visibility such as other clouds, haze, fog, pollutants and so on, which would often limit the distance a good bit. Nonetheless, you can see how the clouds Mike pointed out could easily be seen well up into Virginia in many cases, especially when you consider that many of our towercams are located a few tens of feet (and in the case of our "Tall Towercam" in southeast Wake County, about 1800 feet) above ground. On a very clear day, the distance to the horizon as seen from that camera is around 56 miles.
Jul. 12, 2017 | Tags: atmospheric optics, clouds
Question: What was the June 2017 temperature departure from average at RDU? And when was the last time RDU had a below normal month? — John S
Answer: The mean monthly temperature at RDU for June was 75.6 degrees, and that was .8 deg F below normal for the month. It followed a long string of months with above normal temperatures, including a whopping 8.8 degrees above normal for February! The most recent month with a below normal average prior to June 2017 was May 2016, when the average temperature of 67.5 was .5 deg below normal.
Jul. 11, 2017 | Tags: normals, past weather
Question: What was that crazy thin blue line on the doppler paralleling US-1 from Raleigh to Sanford moving rapidly north-west between 5:30 and 6 pm Monday July 3? All the other local storms seemed to be sitting basically still between Seven Lakes and Fayettville, but this thing caused some intense winds when it passed through Sanford, but only for about 2 minutes. — Todd Lewis
Answer: A bit earlier that afternoon there were intense thunderstorms over northern Cumberland County and Lenoir County that produced intense downdrafts that then spread outward from the storms in a shallow layer of denser air. The dividing line between this denser and cooler air and the ambient warmer and less dense air (as well as between lighter winds outside the layer and a gust front of stronger winds that often accompanies it) is called an "outflow boundary." The change in wind speed and sometimes direction across these boundaries leads to convergence and a resulting concentration of dust, small flying insects and sometimes birds, in addition to creating turbulent variations of density where the differing airmasses are mixing. All of these things combine to create a thin line of weak radar reflectivity that can clearly mark the boundary on radar displays. Since the boundaries are often rather shallow, they are typically only visible fairly close to a radar site, where the beam remains at a fairly low altitude. This one was well-situated with respect to the DualDoppler 5000 and NWS Raleigh NEXRAD radars, and actually followed a curving path northeast across Lee and southern Wake Counties, and then east across southern Wilson County and on into the Greenville area. On occasion, the upward motion caused by the dense layer of air undercutting the surrounding warm, humid and unstable air triggers new convective cells, and this appeared to be the case in a few locations with this outflow, including a cell that popped up over central Lee County at around 5:20 PM after the boundary had proceeded northwest to about the Chatham County border.
Jul. 10, 2017 | Tags: fronts & airmasses, past weather, thunderstorms, weather radar
Question: This is more of a comment about the upcoming total eclipse than it is a question. I have seen two total eclipses (July 20, 1963 and March 7, 1970). Without doubt they were the most spectacular things that I have seen in almost 72 years. I fully intend to be in the middle of the ‘totality band’ for the August 21, 2017 version. I cannot emphasize too much the difference between 99+% and total. I would encourage anyone – everyone – to drive the distance to get into the narrow band of totality. It’s well worth the effort! — Leonard W. White
Answer: We appreciate you sharing your experience, and as you may have seen in one of our earlier answers from a week or two ago, we agree in emphasizing the difference in the seeing the sky, the sun and moon and surroundings during totality, and seeing even a significant, but partial, solar eclipse. A great deal of the experience of a total eclipse is lost even a very short distance outside the path of totality. Good luck in August, and let's all hope for favorable cloud conditions along the path!
Jul. 9, 2017 | Tags: astronomy
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Published: 2007-10-09 14:40:00
Updated: 2014-06-24 16:06:51
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