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Weather Questions tagged “rain”

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Question: When was our last measurable rain in Henderson? — Pam

Answer: At the time you wrote your question, the most recent measurable rain appeared to have occurred on February 15th, while there may have been a trace (less than .01") on the 17th. Since that time, measurable rain has occurred on the 25th and again (heavy at times) on March 1st.
Mar. 7, 2017 | Tags: past weather, rain

Question: I am doing a science project on how climate affects the growth of a tree. I have a cross cut of a tree and have counted the rings to determine it is 90 years old. I wanted to know if you knew where I can find the records for the last 90 years for the recorded annual precipitation in Holly Springs? — Brooke

Answer: It's unlikely you can dig up precipitation totals specific to Holly Springs, but you can find data that far back for the "Raleigh area" using the climate data web page at xmacis.rcc-acis.org. Just mouse over "single station" and select the "Seasonal Time Series" option. Then click on Options and select the variable precipitation. Finally under "Station selection," choose "Raleigh area" and click "Go" to see the data.

Feb. 28, 2017 | Tags: cool sites, past weather, rain

Question: Is it possible for a high pressure system to bring rain? I know that most storms are brought by a low pressure system, but is it possible? — Luke

Answer: This is a good example of how, in order to make weather understandable and more easily communicated, we often greatly simplify what are in fact very complex patterns and processes in the atmosphere. While we commonly associate low pressure with precipitation and high pressure with fair skies (and rightly so in many cases), there are exceptions and there are situations in which interactions of relatively low pressure at one altitude correspond to relatively high pressure at another, sometimes resulting in precipitation. That's a long way of saying that it IS possible for precipitation to occur when there is a high pressure system in place. One common example for us is the "cold air damming" pattern, in which high pressure extends into the state from the north or northeast, creating a north or northeasterly wind flow and bringing a shallow layer of cold, dense air into the region. Sometimes, that air undercuts and lifts warmer, moister air that condenses into clouds and precipitation. In many cases, this only yields sprinkles, drizzle or very light rain, but in some instances the precipitation can be more substantial. Another example would be high pressure building in from the west in the winter, producing strong northwesterly winds for our state. For a lot of us, that is generally a dry pattern. However, it isn't uncommon in the mountains for air flowing up the western slopes within the eastern or northeastern portions of the surface high to be moist enough to produce flurries or snow showers there. One other note is that tropical cyclones often are best organized and set up to intensify if the low pressure center near the surface is lined up nicely with a high pressure center in the upper atmosphere more or less directly above it.
Feb. 10, 2017 | Tags: fronts & airmasses, general meteorology, rain

Question: How did monthly precipitation in 2016 compare to that in 2015? — Tony Stephenson

Answer: We hadn't finished 2016 entirely when this answer was drafted, so some small changes are possible. However, for the entire year at RDU, 2015 brought 57.1 inches, while 2016 had totaled up to 50.9 inches so far. That makes both years quite wet, as the "normal" yearly rainfall (defined by the 1981-2010 average) is 43.3 inches. Months that were especially different between the two years included May, when 2016 saw 3.2 inches more than 2015, July (with 3.1 inches more in 2016), November (in this case, 2016 saw 6.5 inches less than 2015, and was especially dry while the same month in 2015 was especially wet). As of this writing, December 2016 had received 4.3 inches less rain than the previous year.
Jan. 1, 2017 | Tags: normals, past weather, rain

Question: You've given several good answers about percentages, thank you for explaining. I would like to know the geographic boundaries you speak of when you give your forecasts. I love being a biker, but not a wet one. — Tim

Answer: The probabilities of precipitation that we show in graphs, 7-day "pullouts," and as the number you see in the main 7-day forecast on our web site are numbers meant to apply to the Raleigh area. In many cases, they apply reasonably well to most of our viewing area as well, but there are certainly times when a frontal boundary or some other weather feature leads us to expect higher or lower chances for some other parts of the region. In these cases, we might show a map of probabilities that highlights how they vary over both time and horizontally, or might mention verbally (and in the written descri9ption online) that chances will be higher east of Raleigh, lower to the southwest, and so on. In addition, if you change the location of the forecast using the box just above the main 7-day forecast, the probability will be adjusted based on the location you select, and may be higher or lower than the one you see for the Raleigh area.
Dec. 21, 2016 | Tags: maps & codes, rain, wral.com

Question: Who decides what constitutes a 10 or 100-year rainfall event? We seem to be getting a lot of 100-year rainstorms lately, leading me to suspect that our stormwater runoff regulations need to be updated to reflect a new post-global warming reality. — Oak Rapp

