Image: NWS WFO Denver/Boulder Science and Operations Officer Paul Schlatter updating the warning polygon using radar reflectivity on the Gibson Ridge Software program
Discussion: On June 19, 2018, I had the privilege of observing an afternoon of severe weather from inside the National Weather Service (NWS) Weather Forecast Office (WFO) Denver/Boulder in Boulder, CO. What is fascinating about being on the inside of a NWS WFO during thunderstorms is watching the process of weather forecasters issuing watches and warnings for the storms. On this day, nearly 10 severe thunderstorm and tornado warnings were issued for the Denver/Boulder County Warning Area (CWA) and there were three confirmed tornadoes.
To understand the forecasting process, here is a brief overview. To begin, view the current conditions using surface observations, radar and satellite imagery, and most effectively, look out of the window. You have to know where you are starting to understand where you’re going. From there, forecasters look forward into the next several hours using tools such as weather models, maps of atmospheric thermodynamic and dynamic conditions, and analyze air mass movement. Much more detail goes into this process, but these general steps are repeated throughout the day to update a forecast.
Once it has been determined that severe thunderstorms are on the horizon, it’s time to ensure the office is adequately staffed for the occasion. In the case of afternoon thunderstorms, as recently experienced, the forecasters on duty extended their shifts until the late evening and even midnight shifts began. This was primarily to keep forecast continuity and efficiency throughout the event as opposed to taking precious warning time to brief the new forecasters on what’s happening in the area. Now that storms are popping up on radar and satellite, it’s time to evaluate which, if any, storms need warned.
All watches and warnings from NWS are issued using Advanced Weather Interactive Processing System (AWIPS), a weather forecasting package that allows you to display and analyze data. In this case, numerous storms were intensifying such that the WFO assigned two forecasters to keep an eye on separate storms in the CWA. These forecasters mastered the art of multitasking as they not only were forecasting and warning storms, but, answering phone calls to collect weather reports as well. The following description is just a broad overview of the warning process.
Each forecaster used five computer screens to warn the storms. The first screen was dedicated to creating watch and warning statements using the WarnGen feature. Verbiage for these statements was formed by selecting a series of pre-determined phrases that accurately described the current hazards. Then, the statement was automatically generated and disseminated on the web. The next three screens displayed radar and satellite data. On these screens, the forecasters were constantly monitoring the storm development and evolution. They also used the interactive WarnGenfeature to create and edit the warning polygon based on the storm track. After one warning had been issued, the polygon was manually edited and updated as needed so that once the previous warning expired, the succeeding area was ready to be warned. In this situation, it’s important not to jump the gun and warn over a large area at once as a storm track can rapidly change direction and intensity. The fifth screen displayed the Boulder social media pages and NWSChat page of forecasters at different WFOs. NWSChat is a forum for forecasters from different WFOs to communicate in real-time with emergency managers and media, which, as you can imagine, is important during severe weather.
Severe weather days equate to all-hands-on-deck for the WFO. Everyone works together during hazardous weather events to ensure safety of people and their property by answering phone calls from the general public and emergency officials, collecting storm reports from the SKYWARN volunteer severe weather spotters, and frequently update social media on storm conditions while concurrently improving models through verified model conditions. Not only does this benefit the public, it also helps the forecasters verify actual weather conditions. So, the next time you’re under a watch or warning, remember the detailed process of which the NWS forecasters underwent to alert you of incoming hazardous weather!
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© 2018 Meteorologist Amber Liggett
DISCUSSION: During a typical severe weather season, there are a large variety of different forms of severe weather threats which evolve during a given situation based on the given atmospheric dynamics in place. Having said that, in many cases, severe thunderstorm development nearly always involves some sort of dynamic interaction between updrafts and downdrafts. In many cases wherein the convective storm development becomes what is referred to as predominantly “outflow dominant,” these types of convective scenarios consistently involve a storm exhibiting very impressive low-level features. When a convective storm is outflow dominant, this implies that the storm has a strong low-level air stream impinging on the back-end of the storm and forces stronger winds near the front of and/or just out ahead of the storm’s region of peak intensity.
One such feature which is somewhat consistently observed with many outflow dominant severe weather events is known as a shelf cloud. Shelf clouds form as a result of rain-chilled air within a strong downdraft of a strong to severe thunderstorm descending out of the base of the leading edge of the thunderstorm and then forcing relatively warmer and moister ahead out ahead of the approaching storm to be lifted. This lifting of the relatively warmer air is directly due to the fact that this warmer air out ahead of a such a storm is warmer and moister which also means that such a storm has a greater degree of vertical buoyancy.
Hence, as the colder (storm-generated outflow) air descends both down and away from a given storm, this allows for the warmer and moister air to be lifted more easily and then often condense as well as it rises through the lowest levels of the atmosphere. During this process of moisture condensation, there are many occasions during which this moister air condenses and is visualized by way of the development of a shelf cloud. A great example of this is captured in the image attached above (courtesy of Marty Hendickson via Meteorologist James Spann) in association with a severe storm which occurred on the afternoon of June 1st over the coastline of Palm Beach, Florida.
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© 2018 Meteorologist Jordan Rabinowitz
DISCUSSION: As of later yesterday evening and the later night-time hours of Sunday (3 June 2018), there were two distinct convective thunderstorm complexes occurring across portions of both central and eastern Texas. During the course of the evolution of the respective severe thunderstorm complexes which evolved into bowing thunderstorm complexes most often referred to as being mesoscale convective system’s (or MCS’s) these two bowing thunderstorm complexes were on such a trajectory which took them closer and closer to one another as the night progressed. In fact, as time went on, it became increasingly clear that the respective MCS’s were going to collide into one another. Although that reality was fairly certain, one of the primary questions was how this thunderstorm complex collision was going to affect and ultimately influence the intensity and duration of the respective areas of convective storms.
In light of the ongoing progress of the GOES-16 or the GOES–East satellite imager products, there is an even more cutting-edge level of insight which has been made possible now. This newer level of scientific insight has been made possible by GOES-East’s Geostationary Lightning Mapper (GLM) imager. The GLM is a product which observes and archives data pertaining to lightning strikes both from a vertical and a horizontal perspective. The GLM gives a state-of-the-art on the evolution of severe weather it since it allows to gain a better understanding for how particular storms or storm complexes evolve with time during their lifetime.
In addition, this also substantiates how atmospheric scientists and operational forecasters can analyze how a given storm’s severe weather threats evolve during their existence. In the fairly recent example attached above, note how the lightning density count increased notable as the respective thunderstorm storm complexes approached each other and even more so as they merged. However, what you cannot clearly see is that once the respective MCS’s merged, the lightning substantially dropped off in frequency since the interacting updraft cores interacted and likely affected the angle at which the respective updrafts were positioned. More specifically, as the respective MCS’s collided, this collision most likely affected the ability of the updrafts and downdrafts to continue maintaining their most recent intensity prior to the thunderstorm segment collision.
Thus, it just goes to prove that through using modern satellite remote sensing technology, atmospheric scientists have the increased ability to improve the ways by which we study severe thunderstorms of varying types and intensities. The most incredible part is that we are likely only just beginning to tap the ongoing potential of atmospheric science in studying severe weather as we move further into the 21st century.
To learn more about other interesting severe weather events occurring from around the world, be sure to click here: https://www.globalweatherclimatecenter.com/severe!
© 2018 Meteorologist Jordan Rabinowitz