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Question: The topic is "percentage chance of rain". I know that high is bad, and low is good, but I don't know what it means. Does it define location? Duration? Both? — Steve McKinnon

Answer: Do keep in mind that "bad" and "good" in this case depends on context - when rain is badly needed, for example, and high probability of rain would be "good." In any event, the probability of precipitation can be thought of principally as a statement of confidence that a certain amount of precipitation will occur at a given location (this can be a point or a larger area) within a given time span (an hour, say, or a 12-hour period, or an entire calendar day. The percent chance of rain that you encounter most often in the absence of any additional conditions being specified is usually that likelihood that measurable rain (meaning one-hundredth of an inch or more) will occur during the "daytime" or "nighttime" over an area usually consisting of an area of a county or few counties' size, with a zero percent chance meaning rain is not expected at all, while 100% means it is essentially certain to occur. A 40% chance, for example, would mean that measurable rain would be expected to occur about 3 out of 10 times that a similar weather pattern is in place over the forecast area. In most cases, the probability value doesn't provide a lot of information regarding duration or exact locations where rain will or will not occur within the area covered by that forecast value. Usually, there will be additional wording with the forecast to address that - for example, you might hear a reference to frequent showers or a day-long rain, or to isolated showers or a brief shower in spots, that would give some context regarding how often or how persistent rain could be. In some situations, especially shoe involving warm, unstable summertime weather with convective showers or thunderstorms that arise largely from the heating of the day, the probability can indeed also describe roughly how much of the area might be expected to receive the hit and miss coverage, though it's usually difficult or impossible to specify exactly which locations will and will not be rained on.
Nov. 27, 2016 | Tags: general meteorology, maps & codes, rain

Question: Here is a weird weather question. As I sit here in my cool dry house, which we all know is strange for NC. I couldn't help to wonder what would happen if the outside instantly changed to one of our hot humid summer days. Would I hear my house make some crazy noises to settle to the outside weather? If I opened the window would there be some type of weird mini weather phenomenon? I live down near Goldsboro and La Grange. I recently retired from the Air Force and plan to stay here in this great area. Love the WRAL team! — Mike

Answer: Well, you accurately described the question! These kinds of hypotheticals can be a little difficult to assess, but are fun to think about just the same. We don't know exactly how cool and dry your house happens to be, but if it has been that way for some time and then by some means the outside suddenly was near 100 degrees and very humid, it does seem likely you'd hear a number of noises as various component materials of the house underwent thermal expansion at varying rates - some materials may also expand a bit due to absorbing water vapor from the more humid air. Of course, some of that noise might be covered up if your air conditioner kicked on! Regarding the windows, there would be a fairly sharp density and humidity gradient between indoors and outdoors - before you opened the windows, there would be a good deal of condensation on the outside unless the windows were very well insulated. Upon opening them, there would likely be an initial flow of more dense air from indoors out the windows, followed by some inflow of warmer and more humid air into the house. It's at least possible that the mixing of cool air and warm, humid air would produce some brief condensation of tiny droplets at the interface between indoors and outdoors, in effect creating a short-lived cloud at each window. That's about all that comes to mind for now - thanks for the kind words about WRAL, and congratulations on your retirement from USAF!
Nov. 26, 2016 | Tags: general meteorology, humidity/dew point

Question: What exactly is an "upper level low?" Other terms referring to the same thing (I think) are "a pocket of cold air" and "a bundle of energy." There's also a joke that an upper level low is what caused an unpleasant day when the forecast called for nice weather. — Don Eligman

