Forecasting Statistics in Meteorology (credit: World Weather Research Programme, EUMeTrain)
DISCUSSION: No matter how badly a forecaster wants to be 100% accurate, the unpredictability of the atmosphere will guarantee some missed forecasts in their lifetime. But there are a few tools out there that forecasters can use to quantify and take a closer look at how they forecast. This information is invaluable, because it can be used to ultimately improve one's forecasting abilities.
There are many different types of forecasts, and different statistical tools for those forecasts, but for simplicity, the focus here will be on dichotomous forecasts. A dichotomous forecast is a forecast that answers a yes or no question in regard to an event happening. For example, answering questions such as “Will a tornado will occur?” “Will fog will be present?” or “Will it rain before noon tomorrow?” are all examples of dichotomous forecasts. To analyze these types of forecasts, a forecaster can construct a “contingency table.” A contingency table combines the four possibilities that could occur during a dichotomous forecast:
From these tables, different statistics can be calculated to give a forecaster some insight into their own forecasting abilities. For example, if a forecaster wanted to see how often they issue false alarms, they would divide their false alarms by the total times they said “yes, this event will occur.” Using the contingency table, this is just a/(a + b), and this is known as the “False Alarm Ratio”.
These scores can be misleading though, so it is important to clarify which score is being calculated. For example, calculating a forecaster’s accuracy would be the total number of times they got a forecast correct divided by the total number of times they have forecasted. Looking at the contingency table, this is (a + d)/(a+ b + c + d). However, a forecaster usually has many correct negatives, as can be seen in the example contingency table above. Instead of this score, many forecasters use what is called the “threat score” or “critical success index” (TS or CSI). This score is a measure of success, but it does not include correct negatives. Therefore, it is calculated by a/(a + b + c). Since correct negatives are removed from consideration in this score, it is much more sensitive to hits, misses, and false alarms.
As mentioned before, there are many more tools that a forecaster can look at, and a much broader look at forecast verification can be found here!
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© 2018 Meteorologist Joseph Fogarty
A Better Understanding Between Tropical and Subtropical Cyclones (credit: NOAA NWS)
DISCUSSION: As we head deeper into the 2018 Tropical Atlantic Basin hurricane season, there is no question that there are many different questions which loom on the minds of many people around the world. Among them, is how to understand and interpret the critical differences between what is referred to as a tropical storm and a subtropical storm. During any given tropical cyclone season in many different ocean basins around the world, there are often occurrences by which a low-pressure system of a tropical and/or subtropical nature develop. Moreover, people and the general media often confuse people regarding the key differences between what constitutes a tropical vs. subtropical storm.
First off, and above all else, tropical storms are warm-core low-pressure systems which often develop deep convection around the circulation center of the tropical storm. This ring of deep convection is often observed from above and referred to as a tropical storm’s central dense overcast (CDO). A CDO is defined via infrared satellite imagery as the continuous or near-continuous ring of coldest cloud tops which surround the center of the tropical storm. On the flip side, subtropical storms are predominantly cold-core low-pressure systems which extract the bulk of their energy from (Sun-based) diurnal heating cycles which is why the convection around the center of subtropical storms is often most vigorous near and around the peak of the day-time heating hours over a given ocean basin.
In addition, since tropical storms being warm-core low-pressure systems, they are often readily identified by the fact that isobars (i.e., lines of constant atmospheric pressure) which chronologically extend out from the center of the storm (i.e., from the point of lowest minimum central pressure) increase in value which makes sense since a storm’s intensity is weaker as you go outward from the center. This is found to be the same with both tropical and subtropical low-pressure systems. However, the marque difference between tropical and sub-tropical storms is that from a vertical perspective, these isobars tend to dip down near the center of the storm with tropical low-pressure systems which helps to explain why they tend to be weaker with height. On the other hand, isobars tend to bump up a bit near the center of subtropical storms which also explains why subtropical storms (and non-tropical low-pressure systems as well for that matter) tend to generally strengthen with increasing height.
