The following is a very basic guide to forecasting severe weather. Obviously there is way more to forecasting than what I covered here but the idea of this guide is to help with getting people who may be interested in learning pointed in the right direction. Additionally I'll probably put together a "nowcasting" guide (similar to forecasting but looking at the current conditions, where boundaries are, visible satellite, etc etc) to figure out what will happen in the near term. I'll probably add to this guide also with maybe some videos which will help explain things better...
The basics
In order to forecast severe weather you need to know and understand the following basic things:
- Warm fronts: warm fronts are boundaries that typically move northward (or east) and have warmer and typically more humid air behind them. They tend to be a focus for severe weather and tornadoes. They are indicated on a surface map as a red line with scallops pointing in the direction the warm front is moving. Behind warm fronts the winds are generally from a southerly direction.
- Cold front: are fronts that move east and/or south and have cooler and/or drier air behind them. They are typically the focus for severe weather but tend to have more linear storms associated with them. They are indicated on a surface map as a blue line with triangles pointing the direction the cold front is moving. Behind a cold front winds are from a northerly direction.
- Dryline: a line or boundary that separates an area of humid air (in front of it) from an area of dry air (behind it). They tend to be the focus of severe weather and tornadoes. They are indicated on a surface map usually as a brown dashed line. The higher the dewpoint gradient from one side of the dryline to the other is a good indication of dryline intensity.
- Low Pressure: Exactly how it sounds, an area where the barometric pressure is at its lowest. They are represented by a red ‘L’ on surface maps. They are extremely important in the development of storms and severe weather as they cause the lift needed for thunderstorm development. However, sea breezes and mountains/terrain can also cause lift.
- Jet Stream: The jet stream is important when it comes to thunderstorms because low pressure systems typically ride along it and also it creates wind shear which is important in the development of thunderstorms and supercells.
- Capping: Is a warm layer of air aloft that prevents updrafts from further rising and consequently preventing storms from forming.
- Model Runs: Basically the time a model updates in Zulu time (more on Zulu time below).
- Forecast Models: In order to forecast, you have to know and understand what forecast models are and what they are specifically used for. A forecast model is basically computer generated output of what is believed to happen within the time range of the particular model. Some forecast models are typically more reliable than others, but no forecast model is accurate at all times. In general, the closer you get to an actual event the more accurate the forecast models tend to be, but even then they can be pretty inconsistent and at times unreliable. Below I have listed several forecast models for forecasting severe weather (there are others but these are the ones I use and recommend). I have included some basic information about each and provided a link to where you can view them. With forecasting and forecast models, you can have ‘information overload’ with the data the different forecast models are showing you. It can be a fine line between getting enough information to determine what is likely to happen and having too much info and/or over thinking things. In general, it’s usually best to take a simplistic approach in my opinion. Also, since not one forecast model is perfect, it’s best to look for trends and things in common between them. This really is where experience is going to make you better at forecasting. Eventually you will know which models have certain trends that you should pay attention to or ignore. Also, some models are pretty good at some things, but bad at others - again something you will pick up on over time. If you see something the forecast models are consistently showing between several models for several model ‘runs’ (basically updates), then it would be reasonable to think what is being shown has a higher chance of verifying. Also, typically the 06Z and 18Z runs of forecast models tend to be less accurate than the 12Z and 00Z runs so make sure you are keeping this in mind when you are using the forecast models.
Anyhow, here are the different forecast models I recommend and use (there are more than just these though). Keep in mind, the further out in time you go with forecast models the less reliable they will be. In general, anything over 200 hours out is considered completely unreliable. No matter what though, its best to look for trends and/or agreement between the different models and different model runs. You can access all of these different forecast models using the following sites http://weather.cod.edu/forecast/ or http://www.pivotalweather.com/model.php.
- GFS: The GFS is a ‘long range’ forecast model because it goes out 384 hours. This forecast model in particular should be used to look for trends (i.e. the storm tracks or where the jet stream is setting up) and not specific things. The GFS updates every 6 hours (06z, 12z, 18z & 00z).
- ECMWF: Another long range forecast model, the ECMWF (aka the European) is typically known to be more reliable than the GFS. However, like the GFS use it to simply look for trends in the weather. However, access to the ECMWF is somewhat limited unless you pay - for example if you purchase an Accuweather Pro account you will have additional access. This model updates every 12 hours on most free sites anyway.
- NAM: The NAM forecast model is a medium range forecast model that goes out 84 hours. It tends to be fairly accurate but it can also be inconsistent and tends to over estimates dewpoints. This forecast model updates every 6 hours.
- 3KM NAM NEST: Is a medium/short range forecast model that goes out 60 hours. It’s fairly high resolution and worth checking out, especially the day prior to an event and the day of - again watching for different trends. Keep in mind it tends to struggle with over estimating dewpoints. This forecast model updates every 6 hours.
- RAP: Is a short range forecast model. It goes out 21 hours. It is fairly accurate and should be checked in the morning of a day you are planning to chase storms. This forecast model updates every hour.
