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Spring Fever Spreading

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Severe Weather: Nowcasting and Forecasting Informational


**Thread under construction. I will work on this thread gradually as I get time. Please don't post anything until I'm done. I'll state when this is when the time comes.**


I will go over a number of things associated with severe weather and tornadoes, from forecasting to nowcasting. I'll start with the basics and move into more and more advanced topics. 

Table of contents:

Post #1: Sounding/Skew-T

Post #2: Instability

Post #3: Wind shear

Post #4: Forcing mechanisms

Post #5: Storm modes

Post #6: 

Post #7:

Part 1: Soundings/"Skew-T"

What is a sounding?

A sounding is created by the National Weather Service (NWS) by launching weather balloons. NWS offices across the US launch weather balloons at least twice a day, sometimes more if there's highly impactful weather threatening. These balloons record all kinds of data from the atmosphere as they ascend until they pop. Most importantly, they record temperature, dew point, the temperature of an air parcel (if available), wind speed, and wind direction. This data is commonly plotted on Skew-t's, also known as soundings, as seen below. Let's focus on the temperature profile first, which is the top left square of the sounding... just below where it says "bmx". I'll go over the most important parts of the sounding while leaving out more technical parts, such as the dry adiabatic lapse rate and mixing ratio lines



- You can see pressure in millibars on the left side of the sounding ("y-axis") and the height above the surface just to the right of pressure. The surface is at the bottom, the upper limits of the atmosphere is at the top.

- On the bottom of the temperature profile ("x-axis) is temperature in Celcius, labeled from -30 to 50. To trace a line of constant temperature, you don't go straight up; the isotherms are skewed to the right, hence skew-t. The isotherms are the dashed beige line. It's kinda hard to see on these. 

- The solid red line within the graph is the temperature as you go up in the atmosphere. 

- The solid green line within the graph is the dew point as you go up in the atmosphere.

- The dashed dark red line within the graph is the temperature of the air parcel as it goes up in the atmosphere. 

- On the opposite side of pressure is the wind which is shown with wind barbs and the units are in knots (1 knot is approximately 1.1 MPH). The side of the wind barb with either a flag, a line, or half a line, is the direction the wind is coming from. 


- Building off of wind barbs is the hodograph, which is just a way to visualize how winds change with height. 



For the purpose of severe weather, these are really the main things that you need to be able to identify. Now I'll get into the fun stuff.


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Part II: Instability

What is atmospheric instability?

Atmospheric instability occurs when a parcel of air an (air parcel is an imaginary bubble of air) is warmer than its environment. An air parcel starts at a certain point in the atmosphere. If there's severe weather, then it likely starts at the surface. As the air parcel rises, it'll cool at a rate of 10 degrees Celcius per kilometer (10C/km) until it becomes saturated, known as the Lifted Condensation Level (LCL). At which point, the air parcel will cool at a slower rate of 6 degrees Celcius per kilometer (6C/km).

This is important because the environmental temperature will almost always cool at a rate different from the air parcel. The rate at which the environmental temperature changes is measured by lapse rates. The 'average' global lapse rate is 6.5C/km, though it can be significant more or less in a region depending on the circumstances. 

So, yes, the average environmental temperature on Earth cools at a faster rate than an air parcel does. But in the regions where there is instability, this obviously isn't the case. 

How do you get atmospheric instability?

There's multiple ways... but I'll focus on the most common ways you get the instability that goes into severe thunderstorms. 

- Warmth and moisture

Starting at the surface, you need good low-level moisture ("moisture" and "dew point" will be used interchangeably). In the cool season, severe weather occurs most commonly with 55+ degree dew points. In the warm season, anything over 65 degrees can work. Without getting too technical, all I'll say is you'll want the temperature to be relatively close to the dew point. An index used to measure this is dew point depression, which is simply the temperature minus dew point. You want the dew point depression to be less than 20 degrees because this allows for the LCL to be relatively close to the ground. As stated before, this is important because an air parcel above the LCL will cool at a slower rate, thus making it easier or more likely for the air parcel to cool more slowly than the environment.

