Chances are you have witnessed many thunderstorms in your life, heard the unmistakable crack of thunder, seen the bright flashes of lightning, and maybe have fallen asleep to the ever familiar sound of the rain tapping against your window. But unless you live in an arid climate, you have likely not experienced, or maybe even heard of, dry thunderstorms.
Dry thunderstorms are thunderstorms where little to no precipitation makes it to the surface. This can be common in thunderstorms that occur in deserts or areas where the lower atmosphere is very dry. Dry thunderstorms do produce rain, however, as the rain falls through dry layers of air beneath the cloud base, much of the rain then evaporates. The falling rain that doesn’t make it to the ground is also known as virga. As this rain evaporates, it cools the air beneath it causing that air to become “heavier” than the surrounding warmer air. This air can rapidly fall and cause strong winds to fan out at the surface. This phenomenon is also known as a dry microburst.
Dry thunderstorms are also an important phenomenon when it comes to fire weather. Since little precipitation makes it to the surface, dry areas experiencing dry thunderstorms are more susceptible to fire ignition by lightning strikes. In a typical thunderstorm, the rainfall can prevent lightning from igniting fires. For fire weather purposes, dry thunderstorms can be classified as producing less than .1 inches of rain, although this threshold can depend on how dry the area is as well as the amount of vegetation in the area. Both of these factors can determine how much rain is necessary to properly wet the surface and prevent lightning-caused fire ignition.
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©2019 Meteorologist Stephanie Edwards
DISCUSSION: Since severe weather season is now in full force, terms such as elevated and surface based convection will be more common. It’s important to understand the difference between the two when analyzing the possibility of severe weather. Although each one can produce severe weather, the type of weather can vary.
Elevated convection is just as it sounds, convection which is elevated. Convection can be thought of as the transfer of heat between areas of different temperature. This vertical movement of air usually results in the formation of clouds, if there is also enough water vapor present. Elevated convection occurs above the planetary boundary layer (PBL). According to American Meteorological Society (AMS), the PBL is the layer of the atmosphere that is in contact with the earth’s surface. This layer can be hundreds of meters deep and is capped by a temperature inversion. A temperature inversion is one in which the temperature increases with height, typically temperatures decrease with height. Elevated convection will usually occur on the cool side of either a cold or warm front. This is because fronts are slanted as they approach the surface. For example, behind a cold front, the air is cooler and denser. Thus, the level at which an air parcel will be lifted will be higher, and so the top of the PBL will also be higher than at a point that is ahead of the front. As shown in the 850 mb analysis image below, the front is roughly from the Texas panhandle to Dallas to near Shreveport in northwest Louisiana. This level is at about 5000 feet above the surface and is often used to detect synoptic (large scale, such as fronts) weather patterns because it is close to the top of the boundary layer.
Image courtesy of Dr. Greg Forbes
When comparing the 850 mb cold front to the surface cold front, it is obvious that the front is tilted to the northwest with height. This can easily be seen in comparing the surface image below to the previous image above.
Image courtesy of Dr. Greg Forbes
The precipitation associated with this storm system is shown below. It is evident that the precipitation is on the cold side of the front. This would be indicative of elevated convection. The severe threat is not as high with this type of convection. Precipitation is usually lighter, although stronger storms can produce small hail and occasionally damaging winds. Often, most winter precipitation results from elevated convection.
Image courtesy of Dr. Greg Forbes
Surface based convection is just the opposite of elevated convection. It is convection which results from a parcel being lifted from the PBL, instead of above it. This occurs as the surface is heated from the sun’s radiation. This type of convection arises within the warm sector of a storm system and can produce much stronger storms than elevated convection. Most storms which produce large hail, strong winds, tornadoes, and heavy rainfall is the result of surface based storms.
Although elevated convection can produce some severe weather and heavy rainfall, surface based convection is the type we most hear about most because of its greater severity. It's important to realized that both can be problematic, in their own way.
