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Question: How cold does it need to be to snow? — John M. Heering
Answer: The answer isn't as simple as you might think, because whether show will occur doesn't depend only on the temperature at a single level in the atmosphere, but instead depends heavily on how the temperature changes with height. Assuming there is enough moisture and upward motion present to produce precipitation, snow is most likely to occur when most of the atmosphere is at or below freezing. Beyond that, to form snow crystals we also need at least some portion of the air in which precipitation is forming to have a temperature range of about -10 to -20 degrees C (between about 14 and -4 degrees F).
If these crystals and snowflakes (usually a collection of multiple crytals stuck together) fall all the way to the surface in sub0freezing air, then snow can certainly occur. It is also possible to snow with the surface temperature above freezing as long as the above-freezing layer near the surface is shallow enough that the flakes do not melt before they reach the ground. In rare cases with temperatures that decrease very rapidly from the surface upward, snow can make it to the ground with surface temperatures around 40 degrees or a bit higher.
Feb. 2, 2014 | Tags: cold, general meteorology, snow
Question: Can you tell me what the weather was on Saturday, October 12th, 2013? — Michelle
Answer: Observations for that day from the Raleigh-Durham airport indicate it was a cloudy day with a light north to northwest breeze and some very spotty sprinkles. The airport reported .01" of rain for the day, with a low temperature of 61 degrees and a high of 69.
You can find information like this using the "Almanac" page of our web site. There you'll find a "Get Historical Data" section where you can enter a date and click the "send" button to retrieve data from RDU for that date. Once you're on the data page, you can also choose to view a week or a month at a time, change to other weather observation sites, or review a time lapse of archived radar imagery through that day.
Feb. 1, 2014 | Tags: past weather, wral.com
Question: What is lake effect? — Cowboy
Answer: That phrase refers to a process in which the passage of very cold air across the surface of a large, relatively warm lake leads to rapid destabilization and moistening of the cold air (due to heating and evaporation from the lake surface below), producing plumes of convectively-driven, and sometimes quite heavy, snow showers over land areas downwind. Lake effect snows tend to be most prevalent and intense during the first half or so of the winter, then diminish some as lake water temperatures gradually become cooler, so that the difference in water and air temperatures decreases.
Lake effect snow and rain showers are most closely associated for most of us with the Great Lakes and cold arctic outbreaks crossing those large fetches. On occasion, however, we do see similar effects when cold air flows along the Chesapeake Bay, and on rare occasions we can also see plumes of precipitation either generated or enhanced somewhat when cold air crosses some of our local lakes. A recent snow even ended with a band of apparent lake effect snow stretching from Kerr Lake southeastward into the Rocky Mount/Wilson and Goldsboro/Kinston areas.
Jan. 31, 2014 | Tags: general meteorology, lakes and rivers, snow
Question: Not so much a question as a comment. The tale goes that when it thunders in Winter, it will snow 10 days later. I actually remember a couple of time in the past that this has happened. I know it's coincidence, but I would like to point out that we had thunder on Jan 11th. As of today (January 20th) you have the chance of snow showers in your forecast for Jan 21 (10 days later). Now...this happening once or twice is surely a coincidence...but at what point do we start calling it a trend? — Gary
Answer: The day and following night that you're referring to did end up with some snow, with one-half inch at the RDU airport and a band of 1-3 inch totals from around Wilson northward in the northern coastal plain.
There is some physical reasoning behind the old "tale" you mentioned, although it certainly doesn't verify in every case, and even when it does the snow may take the form of unmeasurable flurries. Still, the same large-scale pressure patterns that bring warm, unstable air far enough north for winter thunderstorms here are also those that move cold air most deeply toward the south. Well-timed disturbances might then produce some snow.
The patterns in question involve sharp mid-level pressure troughs and ridges that drive airmasses strongly toward the north and south. This is known as "meridional flow," and tends to produce stormy weather involving large temperature swings. This can be contrasted to "zonal flow," in which airmass transport is mostly west to east, temperatures tend to be moderate, and disturbances are often weak, making both thunderstorms and snowfall less likely for our area.
Jan. 30, 2014 | Tags: folklore, snow, thunderstorms
Question: On the new WRAL weather page, I have trouble telling the overnight low. which night is it for? I know the first number is the high for the day. After the slash bar /, is the 2nd number for that night? For example Friday says 48/33. Does that mean 48 is Friday's high and 33 is the low Friday night and into Saturday morning? — Julie
Answer: Your interpretation is exactly correct. On the on-air "tubes" we use for a 7-day forecast, the low temperature is shown shifted a little to the left on the day it would occur (usually around sunrise). However, this didn't seem to work well for the vertical arrangement used on the main page of the new site design. Here the number on the left is the high for the day in question, then the low temperature that follows would typically be reached the next morning around daybreak, and should match up with the text description when you click the "overnight" button above the text description. As an aside, the on-air format version of the 7-day forecast remains available on our site, located in the "Map Center"/"Other Maps" area.
