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Weather Questions tagged “rain”

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Question: How can you measure the speed of rainfall by looking at radar from one town to another? Does any of WRAL's apps provide means to measure the speed rain is traveling? — Bobby

Answer: The radar sections of our apps do not have a readout or built-in means of showing what the speed of rain areas is, but you can get a sense of that by using the lapse functions either on the Dual Doppler 5000 displays, where you would choose the view you’re interested in and then select the “1-hour” lapse, or by viewing the iControl radar display and clicking the “play” arrow, which will show about a 50-minute long lapse. You can figure a rough speed by following a rain area or storm cell across an area of known distance and dividing that distance by the elapsed time. For example, on the 1-hour lapse, if a band of rain moves halfway from Raleigh to Rocky Mt, that’s about 25 miles, so the band is moving around 25 mph.

Also, in situations where there are severe thunderstorm or tornado warnings in place, you can read the text of the warning and it will usually include a statement that gives the direction and speed of movement of the storm responsible for the warning.

May. 21, 2017 | Tags: rain, thunderstorms, weather radar

Question: Why did you remove the current rainfall amounts from the "Current Conditions" page? — Stefan M

Answer: When the web site was being changed a bit to accommodate changes in the graphics system that we use on-air, we decided it made sense to have the rainfall data consolidated with other kinds of maps in the "Map Center" section of the site. You can find the most recent daily precipitation values (the amount since midnight) for many stations around the viewing area by going to the Map Center page and clicking "other maps" where you'll see "rainfall" as one of the selections. There is a Map Center link in the "Maps & More" box just below the 7-day section of our main weather page.
May. 17, 2017 | Tags: maps & codes, rain, wral.com

Question: In the Sanford area what was the amount of rainfall on Monday night, May 1st? Also, what was the speed of wind gusts for the same day and area? — Joe Russell

Answer: For rainfall in that period, we checked observations from the Sanford-Lee County airport and also radar rain estimates for the surrounding area. The airport recorded .19 inches of rain, with radar indicating a general range of about 1-3 tenths of an inch across the county. Winds were gusty in the afternoon and evening, with the top gust at the Sanford airport reaching 30 mph, but some nearby locations recorded gusts as high as 35-40 mph. In addition, there were quite a few reports of thunderstorm gusts strong enough to bring down trees just to the west, although that band of storms tended to weaken by the time it moved east of about Randolph County. To see a Storm Prediction Center map of reported high wind damage from the afternoon and evening leading into that Monday night, see www.spc.noaa.gov/climo/reports/170501_rpts.html.
May. 14, 2017 | Tags: cool sites, past weather, rain, severe weather, winds

Question: Could the answer to Nadine's question referring to flooding along the Tar as well as other rivers, be attributed to the explosive growth of the Triangle from new roads, house construction, etc, causing major runoff into our streams and river basins? — Frank

Answer: The factors you mentioned involving increases in impervious surfaces that lead to greater runoff rates in heavy rainfall events are important to consider and have certainly played a role in how streams and rivers respond to these situations, and how that response evolves over time, so you raise a good point. However, we think it is unlikely that changes during the 6 months between Matthew and the April storm were enough to have a strongly noticeable added impact, while the localized differences and similarities in amount and distribution of rain between the two events seems to reasonably account for the differences in flooding levels that resulted.
May. 11, 2017 | Tags: flooding, lakes and rivers, rain

Question: If the hourly forecast is 0% chance of precipitation and it is raining, does that make the forecast 100% wrong? — David Buffaloe

Answer: It's certainly hard to argue otherwise, although the converse isn't necessarily true. Ideally, we try to reserve the use of a zero percent rain chance to days we're very confident of dry weather for the great majority if not all of our viewing area. However, in order to allow for some variation across the region and to allow for tailored forecasts for specific towns or zip codes, etc, including anywhere in the U.S., our hourly forecast info is automated and tied to a gridded forecast database that is strongly influenced by computer model projections. Because these occasionally show a zero percent chance when we would probably lean more toward something in the 10-20% range, we encourage you to use the hourly forecasts in conjunction with the parts of our web site that include our written description of the forecast along with a daytime or nighttime probability of precipitation that we select and enter directly. That will often give you a better sense of whether we think a few isolated showers are not out of the question, even if an automated hourly forecast shows 0 percent for some portion of the day. On the other hand, a 100% chance of rain for a given valid period (whether an hour or a day, for example) wouldn't be wrong simply because rain isn't falling at a given moment, but would be wrong if the entire hour or day went by with no measurable precipitation. It means that precipitation is definitely expected to occur within the valid time of the forecast, but not every minute of that forecast period.
May. 10, 2017 | Tags: maps & codes, rain, wral.com

Question: It would seem the national weather service nor other forecasters did a great job of forecasting the 8 inches of rain we received from Saturday to Tuesday. What happened? Were there forecasts that I should have listened to but ignored? — Dennis Thompson

