Understanding Severe Weather Outlooks: Part One

During times of potential severe weather in Central Florida, you’ll often see us refer to a series of products produced by the Storm Prediction Center (SPC). They’re called “Convective Outlooks,” and focus on the development of thunderstorms and the risk of severe weather. Sometimes you’ll hear these referred to by us geeky types as “categorical” products, because they classify the risks into a few different categories. These products are a great way to understand how primed the atmosphere is to produce damaging weather, as long as you understand the inherent limitations.

Before we go any further, please understand that severe weather does not care what pretty little colors the SPC drew on a map that day. Storms gonna storm, no matter what the prediction was. For example, on a recent day in which multiple severe thunderstorm warnings and a tornado warning were issued for the attractions area, the SPC had not included the central Florida area in any area of increased risk. The SPC Convective Outlook that day included only “thunderstorms,” which were generally expected to be non-severe. On the flip side, just because the SPC declares an area to be in high risk, that doesn’t mean that there will absolutely be a severe storm where you live. The products are intended to provide a layman’s view of how capable the atmosphere is for bad weather in a particular area on a particular day but the actual outcome depends on a lot more than can be condensed into a few categories. OK, provisos out of the way… on with the show…

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Risk Categories

The SPC breaks its risks out into six categories:

Thunderstorms
The base-level category (which honestly we don’t even bother to report here because it is Florida, after all) is “Thunderstorms.” You’ll see it abbreviated on some official SPC products as “TSTM.” When we make maps here, we list them as “Non-Severe” just to better clarify what they mean. Officially, the category refers to an area where there is at least a 10% chance of thunderstorms but they are generally not expected to be severe. The SPC still cautions that any thunderstorm carries a risk of lightning and flooding; and of course there’s always a risk that one could go rogue and bubble up into something more than expected.

Marginal Risk is the first category which refers directly to a risk of actual severe weather. On SPC maps, it’s sometimes abbreviated MRGL. The key points to this risk level are that isolated severe storms are possible. They are generally expected to be limited in longevity and organization, have very little geographic coverage, and relatively low intensity. This type of outlook is not uncommon in the attractions area. It’s not even necessarily a reason to cancel any outdoor plans, but it’s the first sign of a day you might want to keep an eye on the weather as you go about your day.

Slight Risk is the second level of risk. On SPC maps, you may see it abbreviated as SLGT. The one-sentence summary of this one is that scattered severe storms are possible. It means that geographic coverage may be a bit greater than in the Marginal Risk areas, but still not necessarily widespread. The storms will probably be a bit more organized, but most of the storms that do qualify as severe will still be relatively weak and shouldn’t last that long.

Enhanced Risk is the third level of risk, and the level at which you should really start paying a bit more attention to the weather than you normally would because days like this don’t happen often. Abbreviated on some maps as ENH, this category is one in which numerous severe storms are possible. Storms will be more widespread, better organized, and generally could be stronger than in the previous categories. This, again, is not necessarily a reason to cancel everything and stay locked in your safe room… but it’s a day to be extra weather aware and make sure you have several ways to hear warnings.

Moderate Risk is the fourth level of risk, Moderate is actually a bit of a weak name for a relatively strong level of risk. Some maps show this as MDT. The summary of this is simple: widespread severe storms are likely. Note the switch from “possible” to “likely,” which is a term the SPC does not use lightly. On days with this designation, widespread severe weather is forecast, with “a few tornadoes and/or numerous severe thunderstorms.” Storms that form are often of the supercell type, known for large hail and potentially intense tornadoes. Alternately, they may be embedded in an approaching squall line, which can produce damaging straightline winds.

High Risk is the fifth and highest level of risk, and something to take very seriously. Designations like this are pretty rare, especially in the Central Florida area. So when they’re issued, it’s time to act. On these days, widespread severe storms are expected. The expected types of storms can be “either numerous intense and long-tracked tornadoes or a long-lived derecho-producing thunderstorm complex that produces hurricane-force wind gusts and widespread damage.” In other words, “long-lived, very widespread, and particularly intense.” These days are fairly rare nationwide but can occur even in Florida. If a High Risk is issued, it’s imperative that you know what’s going on and have a warning system that can wake you even when you’re asleep, such as NOAA Weather Radio and alerting mobile apps.

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Mapping the Risk

Risk areas are shown on a map by drawing various colored areas that correspond to the areas of greatest risk. Here’s the map for the attractions area on January 22, 2017.

If we zoom out a bit further, you can see the full spectrum of severe risk categories on the map.

You’d be forgiven for not really remembering 1/22/17 as a red letter severe weather day. It was one of those days where even though the atmosphere was juicy and ready to go, much of the forecast was somewhat of a bust and didn’t verify. There were plenty of storms and a few reports of hail under 2″ in size and some widespread severe wind gusts, but no tornadoes in the immediate area. In case you’re wondering, the correct way to react to such a mismatch between the forecast risk and the actual result is “we’re lucky that we really dodged a bullet today,” not “those bozos never get anything right.”

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In our next installment…

So far we’ve learned about the different categories of severe weather risk. But how does the SPC determine what category to assign for each day? We’ll take a look at that in our next installment on Monday.