Severe weather can occur at any time of year, but for this exercise imagine it’s late June. It’s 90 degrees with the usual stickiness that accompanies summer in the Mid-Atlantic. There’s a chance of thunderstorms in the forecast because it’s a cold front coming in, the Storm Prediction Center has issued an “enhanced” risk for the area for the afternoon. The news comes through with word that a severe thunderstorm watch has been issued until 9. What do all these terms even mean?
In general NWS terminology “watch” means the weather hazard *might* occur and “warning” means the hazard is about to occur or is occurring right now. In severe weather, the terms have another dimension because what qualifies as a severe thunderstorm has two set criteria, and the storm should at least have the potential for one or both. The storm must be capable of producing downdraft winds of 58mph or higher (50 knots) and/or have hail 1″ or larger in diameter. The rate of lightning or rainfall has no bearing on the official definition but storms that qualify as severe tend to have heavy rain and/or frequent lightning. Severe thunderstorms can also rotate and produce tornadoes.
The Storm Prediction Center (SPC) in Oklahoma is NWS’s forecast arm for severe weather and they have developed a five-tier risk system for severe weather. It’s rooted in probabilistic forecasting of “x chance of severe weather events in 25 miles of y point”, but that’s complicated to explain. Oversimplifying it, it’s a level 1-5 scale with each step being successively worse and more dangerous. It goes Marginal–>Slight–>Enhanced–>Moderate–>High.
- At the bottom of the scale, we have the “Marginal” (MRGL) risk. This is saying there could be a few storms and they could produce some widely isolated severe weather.
- One step up we have “Slight” (SLGT) risk. More coverage than marginal, but not an outbreak. In the summer, this would be the more run-of-the-mill severe weather. A few storms causing localized damage. Again, the risk posed from the three modes is localized and low
- “Enhanced” (ENH) risk starts jumping up on the scale. This is even more widespread, and also generally more intense. You might see this with a front sweeping through and pushing a good line of storms in.
- “Moderate” (MDT) risk starts getting towards the high-end, high impact severe weather events. You’re usually dealing with widespread severe weather. The derecho of 2012 was a moderate risk event (though there’s a debate over whether that was sufficient)
- “High” (HIGH) risk is reserved the most dangerous severe weather events. An interesting sidenote here is that a high risk has not been issued in the mid-atlantic states yet
this chart simplifies it
So back to you, and your lunch (how is it by the way?). You see that the severe thunderstorm watch is out. This is the next step up on the ladder. This means that SPC, in coordination with the local forecast office is now specifically highlighting an area where they think severe weather is possible. The watch will cover states or parts of states, issued county-by-county and it will have an official text that names the threat more specifically. The watches can, and often are, preceded by what are called mesoscale discussions (MDs). The MDs are technical discussions about an area SPC is interested in for issuing a watch. They’ll describe how the environment is setting up and what SPC’s thinking is. They’ll also put a chance of a watch in it, giving you an idea of how likely a watch is.
A typical watch will be posted for 6-8 hours and will be trimmed back locally as severe weather passes and the atmosphere becomes unfavorable for severe weather.
IMPORTANT: Just because a watch is issued does not guarantee that severe weather will occur. It just means the potential is there for it.
IF severe weather does start occurring, then it’s over to the local NWS office for warnings. Warnings are usually issued for up to 1hr for a box tracking the storm’s projected path and they’ll usually detail what the main threat from it will be. Around here, that’s usually damaging wind, though it can be in conjunction with hail. If the radar is detecting rotation in a storm, then they’ll issue a tornado warning.
If a severe thunderstorm warning is issued, it’s best to head indoors and wait for the storm to pass. You may need to find your way into even more indoors for really severe weather.
If a tornado warning is issued and you’re outside. Stop whatever it is you’re doing and head inside. Do not try to find these things. Typically tornadoes in the mid-atlantic are short-lived and rain-wrapped making them dangerous and hard to see, so don’t try.
Once inside, or if you’re already inside, get to the interior of the lowest floor of your house, office, or safe space. A basement is preferable but if you do not have one, an interior lower floor bathroom or closet is another good place to seek shelter.