This is Part 2 of our two-part series on flash flooding. Part 1 can be found here.

Here in the Washington-Baltimore area we usually avoid the violent tornadoes seen in the Midwest, or the direct hits from hurricanes felt along the East Cost and Gulf of Mexico. Flash Flooding, however, is a hazard that tends to rear its ugly head from time to time during the course of an typical year, much as it did last night.

Let’s go in-depth about flash flooding in this region, what month and time of day it’s most likely to occur, where flash flooding has been most common during the past decade or so, and tips to stay safe from this deadly hazard.

The premise is pretty simple – the more intense the rainfall and longer it lasts, the more potential there is for a flash flood to occur. Most flash floods are caused by heavy showers or thunderstorms that are slow moving or “train” (line up one right after another) across a region. Rainfall is typically more intense and flash flood warnings are more frequent during late spring through early fall, when the atmosphere is usually more moist and unstable than during the colder months (see monthly graph of flash flood warnings below).

Warning tallies were found via Iowa Environmental Mesonet’s watch/warning archive. A complete gap in flash flood warnings was noted between 2005 and 2007 in their database. To build the fullest picture possible based on the current warning type (polygons not limited by county borders) 2003 and 2004 were included in the monthly and the hourly data survey. Prior to that period, flash flood warnings were county based. (Ian Livingston using IEM data)

The September peak is probably at least in part due to that being when tropical systems are most likely. The secondary maximum in June may be due to the fact that fronts are stronger in June than in July or August, increasing the chances of having a larger organized line of thunderstorms.

While the majority of flash floods across the country occur during summer months, many floods and flash floods along the West Coast occur during cold season when a so-called “atmospheric river” sets up and brings moisture northward into the mountains, creating what’s known as the “Pineapple Express” pattern. Early-spring and late-fall flash floods are also relatively common across the South closer to the Gulf of Mexico.

What time of day do our flash floods usually occur?

Flash floods in our area typically develop during the afternoon and evening (see graph below) since that’s when the atmosphere is usually warmest and most unstable, and thus most conducive to heavy showers and thunderstorms. Some flash floods in our area develop during the overnight as well, but nocturnal thunderstorms and flash floods are much more common in the Plains, where the low-level jet strengthens at night and helps fuel organized thunderstorm systems.

The same data analysis method is used for this hourly graph as described for the monthly graph above. Warnings are tabulated by the hour (UTC) they were issued. (Ian Livingston using IEM data)

Impermeable surfaces like asphalt promote rapid run off and make urban areas particularly susceptible to flash flooding. We constructed the graphic below showing the frequency of flash flood warnings (the darker the shading the greater the number of warnings that were issued) and the actual flash floods (the green dots) during 2008-2012 based on data from the National Weather Service forecast office in Sterling, Va.

The map above covers the past five complete years of flash flood warnings and flash flood reports as archived by the Iowa Environmental Mesonet. No location on the map was unwarned during the period, and darker shading indicates the most warnings, while dots are reports to the NWS of flash flooding. This data grab does not include 2002-04 due to apparent errors (likely incorrect GIS entries at the database level) with storm reports during that period. Though the map is an accurate representation of warnings from 2008 to 2012, five years is a short window to draw uncontestable conclusions from. (Ian Livingston using IEM data)

Note that flash floods were heavily clustered near the urban areas around Washington and Baltimore (see dots on the figure above). Part of that clustering may be due to the fact that there are more people around to report flash floods, but it’s also due to the large expanses of impermeable asphalt and concrete around cities and the lack of heavy vegetation to slow the movement of waters.

Moving water can exert a tremendous amount of force and can shift large boulders. Just one foot of water can displace 1500 pounds. Six inches of rapidly moving water can knock a person off their feet. As little as 18 inches of water can float a vehicle and two feet of water will float a bus. The pictures seen here illustrate the incredible power of moving water.