To elaborate a bit more on this issue, we will revert to the official definition of a subtropical cyclone. Per the definition from the National Oceanic and Atmospheric Administration (NOAA), “A non-frontal low-pressure system that has characteristics of both tropical and extratropical cyclones. Like tropical cyclones, they are non-frontal, synoptic-scale (i.e., often systems which have life-cycles of between 5 and 7 days or so) cyclones that originate over tropical or subtropical waters and have a closed surface wind circulation about a well-defined center. In addition, they have organized moderate to deep convection, but lack a central dense overcast. Unlike, tropical cyclones, subtropical cyclones derive a significant portion of their energy from baroclinic sources and are generally cold-core in the upper troposphere, often being associated with an upper-level low or trough. In comparison to tropical cyclones, these systems generally have a radius of maximum winds occurring relatively far from the center (i.e., usually greater than 60 nautical miles), and generally have a less symmetric wind field and distribution of convection.” Hence, there are some similarities as well as some key differences between tropical and subtropical storms.
Attached above you will find a recent still image of the ongoing Subtropical Storm Alberto from 3:31 AM EDT on 27 May 2018 and then just below it, a brief corresponding infrared satellite imagery loop.
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© 2018 Meteorologist Jordan Rabinowitz
Is “Heat Lightning” a Myth? (Credit: The Weather Channel, The Farmer’s Almanac, WeatherWorks)
The belief that “heat lightning” is a real thing, is simply not true, but rather a misunderstanding of the term. Most commonly, people think that heat lightning is lightning that occurs in the clouds when it’s hot outside. When, in fact, the term heat lightning or silent lightning is used to explain cloud-to-ground lightning that occurs very far away.
Cloud-to-ground lightning occurs within a thunderstorm when lightning comes from the clouds and touches the ground - it’s pretty self-explanatory - While sound travels through the lowest layer of the atmosphere, the troposphere, with height, the temperature and density change which results in the sound of thunder being refracted or bent. When thunder is refracted in the troposphere, the sound waves are bent by bodies of air at different densities. Thunder also reflects off the Earth’s surface. Since Earth is round, when it reflects off the surface it sometimes doesn’t make a noise in places that we see lightning. With refracting and reflection, we are left with places that don’t hear any noise when they see lightning. You’re most likely to hear thunder if you are within 10 - 15 miles of the lightning strike.
When you are observing “heat lightning,” you are usually more than 10 - 15 miles away from the storm. In fact, you can see it from up to 100 miles away! The lightning is still visible because light travels faster than sound and isn’t refracted in the troposphere. It is easily seen during the hot, humid nights of July and August. When the sky gets hazy on a hot night, the light from intense thunderstorms can be reflected off this haze, which lights up the sky and can be seen from miles away. Since the sound of thunder doesn’t follow the lightning, we tend to label it as “heat lightning.”
Always remember, that within a thunderstorm, if you can see and hear lightning and thunder respectively, seek shelter and remain cautious. Even though “heat lightning” isn’t exactly real, it’s completely safe to watch since the storm is very far away. One of the most common places this phenomenon occurs is in Florida over the water at night. The remnants of storms that could have formed during the day coming from the opposite coast causes thunderstorms above the oceans and water.
To learn more about other neat educational topics in meteorology, be sure to click here!© 2018 Weather Forecaster Allison Finch
On May 20th, 2013, an EF5 tornado decimated the town of Moore, Oklahoma, killing twenty-four people and inflicting two billion in damages. The Moore tornado is estimated to have been over a mile wide in diameter, staying on the ground for forty minutes and traveling over fifteen miles. This disaster has left many wondering, to this day, how we can better predict these deadly acts of nature. The reality is, we still know very little about tornadoes.
The American Meteorological Society defines a tornado as a “violently rotating column of air, pendant from a cumuliform cloud or underneath a cumuliform cloud, and often (but not always) visible as a funnel cloud”. According to The National Oceanographic and Atmospheric Administration (NOAA), most tornadoes form from large, rotating thunderstorms known as supercells, which are a combination of these specified factors: instability, moisture, lift, and wind shear. Though the factors are known, no one really knows what magical combination of the aforementioned factors result in a tornado. The Storm Prediction Center, partnered with NOAA), report that numerous weather combinations can lead to tornadoes, but – and this is a bit crazy – the same combinations frequently fail to produce any tornadoes whatsoever. The only reliable way to know whether or not given weather conditions will produce a tornado is if one actually appears.