- Environmental lapse rates

Another way to get instability is to have a rapidly cooling environment with height... in other words, steep lapse rates. This is especially important above the LCL where the air parcel is already cooling relatively slowly. This is achieved through differential advection... a topic I'll address later with Elevated Mixed Layers. 

- Ways of measuring instability

The most common way we measure instability is called Convective Available Potential Energy, or CAPE. The amount of CAPE is the area between the air parcel temperature and the environmental temperature.

Less than 1000 J/kg of CAPE is often called marginal

1000-2000 is called unstable 

2000-4000 is very unstable

4000+ is extremely unstable.

2000 CAPE is typically the threshold for where severe weather begins becoming more probable. You may need more or less based on the amount of wind shear. You can need as little as 200 J/kg CAPE if you have extremely strong wind shear.

Elevated Mixed Layer (EML)

EMLs are an important part of severe weather, and is one of the bigger reasons why the US is the global capitol of severe weather and tornadoes by a long shot.

EMLs are characterized by steep lapse rates and very dry air aloft. EMLs originate from high terrain deserts...in the US, they come from the high, arid climates of the western US, but EMLs can be found in Bangledesh and in the Middle East. Environments in a desert are characterized by hot and dry surface temps with very rapid cooling aloft. Under the right conditions, this airmass can be advected east... and as it gets advected, it gets pushed aloft. This can be seen in soundings where there's a classic EML plume; the environmental temperature is almost identical from location to location.

What separates an EML from just an arid atmosphere is the presence of low-level moisture below the desert layer. The low-level moisture ensures a relatively low LCL. The EML then starts with a "cap", also known as a temperature inversion, or an area where temperature briefly increases with height. When a rising air parcel reaches a cap, it can't rise anymore, so it sinks. So, effectively, a cap will inhibit convection. Inhibiting convection is important because it allows for surface heating to do its work, thus increasing instability. If there's a capped atmosphere with moist low-levels, there will likely be vertically shallow cumulus clouds... if any. 

Eventually, this cap can be broken down by mixing (natural cycle of warmer temps coming down to the surface, cooler temps going aloft), an approaching system, or if there's been enough heating to overcome the cap. At which point, the air parcel can rise freely through the atmosphere. This parcel will likely accelerate as it moves through the EML due to the rapidly cooling temps with height in addition to the fact that the air parcel itself is cooling a few degrees slower than the environment (the lapse rates in an EML can range from 8C/km to 10C/km). An air parcel can overcome a cap if it's forced up above the cap, but that would require the EML to be pretty close to the surface and, I think, is unusual in significant severe events.

Therefore, an uncapped atmosphere with an EML can become highly unstable and has been observed in a great majority of major tornado events and most severe weather events due to the ability for it to inhibit convection until maximum heating has been achieved and for its ability to enhance lapse rates. It's therefore pretty rare for 4000+ CAPE to be achieved without an EML.EML.PNG


EMLs can be broken down by any precipitation. Therefore, EMLs are most commonly found in the Great Plains due to their proximity to the desert. EMLs can uncommonly be found in the Ohio Valley and southern US, and rarely in the east coast states. EMLs are most common in the spring and summer, though they can occur at any time of the year under the right conditions. 

Instability is a very important factor for severe storms, but having an unstable atmosphere of any caliber is only half the battle...

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Part III: Wind Shear

What is wind shear?

Over land, wind will be weakest at the surface due to the frictional effects of land. Wind shear describes how wind changes (i.e., increases) with height. There are 3 reference points in the atmosphere that Meteorologists use with wind shear; 0-1km, 0-3km, and 0-6km (i.e., surface to 1, 3, and 6 kilometers above). There are two types of wind shear; speed shear and directional (AKA vertical) shear. 

Why these points? East of the Rockies, 1km above the ground translates to 850mb. This level is important because that's typically where the low-level jet (LLJ) is found. The LLJ can be found in the warm sector of a system and is responsible for enhancing warmth and moisture transport. It can also enhance tornado potential given the right circumstances. 0-6km is important because that translates to somewhere between 450mb and 500mb, which contains the jet stream.

