 Discussion: Thunderstorms are a powerful yet beautiful display of nature and can bring needed rainfall or devastating wind and hail. While they are most common in the afternoon during spring and summer, they can occur at any time of day, and in any season of the year. While thunderstorm formation is a complicated subject, meteorologists have become increasingly skillful in forecasting thunderstorm activity days in advance. When you think of a thunderstorm, you typically think of a warm and humid day. There is reasons behind this is chiefly due to three necessary ingredients for thunderstorm formation; instability, moisture, and a lifting mechanism.
The environment that thunderstorms thrive in consists of high moisture content especially closer to the surface of the Earth. It probably makes sense why this is the case. Without moisture, you cannot condense moisture into clouds and therefore no thunderstorms can be produced under such conditions. Typically, meteorologists use moisture variables such as the dew point temperature (the temperature at which air becomes saturated with respect to water vapor) to forecast if the atmosphere contains enough moisture. The next ingredient is instability, and to think of instability, we must imagine a parcel of air. Meteorologists use the concept of a parcel because it is easier to visualize the intricate processes of the atmosphere. Now imagine you were standing on the ground and you released this parcel into the atmosphere. What happens to that parcel at this point can tell us if we have a stable or unstable atmosphere. A stable atmosphere is one in which a parcel will want to sink back down to the surface and resists vertical accelerations. This is typically what you see underneath the influence of high pressure systems. An unstable air mass on the other hand is one in which a parcel will want to keep rising because the temperature of that parcel is warmer than the air around it. Back to our parcel visualization, if we release this parcel and it keeps rising, then we know we have an unstable air mass. Meteorologists utilize a variety of tools to diagnose the stability of the atmosphere and even have equations to help us identify instability. One such that you may have heard of is Convective Available Potential Energy or CAPE. CAPE combines the moisture content of the atmosphere along with the stability to give us a proxy for environments conducive to thunderstorm formation. These values have a wide range that is dependent on season, and synoptic (or larger-scale) environment. Now, we have a moist atmosphere, and we have a situation where air parcels will want to rise freely on their own. However, usually the atmosphere is not that simple. In fact, there can be a layer of stable air close to the surface underneath the more unstable air. We must find a way to push those air parcels above this layer into the unstable air. This is why thunderstorms usually form out ahead of a cold front. The cold front gives those air parcels the initial push they need to overcome the stable layer and rise freely on their own. There are other lifting mechanisms other than the cold front such as warm fronts, sea-breeze fronts, and even the cold-dense outflow produced from another thunderstorm. Thus the formation and sustenance of convection is simple from a standpoint of forecast operations. To learn more about other interesting and high-impact topics in severe weather from around the world, be sure to click here! ©2018 Meteorologist Allan Diegan
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 DISCUSSION: Although it is one day after the anniversary of the Super Outbreak of 2011, this event still has me in disbelief, and in awe, that such an event transpired. This article will feature some details and explanations on this incredibly memorable weather event focused on Mississippi and Alabama and nearby locations as shown above to have been impacted by dozens of tornadoes. This article will focus on the four main features needed for severe weather: moisture, a lifting mechanism, instability, and wind shear. Let’s first assess the wind shear present in the atmosphere on the 27th of April, 2011.  When analyzing severe weather, wind shear is a common variable assessed to determine the type of convection that may develop in the atmosphere. The 0 – 6 km bulk shear is the most typical quantity with values greater than 40 knots indicating most of the convection likely to be supercellular. Above is the analyzed 0 – 6 km wind shear during the mid-afternoon (21Z) on April 27th 2011 overlaid with the tornado tracks and intensities developing at around the same time. Values where most of the tornadic activity is developing ranges anywhere from 50 – 80 knots of 0 – 6 km wind shear, easily eclipsing the threshold leading to essentially solely supercell storms at this time.  Next, let’s assess the instability. The same time that is analyzed in the previous section shows substantial instability present across Mississippi and Alabama where most of the violent tornadoes are developing during this time. 2000 – 3500 J/kg of surface based convective available potential energy (CAPE) shows plenty of potential for strong rising air and predominantly supercell convection, as we concluded in the previous section. However, just because there is a lot of CAPE does not necessarily mean severe weather, there needs to be a lifting mechanism to get the ball rolling, to allow for this air to rise on its own.  