Answer: The values of "average recurrence interval" or "return period" for rainfall over a given duration are statistical calculations based on historical data, and are compiled and published by the National Oceanic and Atmospheric Administration's National Weather Service in a series of documents called NOAA Atlas 14. The results can also be accessed and viewed online via a web page called the Precipitation Frequency Data Server (easily found with a web search), where you can drill down to any particular location to find the interval associated with any given rainfall amount and duration, along with a confidence interval of what the true return interval is likely to be based on the available period of record and the uncertainties associated with such analyses. Of course, as conditions change over time, those numbers will be recalculated to reflect new data. It is worth keeping in mind that while the terms like "return interval" imply that a 100-year rain rate should only happen every 100th year, it really means that in a given year, there is a one percent chance of that rate occurring. That doesn't preclude a similar event happening just a few years later. Of course, if they continue to happen at rather frequent intervals, that will eventually cause the calculated recurrence intervals to become smaller. Climatologists have to balance the desire to have such calculations reflect current reality against the possibility of changing the numbers too much based on what may turn out to be a short-term anomaly that isn't representative of longer term trends.
Dec. 9, 2016 | Tags: cool sites, normals, rain

Question: We live near Wake Crossroads in NE Raleigh. Like today, when storms come through Raleigh, it will be raining all around us, but we will not get any rain. Is there something unique about this area that it misses a lot of rain storms? — Jan

Answer: We haven't seen anything to indicate that should be the case, as average monthly and yearly rainfall totals for that general area appear similar to most surrounding locations. Of course, in any given system passing across the region, there can be considerable variability in rainfall amounts due to the typical fluctuations over time and distance associated with many weather features. On the Sunday afternoon that you wrote in, for example, dry air in the lower atmosphere evaporated a lot of the precipitation that fell during much of the daylight hours, especially from around Raleigh northeastward, but as the lower atmosphere became more humid, rain became steadier and heavier heading into the evening and overnight. Observations from a pair of personal weather stations in your immediate vicinity, in agreement with radar rain estimates for that period, indicate you should have ended up with around 3-6 tenths of an inch of rain, similar to much of our area with that disturbance. We've found that the variability of rainfall amounts, and the availability of radar displays to track systems passing through the area (often affecting part but not all of the region) has led people from pretty much every part of our viewing area to have the same perception - that something routinely and uniquely blocks precipitation, storms, etc from their particular location.
Dec. 8, 2016 | Tags: general meteorology, rain

Question: How did rainfall totals compare between Hurricane Matthew and Hurricane Floyd? — Dustin R

Answer: There are some similarities and important differences, especially with regard to distribution. The areas that received over a foot of rain from Floyd were focused over the northern coastal plain and along to just inland of the southern coast. Those kinds of totals from Matthew were mainly focused over the southeastern Sandhills and southern coastal plain. In each case a large section of the eastern third or so of the state received over 8 inches, and there were sizable pockets with over 15 inches. You can compare similar contour maps of rain from each storm by going to www4.ncsu.edu/~nwsfo/storage/cases/20161008/precip.20161008.png for a Matthew map, and scrolling dow a bit at www4.ncsu.edu/~nwsfo/storage/cases/19990915/ for a Floyd map.
Nov. 30, 2016 | Tags: cool sites, hurricanes, past weather, rain

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: I have a question about the cumulative rainfall amounts. Does being nearer the center of the city affect the rainfall? For example, the RDU rainfall shows little rain since Matthew, and that is what we have here. However, before that, from 8/15 - 10/10, it shows somewhat below average but a reasonable ratcheting up. However, where I live, about a mile south of I 540 north, we had only a few sprinkles in that time and I barely had to cut my grass. Right now, it looks like my azaleas are under significant stress and may not make it, something I would not really expect for a cumulative rainfall that is above normal. — Margaret

Answer: The cumulative total graphs on our web site show the values specifically for the RDU airport. While these often even out reasonably well for the surrounding region over time, for a few weeks or a few months it is quite possible for amounts to vary a good deal over fairly short distances. Looking over radar and gauge based estimates for the general area around the airport does make it appear pretty substantial rainfall fell in most places during a number of periods through August and September. As you noted, since the very heavy rains of Matthew ended there have only been 2-3 additional episodes with rainfall, each of which were quite light and scattered. That has left what was initially very wet topsoil and subsoil values across the region on the decline. By the time you wrote to ask, crop soil moisture readings were down to the "slightly dry to favorably moist category" and evaporation rates had notably exceeded precipitation rates in recent weeks. With the uppermost soil levels drying at varying rates across the area, it seems possible your azaleas are responding to a short-term lack of water, though we can't really address whether some other factor may be affecting your particular plants.
Nov. 14, 2016 | Tags: rain, wral.com

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