Speed shear

Speed shear describes how wind changes in magnitude with height. There are 2 common levels we look at for speed shear... 0-1km and 0-6km. Which means we're looking at how the winds increase from the surface to 1km (the LLJ) and also how the winds increase from the surface to 6km (the jet stream).

Directional shear

Directional shear describes how winds change direction with height. There are 2 common levels we look at for directional shear... 0-1km and 0-3km. This means we're looking at how the winds change direction from the surface to 1km, and from the surface to 3km (~700mb). When winds turn clockwise with height (become more westerly), we say this wind profile is veering. Veering indicates warm air advection (WAA) and creates cyclonic spin in a storm. When winds turn counterclockwise with height (become more easterly), we say the wind profile is backing. This indicates cold air advection (CAA).




Why is this important?

Speed shear creates a horizontal spin in the atmosphere. Imagine having a pinwheel and blowing on the top of it. Obviously it'll create a spin. This is the same idea; when there's stronger wind on top than below, it'll create a spin. When you add vertical wind shear to it, it helps make the updraft rotate, which by definition, is a mesocyclone. Let's break it down:

Part 1: Weak speed shear, weak directional shear

In the summer, winds are generally very weak from roughly latitudes 40 and south. The atmosphere is usually very unstable and storms are free to form. However, they often are very short-lived. Why? A lack of wind shear. 

Since there's no wind shear, storms pop up and build up vertically since there's no wind to make them tilt. The result of this is that the updraft lies in the same area as the downdraft... so eventually, the updraft gets rained out and the storm basically just kills itself. These storms usually just have heavy rain and strong winds. The winds can sometimes become severe if there's substantial instability. But hail is rare from them due to the very high level freezing level... though, again, substantial instability can result in hail.


Here's what a lack of wind shear looks like in a sounding (July 6, 2012 in Wilmington, OH) 


Part 2: Speed shear without directional shear

This is sometimes called undirectional winds. This is when there's very little change in the direction of the winds but sufficient increase in wind speed with height. The dominant severe mode with this is bow segments and squall lines. Since there's no directional shear, tornadoes are a low threat because the only rotation in the storm is horizontal; there's nothing to tilt it vertically. However, since the wind is out of the same direction throughout the atmosphere, it can enhance the damaging wind threat.

An example of this (July 26, 2012 in Wilmington, OH)



Part 3: Speed shear and directional shear

This case is when the most significant severe weather happens. Although it's not necessarily rare to have good speed and directional shear, it's rare to have it coincide with good instability. The speed shear in this sounding creates the horizontal spin, then the directional shear tilts this into the vertical, creating a mesocyclone. Situations with instability, speed, and directional shear create the most dangerous conditions because they can create supercells that can do everything; they create almost all 2"+ hail reports and almost all EF3+ tornadoes. You can also get line echo wave patterns (LEWPs) which I'll discuss later.

An example of this (April 3, 1974 in Dayton, OH... ~3 hours after the F5 Xenia tornado)


Where wind shear can go wrong - veer-back-veer (VBV) profiles

Sometimes the vertical stacking of a trough can get weird and create an area of veering winds with a brief layer of backing winds on top of it, followed by veering again. This is problematic because the area of backing disrupts the rotation created by the veering. Think of stirring a pot of water... you achieve a nice rotation. But then you suddenly start stirring the other way and the pattern gets completely disrupted. That's analogous to what happens in a VBV profile. You can have an otherwise great setup for severe weather, but the VBV profile will discourage supercells and tornadoes. What usually results from a VBV profile is messy supercell structures followed by a transition to a linear storm mode. Any tornadoes that do form will be short-lived. VBV is often overlooked and/or underestimated when forecasting severe weather and cause for busts if ignored.

An example of VBV (November 17, 2013 in Wilmington, OH)


Next time I get time I'll be talking about forcing mechanisms which should be pretty brief... then we get into the fun stuff like storm modes.

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