Credit: NOAA, Dr. Greg Forbes
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@2019 Meteorologist Corey Clay
DISCUSSION: On May 20th and 21st, a low-pressure system swept across the southern Central Plains and into the southern United States bringing tornadoes to parts of the Plains and Mississippi River Valley. The system began as an area of low pressure over the Colorado Rocky Mountains with a warm front stretching into New Mexico, through the middle of Texas, the most northwestern tip of Louisiana and into Arkansas. The warm front gradually moved slowly northward through Texas aided by a southerly wind which brought with it very warm moist air from the Gulf of Mexico.
The warm front interacted with the colder drier air in north Texas and southern Oklahoma that were influenced by a ridge of high pressure that was over Minnesota and Wisconsin on the morning of the 20th. The warm moist air from the South had Convective Available Potential Energy (CAPE) values above 2000 J/kg according to the soundings at locations such as Amarillo, Texas and Ft. Worth, Texas. CAPE is how much buoyancy is available in the atmosphere over a certain location with higher values leading to more severe thunderstorms, bigger hailstones and even tornadoes. The normal minimum amount of CAPE required for a tornado to be possible is about 2500 J/kg. The Storm Prediction Center (SPC) on the morning of the 20th had issued a convective outlook which included the highest risk level on their scale “High” to signify that there was a high level of confidence that widespread extreme thunderstorms and strong tornadoes are possible. The extremely high risk area covered parts of Oklahoma. According to the storm reports on the 20th, there were about 20 tornadoes reported with the strongest one being an EF-3 near Midland, Texas. There were some reported injuries but no deaths have been reported.
Then, on the 21st, the warm front proceeded to move to the South as it crossed Arkansas, Tennessee, and Kentucky towards Missouri and the Ohio River Valley. However, a cold front snuck in from the Rockies and connected with the low pressure system. The interaction between the two fronts led to the forming of an occluded front along the Kansas- Oklahoma border and grew as the cold front started to catch up with the warm front. Part of the warm front remained a warm front in Missouri into the night of the 21st, while the part of the warm front that was over Kentucky and Tennessee became a stationary front due to a blast of cooler air coming down from the Northeast as a cold front was passing through the Carolinas. On the 21st, the storm reports had tallied over 17 tornadoes across Kansas, Oklahoma, Missouri and Iowa with the strongest being an EF-3 in Bern, Kansas. One death has been reported from all these storms coming from the sole tornado in Iowa. This outbreak is just the beginning of a series of outbreaks that last through the end of May.
At the GWCC, we would like to remind you to be prepared for tornadoes. Some of our tips include getting a NOAA radio as well as batteries, flashlight, unperishable food and water. We recommend that if the National Weather Service (NWS) issues a tornado watch or warning that you either go to the basement or storm cellar, if you have one. If not available, then head to the lowest level possible in the house and get to one of the most inmost rooms without a window as a window would break and spray glass resulting in injury.
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©2019 Meteorologist JP Kalb
Summer break has just started for most schools across the United States and coupled with that is the awakening of Tornado Alley. Last year, the number of tornadoes that occurred was “severely lacking” according to many avid tornado chasers that frequent this area over the months of April, May, and June. This year is a completely different story. Tornado Alley has had quite an awakening this week (May 20-24) with the Storm Prediction Center issuing a slight to moderate risk every day since May 17 and will be continuing to issue them into the beginning of next week (May 27) as well. This has been an incredible issue when it comes to flooding and communities being able to recover and rebuild from the tragedies that have struck. Tornadoes are a dime a dozen during the day the week of May 20. However, there was one day that stuck out to meteorologists alike, and that was Monday, May 20.