Shown above is the surface analysis for the 27th at 21Z which shows a sub 1000 mb low pressure system along the Tennessee and Arkansas border with the surface winds out of the south in head of one of the lifting mechanisms, being the cold front. At this time the front is situated somewhere in Mississippi indicated by the shift in the surface winds. This strong cold front is providing a lifting mechanism for these storms to fire across Mississippi and soon after Alabama, throughout the overnight, and on the 28th across much of the rest of the Southeast and Mid-Atlantic. A surface trough out in head of the cold front was analyzed by the WPC at this time increasing the convergence across the area of interest providing another source of lift and initiating storms. The strong surface winds play another role is this setup, which is advecting that warm and moist Gulf of Mexico air into the Southeast enhancing the lift out in head of the cold front and destabilizing the atmosphere around the surface trough. The moisture is the last ingredient needed for severe weather and will be discussed below.  And here we have the surface dewpoint temperatures in degrees F. Dewpoints in the upper 60s to lower 70s throughout the region, especially across Mississippi and Alabama, is a strong indicator of severe convection with tornadoes possible. It should be noted that 850 mb dewpoint temperatures were also quite impressive in the upper 50s and even a few lower 60s which is indicative of an incredibly moist lower troposphere which enhanced the potential for a widespread severe weather outbreak.  Taking a quick look at a sounding from Birmingham, Alabama at 00Z on the 28th, 3 hours after the analysis from above, shows everything that has been pointed out thus far. The large amounts of instability in the form of CAPE, wind shear numbers off the charts, the moisture at the surface and at 850 mb, as well as a good indicator of the overall spin in the atmosphere. This can be assessed by looking at the hodograph which shows a nice “fish hook” meaning lots of storm relative helicity present. All of these ingredients developed in a spectacular, yet deadly fashion. Take a look below at this incredible radar loop of the storms developing. The amount of rotation in the atmosphere is quite notable just by looking at the reflectivity and noticing all the hook echo’s across the region (northern Alabama and Mississippi). An incredible meteorological event, but also incredibly deadly. Every so often mother nature reminds us that “she” can produce in a spectacular way when just the right condition are in place.
Stay tuned to GWCC, and if you want to read more severe analyses, click here! ©2018 Meteorologist Joe DeLizio 
During the month of April, the frequency of tornadoes in the United States tends to increase especially in the Midwest or the region known as “Tornado Alley”. This is all thanks to a gradual lift in the jet stream, which allows the warm moist air from the Gulf of Mexico to lift into the Midwest. This warm moist air then clashes with the cool air sinking down from Canada, creating a prime tornadic environment for thunderstorms in the Midwest. The story this year has been the complete opposite as old man winter has held a firm grasp over much of the United States. As the jet stream has stayed put in its late winter position, letting the cool air from Canada spread across most of the continental U.S well into late April. This leaves the warm moist air from the Gulf of Mexico suppressed to areas across the deep south. Hence why we have had more severe weather events occurring down in the south instead of the Midwest. In a sense, the Ohio and Mississippi Valley's have become the "new" tornado alley in 2018. As of a result of this, both Oklahoma and Kansas have yet to record a tornado in 2018. In comparison, California averages about 10 tornadoes per year, and so far, has recorded 5 tornadoes in 2018. On April 22nd, 2018, a few waterspouts came ashore near Fort Walton Beach in the Florida panhandle causing a few tornado reports to emerge. In fact, 19 other states have recorded a tornado this year as shown by the graphic above with neither being Oklahoma or Kansas. Each red dot is a tornado report confirmed by a survey team from the National Weather Service. You can see that Oklahoma and Kansas have had a few close encounters will a tornadic thunderstorm, but never has it touched down in those two states. The latest first tornado on record in the state of Oklahoma is April 26th. As for Kansas, the latest first tornado report is May 28th, 1980 per the National Weather Service office in Topeka. Although the record for Kansas will not be broken, the record for Oklahoma has already been broken as it seems to stay quiet severe weather wise during the final week of April. The next tornado that is confirmed in Oklahoma will set a new mark for the latest first tornado in Oklahoma since records began in 1950. To close, there were 651 tornado reports spanning from January to the end of April last year. This year, the Storm Prediction Center has documented only 268 tornado reports, a 41 percent drop from last year. To learn more about other interesting and high-impact topics in severe weather from around the world, be sure to click here! © 2017 Meteorologist Joey Marino  DISCUSSION: As April ends, tornado season approaches its peak. However, this year so far, there has been fewer tornadoes than in the past 3 years. This pattern is expected to continue as the Storm Prediction Center’s (SPC) latest outlook has only slight chance for severe thunderstorms for the upcoming