In addition to the tornado’s elusiveness, there’s also no timeframe as to how long or short a tornado’s lifespan may be. Generally, tornadoes die within a few minutes; the destructiveness behind the Moore tornado was due to its longer-than-average lifespan and higher-than-average wind speeds, topping out at 210mph, making it a monstrous EF5 tornado. A tornado’s strength is measured on what is now called the “Enhanced Fujita Scale”, taken from the simpler Fujita scale, (“F” Scale”), in 2007. This scale was created by Dr. Ted Fujita, a Japanese-born American meteorologist whose focus was on severe weather. He felt as though the Beaufort scale, which was a method back in 1805 that estimated wind speeds by studying how man-made objects behaved in a breeze, was inadequate and unable to properly measure a tornadoes strength. Beaufort’s scale begins at zero wind speed and ends with a hurricane, which is defined as, “…maximum sustained winds reaching seventy-four miles per hour”, according to NOAA.
Dr. Fujita was well aware of other empirical measurements capable between seventy-four miles per hour and the speed of sound, (seven-hundred and sixty miles per hour). He also knew that these measurements would be imperative in measuring a tornado’s destruction. Fujita published a paper in 1971 entitled “Proposed Characterization of Tornadoes and Hurricanes by Area and Intensity”, using results from a three-year study of tornadoes funded by NASA. His paper laid out the basic proposal of the “F” scale: estimating the peak wind speeds of a severe storm by categorizing the damage that is left behind. This scale ranged from an F1 all the way up to an F12, which theoretically would pack winds equivalent to the speed of sound, (though never seen on Earth, winds to this magnitude are common on the planet Neptune).
As with any great idea, there tend to be flaws: It was based on observation, not measurement. Tornadoes are exceptionally destructive; normal weather instruments were incapable of operating, let alone surviving a tornado. Tornadoes are also incredibly unpredictable, making it near impossible to deploy a sturdier instrument in its path. Another issue being that this scale did not take the multitude of varying building materials into consideration: a sturdy home could withstand the winds of a small tornado whereas a mobile home would more than likely be flipped over. Lastly, the scale did not account for time. A brief, rapidly moving storm with excessive winds could actually cause less damage than a slow-moving, long-lasting storm with exponentially slower wind speeds. Realizing these issues, scientists from Texas Tech University suggested changes to the Fujita Scale, coining the Enhanced Fujita Scale. This newly enhanced scale featured tornado damage assessed by meteorologists and engineers across twenty-eight different types of building materials and natural structures. The horrific damage inflicted upon buildings like Briarwood Elementary School, as well as the loss of numerous single-family homes, enabled scientists to originally categorize the Moore tornado as an “EF5”. The Enhanced Fujita Scale came into play in the year 2007, and is now the only true way to judge the impact of a tornado – at least on a physical level.
©2018 Meteorologist Ash Bray
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Understanding the Importance of the Dry Line in Severe Weather Threats (credit: Andrew Pritchard)
DISCUSSION: As we get into the heart of the climatological severe weather season across the Central Plains region of the United States, many people get more and more engaged with the details pertaining to severe weather forecasts. Of the many parameters and details which control the timing and magnitude of severe weather events across the South-Central Plains region of the United States, one of the biggest players is the seasonal feature which is most commonly referred to as the "dry line." The dry line is effectively the quasi-linear feature which seasonally separates regions characterized by drier and hotter desert-based air masses from much warmer and moister tropically-oriented air masses. Thus, the dry line is effectively a convectively-favorable "battle zone" from the early Spring to Summer time frame and can often act as a catalyst for convective storm development.
It is also important to note that dry lines are (as previously noted) a seasonal feature such that dry line influences only exist during the Spring and Summer-time months more often than not. However, it is worth noting that dry line influences can occur outside of those seasons as well which can catch people by surprise outside of the Spring and Summer-time months. Another interesting thing about dry line dynamics is the fact that when dry lines do form, they often slide eastward in accordance with daytime heating and then retrograde back westward during the overnight hours. Therefore, although most convective storm activity which is instigated via the presence of the dry line during daytime hours in a given situation, it can be cyclic for multiple days at a time (if the conditions are right for a long enough period of time).
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© 2018 Meteorologist Jordan Rabinowitz