The sounding shown above is from a notable day in history, April 27, 2011, in Alabama. Note that in this image, there are winds backing with height, a large dry pocket, the atmosphere is saturated, but the surface also contains another large dry pocket with a low LCL (lifted condensation level). The LCL matters because that is the level where an air parcel reaches maximum saturation as it is lifted through the atmosphere. Low LCL’s are ideal for tornadic development. Something also to note on this sounding are the CAPE and lapse rate values. CAPE stands for Convective Available Potential Energy and describes the instability of the atmosphere (the larger the CAPE, the more unstable the parcel, the more likely it is to produce severe weather, thus why it is also a big factor in tornado genesis). Lapse rate values depict how fast the parcel temperature is falling with height (the steeper the lapse rate, the more instability added due to an even larger temperature gradient, more severe weather, etc). CAPE values for that day reached 3014 J/kg, which is a significant number as the normal CAPE on any given day in the South is anywhere from 900-1400 J/kg. The lapse rates recorded this day were 8°C/km, which is about a 46.4°F decrease per kilometer! So, in this sounding, there is an atmosphere that is incredibly unstable, with temperatures rapidly decreasing with height, and a lot of shear to back it up. With wind shear, a favorable atmosphere with tornadoes would have both directional and speed shear. What this means is that tornadoes are favorable when winds are “backing” with height (this means that winds are going in a counter-clockwise directional pattern with height) and with increasing wind speeds as the column increases with height. This day had significant shear values with an inflow shear of 56 knots (~64 mph). This day also was denoted as a “High Risk” day from the SPC (Storm Prediction Center) and that day delivered and will forever go down in history as one of the deadliest outbreak days of the Super Outbreak of 2011.
This sounding (Fig. B) was taken in Oklahoma after SPC issued a “High Risk” for almost the entire state (excluding the panhandle). The main difference between May 20, 2019 and April 27, 2011 is that this forecasted outlook ended up being a “bust”, sparing Oklahoma a day of tragedy.. The environment in this sounding would have made many meteorologists agree with the issue of “High Risk” quality. To begin with, the LCL was half that of the 2011 sounding, with the cloud deck appearing to just about scrape the ground and CAPE values topped out at 4,143 J/kg. This is an incredibly unstable environment already, but the lapse rates weren’t as high, with rates topping out at 5.3°C (41.54°F) per kilometer and wind shear was also lacking due to the dry pocket not being as large and the saturation of the environment being closer to the surface. What ultimately led to the demise of this spooky sounding was a cold pool which came in and cut off inflow to the supercells too quick to the majority of the area in the high risk. There were still 19 tornadoes that day. While it may not be 2011 worthy, it is still an incredible number of tornadoes in a short span of time.
When looking at similar soundings, anything that can come close to a sounding of the Super Outbreak of 2011 will (and should) raise flags and be approached with extreme caution. The SPC was being cautious and wanted to ensure the public was weather-aware and prepared to do whatever they had to if that day unfolded as it did in the past.
© 2019 Meteorologist Ashley Lennard
Figure 1: Convective storm system in the desert during the North American monsoon season. (Image courtesy of Zack Guido, University of Arizona)
As we approach summertime we will discuss a topic about deserts, which are often associated with hot and dry conditions. In a desert region, we find large diurnal temperature differences, generally clear skies and a lack of significant plant and animal life due to little amounts of precipitation. Considerably deserts are found all over the globe from North America to Asia, and surprisingly even cold deserts exist, such as Antarctica. For the purpose of this article, we will focus on deserts that experience significant heat and what seems like an endless forecast of precipitation-free summer months
The main focus is a phenomenon that brings isolated rain and storms to deserts as a result of extreme heating, that being a Thermal Low-Pressure system, otherwise known as a heat low. Storm systems need a variety of ingredients to form, one of those is a heat source that generate rising air that will eventually give way to the formation of clouds. Figure 2 is a cross section graphic illustrating heights measured in millibars (mb) starting with 850 mb being closest to the surface. This shows the hottest area allowing for expansion as heat rises and eventual generation of clouds located at the red ‘L’ that symbolizes a low-pressure system.
Figure 2: Cross-section of Surface to 300mb height that shows low pressure formation in the hottest environment on the surface. (Image courtesy of the United States Navy)
Heat is a leading contributor to instability that aids in formation of a low-pressure system. As a matter of fact, we see similar mesoscale changes to weather systems in events like sea breezes (cold air from the sea meeting warm air over land) and lake-effect snow (cold air meeting warm lake water) both of which ultimately lead to rising air, instability and eventually low pressure systems that bring precipitation.
Multiple factors play a role in the formation of low-pressure systems that bring precipitation, but in the case of thermal lows, heat is the main lifting mechanism since heat rises. Ideally, we would want to see moisture and surface convergence that will add fuel to a low pressure system. While a low pressure can form from extreme heating, resultant precipitation does not always reach the surface. Due to the dryness of a desert region, precipitation can form, but may fall as virga — precipitation that evaporates before reaching the surface. Another factor that can hinder the production of precipitation during these events is a capping inversion. This means that parcels of air that rise to a certain level cannot rise further because the surrounding air is the same temperature or warmer.
