Today's Topic: Understanding Storm Prediction Center Convective Outlooks

For Day 2 of Severe Weather Awareness Week, we're going to focus on breaking down the SPC Convective Outlooks, or what's better known as Severe Weather Outlooks.

Because this is such an expansive topic, I've decided to break down today's topic into 2 parts. In this article, we'll discuss both parts but I would also encourage all of you to watch, at least, Part 1 of today's topic on YouTube here. Watching Part #2 is also highly encouraged but it does get a little bit more in-depth.

All materials used in the creation of this article and associated YouTube videos, as well as the most up-to-date severe weather outlooks, can be found on the SPC website at https://www.spc.noaa.gov.

Today's Video (Part #1"The Basics"):

Today's Video (Part #2 "The Not-So Basics"):

What Is The Storm Prediction Center (SPC)?

The Storm Prediction Center is a branch of the National Weather Service. Stationed in Norman, Oklahoma this office is responsible for issuing all Severe Thunderstorm and Tornado Watches, Fire Weather Outlooks, Mesoscale Convective Discussions, and Winter Mesoscale Discussions. This is also the office responsible for issuing Convective Outlooks, better known as Severe Weather Outlooks, for the each day, as well as the next 7 days in a given forecast period (more on this later.)

What Is A Severe Thunderstorm?

The severe weather outlook would need to be issued when a risk of severe storms exists. Severe Thunderstorms are storms that produce damaging winds over 58 MPH, large hail of at least 1 inch in diameter, or a tornado.

What Are Convective Outlooks?

Convective Outlooks, or what are better known as Severe Weather Outlooks, are a mechanism that allows the National Weather Service to provide advanced notice of severe weather potential before Severe Thunderstorm Watches or Tornado Watches are issued.

These outlooks denote the overall risk of severe weather within 25 miles of any given point on a map (example shown to the left) by overlaying different levels of risk shown by 6 different colors.

We'll discuss what each color is indicative of, how the decision to put each area under what color is made, and what the underlying specific severe weather hazards may exist within each different risk area. To do this we'll start by focusing on 4 main elements/components of each different categorical risk level on the Severe Weather Outlook map. The 6 different risk levels are shown in the graphic below in order of least threat to greatest threat. At the bottom of the graphic, I've listed the 4 elements that we'll be focusing on for each risk level.

What Is A General Thunderstorm

(0/5 - Light Green) Outlook?

The first color shown on the map (Light Green) is indicative of a category known as "General Thunderstorms" and in simple terms indicates that there is no severe weather risk within the area highlighted.

Within the light-green shaded areas on the map standard summertime thunderstorms are expected. Occasionally, strong storms - that is to say; storms that produce wind gusts between 40 and 58mph or small to medium-sized hail less than 1 inch in diameter - can occur within these areas, and in even rarer cases, weak severe weather (winds of around 60mph, large hail around 1 inch in diameter, or weak/brief tornadoes) can occur in these regions. Generally, however, this is just a "risk" for standard thunderstorms which will bring rain and lightning to the area.

What Is A Marginal (1/5 - Dark Green) Risk?

The next category (dark green) is known as a "Marginal Risk" and is the first tier of risks the SPC issues to highlight severe weather potential within those shaded areas.

This risk category features a severe threat that is generally limited in all aspects.

• Geographic coverage of severe storms will be limited with only a couple of isolated, weak severe weather reports possible.

• The forecast with this type of risk is also not going to be particularly sound in most cases with several uncertainties likely existing in terms of storm mode (supercells vs bowing-line segments vs a messy conglomeration of storms), timing of storm development or arrival, the ability for storms to mature into severe storms if they do develop.

• As mentioned above, the intensity of severe storms within this type of risk will likely be low-end with only strong storms (storms that produce wind gusts between 40 and 58mph or small to medium-sized hail less than 1 inch in diameter) or weak severe weather. Significant severe weather with this type of risk is extremely rare.

What Is A Slight (2/5 - Yellow) Risk?

