Ask Greg

Recent Questions

Question: I love following the wet weather with Doppler 5000 online. If I want to see if the storm is about to hit, or watch when it will be over. Thank you for that service. My question: if there is nothing showing on radar as things move say west to east...appears clear...then all of a sudden a rain pocket shows up sometimes to the red. Why is that when the surrounding skies were clear? — Margie

Answer: There are a couple of ways this can happen. In rare instances, it is possible there was some kind of radar outage so that showers or storms are not visible, then the outage is resolved and all of a sudden any echoes in the vicinity become apparent.

The more likely and frequent explanation, though, especially in the late spring through early fall, is that a shower or thunderstorm simply developed very rapidly in an area that was previously rain-free. When the atmosphere is quite humid and unstable (meaning a strong decrease in temperature with increasing height, especially if there is also a drop-off of humidity with height as well), strong updrafts can develop that lead to precipitation that can go from non-existent to quite intense in a matter of just a few minutes. This can occur from buoyancy produced by uneven heating of the surface by strong sunshine, or in some cases the new showers or storm cells can be triggered by the passage of a fairly weak upper-level disturbance, or a low-level feature like a weak front or a leftover outflow boundary from a distant or even already-dissipated thunderstorm.
Jul. 17, 2016 | Tags: thunderstorms, weather radar,

Question: A number of times in the last few days I've noticed on iControl radar indications or lines moving quickly from northeast to southwest while all other indications are moving in their usual ways. Are these aircraft trails (similar to vapor trails)? — William Rothwell

Answer: We can't be quite certain from your description, but we strongly suspect you're noticing some echoes produced by thunderstorm outflow boundaries. These are effectively shallow cold fronts associated with rain-cooled air that descends rapidly in thunderstorm downdrafts, spreading rapidly outward from the storm when the cooler, dense air reaches the ground. Depending on the structure of overall ambient winds, these outflows may preferentially move in one direction, or they may spread out in a concentric ring or oval around a slow-moving storm. The boundaries usually appear as thin lines of rather light echoes, and they appear on radar because the winds flowing out from the storms cause a convergence when they run into the surrounding airmass, causing a concentration of dust and flying insects, and also leading to turbulent mixing along the boundary that creates pockets of varying temperature and humidity, all of which can act as weak radar reflectors. One thing that's a little tricky regarding these boundaries is that they tend to be shallow, extending just a few hundred to a few thousand feet above the ground. At these heights, the boundaries may only be visible when they are fairly close to a radar site, and can appear to dissipate when in fact they have simply moved out of range so that the radar beam is passing over top of them. Regarding your question as to whether these might be aircraft, that is unlikely to be the case. Most contrails left behind by aircraft are made of particles of ice that are too small to produce noticeable radar echoes.
Jul. 16, 2016 | Tags: general meteorology, thunderstorms, weather radar,

Question: How rare is a tornado around the Smithfield area? — Scott

Answer: Tornadoes are very rare for most any particular location in our state. If we take a bit of a wide view and consider tornadoes that have occurred in Johnston county to be "around Smithfield," we find that the National Center for Environmental Information's Storm Events Database shows there have been 21 tornadoes reported in the county since 1950. Those were clustered into 13 days during that period (which covers about 24,180 days) with a tornado occurring in the county, for a statistical rate of one somewhere in the county every five years or so. Most of the tornadoes were of the EF-0 and EF-1 or equivalent categories, with 2 rated EF-2 (in Nov 2008 and Apr 2011) and one F3 (in Nov 1992). One fatality has resulted in the county, that one from the Nov 2008 event.
Jul. 15, 2016 | Tags: past weather, tornadoes

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
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,

Question: Not a question per say, only that I have been working at the RDU FSS (flight service station) since 1989. The official thermometer is just outside our building, so when you say it's 105 (as it was THREE time this week back in 2012!) we know it's where WE work. — Michael Hale Gray

Answer: There was indeed some big time heat that summer! Just to clarify, though, the three instances of highs reaching 105 then didn't quite fall within one week. The high at RDU was 105 on June 29 and 30, and then again on July 8th. That's not much relief, though, since the temp did reach 100 or greater on 5 of the 7 days in between those readings (only staying sub-100 on July 2nd). One other point to note is that, unless we're not correct in our understanding of where the FSS is located, there shouldn't be an official temperature sensor located just outside of it (perhaps a supplemental thermometer is positioned there instead?). The NWS/FAA Automated Surface Observing System that provides the temperature data for official observations from the airport is located a significant distance from any major buildings, as part of a group of sensors positioned just a little west of the northern portion of Runway 23 Right.
Jul. 11, 2016 | Tags: heat, instruments, maps & codes, past weather

Question: There is a trend going on right now that at some day of every week that we have had there is a storm chance on that day, sometimes it is multiple days. Can you explain to me why this is happening? — Scott Stephenson

Answer: In recent weeks we've had a couple of different patterns that contribute to that situation. For a couple of significant stretches, we had a broad, recurring upper-level trough set up over the midwest and the Tennessee Valley area, which leaves us on the east side of the trough in a region that is generally favorable for upward motion, corridors of enhanced moisture, and the periodic showers and thunderstorms that can result, especially as smaller disturbances rotate through the larger upper trough. A couple of other stretches of time have involved more of a standard summer pattern with a surface "Bermuda" high pressure area stretched into the southeastern U.S. with its axis a little south of us. This often leads to lots of heat and humidity in the lower atmosphere that can bring afternoon and evening showers and storms on a fairly regular basis. These are the kinds of storms that may not affect everyone every day, and often only result in a short period of rain in any given area through the course of a day, but the potential is there and we often have about a 30-40% chance of rain in these situations and therefore include that in several days of our upcoming forecasts.
Jul. 10, 2016 | Tags: general meteorology, thunderstorms

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: I've been living in NC my whole life. I'm now 31 years old. For the past couple of years we have not seen a hurricane that was able to come inland, passed Raleigh. I know that is a great thing. When can you tell if the weather is right for a hurricane to form? Is the water too cold for one to come? — Carter A Mickens

Answer: There are several factors meteorologists keep an eye on in regards to potential formation of tropical cyclones. The development of these storms typically requires a combination of high humidity, low vertical wind shear, and warm sea surface temperatures (generally around 80 degrees or higher). The shear and moisture conditions, along with potential weak disturbances involving some spin in the lower atmosphere, can vary a good bit day-to-day or week-to-week, leaving some areas favorable for supporting development and others unfavorable at any given time. Ocean temperatures tend to vary more slowly. Those have been, and should continue to be, favorably warm from the southern NC coast east and southeast, as well as over the Gulf of Mexico.
Jul. 8, 2016 | Tags: general meteorology, hurricanes

Questions 41 - 50 of 5041.

Ask Greg Your Question Now!

Please understand that the volume of Ask Greg questions makes it impossible to answer every one or to list them all here. You may find it helpful to search for your own question using the form at the top of this page to see if it has been posted in our database.

When you submit a question you understand that your question and e-mail address will be sent to our editorial staff. Accordingly your question will not be subject to the privacy policy of this site.