Part 3 - Severe Weather Basics

In order for a storm to be classified as severe, it needs to meet at least one or more of the following criteria:

• Hail 1 inch or greater (quarter size or bigger)
• Winds of 58 mph or greater (50 knots or greater)
• Tornado

Before we dive too much into the details of severe weather, we need to go over the difference between a watch and a warning. This is important terminology that should always be kept in mind when dealing with any kind of hazardous weather, especially severe weather.

Watches are issued for larger areas over a multi-county region. Its purpose is to give a heads up that there is a potential for severe weather. Conditions are favorable for severe storms to form/occur. Watches can be issued for severe thunderstorms, tornadoes, and flash floods. "Be prepared" is the mindset to have when a watch is issued.

Warnings are much smaller as they generally encompass an individual storm. It means that severe weather is either occurring or is imminent. NOW is the time to take action and take shelter in the place that you, hopefully, already thought of when the watch was issued.

Another way to think about the difference between a watch and a warning is through food. When a watch is issued, it means the ingredients for severe weather are there but not everything has come together yet. So - a cupcake or pizza watch would mean that we have the individual ingredients for cupcakes or pizza but we don't have the final product yet. As forecasters, we have "ingredients" that we look for when forecasting severe storms and if those ingredients are present, that's when a watch is likely to be issued. When a warning is issued, the ingredients have all come together to create the severe storm. In terms of a cupcake or pizza, the ingredients have been put together and the final (delicious) product is there.

• Moisture - Preferably in the lower to mid-level portions of the atmosphere. For the West Central Texas area, there are 3 typical sources for moisture:
  • The Gulf of Mexico
  • Monsoonal moisture from the southwestern U.S. and Baja California region
  • The Pacific Ocean - Pacific moisture for our area is usually found in the mid to upper levels since it has to traverse the Rockies before reaching our area. Due to the prevailing wind pattern over the United States going from west to east, Atlantic moisture typically won't make it far enough west to impact our area.

• Instability - Ability for air to accelerate upward or downward when started up or down (set in motion). What creates the best conditions for this?
  • Warm air near the surface with cold air aloft
  • An increase in low level moisture

• Lift - The agent which lifts the moist, unstable air and starts the thunderstorm. Here are some sources of lift:
  • Frontal (any type of front can create lift but warm/cold fronts are more common around here)
  • Gust fronts/outflow boundaries from other thunderstorms
  • Drylines
  • Orographic - this doesn't play an impact as much here as we don't have much in the way of terrain but areas with mountains and more pronounced changes in terrain see orographic lift more frequently.
  • Any place where air is converging and hence being lifted upwards

Without lift, moisture and instability alone can't necessarily create storms. This only occurs in specific cases when the ambient temperature reaches its convective temperature (ConvT or CT) - this is the temperature the air at the surface must reach before lift (cloud formation) can occur without any forcing mechanisms (mechanical lift such as fronts or boundaries)

• Another important ingredient for severe storms in particular is shear. Shear is the turning of the winds in the atmosphere. Shear is useful in helping to maintain/organize storm activity.

The Storm Prediction Center, also referred to as the SPC, is a part of the National Weather Service in a branch called the National Centers for Environmental Prediction (NCEP). They provide forecasts and watches for severe thunderstorms and tornadoes. They also monitor hazardous winter weather and fire weather events across the contiguous United States.

Convective Outlooks - These are forecasts of both severe and non-severe thunderstorms for a given day, showing where they will likely occur. Risk categories for severe weather are given ranging from Marginal (Level 1/5) to High (Level 5/5) depending on the coverage and intensity of the storms expected in a given area.

• Outlooks go out 8 days with greater detail included in the first 3 days.

• These outlooks help the individual forecast offices make important staffing decisions in case high-end severe weather is anticipated in their given area
• We will often post graphics with the overall convective outlooks included to describe the severe weather potential for the West Central Texas area. Depending on the height of the severity (sometimes slight, always enhanced or higher), conference calls will be coordinated with local partners to give a more in-depth analysis of what we are expecting on a more local scale

• Convective outlooks on Days 1 and 2 will give probabilities for the individual severe threats (tornadoes, large hail, damaging winds) and include areas where potentially significant events (defined as tornadoes rated EF2+, winds 74 mph+, or hail 2+ inches) could occur.

Mesoscale Discussions - As the event gets closer, the severe threat will often become more defined in both time and space. Mesoscale discussions (MDs) are written by forecasters at the SPC to describe how the severe threat is expected to evolve over the next few hours. It helps meteorologists understand the reasoning/causes for what is potentially about to happen and will allow for additional preparation

Convective Watches (Severe Thunderstorm or Tornado) - When severe thunderstorm development appears likely in the coming hours (or is imminent), SPC will coordinate with the local NWS offices to issue a watch. These watches let people know that conditions are favorable for severe weather and now is the time to be prepared. SPC does not issue local warnings - that is in the hands of the NWS meteorologists at the local weather forecast offices (WFOs).

SPC keeps track of storm reports issued from every office for all severe weather events.

Here is a quick summary of Texas storm reports for 2021 calendar year:

• 118 Tornado Reports
• 688 Hail Reports
• 656 Damaging Wind Reports
• 1462 Total Severe Weather Reports for the state of Texas. Texas had the highest number of severe weather reports of any state in 2021, though much of that is likely due to our size.