Answer: When you hear reference to an upper-level low, a "shortwave," or "energy aloft" of that sort in a weathercast, it usually is a short-hand way to reference enhanced areas of cyclonic rotation (known more technically as vorticity), usually tracked most closely in the altitude range of about 15-25,000 feet above the ground, with some exceptions. These areas can be due to strongly curved flow (up to and including "closed" upper lows, in which there is a completely circular flow around the low pressure center), to strongly sheared flow along a horizontal gradient of wind speeds, or, in many cases some combination of both. When these areas make forward progress (usually in the form of moving with a west to east component at our latitude range) they tend to induce diverging flow and strong upward motion on their downwind side east of the disturbance, which can lead to the development or enhancement of low pressure near the surface, along with formation of extensive cloud cover and precipitation, assuming sufficient moisture is available. Due to the manner in which the atmosphere tends to be structured, these upper-level lows, short wave troughs and such tend to correspond to areas with cool temperatures relative to the same altitude in surrounding locations, thus the occasional "cold air pocket" reference. In some situations, the arrival of this colder air aloft over relatively warm, moist air below can enhance instability and trigger the development of convective showers or thunderstorms, even when the upper disturbance may not have otherwise been intense enough to result in precipitation.


Nov. 25, 2016 | Tags: general meteorology

Question: I was wondering what are the oldest high temperature record and the newest low temperature record and what year, on average, was each type of record set for the entire year. — Michael

Answer: We're answering your question based on data from the RDU airport rather than a longer database that joins that data with reports from several other sites that served as the "official" Raleigh site back into the late 1800s, so that all the records would apply to roughly the same physical location. That being the case, we found that the oldest high temperature record still standing at RDU (where records began in the mid 1940s) is the 94 degree reading for May 21st, which was set in 1944. The newest low temperature record at the airport was the 32 degrees that was measured on October 19th in 2015. Assuming we interpreted the second part of your question correctly, we found the average year of maximum daily temperatures records for RDU to be 1987, while the average year of minimum temperature records is 1976.
Nov. 24, 2016 | Tags: past weather, records/extremes

Question: Why does your hourly forecast rain prediction (% chance of precip) not match the corresponding graph? — Nathalie

Answer: We assume you're asking about the first page that appears when you click on "Hourly" from any of our weather pages, or "View Hourly Forecast" just above the main 7-day forecasts display. If so, there is a list of hourly temperatures, sky condition symbols and precipitation probabilities, and then farther down the page is a graph with a box above it that allows you to select from several different variables to display. The default page for that graph shows traces of both temperature (in dark blue) and precipitation chance (in light blue). The code for that page is such that the values shown on the graph should match those in the hourly listing above. If you notice them showing different values again, please consider capturing a screen shot and noting the date and time so we can troubleshoot. Also, note that if the precipitation chance is zero, the light blue trace nearly blends in with the bottom of the graph. Do be sure to check that you weren't reading precipitation chances from the dark blue (temperature) line by mistake. The graph has a temperature scale on the left side and a precipitation probability scale on the right.
Nov. 23, 2016 | Tags: maps & codes, wral.com

Question: Should someone with asthma be outside tomorrow? Is this smoke danger what would cause difficulty breathing for them? Is there an alert out for this? — Judy

Answer: Unfortunately, this is the kind of question that we can't necessarily get to and post in this forum in time to address the particular instance being asked about. However, we wanted to pass along that there were some air quality alerts that stretched into central NC associated with smoke from the wildfires to our west, and that the concern was that those with serious respiratory illnesses, along with the elderly and very young children, should avoided strenuous exertion outdoors in those conditions. For anyone with questions about upcoming air quality issues, we maintain a couple of sources of information that can help. First, there's a link on our "Weather Resources" page (www.wral.com/weather/asset_gallery/13051669/) titled "NC Air Quality Forecast." Also, you can click the "Alert Center" link near the top of any page on our site, then click the "View active alerts" box to see every current weather or air quality alert in the state.
Nov. 22, 2016 | Tags: air quality, wral.com

Question: How do you see and report a fireball on 11/20 when today is 11/16? — Tim

Answer: That's a very good question, and the answer of course boiled down to a typographical error. The folks who wrote in to ask us about that fireball mis-typed the date. We were able to determine pretty confidently based on their description and the timing of their observation, that it matched up well with the fireball of late Saturday evening on November 12, as logged by the American Meteor Society at www.amsmeteors.org/members/imo_view/event/2016/4422. Our intent was to fix the date in the posted question, but after copying their question into the AskGreg database, we forgot to make that edit. We apologize for the mistake.
Nov. 21, 2016 | Tags: astronomy, cool sites, wral.com