Because thermal low pressure systems form during significant heating, we typically see them in the climatologically warmest times of the year. For example, the desert region extending through the southwestern United States and portions of Mexico experience a Monsoon season in mid-summer (July/August) as a result of this phenomenon.
While the desert regions of the United States are typically dry, the proximity to the surrounding bodies of water can allow for moisture transfer that feeds into the surrounding air, which ultimately allows for more precipitation. Below is a graphic that shows sources of moisture that transfer towards the low-pressure system.
Figure 3: The southwestern United States and portions of Mexico experiencing thermal lows with the aid of moisture transfer (highlighted in green). (Image courtesy of NOAA)
Topography can play a role in aiding storm development in thermal lows allowing for orographic lift, or forced lift due to higher elevation like mountains. Pollution can also be spread through these systems because of elevation differences along with mid-level pollutants like ozone that can be carried with thermal lows.
As a result of the above, forecasting for thermal lows can be complicated because of the above factors playing a role in which areas will receive precipitation or if the precipitation will reach the ground.
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© 2019 Meteorologist Jason Maska
DISCUSSION: When it comes to anticipating the evolution of severe weather events across the United States of America or any other nation on the planet for that matter, there is no debate that the inherently fluid dynamical aspects of severe weather events can be incredibly complicated, to say the least. Having said that, atmospheric science research has come a long way in being able to find ways to understand the dynamics of various types of severe weather. One of the more complicated forms of severe weather will often come in the form of severe thunderstorms capable of producing tornadoes. The most notorious form of severe weather where there are often concerns for potential tornadic development come by way of severe thunderstorms known as supercell thunderstorms, wherein they are unique by way of the fact that they have a rotating vertical updraft as opposed to a straight (i.e., up and down) vertical updraft structure.
Whether it is behind the desk at a given National Weather Service office or if you are out in the field trying to observe and study storms in real-time, there are a number of different features which can be incredibly hard to diagnose moment to moment, but can often be much easier to study and understand after the event has concluded by way of a time-lapsed video. One such example of a more complicated feature to diagnose in the field on a moment-to-moment basis would be visualizing the presence of an inflow sector feeding into the main updraft of an intensifying severe thunderstorm.
As you can see in the imagery attached above from the Twitter post courtesy of @BenV_, you can clearly see on the left so the image of arrow seems to rise and condense into clouds somewhat rapidly. This process of apparent condensation of moisture into low-level cloud features at the base of the thunderstorm is a direct result of the updraft effectively “sucking” in moisture from the surface to continue fueling the maintenance of the storm’s existence. Thus, when such visual features and overall imagery is time-lapsed over a sufficiently long period of time, this can allow for a more effective diagnosis of strong to severe thunderstorm evolution for any given situation. In this same brief loop of time-lapse storm footage, you can also see how there were several instances reach Bonito tried to and eventually formed on the far-left side of the storm footage clip. Granted, in real-time, the evolution of a developing tornado would be much easier to diagnose in the majority of situations since there often would be a visual manifestation of a condensation funnel and/or dirt being picked up at the surface and being sucked up into the strengthening tornadic circulation.
This just goes to show that although strong to severe storm dynamics can be quite complicated, they are still comprehensible under the right circumstances.
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© 2019 Meteorologist Jordan Rabinowitz
DISCUSSION: When it comes to trying to forecast or at least anticipate when there will be a larger threat for severe weather across the contiguous United States (or other parts of the Northern Hemisphere) during the Spring-time season, there is no question that there are most definitely times where concerns increase. More specifically, during the period from April to mid-June in the Northern Hemisphere and across a good portion of North America for that matter, there is often the greatest climatological threat for the development of larger-scale weather patterns which are conducive for the threat of severe weather events.