Most flash flooding deaths occur when people drive their cars into moving water that often only looks a few feet deep. Some people have the mistaken impression that they are safe in a large SUV, minivan or pickup truck. The truth is, moving water can push virtually any vehicle into a ditch or stream where, it often flips over and traps individuals in the car.

More than half of all flooding and flash flooding deaths are associated with people getting caught in vehicles after driving into water that often doesn’t look too deep. When water is running over a bridge, there is a good chance that the structural integrity of the bridge may be compromised and the bridge may not hold your car even if you think you can drive through the water.

According to NWS statistics, almost two-thirds of flash flood fatalities are male. 18-35 year old drivers are the ones most likely to make the fatal mistake of driving into a flood. Males are even more likely to make a fatal error in judgment if another male is in the car. The NWS adage to “Turn Around, Don’t Drown” is a good one.

Six flash flood safety tips from the National Weather Service

• In hilly terrain, flash floods can strike with little or no advance warning. Distant rain may be channeled into gullies and ravines, turning a quiet stream into a rampaging torrent in minutes. Never camp on low ground next to streams since a flash flood can catch you while you’re asleep.

• Do not cross a flowing stream on foot where water is above your ankles.

• If you are driving, don’t try to cross water-filled areas of unknown depths. If your vehicle stalls, abandon it immediately and go to higher ground. Rapidly rising water may sweep the vehicle and its occupants away. Many deaths have been caused by attempts to move stalled vehicles.

• Be especially cautious at night. It’s harder to recognize water danger then.

• Don’t try to outrace a flood on foot. If you see or hear it coming, move to higher ground immediately.

• Be familiar with the land features where you live, work, and play. You may be in a low area, near a drainage ditch or small stream, or below a dam. Be prepared!

How does the National Weather Service forecast Flash Flood Watches and Warnings?

Anticipating where a flash flood warning is needed is a tremendously difficult undertaking that requires lots of knowledge and experience. Precipitation forecasts are typically much more accurate for large-scale weather systems than they are for areas of showers and thunderstorms.

The 12 NWS River Forecast Centers (RFCs) spread across the lower 48, along with the NWS Weather Prediction Center and local NWS forecast offices, work together to asses flash flood risk and issue warnings. For example, the RFCs provide local NWS forecasts offices with guidance on how much rainfall is needed to produce flooding in 1, 3, 6, 12 and 24 hour periods, while the Weather Prediction Center prepares a guidance product that attempts to quantify the probability of excessive rainfall occurring.

Bob Davis, NWS forecast and flash flood expert, cautions that in certain circumstances the amount of rainfall required to produce flash flooding may be lower than those initial estimates. He also notes that county-based flash flood guidance may overestimate the amount of rain needed to flood urban areas.

The local NWS Forecast Office in Sterling, Va., is responsible for sifting through the various guidance products and deciding when and where to issue a flash flood watch.  Watches are generally issued when precipitation is expected to exceed the flash flood guidance values at some time interval.

Flash flood warnings are issued when flash flooding is imminent or is already occurring.  They may be issued upon reports from individuals who have noted that streams are overflowing or roads are flooded.  They may also be issued based on rainfall reports from gauges and radar estimates of how much rain has fallen. Radar estimates are especially important in areas where the density of gauges may not be high enough to pick up the maximum rainfall intensity.

However, the relationship between radar reflectivity and rainfall amount is not as simple as it may seem, as it depends on the size of the droplets.  On radar the reflectivity from a storm producing a modest number of very large droplets can look the same as a storm producing a much greater number of smaller droplets, even though the latter tend to be associated with higher rainfall rates. A larger potential problem is that during heavy showers with low cloud tops, the radar may overshoot the most intense echoes because of the tilt of the radar beam.  Also, mountains can block the beams and limit the usefulness of radar estimates in much of the West (see more on radar estimates).

Despite these limitations,  radar rainfall estimates are critical in identifying many flash floods. The flash flood warning system has been recently improved with the introduction of an automated system that monitors the radar and gauge rainfall estimates and compares them to stream basins and sub-basins.  The system allows forecasters to develop more stream-specific data in warnings.