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Weather Questions tagged “rain”

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Question: What is an areal flood? — Arr Schaller

Answer: It's probably easiest to describe by comparing it to the other common flood warnings that the National Weather Service may issue, since they differ largely in the sense of how quickly the onset of flooding may occur, and how localized flooding is expected to be. They define flash flood warnings and areal flood warnings in the following ways:

A Flash Flood Warning is issued for flooding that normally occurs within six hours of heavy or intense rainfall. This results in small creeks and streams quickly rising out of their banks. Dangerous flooding in areas near these creeks and streams, as well as low-lying flood prone areas, develops very quickly and is a significant threat to life and/or property.

An Areal Flood Warning is normally issued for flooding that develops more gradually, usually from prolonged and persistent moderate to heavy rainfall. This results in a gradual ponding or buildup of water in low-lying, flood prone areas, as well as small creeks and streams. The flooding normally occurs more than six hours after the rainfall begins, and may cover a large area. However, even though this type of flooding develops more slowly than flash flooding, it can still be a threat to life and property.

A third type to be aware of is the River Flood Warning, which is issued when runoff from extended periods or large areas of heavy precipitation cause rivers to reach or exceed established elevation levels that correspond to minor, moderate or major flooding levels at designated gauge locations along the river. While the other warnings typically remain in place for a few to several hours, river warnings can sometimes stay in effect for days.
Sep. 25, 2016 | Tags: flooding, preparedness, rain

Question: Take a look at the WRAL 24-hr rainfall map. What is up with the rainfall in Greenville? It looks like they are reporting rain from Hermine in centimeters rather than in inches. — Dave Salman

Answer: We noticed that, too, and in fact there were a couple of different locations that we were not directly plotting the values on our maps where we saw that color contours in the background had areas that were obviously contaminated with readings that were too high, so that we turned off that background layer for on-air purposes. We aren't absolutely sure what the problem was yet, but have seen instances in the past with reports from the type of automated sensor used at the Greenville airport in which display software adds up rainfall cumulative rainfall totals as if they were sequential un-related values instead, leading to a significant over-calculation of the reported amount. Other nearby stations, along with radar-based estimates, make it appear most areas in the vicinity of Greenville received around 6-8 inches of rain Thursday through Saturday from the combination of a frontal boundary and Hermine.
Sep. 7, 2016 | Tags: instruments, maps & codes, rain

Question: Will it ever, ever rain in Raleigh again?? Will the air in Raleigh ever be cool again? — Candy

Answer: Rain has certainly been less widespread and frequent for the past few weeks, and that general pattern may not change all that much soon. However, by the time you read this response, it is quite possible there will have been some showers and storms associated with a passing cold front, and some rain generated by an interaction of that front and a tropical storm moving by to our southeast. Those systems didn't absolutely guarantee substantial rain everywhere in the vicinity of Raleigh, but they did involve a high probability of having some, and also a good chance of temperatures running notably cooler, with less humidity, for a few days. This may be followed by another round of fairly hot weather, but we'll be heading into the fall over the next few weeks and you can be assured that, with a few ups and downs along the way, we'll trend toward cooler weather.
Sep. 3, 2016 | Tags: general meteorology, heat, rain

Question: What does a double rainbow mean? Thank you. — Pat

Answer: The principal indication given by a double bow is that the raindrops that are refracting and reflecting sunlight are of medium to large size, rather than being very small or tiny droplets. The larger droplets produce narrower and brighter bows, making the secondary bow more visible than it would be with smaller drops. The primary bow, produced by light rays reflected once within the drops, occurs along a circle that is about a 42-degree angle from the center of that circle, while the secondary bow, produced by light rays that reflect twice within the drops, occurs at about a 51-degrees angle. One thing to notice when you see a double bow is that due to the second reflection the colors are reversed - the primary bow is red on the outside edge, while the secondary is red on the inside.The secondary bow is a good deal fainter than the primary, so that when the drops are small and the bow is faint, it may not be noticed at all. There's a lot of great information about rainbows, along with diagrams explaining how they form and photographic examples of the many different forms, at http://www.atoptics.co.uk/bows.htm.
Jul. 14, 2016 | Tags: atmospheric optics, rain

Question: Storm just rolled through and at 9:10PM the temp has dropped to around 74 degrees. On your hour by hour forecast, it is predicted to be around 80 degrees about now. When temperatures drop this time of night due to rain, will the temperatures rise even though there is no sun to heat the air back up? Also, can we expect the overnight temperature to be lower since we are now cooler than expected for this time of night. — Joseph

