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Question: A recent 7-day forecast showed "tomorrow" (Feb 20th) with a high of 22 and the day after with a low of 38. Unless there is an instantaneous jump of 16 degrees at midnight, this seems impossible. How am I supposed to interpret that data? — William Faulk

Answer: Looking back at the actual progress of temperatures in that stretch of time shows the first day ended up with an actual high of 26, the following day had a low of 18 and then the second day high was 40. We suspect there was a data entry typo on our part in which the forecast high for the second day was entered into the space where the second day low should have gone and this caused the odd sequence on the displayed 7-day.

While that was likely a simple error, the weather does make it tough to choose which numbers to show in those spaces on occasion. In providing a forecast for rough planning purposes, we usually shoot for showing the high and low that most people are likely to experience in typical day to day activities rather than strictly meeting the 24-hour (midnight to midnight) definition used to maintain climate records. There are occasions when this creates an odd-looking 7-day in which the afternoon high may be the same value as the preceding or following low temperature. This usually means that we expect temperatures to be climbing or falling at a substantial rate in a non-typical manner (usually, lows occur near or a little after sunrise and highs around mid-afternoon). For example, if you see a low of 40 with a high of 40 the same day, you may find that our on-air or web forecast explanation for that day might show that the temperature will be around 40 at daybreak, then fall as the day continues. There are rare occasions when this general protocol leaves us with few really good options on how to set those numbers, but we still feel it's a better idea in general than showing numbers that might only be applicable at, say, 1 or 2 AM.
Mar. 6, 2015 | Tags: maps & codes, wral.com

Question: My wife Pam and I have been debating what makes ice slippery. Do you know? — Bill Mandulak

Answer: That has actually been the subject of some serious study and thought by chemists, physicists and others, and has not been as easy as you might imagine to pin down with certainty. However, the most recent prevailing theory has to do with the existence of a very thin liquid-like layer at the interface between ice and other objects, a layer that exists even when the temperature is below-freezing. This has been referred to as a "pre-melting" layer and attributed largely to the fact that the water molecules at the surface are exposed on one side and not bound together chemically the way molecules in the interior are, allowing them more freedom of movement, and this movable layer leads to some loss of friction between any object in contact with the ice and the bulk of the ice below. This effect is enhanced by the application of pressure to the ice (for example, by a person's shoes, tires of a car or skate blades), which lowers the melting temperature a bit to help form an actual thin layer of liquid water, making the interface even more slippery. In addition, to pressure, frictional heating caused by the movement of another object across an ice face can heat the ice a bit, again producing a thin, slick layer of meltwater.
Mar. 5, 2015 | Tags: cold, general meteorology

Question: As a follow-up to the recent question about cold weather in Rocky Mount area in winter of 69-70, what is the longest period of below 32 degrees in Raleigh? — Dave Crotts

Answer: The answer isn't quite as easy to track down with certainty as you might think, but here is what's available. We checked back through data from the Raleigh-Durham airport, and also asked for a check of past hourly records by the State Climate Office and received a search result from the Southeast Regional Climate Center as well. They found a stretch of 157 straight hours at or below freezing (that is, 32 or lower) ending on January 16, 1982, and a streak of 100 consecutive hours below freezing (31 or lower, which is what your question asks) ending on Feb 5, 1996. Their databases, however, rejected observations from 1966-1971 from the search based on quality control checks, due to observations being archived only every three hours in that time frame. While we could not narrow it down to an exact number of hours, a search of some different archives turned up what appears to be a similarly long stretch of sub-freezing temperatures in January 1970, with the temperature falling below freezing on the 19th and rising back to 32 and above on the 24th. The data in that set is only available every three hours, so there is some uncertainty about exact numbers, but it appears there were at least 124 consecutive hours below freezing in that event.
Mar. 4, 2015 | Tags: cold, past weather, records/extremes

Question: What effect, if any, does wind chill have on freezing water pipes? — J.P. Gibson

Answer: The wind chill value per se does not apply to non-biological objects like water pipes. So, for example, if the air temperature is 35 degrees but the wind is blowing such that the wind chill value is 28, that doesn't mean the pipes will freeze, because the temperature of the pipes and the water in them will remain at 35 degrees.

