Posts in Category: Education
Flash Flooding – Staying Aware
Flash flooding is often an overlooked but very real danger with virtually any thunderstorm. What makes flash flooding so unique is the speed with which it can occur, often catching people by surprise. What makes it even more dangerous is that you don’t really need a special storm to produce it. Your average garden variety thunderstorm can produce flash flooding if the conditions are right.
Cars crossing a flooded stretch of road in Toronto
So what’s the difference between flooding and flash flooding?
Flooding is basically the process where water covers land that is usually dry. We often hear about channel flooding (large rivers) on the news where snow melt, an ice jam or days of heavy upstream rainfall cause a river to overflow its banks. These types of flooding events are generally slow in nature and to some degree well predicted with a good estimate of peak flood levels.
Flash flooding on the other hand is any form of sudden flooding. It’s often very poorly forecasted with little to no warning and extremely difficult to model or predict. Flash flooding usually involves channel flooding (rivers, creeks, tributaries), urban flooding (failed or over capacity infrastructure) and areal flooding (ponding, pooling).
Overland flooding in Colorado
There’s no easy way to know when or where flash flooding will occur or how serious the situation will become. But there are certain elements that make an environment more prone to flooding.
The first factor flash flooding requires is heavy, persistent rainfall. A storm that is slow moving or one which back builds is a prime storm to produce flash flooding. Rainfall rates in storms can easily reach and exceed 60mm/hr or greater, and to put that into perspective that’s 1mm of rainfall per minute on the low end! In fact most thunderstorms actually produce peak rainfall rates of 100 to 300 mm/hr or about 1.6 to 5 millimeters of rain per minute. That’s why you can easily get 20 millimeters of rainfall in as little 10 to 15 minutes as a storm passes over. So if a storm produces these high rainfall rates over a specific area for any length of time flooding will occur.
Field flooding in Texas after a very slow moving storm
The second factor is the layout of the land and it’s ability to absorb water. For example, the rainforest sees so much rain from heavy thunderstorms it’s basically an environment that’s adapted to absorb all that moisture without just flooding and falling apart. On the other hand, deserts are known as high risk areas with even a small amount of rain leading to potentially catastrophic flooding because the soil simply is not good at absorbing water.
Flash flooding in Burlington, Ontario at Dundas St & Guelph Line
Hilly and flat areas have their own problems. Flat areas experience areal type flooding where water will build in say a field forming an impromptu pond and then suddenly move as a large mass (known as overland flow) from one area to another (very dangerous!). This type of flooding can strike extremely quickly with virtually no warning and very large masses of water stretching long distance can easily cover kilometers of roadway in a single swoop. Hilly or mountainous areas exposed to flash flooding pose the threat of mud and landslides which can occur with no warning and move very quickly destroying everything in their path. Channel flooding which occurs in mountainous or hilly terrain is also extremely dangerous with flood waters achieving high velocities due to large angular slopes. It’s important to remember that in all landscapes benign waterways can potentially become raging rivers.
The third factor with flash flooding is the current state of the environment with regards to saturated soil, water table depth and pre-existing water levels in channels. An environment that is already saturated with high water tables is very susceptible to flash flooding.
What are the dangers of flash flooding?
Everything from the loss of property to death is a potential result of flash flooding.
What do I do if I’m caught in a flash flood?
Turn around, don’t drown! Prevention is the best cure, if the roadway in front of you is flooding or you see that things are flooding around you, get out of the area and seek higher ground if possible. Most people who get into trouble in flash floods attempt to cross flooded roadways.
A tributary swallows a roadway in Kansas
This video below is a great example of why crossing a flooded roadway is bad, dangerous idea!
Fortunately for this individual, they we’re okay, but it’s a great video that demonstrates why things aren’t always as they appear. It’s easy to look and assume that there is a road under that water, except if your assumption is wrong, you’re in big big trouble! The safest way to get out of a jam is not to get in one in the first place. If you see the road ahead is flooded, slow down, stop and turn around. If you start driving into a flooded roadway and decided you want out, the safest way out is the same way you got in there, just backup if you can safely do so.
A flood guage in Texas tells drivers the depth of the water crossing the road
I’m trapped in my car!
If you find yourself in a position where you’re trapped on a flooded roadway, don’t panic. Stop, breathe, assess the situation. Firstly, grab your cell phone and keep it away from water. Second, roll down at least one window & unlock all your doors so you can easily escape your vehicle if need be (it evens out the pressure). Now assess the situation, Where am I? How fast is the water moving? How fast is it rising? How deep does it look? Can I stay dry? Is there high ground/safety nearby? What’s around that I can try to hold onto? Do I have anything in here that’ll help me float?
If the water is calm and appears waist deep, you’re probably okay. Try to stay dry, call for help on your phone and follow any instructions. If you can swim, and feel confident wading through the CALM water to an elevated portion of land and it’s warm enough where HYPOTHERMIA will not be an issue then do so very carefully but wholly at your own discretion.