From there we move to a Slight Risk for Severe Weather. This risk features at least some increase in at least one of the following:

• Forecast Confidence

• Storm Intensity, Duration, or Coverage

While this risk could still feature certain limited aspects it is typically reserved for events where there is more of a risk for higher storm intensities with the possibility of significant severe weather, more widespread severe weather with scattered reports as compared to isolated reports, or a slightly longer event duration as compared to a Marginal Risk.

While less common in our experience, a Slight Risk may also be issued when there is an increase in forecast confidence as compared to a Marginal Risk but typically there is still a decent amount of uncertainty surrounding the forecast for a slight risk event.

What Is An Enhanced (3/5 - Orange) Risk?

This next risk category is known as an Enhanced Risk and features a greater risk for a more widespread and/or persistent severe weather event as compared to a slight risk.

This risk level may also have a slightly higher chance for significant severe weather but the focus for this upgrade, again, is typically on the overall amount of geographic storm coverage and/or threat duration.

Numerous severe weather reports are expected across the region and a more extensive duration with the threat lasting for several hours or more. Forecast confidence at this stage is typically pretty sound as well with little uncertainty left in question.

What Is A Moderate (4/5 - Red) Risk?

This is the first category where we really start to worry about a significant severe weather event that poses an elevated life safety threat. A major severe weather outbreak may be possible with this type of risk.

In addition to the higher risk to life safety resultant of the threat for more widespread significant severe weather events, this risk also features high forecast confidence with few doubts, if any, about severe weather occurring, as well as a more prolonged threat duration closer to 12 hours or more.

What Is A High (5/5 - Pink/Magenta) Risk?

This is the final and most dangerous severe weather risk of them all. A High Risk is only issued when a tornado outbreak or derecho is expected and poses a significant threat to life safety due to the threat for several significant, destructive damaging wind gusts and/or strong/long-track tornadoes.

In addition for the extremely high probability of significant severe weather, this risk will also exist for 12 hours or more, cover an extremely widespread geographic area, and features a very strong degree of forecast confidence.

How Does The SPC Decide What Colors To Use?

The Difference Between Categorical & Probabilistic

So we've broken down each category of risks. Those risks come together to create what's known as the Categorical Outlook (shown in the top left of the graphic below).

But where does the Categorical Risk come from? The answer is all in the Probabilistic Hazard Outlooks. There are 3 different probabilistic outlooks, one for each severe hazard; tornadoes, wind, and hail. These 3 outlooks come together to cumulatively create the Categorical Outlook based on the overall life safety threat from those individual risks.

Probabilistic Tornado Outlook

The Tornado Outlook denotes the probability of a tornado occurring within 25 miles of a given point on the map.

Probabilistic Damaging Wind Gust Outlook

The Damaging Wind Gust Outlook denotes the probability of a damaging wind gust of at least 60mph occurring within 25 miles of a given point on the map.

Probabilistic Large Hail Outlook

The Hail Outlook denotes the probability of hail of 1 inch in diameter or greater occurring within 25 miles of a given point on the map.

When Are SPC Outlooks Issued?

The Storm Prediction Center issues severe weather outlooks each and every day for the next 8 days in a forecast period. If we use today (March 18th 2024) as an example for the start of our forecast period.

• The Day 1 Outlook would be for today (Monday 03/18/2024)

• The Day 2 Outlook would be for tomorrow (Tuesday 03/19/2024)

• The Day 3 Outlook would be for the day after tomorrow (Wednesday 03/20/2024)

• The Day 4 Outlook would be for Thursday (03/21/2024)

• The Day 5 Outlook would be for Friday (03/22/2024)

• The Day 6 Outlook would be for Saturday (03/23/2024)

• The Day 7 Outlook would be for Sunday (03/24/2024)

• The Day 8 Outlook would be for next Monday (03/25/2024)

This is the schedule of outlook issuance:

• The Day 1 Outlook is issued at 2am EDT and is updated at the following times:

  • 9:00am EDT

  • 12:30pm EDT

  • 4:00pm EDT

  • 9:00pm EDT

• The Day 2 Outlook is issued at 4am EDT and is updated one time:

  • 1:30pm EDT

• The Day 3 Outlook is issued at 3:30am EDT.