Jan. 29, 2014 | Tags: maps & codes, wral.com
Question: What years did the five deepest snowfalls occur at RDU airport since 1920? — Ed
Answer: Records for the airport begin in the mid 1940s, so we used a relatively new database called Threadex that strings together a series of "official" Raleigh-area reporting sites (the latest of which is RDU) that stretches back to 1887, and checked values from that database since 1920. Most of our biggest snowstorms are spread over parts of a couple of days and occasionally more, so we looked at the highest accumulations in any three-day stretch during that time. The results came back in the order January 2000 (20.3 inches), March 1927 (17.8 inches), February 1948 (14.5 inches), March 1980 (11.1 inches) and January 2002 (10.8 inches).
Jan. 28, 2014 | Tags: past weather, records/extremes, snow
Question: First off, thanks for years of professional and educational weather reporting and forecasting. Now for my question: Years ago, and I mean YEARS ago, you first introduced us to "isobars" and you fitly described them as "lines of unequal pressure", which makes sense, since unequal pressure produces wind speeds of varying degrees. But lately (maybe last six months) I've notice that when you are explaining weather map details and icons, you define the isobars as "lines of equal pressure". Hmmm, it seems that if the pressure were equal across a certain geographical region, there would be no variance in wind speed, hence no lines to separate one amount of wind from a differing amount. Do you see why I'm confused? — Steve Hester
Answer: It seems likely there may have been a simple slip of the tongue on-air sometime long ago that you happened to hear in regards to isobars being lines of unequal pressure, because it has always been the case that they are lines of equal pressure, that is the lines run along locations where the pressure is the same in order to outline areas of relatively high or low pressure, making pressure centers, troughs, and ridges evident on a weather map in the same way that contours of elevation on a topographic map (which could technically be called isohypses) help us visualize hills, depressions, valleys and ridges in terrain.
On a terrain map, a series of near-parallel isohypses close to one another represent a steep slope perpendicular to the contour lines. Likewise on a weather map, a number of isobars spaced closely together indicate a strong gradient of pressure crosswise to the isobars, and is usually an area where the strong horizontal pressure difference translates into rather strong winds, while the opposite applies for isobars that are widely spaced from one another.
Jan. 27, 2014 | Tags: general meteorology, maps & codes
Question: Did it hail in either late June or early July of 2013? — Rhonda Coffer
Answer: Your question didn't specify a location, so it's a little tough to give a confident answer that would apply in any area, since hail is often very localized in nature. In addition, hail may occur somewhere other than over a weather reporting station, such that most of the reported occurrences are associated with larger hailstones that meet severe storm criteria. Given those caveats, we took a look at severe storm report maps from the Storm Prediction Center from June 15 - July 15, 2013 and made note of hail reported in some portion of NC on the following dates: June 25, 26, and 28, along with July 9, 11, and 12. You can see those same maps, which include listings of the reports in a text table beneath the map, at www.spc.noaa.gov/climo/online/, where you can find the "Past storm reports" section and search for the date you'd like to see. Also, the State Climate Office has a storm reports search function where you can check more specifically for the location you're interested in. That page is located at www.nc-climate.ncsu.edu/lsrdb/index.php. Here3, you can use the "advanced options" section in the blue box to choose a range of dates to search, and can expand the "thunderstorm hazards" section to limit your searches to only show storms that produced large hail.
Jan. 26, 2014 | Tags: cool sites, hail, past weather, severe weather
Question: Has it ever snowed after Easter in N.C.? And do you think we will see any of the white stuff this year? — O.B. Autry
Answer: It has snowed later than Easter in North Carolina and is probably not an unusual occurrence for parts of the mountains. It is tougher to find such events farther east, though, but as an example, the RDU airport recorded 1.8" of snow on April 18, 1983, a year in which Easter fell on April 3rd. Likewise, at the same site .3" of snow was recorded on Apr 11, 1989, and Easter was on March 26th that year. We've already had a bit of snow in the area this season, but it remains to be seen whether cold air, moisture and lift can combine in the right way for a substantial snow in the coming weeks. There aren't any good ways to predict that reliably beyond a few days in advance.
Jan. 25, 2014 | Tags: past weather, snow
Question: Where can I find that map which showed rainfall amounts during the previous 24 hours? I have referred to it often. — Henry Nuttle
Answer: The new site design collected a lot of the maps into one selection page, and that's where you can find the 24-hour rainfall map. Just click "Map Center" and then click the "other maps" tab. One of those is "rainfall," and it is the map you are looking for.
Jan. 24, 2014 | Tags: maps & codes, rain, wral.com
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Published: 2007-10-09 14:40:00
Updated: 2013-08-13 13:37:27
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