If the water is calm but rising rapidly, call 911 immediately! They will likely either tell you to stay with your vehicle unless you can reach safety quickly without getting into further trouble. Don’t panic, if you have anything around you that floats or will help draw attention, USE IT.
A woman dangerously attempts to cross a flooded roadway with fast moving water in Burlington, Ontario
If the water is flowing quickly and rising call 911 immediately. You will need to eventually escape your vehicle (assuming it’s being swept away), look for something to hold onto! Call out for help!!! If you’re trapped in the flow, swim with it (not against it) and try to move diagonally towards a shoreline or any area that may allow you to escape or slow down. Conserve energy, if possible try to lay in a position that allows you to float, hold onto anything that may help you safely float and keep calling for help! Even if you don’t see rescuers in your immediate area, they may be just downstream preparing to save you and you’ll need all your energy for rescue! Don’t give up, keep looking for a way out.
If the water is flowing quickly but not rising call 911 immediately. If your vehicle is not being swept away, but the water is moving quickly, STAY PUT. You’re safer not moving and waiting for rescue, if you try and self rescue you may be swept away!!! Try to stay dry, warm, and aware! Follow any instructions 911 gives you. Do not be lured out of the vehicle at all costs, stay put until you are rescued!
I escaped the flood but I’m soaking wet & cold
Take your clothes off immediately (get naked!), hypothermia is a major danger. Try and dry off with towels, seek warm shelter/fresh clothing and emergency medical attention immediately. Allow your body to shiver (it generates heat) and keep standing up. Cold blood returning to your core from extremities such as your feet is very dangerous, standing slows this process and prevents shock.
I’m a bystander, what should I do?
Don’t encourage people to do anything! Call 911 and tell them what you see, who is trapped, where you are. Reassure the trapped individual(s) they’re not alone, help is coming. Keep an eye on them and always have them in sight, be aware of their direction of potential travel downstream and keep an eye on the clock, exact times are very important with rescues and fast moving water. If you can somehow give them something helpful (life-jacket, PFD, pool noodle, etc) be very careful doing so. Don’t try and rescue people, the professionals will do that! You might make it worse. As a bystander the best thing you can do is keep the emergency services aware of the situation as they respond, you have a working phone, the victim may not, keep your phone working. Above all, stay calm, being calm allows for better rationalization. Only offer the emergency services help if asked, don’t go and get in the way!!
Spillover flooding as the Don River engulfs a portion of the Don Valley Parkway in Toronto
Okay, my car stopped running, I’m stuck in water and it’s shallow?
Chances are you choked the intake. Assuming you’re in no position of danger from drowning or anything, DON’T TRY AND START THE ENGINE!!! Your car might be okay, you just need a tow. If you try to start your engine when it’s full of water (hydro locked) you can destroy it. When water gets into your vehicle’s cylinders and you try and start the engine the water does not compress, instead the piston rods BEND since the piston cannot compress the water unlike the gas/oxygen mix normally in there which does get compressed. When the rods bend, bye-bye engine. People make this costly mistake all the time! If you choke your engine on a flooded road, get it towed to a mechanic before you dare attempt to start the engine.again.
Hail Storms – An Educational Primer
Before I dive into the topic of hail and hail storms, I would like to clarify that there is not a singular storm type known as a hail storm. Hail is a product of convection and vertical growth cloud just as lightning, tornadoes, heavy rain and other thunderstorm phenomenon are part thereof. Storms which are called “hail storms” by the media, literature and other sources are noted for their production of large quantities of hail in addition to other things such as rain and lightning. So remember that hail is a part of a parent thunderstorm and not a storm in and of itself.
Now you might be asking yourself how bad can it possibly get inside the storms core?
Hail Covered Road
Well that really depends on where you are, the type of storm and the atmosphere. In Southern Ontario thunderstorms tend to grow in moisture rich environments with high humidity. Storms that are typically found in moist air masses will produce less hail and more rain. There are a number of working theories but for one, moist air tends to be deeper and retain warmth so the effects of evaporative cooling aloft are diminished. Secondly, the updrafts become moisture-full sooner and the upward velocities are diminished limiting overall lifting power. Third, moist storms tend to begin dropping precipitating sooner, if not almost immediately compared to drier storms limiting the time for hail growth.
Classic Okalhoma Dryline Supercell with 4+ inch hail in the clearing / vaulted region just right of center
What has been observed generally is that dry environments and especially those with high elevations and low freezing levels produce more hail than moist environments with high freezing levels. In addition there are many theories around cloud microphysics that suggest atmospheres with less dust or particulate matter contain less nuclei for rain to form on and hail tends to be favoured.