Answer: The situation you describe has enough variables involved that there isn't a single yes or no answer. As with many situations in weather, "it depends" on a number of factors, and there will be times when the rain-cooled air will regain some of the lost temperature (if, for example, the cooled air is rather shallow and mixing brings warmer air down from above, or the rain-cooled air covers a small horizontal area and advection causes warmer air from another location to flow back over the area that had cooled some, or if the ground had become very warm during the day and some stored heat is released that is able to overcome other cooling effects during the night). On the other hand, absent some of those effects, it is also possible for the temperature to remain near, or to simply fall from, the newly reduced level. The most rapid fall would typically occur if skies clear out after the rain and winds remain light. A similar set of considerations affects whether the rain-cooled air will eventually fall to a lower temperature than it would have in the absence of the rain - in some cases, yes, in some no. On the night you asked about, the temperature at RDU fell from 82 just before the rain to 75 just after (at about 9 PM), then ticked up to 76 by 1 AM before dropping to 74 at 2 AM, only to edge back up to 75 at 3 AM. After that it drifted as low as 72 right around sunrise.
Jul. 13, 2016 | Tags: general meteorology, rain

Question: One of the questions references an almanac and rainfall amounts. Is that Online? — Lyn Triplett

Answer: It is indeed. The particular product showing how rain at RDU compares to "normal" over the past five years is located in the "Almanac" section of our web site. Just look for the link in the upper part of the web page. Then, on the Almanac page, you'll see a link for "RDU rainfall Charts," where you'll find graphs of cumulative rain totals versus cumulative normals for several periods ranging from 30 days to 5 years.
Jul. 12, 2016 | Tags: past weather, rain, wral.com

Question: The local pool certainly seems cooler after a significant rain. Are there any generalizations you can make about the temp of the rain during a typical summer storm? — Nadine

Answer: There are, although the situation is complicated enough that some generalizations are indeed necessary, since the temperature of rain reaching the surface depends on its initial temperature in the formation region aloft, the transfer of energy into the drops (warming) as they fall toward the surface, the cooling of the drops as they evaporate (offset in some cases by condensation of additional moisture onto the drops as they fall through warmer, humid air), the size of the drops, and the presence or absence of hail in the storm (even if the hail melts before reaching the surface. All that said, it is common in summer storms for rain temperatures to range anywhere from mid 50s to around 60 degrees in the presence of significant hail, to upper 60s to mid 70s otherwise. Of course, given that summer storms often develop or roll in when temperatures near the surface are in the 80s or 90s, this can indeed have a sharp cooling effect!
Jul. 9, 2016 | Tags: general meteorology, rain, thunderstorms

Question: Looking at the almanac, NC has had almost 40" above normal rainfall in the past 5 years. Even the 30, 60, 90-day almanac shows consistently higher than normal amount of rainfall here. I thought it was just my imagination but we really are getting a lot more than normal rain here in NC. What's up with that, in your opinion? — D.

Answer: You're correct, but it is really pretty difficult to provide a satisfying answer beyond the fact that "normal" is a 30 year average value (ending with the most recent "zero" year and updated every ten years) that results from years having a fairly wide spread of rainfall amounts, and in which some multi-year longer scale variations are evident when you look at a really long-term plot of annual totals. in the case of the past 5 years, for example, 2015, 14 and 13 all were notably wetter than normal, but the 4 years prior to that were all at or below normal. Historically, in records going back to 1887, we see some stretches with a steady average but large swings year to year, interspersed with a number of stretches of 4-6 years that favor more frequent above (late 1940s to early 50s, mid 1990s to around 2000) or below (1880s to early 1890s, mid 1960s to around 1970) amounts. We'll have to wait a few more years to see how this year and the past three stack up in that historical sense, and whether any further long-term patterns become evident.
Jul. 6, 2016 | Tags: past weather, rain

Question: Hi! I know this is a more specific question, but I'm not sure how to get the answer other than to go to "The Fish!" I am trying to find out what time rain entered the Raleigh/Knightdale area on 6/28/16. — Erica Hinton

Answer: An exact time for a particular spot is touch to pin down due to a few scattered sprinkles and showers that occasionally formed and dissipated out ahead of a more widespread, steady band of rain and showers that morning. Based on radar and surface station observations, however, for Raleigh the first sprinkles and patches of rain tracked in from the west and southwest around 8 AM or just after, while the steadier rain began to pick up closer to 9 AM. In both cases, the timing would have been about 15 to 30 minutes later for Knightdale.
Jul. 2, 2016 | Tags: past weather, rain, weather radar

Question: I need daily rainfall totals for research I am doing near the Arboretum at NC State University. Is there a database that exists with this kind of information? — Joel Burley

Answer: Our primary recommendation for you would be the version of the Applied Climate Information System at xmacis.rcc-acis.org/. There are a couple of functions under the single-station, like "Daily Data Listing" and "Daily Data for a Month," that you may find very helpful. There is a cooperative station at or very near your location of interest. You can access that information in the "Station Selection" drop down by choosing "Raleigh State Univ." If that doesn't appear in the list, make sure the "Change CWA" section has been set to "RDU - Raleigh."
Jun. 26, 2016 | Tags: cool sites, past weather, rain

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