  • In rare situations, an update may be issued if forecasts have significantly changed.

• The Day 4 - Day 8 Outlooks are issued between 3am and 6am EDT.

How Do SPC Outlooks Evolve As A Potential Event Draws Nearer?

During the Day 4 through the Day 8 period, we get outlooks that denote the overall probability of any severe hazard occurring within 25 miles of a point in one of 4 ways:

• Potential Too Low - Indicates that conditions are not conducive for the development of severe weather.

• Predictability Too Low - Indicates that conditions could be favorable for the development of severe weather but there is not enough certainty to define specific geographic areas in a risk at the time of issuance.

• 15% Risk of Severe Weather - Indicates a 15% Risk for any Severe Weather Hazards within 25 miles of a point. This risk typically results in a Slight Risk or higher Categorical Outlook for Day 1.

• 30% Risk of Severe Weather - Indicates a 30% Risk for any Severe Weather Hazards within 25 miles of a point. This risk typically results in an Enhanced Risk or higher Categorical Outlook for Day 1.

With the Day 3 Outlook, we start to get more familiar with Categorical Outlooks from the SPC. We see our 5 risk levels with up to 4 of them being used in the Day 3 Outlook (due to the uncertainty of reliably forecasting significant severe weather 3 days out a High (5/5 - Pink) Risk cannot be issued for the Day 3 Outlook. The maximum possible risk is a Moderate (4/5 - Red).

We also get our first probabilistic outlooks on Day 3. The difference with this probabilistic outlook as compared to day 1 or day 2, is that it only outlines the probability for any severe weather threat instead of each individual type of severe weather threat; tornadoes, damaging wind gusts, or large hail.

The Day 2 Outlook is when we really start to get a good idea of the individual risks for each type of severe weather. While extremely rare, a High (5/5 - Pink) Risk could be issued for a Day 2 Outlook.

By Day 1 we're really starting to get the best picture of what to expect. We get updated categorical and probabilistic outlooks at least once every 5-7 hours.

How Accurate Are These Outlooks?

Generally speaking, these outlooks are extremely accurate. The meteorologists at the SPC are truly brilliant and they do a phenomenal job of forecasting severe weather. Let's take a look at the severe weather reports from March 31 of 2023 - the outbreak of severe weather, and it's associated outlooks, that we've been referring to repeatedly today and see how well they match up to the SPC Risk Areas.

I'll let that graphic speak for itself. Very impressive.

However, having said that, meteorology, especially when it comes to severe weather forecasting, is still not a science that us humans have completely "down to a T" and Mother Nature still throws the weather community a Curveball every now and then.

For example, let's take a look at February 27th of 2024:

This day featured 2 separate areas of Enhanced (3/5 - Orange) Risk driven by a 10% hatched tornado risk. The northern risk area across far southeastern Wisconsin and parts of northern Illinois did fairly well in terms of cumulative severe weather reports. However, the southern risk centered on Kentucky did not do well at all with only 2 wind reports for that area.

Furthermore, when we specifically focus on the probabilistic tornado risk from that day we see that not a single tornado was reported in either of the two highest-risk areas for tornadoes.

Taking a look at the two tornadoes that occurred in the 2% Tornado Risk area in lower Michigan it is important to note that the southern tornado was almost rated EF-2 (a significant tornado) with estimated winds of 110 MPH and the northern tornado was rated EF-2 with estimated winds of 115 MPH.

We highlight this risk from February 27th, not to disparage the SPC or our local NWS Forecast Offices, but rather to call to mind that tornadoes, even significant ones, can occur even on days when there is an overall low risk for tornadoes. Especially in Michigan, we see this type of scenario play out over and over again.

Make it a priority this year to take these risks seriously. It never hurts to be cautious!