week which includes the beginning of May. April has generally been thought as the main start of the tornado season in the United States with May being the main peak. This April has been below average as the SPC has only reported a preliminary total of 47 tornadoes for the month alone which is far from the 3-year average of 174 tornadoes. One of the biggest reasons there has been a low number of tornadoes so far in April has been a combination of upper-level and low-level winds. The southern plains have been receiving dry warm winds from the southwest which with help from multiple low pressure systems from Canada become westerly. In addition, there has been a dominant cold dry northwesterly flow across the northern plains. This lack of a moist southerly flow especially in the upper levels of the atmosphere removes one of the crucial ingredients of a tornado since tornadoes are typically formed because of a collision of the dry cold northerly winds and a warm moist southerly from the Gulf of Mexico. The combination of these two dry winds especially at the surface makes the surface a bit drier which increases the dewpoint depression. The dewpoint depression is important in severe weather as it is used to determine the level of condensation (LCL). The LCL is the level of the atmosphere where if you take a parcel of air from the surface that has the moisture condensing until the moisture in the parcel is fully condensed. After you take a parcel of air from the surface to the LCL, the parcel follows the moist adiabat, which is the temperature change of a parcel full of condensation, all the way up the atmosphere to about 53,000 feet (100 mb). If the parcel crosses the temperature of the surrounding environment, there is Convective Available Potential Energy (CAPE). CAPE is the measure of buoyancy in the upper atmosphere and is related to the vertical velocity. However, an increase in dewpoint depression leads to a higher LCL and a lower moist adiabat which decreases CAPE. Also, a temperature inversion creates difficulties for CAPE due to the increase in temperature. The combination of the temperature inversion and the higher dewpoint depression is one of the reasons that there is not such a large number of tornadoes. Another major reason there has been few tornadoes is a result of little directional shear in the winds as they reach higher in the atmosphere across much of the Central U.S. Directional shear is necessary to jumpstart rotation in order to form a tornado. To sum it up, the lack of tornadoes this year so far is due to the directional multi-level winds. To learn more about other interesting and high-impact topics in severe weather from around the world, be sure to click here! © 2017 Meteorologist JP Kalb  A tornado northeast of Cheyenne, Wyoming on June 12, 2017. As spring is spinning up, tornado season is already at an active start. Despite deadly winter outbreaks earlier this year, the U.S. is running behind on the number of tornadoes this year- 58 through February 28th (compared to the average of 107). A strong blocking pattern over the western Atlantic (North Atlantic Oscillation) has allowed cold fronts to drop into the South, limiting the flow of warm, moist air from the Gulf and keeping instability low.
PDO (Pacific Decadal Oscillation) remained somewhat positive throughout the winter, helping California, the Southwest, and even the Great Plains to be drier than normal (negative PDO regimes allow storms to dig southward to bring moisture across the central and eastern U.S.). Drought conditions can lead to greater dew point depressions at the surface (having been warmer and drier than normal) and higher cloud bases. The higher the cloud base, the harder it is for a circulation to reach the ground to become a tornado. In addition, a moderate La Niña can favor increased tornadic activity, as the jet stream shifts farther north and drags up moist air from the Gulf. This pattern of a positive PDO, borderline moderate La Niña, and positive NAO leans towards a below to near normal tornado season and favoring an active start of tornado season across the Southeast and Mississippi Valley. A quieter than normal May is not out of the question for prime chasing territory across the Great Plains due to the weighting of a below normal season so far. But, what would tornadogenesis look like in the future? Climate change may affect the frequency and intensity of tornadic events, as a warmer atmosphere can hold more water vapor and changing the amount of energy available in these storms. Above normal winter temperatures can also be an indication of events to come. Since climate change influences the onset of seasons, tornado season may shift earlier in the year and continue longer than normal. A strong tornado outbreak in late January of this year producing 81 tornadoes across the southeastern U.S. was the second largest January tornado outbreak and the third largest winter tornado outbreak since 1950. Climate change could be the only plausible explanation as to why extreme events, such as widespread tornado outbreaks, are happening with increasing frequency, but a single event cannot necessarily be attributed as a direct result of these changes. This discussion is only meant to be a best-guess outlook and should not be used as a basis for interpretation without checking your local short-term forecast first. To learn more about severe storms (and their climatology), please click here! ©2018 Meteorologist Sharon Sullivan |
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