The reason for this increased severe weather threat is the fact that the period between April and mid-June is the period during which there is a bolstered contrast between cooler air masses (i.e., polar-originating air masses) and warmer air masses (i.e., sub-tropical/tropical-originating air masses). As a result of this increased propensity for air mass “clashes”, there is often a corresponding increase in the potential for increased atmospheric instability and severe storm potential. This severe storm potential increase is essentially a consequence of the atmosphere trying to restore a state of stability to what is a much more unstable low to mid-level atmospheric environmental set-up.
One such example of this concept is defined by the convective storm outburst which unfolded earlier in the day on Sunday (5 May 2019) in which strong to severe convective storms fired to the east of a developing low-pressure system. In this case, the dry-line feature which often is present during the Spring to Summer-time seasons across the South-Central Plains region, helped to generate deep convective storms which stretched from southwest Texas to eastern Nebraska with intermittent discontinuity between the areas of deepest convection in that stretch of the United States. In the simplest context, a dry-line is an arbitrary atmospheric boundary which separates hotter and drier desert air from warm, moist air which often originates from the Gulf of Mexico. To learn about what dry lines physically are, we welcome you to search the home page of our website for “dry line” since we have more details on that atmospheric topic with interactive graphics.
Thus, this example of real-life convective storms goes to show that the atmosphere is often capable of generating strong to severe storms under the correct environmental circumstances.
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© 2019 Meteorologist Jordan Rabinowitz
TORCON: Are Scientists Becoming Better at Predicting Tornadoes (Photo Credit: NOAA’s Storm Prediction Center, Dr. Greg Forbes)
SPC Day 1 Convective Outlook (left) and Dr. Greg Forbes’ TOR:CON index (right) for the late-winter tornado outbreak of February 23-24, 2016. 61 tornadoes were reported across eleven states and three deaths. Issued at 11 pm CST on the 22nd and 9 am CST, respectively.
Now that spring is upon us, odds are that millions of Americans will find themselves under a TORCON of 6 or above or even a “slight” or “moderate” risk of severe weather. But, what does this mean exactly? TOR:CON, or the Tornado Condition Index, was created by Dr. Greg Forbes of the Weather Channel using a rating system describing the likelihood of a tornado within 50 miles of a location.
The three main parameters used when assigning numbers in the TOR:CON are based off of instability (when warm humid air is overlaid by cold dense air above), wind shear (changes in wind speed and direction with height- higher values close to the ground usually represent a greater risk for tornadoes), and a source of lift such as a dryline or frontal boundary. TOR:CON values range from 0-10 with 0 being near-zero chance of a tornado or severe thunderstorm, 2 low chance of a tornado, 4 moderate chance of a tornado nearby and/or hail and high wind gusts possible, 6 high probability of a tornado, 8+ very high probability of a tornado.
The SPC categorical risk maps, also known as convective outlooks, are used to describe the likelihood of large hail, damaging winds, and/or tornadoes ranging from marginal risk (a stray severe storm is possible), enhanced (slight risk of severe storms but not widespread in coverage), enhanced (greater risk of severe storm coverage and intensity), moderate (widespread severe weather with several tornadoes likely), and high risk (a severe weather outbreak is expected from numerous intense tornadoes). A Day 1 convective outlook would show a 30-60% of significant severe tornadic activity coinciding with a high risk category.
Some caveats, though. High TOR:CON values do not reflect a forecast of how strong tornadoes will be, nor are a predictor of the number of tornadoes within a geographic area. Low TOR:CON values may also mean that the environment is locally favorable for tornadoes. A TOR:CON index of 4 means there’s could be a 40% chance of a tornado within 50 miles of any location in the indicated area, but it also means there is a 60% chance of a tornado will not occur in your area, whereas the analogous SPC tornado probabilities apply within 25 miles. A separate concern with SPC risk categories is the extent of severe storms don’t necessarily correspond with how strong they are. Sometimes, there is a widespread wind or hail threat to merit a moderate risk, but there are few tornadic supercells and/or little damage.
Since tornadoes are such small-scale events, TOR:CON is a fair way of showing the general threat area for tornadic activity. TOR:CON is more of a “heads up” as to what can be expected and if these storms can spawn tornadoes. A high TOR:CON value doesn’t necessarily mean to spend the night in the basement, but it’s important to be weather-aware through different media avenues.
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©2019 Meteorologist Sharon Sullivan