Dry supercell storm with golf ball to baseball sized hail in Western Nebraska – hail shaft centre left with really big hail towards clearing on the left
Hail growth itself revolves around three primary theories. The first theory suggests hail rises and falls within a storm several times before falling to the ground. This up and down theory is supported by smooth stones and through the observed concentric rings of ice growth within a hail stone suggesting cycles of melting and freezing. The second theory is the suspension theory, where a hail stone sort of floats in an updraft gradually becoming heavier as it grows and eventually descending, then falling to the surface while banging into other lighter stones on the way down. These smaller stones and supercooled rain drops may stick to the larger hail stones creating a spiked appearance. The last theory is that a hail stone begins its growth low in the updraft while being thrown upwards rapidly and experiences rapid growth in the supercooled sub-freezing zone before eventually getting ejected out with downdraft and other precipitation. Stones that have a very cold soft white appearance and feel “soft” or graupel like support this theory.
Powerful dryline Panhandle Texas Supercell Storm – gigantic 3 inch hail just to the center right of the image
Unfortunately not one of these theories has been proven because hail is extremely hard to observe and there’s simply no easy, safe or conclusive means to “watch” hail growth in a storm. We can use radar to see a hail region develop but it’s hardly conclusive evidence when it comes to the actual formation process since the cloud microphysics in relation to the updraft structure and environment are critically important. I would argue that all three theories are likely correct and probably occur concurrently with one another in a storm through its life cycle.
Golf ball to baseball sized hail coated the roadside near Dumas Texas
There are also a number of geographic nuances we deal with here in Southern Ontario that places such as a the high plains of the U.S don’t deal with. The elevation across all of Southern Ontario is much lower ranging from 80 meters above sea level near the Southern Great Lakes to about 500 meters peak elevation in the Dufferin Highlands. Elevation changes around the bulk of the land mass lies in the 200-400 meter range. The average elevation of the plains in Colorado is around 1500 meters above sea level, that’s a whole 1km higher than the highest part of Southern Ontario and some parts of Colorado that are “flat” are actually +1700 meters ASL. The forecast implications of this high geography is immense since the 850mb level in Colorado is basically the ground, while here in Southern Ontario it’s 1.5km above the ground. Because of the elevation in Colorado the ground is much closer to the freezing level and hail has a far better chance of reaching the ground than it does in other places.
Golf Ball to Tennis Ball Sized Hail in Oklahoma – May 2011
When you consider all the above factors, the largest hail you will typically see in Southern Ontario is 1cm or less with 2.5cm (quarter sized) hail occurring on occasion. Golf ball sized hail which is 4.5cm in diameter is possible and does occur a few times each year but only with the most powerful storms. The largest photographically documented hail in Southern Ontario fell on the afternoon of July 8th 2007. Fellow storm chaser Dave Patrick of www.ontarioweather.com was chasing a violent supercell which had produced several tornadoes and came across shredded trees between Mildmay and Listowel. When he ventured out to have a better look he found baseball sized hail which had cratered the ground. The hail was sitting for 20-40 minutes before he arrived and the largest stones were likely 3.25 inches / 8.3cm (teacup sized) before melting. His full chase log is online here.
Storm Chaser Dave Patrick holds a baseball sized hailstone near Mildmay Ontario
It’s also important to remember big hail does not necessarily need a powerful supercell storm if the right elements come together. June 16th 2008 was a perfect example, the surface dew points were low so the air was pretty dry and the atmosphere above was super dry, very cold and the lapse rates were steep thanks to strong daytime heating! One storm popped up along a lake breeze boundary and began to rumble south into Vaughan. The storm produced tons of spiked 2-3cm hail denting cars and damaging crops. This storm formed in an environment that was far more reminiscent of the high plain and and so too was the resulting weather.
Ping Pong to Golf Ball Sized Hail in northern Vaughan – June 16th 2008
So you might be wondering where do you find hail in a storm?
If it’s a squall line, the hail will likely blow in quickly on the leading edge with the heaviest rain and then quickly taper off. Single cell storms may produce heavy rain followed by a brief burst of hail which will diminish. Multicell storms will have various pockets of heavy rain/hail and vary a great deal. Supercell storms produce the biggest hail just to the north of the updraft core at the rear of the storm. A general rule of thumb is that the largest hail is going to be between the highest towers of the storms and the dark rain core.
Hail bow – large hail center and larger hail to the right of center
How do you spot hail from a distance?
Typically hail will show up as a bright, band or streak in a darker rain core closer to the towering updrafts. This bright zone is referred to as a hail shaft. As a rule of thumb, the brighter the hail streak the larger the hail. Once a hail streak becomes invisible watch out! The general rule is that as hail size goes up, the visibility of the hail zone also increases because there is less diffraction of light. The spacing between the stones also increases further helping visibility get better. This is somewhat dangerous because unsuspecting motorists, chasers and others may wander into this clear zone when trying to escape the storms heavy rain core only to get pummelled by big hail. Another consideration is that anytime you’re sitting under the flanking line, especially when you’re closer to the area where the flanking line and parent storms begins precipitating then you’re at risk of getting hail, even if the sky appears to be clearing.
Largest hail center right towards blue sky – Golf Balls falling in that zone
Nebraska Supercell Storm Radar Signature – purple area is very heavy rain and hail