On the other hand, if the temperature falls to, say, 20 degrees and wind is blowing strongly across an exposed pipe, it may cause the temperature of the pipe (and eventually the water inside) to fall to 20 degrees more rapidly than it would if the wind were calm.
Mar. 3, 2015 | Tags: apparent temperature, cold, winds

Question: Could the term back door cold front be explained? I keep hearing that but it is not clear exactly what that means. — Kent Riedling

Answer: The majority of cold fronts that cross our area, and the eastern U.S. in general, are oriented in a line that runs from north to south or northeast to southeast, and they tend to move through here from the west or northwest. This being the most common mode of arrival, the cold air behind the fronts can be figuratively said to enter through the "front door." Less commonly, a cold front will have more of a west to east orientation, and will push across our area from the north or the northeast, often because of a strengthening cold high pressure center over eastern Canada or New England. When the cold air arrives in this manner, the front that marks its leading edge is called a "back door" front.
Mar. 2, 2015 | Tags: folklore, fronts & airmasses, general meteorology

Question: Explain thunder snow heard in Sanford on Wednesday 2-18-2015 at 1715 as the snow came down. — Anonymous

Answer: Even though temperatures became quite cold on that day, we had a strong cold front moving in and some very cold air aloft, so that the lapse rate, or decrease in temperature from the surface to about 10,000 feet above the ground, was very sharp due to extremely cold air aloft. That difference made the lower atmosphere unstable, in quite a similar way that we form summertime thunderstorms. In addition, there was strong lift associated with the approaching front, and the line of snow showers swept through in a similar structure as a convective season squall line. That structure was enough for strong vertical motions to separate charges within the clouds enough to generate a few lightning strikes here and there, producing the thunder that you and some others heard.
Mar. 1, 2015 | Tags: snow, thunderstorms

Question: Is there a scientific or official definition for what some call an "Indian Summer"? What causes them? — Jennifer Burns

Answer: The Glossary of Meteorology defines Indian Summer as a period of abnormally warm weather, with mainly clear skies and cool nights, that occurs after a substantial stretch of seasonably cool weather, usually after the first killing frost of the fall season. Indian Summer can occur multiple times, or not at all, in a given year. In Europe, comparable terms include Old Wives' Summer and Halcyon Days. The cause is usually development of a slow-moving ridge of high pressure aloft over or just east of the area experiencing the warm-up, one that allows for a southerly or southwesterly flow that results in fair skies and temperatures well above normal. There isn't anything unique about this pattern, other than the time of year that it occurs. A similar warming in mid-winter is often referred to as a "January Thaw."
Feb. 28, 2015 | Tags: folklore, general meteorology, heat

Question: What is the difference between the Radar maps and the Doppler maps? — Darlene

Answer: The general "radar" maps in the Map Center section of our site are regional maps that use composites from multiple National Weather Service Doppler radars around the country. These allow us to follow precipitation areas anywhere in the U.S. and allow for covering systems that are too large for a single radar to encompass. The down side can be that in some cases the displays make it appear that precipitation might be reaching the ground over a much larger area than is really the case, because they include distant echoes at high altitudes that in some cases may be evaporating before reaching the surface.

The "Dual Doppler 5000" and "Fayetteville Doppler" images, and the live stream that features both of those WRAL-owned radars, show only echoes from those two. They tend to be more up to date and more detailed than the regional radar data, but they do not have color coding for precipitation type (only intensity). There may be times when precipitation aloft that is shown on the regional radars, but does not reach the surface, is not visible on the local radars because it has evaporated at a higher altitude than the local beam is traversing, or because of different setting for the lowest threshold of echoes that are displayed (we occasionally adjust this so as to emphasize more substantial precipitation areas and reduce the display of ground returns and other non-meteorological returns such as insects and birds, etc, but usually keep it in a range that captures rather light precipitation as well). Of course, on rare occasion it is also possible there's an outage or other discrepancy with the local radars that makes precipitation less visible than it should be.
Feb. 27, 2015 | Tags: weather radar, wral.com

Question: There's a strong, blustery wind that blows east down Jones St in Raleigh almost every morning. I was walking in one morning with a gentleman who works in the tower at RDU and he said that that wind had a particular name, and he told me what it is but I have forgotten. Do you know? — Scottie

Answer: We aren't aware of a name specific to a westerly wind blowing down Jones Street, but would be interested to hear if someone knows of one. We do wonder if the gentleman may have referred more generically to winds that are strengthened beyond their ambient intensity by being funneled along between a series of buildings in such a way that Bernoulli's Principle and the Venturi Effect come into play, resulting in an acceleration along what's sometimes referred to as a "street canyon" or "urban canyon." Perhaps one of those terms, or something closely related, is what you remember.
Feb. 26, 2015 | Tags: winds

Question: Is there a way to search the weather page for stats on coldest recorded temperature, highest temp, longest drought, etc, as it pertain to RDU? — Dorothy

Answer: Through our Almanac page, you can pull up observed data for RDU that will will include the coldest and warmest records for the date in question, but we do not have a function on the site for searching out more extensive extremes of the kind you're asking about. There are some great sites out there where you can find that kind of information though, either in short summaries of extremes or by way of querying databases. One good example is the State Climate Office site at www.nc-climate.ncsu.edu/climate/nc_extremes.php (there is also a "drought" area on their site. You might also like exploring the Southeast Regional Climate Center site at www.sercc.com (be sure to check the "climate perspectives" section).
Feb. 25, 2015 | Tags: cool sites, drought, records/extremes

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