Question: I have been watching you guys for 30 some years now. I love watching the weather because you are so accurate. My question is how do you think our Winter will look here in NC? I live in Fayetteville and was just wondering. — Jutta

Answer: We wish we could give you a good definitive answer, but seasonal outlooks by nature are quite general and somewhat low in confidence, and this year the signals that play into modulating our wintertime patterns are somewhat weak and mixed in nature. There is a weak La Nina pattern under way that appears it may weaken further and dissipate around mid to late winter. La Nina patterns tilt the odds toward an overall warmer and drier than normal result in our area, but is not a perfect predictor by any means, and also doesn't correlate very well in terms of seasonal snow which depends highly on short-term weather patterns and small variations in storms tracks. Another possible influence is the areal coverage of snow pack across Eurasia, which can affect the jet stream and resulting strength of the Polar Vortex. The snow cover is well above normal there this fall, which has been related to colder than normal winters for the northeastern U.S. - this effect is stronger off to our north, and leaves a lot of variability as to the results in central NC, so all we can really do is note that if La Nina dominates, we could well have a winter with temperatures a little warmer than normal overall, and perhaps somewhat suppressed precipitation, but even if this happens it tells us very little about how much snow or ice could occur.
Nov. 20, 2016 | Tags: el nino/la nina, winter weather

Question: Please explain why the Cape Fear did not have significant flooding from Matthew, while river basins to the north and South did. — Marsha O'Hare

Answer: We should start by noting that flooding along the Cape fear was quite substantial in some areas, especially southeast of Fayetteville and Elizabethtown. Most gauges from Fayetteville downstream recorded water levels that reached in the the "major flood" category, though for Fayetteville it was a brief excursion. On the other hand, a gauge just upstream from Wilmington recorded its second highest level historically. Upstream from Fayetteville, most gauges topped out for a short time in the moderate flood category. The rivers to the north and south in the Neuse and Lumber basins did have more extensive major flooding that lasted a bit longer. All of this relates to the shape of the individual watersheds and the pattern, or footprint, of the heaviest rainfall. Looking at contours of rain shows that while our entire region got very heavy rain from Matthew, in a relative sense some of the lower amounts fell near the headwaters of the Cape Fear, and along the Cape Fear basin as far east as about Fayetteville, with higher totals concentrated farther north and south in the adjacent basins. Those higher amounts did extend a bit into the Cape Fear basin from around Fayetteville eastward, helping to explain why major flood levels were more likely downstream as water in various tributaries collected into the main stem river.
Nov. 19, 2016 | Tags: flooding, hurricanes, past weather

Question: Does all the Hurricane Matthew runoff dilute ocean salinity? Does reduced salinity affect weather? — Chris

Answer: We took a look at salinity measurements from a couple of offshore stations in the period following Matthew and did find that there appeared to be a dip of around 1-2 Practical Salinity Units (PSU, from around 36 to 34) that was somewhat delayed, as you might expect from runoff that took some time to make its way from the areas that saw the highest rainfall totals. That dip was followed by a recovery to previous, pre-storm levels after a week or so, by late October. We suspect more notable decreases in salinity probably occurred in the sounds, but did not have access to similar salinity readings there. Generally speaking, less saline water is not as dense as saltier water and will tend to remain near the surface initially. In theory, this might reduce evaporation rates for a given temperature, and have some small effect on cloud formation or stability, but we doubt there were major changes in weather that resulted due to the temporarily lowered salinity values. It seems more likely that larger impacts from the Matthew runoff would have been biological in nature, especially in the shallower sounds where salinity changes, variations in nutrient levels and dissolved oxygen, and sedimentation from runoff would have been more concentrated and perhaps persistent. While updated info on these impacts from Matthew are not immediately available, significant impacts were noted in the wake of Dennis and Floyd in 1999, with salinity in the Pamlico Sound, for example, decreasing by about three-quarters, from around 20 PSU to 5 or so, for a couple of months following those storms.
Nov. 18, 2016 | Tags: Floyd, hurricanes, water resources

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