- HRRR: Is also a short range forecast model. It goes out 15 hours (though more on some sites) and is also fairly accurate with short range data. This forecast model updates once per hour and you should also check this regularly on days where severe weather is expected. It struggles with estimating how strong the cap is though so sometimes it shows storms forming even though the cap is actually too strong for them to form. This model updates hourly.
When it comes to weather, forecasters use GMT/Zulu/UTC time for specifying times. An example of this would be the "12z" run of the NAM forecast model. The ‘Z’ in 12z stands for Zulu time. In the central part of the United States, we are in the GMT -5 time zone during the severe weather season (we are 5 hours behind where GMT technically occurs). What this means is 12Z would be 12 minus 5...so basically 12Z equals 7AM in the central part of the US. I realize this probably sounds more confusing than it is but it won’t take long for you to pick up on it I’m sure. When it comes to thunderstorms, often times you will hear people talk about storms occurring between 21Z and 00Z (which would be between 4PM and 7PM in the central US). To help, you can go to the following page and it will show the current time in "Zulu" time - http://weather.cod.edu/forecast/
What to look for when using forecast models:
There are several basic elements to look for when it comes to forecasting severe weather when looking at forecast models. In essence, what you are looking for are the areas with the "best conditions" or highest probabilities which is something that will take time to know exactly what to look for. Keep in mind, it’s not often that a setup looks absolutely perfect. That obviously doesn’t mean that a non-perfect setup won’t produce though.
These basic elements are listed below:
- CAPE: Stands for ‘convective available potential energy’. In essence, CAPE represents instability in the atmosphere. Typically the higher this number, the better your chances for thunderstorms and severe weather. Depending on the setup and time of year, typically you need a CAPE value of at least 750j/kg to have thunderstorms develop. Often a large CAPE value will lead to storms producing large hail.
- CINH: Stands for ’convective inhibition’. It is a representation of the cap on the atmosphere. The higher this number (usually it’s a negative value so a value of –200 is worse when it comes to storm formation than a value of –50), the more likely it is that storms will not develop or will not be able to sustain themselves or become severe. In general, for severe weather you want little or no CINH.
- Dewpoint: Represents the amount of moisture in the atmosphere. For severe weather, typically you need a deep layer of moisture or in other words moisture not only at the surface but also at other levels of the atmosphere - which is usually an issue early on in March, April, etc. Also, a surface dewpoint of at least 55F is typically needed for severe weather. However, this can vary greatly depending on what part of the Country and what time of year you are looking at. In Colorado, dewpoints do not need to be very high to get storms. Conversely, during July in Iowa dewpoints are typically very high (not uncommon for them to be in the upper 60s and 70s Fahrenheit at the surface) and still not get thunderstorms (usually due to capping issues or lack of adequate forcing).
- Bulk Shear (0-6 km): Is important for the development of thunderstorms and severe weather. Typically the higher this number, the more favorable the conditions are for severe weather and some- times tornadoes. In general, you want bulk shear values of 30 knots or more for severe weather. When looking at forecast models, you can see this shear at the 500mb level.
- Speed Shear: A significant increase in wind speed with height will help tilt an updraft into the vertical. Tilting the updraft is important as it will prevent the downdraft in the storm from cutting off or interfering with the updraft.
- Directional Shear: Will aid in the development of a rotating updraft. This in combination with high helicity values will typically increase the tornado threat in storms that form.
- Speed Shear (0-1 km): Is shear that is at a different vector then Bulk Shear and is particularly important in the development of tornadoes as it affects the low level features of a storm. Usually 20-25 knots of this kind of shear should be enough for tornadoes. When looking at forecast models, you can see this shear at the 850 MB level, which is also where the low level jet is located (I will discuss this topic in more depth later in this chapter).
- EHI (0-3 km): One of the main ingredients that I look for when looking at forecast models. The higher this number typically the more favorable the atmosphere is for supercells and possibly tornadoes. You want to look for areas that have EHI values of 3 or higher and look for storms that would form near these areas (usually just north of the areas of highest EHI).
- 700 MB Temps: Another way to look at the cap on the atmosphere. Depending on the time of year, typically you want a number of 12°C or less for thunderstorms to develop. However, later in the year (July/August) it isn’t completely uncommon for storms to occur if they are 13°C and sometimes as high as 14°C. Anything higher than this though and storms will more than likely not form.
- Surface Temps/Wind: This is one of those concepts that will take time to learn how to identify all of the different features that will be important in regards to severe weather. In general, you will want to look for areas where the lowest pressures are found, which are typically along warm and/or cold fronts. Look at temperatures and winds along a warm front, which are typically out of the east or southeast. Depending on the setup, the warm front, dry line (a boundary that separates warm humid air from warm dry air) and/or ‘triple point’ (the area where a warm front, dry line and area of low pressure come together) are ideal locations to target if your goal is to see tornadoes.
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