Answer: By their nature, extreme weather events like the heavy rainfall in this case are difficult to accurately forecast, but the idea that heavy rain was on the way and that flooding would be a significant concern was in the forecast from NWS and the rest of us for days ahead of this system arriving. A flood watch was in effect days ahead of the actual flooding, with forecasts that called for rainfall totals in the watch area, which covered almost all of our viewing area, in the 2-6 inch range. While this was a little on the low end of the actual range of 3-8 inches for most of the area, it really wasn't bad considering how far out of the norm such rainfall amounts are, and we commend the NWS for accurately having those flood watches in place well ahead of time. One aspect of the forecasts leading up to the event that did amount to a discrepancy was a tendency on the day the heaviest rains began to move in (Monday) for most of the major computer forecast models to shift the axis of heaviest rain off to the southwest, indicating areas from around the Triangle north and east might only see about 1-3 inches, while the higher amounts would end up southwest. This was a temporary shift, and reality was closer to earlier outlooks with the axis of heaviest rain ending up stretched from around the central coastal plain west-northwest into the Triangle area, leaving the Fayetteville area and points west with somewhat lower totals. This led to the most significant and sustained river flooding occurring along the Tar and Neuse rivers, while the Cape Fear, Little and Haw rivers farther south were somewhat less impacted.
May. 5, 2017 | Tags: flooding, lakes and rivers, rain

Question: What was the official rainfall totals for this past event for RDU and Wake Forest? — Tracey Timmerman

Answer: At the RDU airport, the three-day total for April 23-25 was 7.45 inches. While that total comes from a weather site maintained by the National Weather Service, for the more immediate Wake Forest area we could only sample some personal weather stations that are posted online and some stations that are part of the "CoCoRAHS" network available at www.cocorahs.org, where we checked a sample of three Wake Forest area stations from each source. The storm total rainfall over roughly the same period for those six sites ranged from a low of 5.6 inches to a high of 8.1 inches.
May. 2, 2017 | Tags: cool sites, past weather, rain

Question: Why is it relatively common for central NC to receive several inches of rain from one system but incredibly rare (if not impossible) for Central NC to see a system carrying snow with liquid amounts equal to several inches? Wouldn't that mean we receive several feet of snow? — Evan Carter

Answer: The principal difference is that during the winter colder temperatures mean that the atmosphere is carrying less water vapor and is generally less moisture-laden than during the warmer months of the year. In addition, when strong systems in the winter do bring in very significant amounts of moisture, they commonly pull warmer air in at the same time and either prevent snow or limit snowfall totals by changing snow to rain, or at least mixing any frozen precipitation with rain. Of course, we do have very occasional large snowfalls in central NC, but even in those cases, the 15, 20 or 25 inch snowfall totals would equate to a total of around 1-2 inches of liquid water. A system capable of producing rain on the order of 4-8 inches as we saw recently would almost inevitably pull sufficiently warm air into the region to prevent anything close to all of the precipitation falling as snow.
Apr. 29, 2017 | Tags: general meteorology, rain, snow

Question: Guess who is planting Bermuda on one acre!! That's why we're wondering if there are long range forecasts for say the next month that may give us general ideas on when the rains may be more consistent??!! Sure would help us in seed germination without a huge water bill! — Jennifer Niemiroski

Answer: Our forecasts focus on the next 7-days, and while we do highlight some longer trends occasionally on the air or in posts to our web site, forecasts that are really helpful with the kind of planning you're looking to do remain pretty limited in detail and reliability. We can suggest a few resources to check into that might help some, but they are subject to significant uncertainty and in some cases, by necessity, are quite generalized. First, the Climate Prediction Center issues outlooks for whether the overall odds appear to tilt toward below, near or above normal rainfall for periods 6-10 days, 8-14 days and 3-4 weeks in the future. You can find these maps at www.cpc.ncep.noaa.gov, where you can just mouse over the "precipitation" link under each of those categories. You might also find an "ensemble" forecast product at weather.gc.ca/ensemble/naefs/EPSgrams_e.html, where you would use the box at the upper right to select "USA" and "Raleigh." The graph that shows up is a 15-day forecast that includes a precipitation sections. You'd want to look for stretches of time when the yellow boxes are mostly above zero (indicating a good chance of measurable precipitation) frequently over the course of a few days. Good luck!
Apr. 26, 2017 | Tags: cool sites, maps & codes, rain

Question: I was wondering on the "drought" we are seeing if it is really somewhat misleading. I know we are down for the year in rainfall since January 1, but we had a mega surplus of rain thanks to Mathew from last October.... so are we really that dry? — David Jones

Answer: The most recent U.S. Drought Monitor and NC Drought Management Advisory Council information at the time we answered your question indicated abnormally dry, but not drought, conditions from around Raleigh south and southwest, while areas northwest of Raleigh were designated as moderate drought. You make a good point about the large amounts of rain from last fall, and in fact that is thus far translating into near-normal water supplies in major reservoirs. On the other hand, lower than normal rainfall over the past few months has led to stream flow readings over much of central and western NC that are in many cases in the lowest 10th percentile of historical observations. Given this, soil moisture may trend down from current mostly adequate to slightly dry levels, and the drought designation would appear to be based on these "short-term" dry conditions and potential impacts (mainly to soil and agriculture), while the western third or so of the state is considered to be in long-term drought that is severe to extreme for the southern mountains and southern foothills. This implies impacts not just on agriculture and soils, but hydrologic and ecologic impacts (water supplies, power production, effects on wildlife, etc) as well.
Mar. 31, 2017 | Tags: drought, rain

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