The atmosphere is a little bit like a giant pot of water, only the water happens to be in gas form. Now, the atmosphere consists of many different gasses, from hydrogen to oxygen to nitrogen to ozone, but water vapor is the dynamic gas that makes weather as we know it happen. Every cloud in the sky contains water in some form or another, and it is the thermodynamic reactions of water changing from gas to liquid to solid that is responsible for virtually every weather phenomenon we're familiar with, from a rain shower to a thunderstorm to a blizzard. Every thunderstorm is like a sea of boiling water vapor, with heat needed to initially make the pot boil.
That being said, nowhere in the atmosphere does the temperature reach a steady 212°F. Quite the opposite; most of the upper troposphere (where all the action occurs) is below freezing. What needs to happen to get clouds is that warm moist air from the surface has to rise into surrounding air that is cooler. As the air cools, it condenses, and dense pockets of water vapor form clouds, while water that gets cold enough to become liquid or solid form both clouds and precipitation. The cooler the air that surrounds a lifted parcel, the further it can rise freely. The quicker it condenses depends on how moist it is. If the air is sufficiently moist, water vapor can begin condensing at a very high temperature, even above 70°. This is the dewpoint that you hear me talk about so much. Once a parcel of air hits the dewpoint, it begins to cool more slowly than when it was dry. This allows it to rise further in the atmosphere, provided that the surrounding air it rises into is cooler. This is what I mean when I refer to atmospheric instability - surrounding air that the parcel rises into is cooler. Warm air rises, cold air sinks. If warm air beneath cold air, you get this unstable environment.
Now, you can have plenty of instability, but you may not have any action whatsoever. Imagine a pot of boiling water. Keeping a lid on the pot (cap) can be an effective way to keep it from boiling over. As long as the temperature within the pot doesn't rise high enough to overcome the cap, you won't have an issue. Even if it does, you may only get a small simmer-over provided you get to the pot in time and turn the temperature down. However, imagine the havoc that would be created by pouring a bucket of cold water into that boiling pot of water on your stove. You're going to almost certainly turn your kitchen into a flooded mess. This is what happens when you add a cold front to the mix of a warm, unstable environment. The strength of the cold front can have a huge impact on how much water boils over; if the cold front is weak, you may only be pouring a cup or so of cold water into your boiling pot, or be pouring it slowly enough that it won't cause too much havoc.
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| A positively tilted storm system, and associated cold/warm fronts. |
This is what you get with positive and negatively tilted storm systems. A positively tilted storm system usually consists of a low pressure center that isn't quite up to snuff. It takes a fair amount of warm moist air ahead of this type of system to make severe weather occur. That isn't to say it doesn't occur; quite the contrary, most severe weather events are driven by positive to neutrally tilted storm systems, and their associated cold fronts. Even in the complete absence of a proper cold front, severe weather can occur if that pot is boiling like an angry demon and just a drop of cold water gets tossed in. This is exactly what happened yesterday; there was a great deal of warm unstable air in the lower atmosphere, and lake breezes caused some very severe storms to pop up. They weren't widespread, but they dropped some severe hail, produced damaging winds, and possibly even a tornadic waterspout over the Saginaw Bay if radar was to be believed. Extreme instability helped fuel the August 2nd event, too, as a moderately strong cold front descended from the north. A cold front doesn't need to be on the extreme side to produce severe weather in that kind of an environment.
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| A negatively tilted storm system and associated cold/warm fronts. |
If, however, the cold front is very strong, then it's the equivalent of taking that entire bucket and dumping it into the pot on the stove; even a pot that's only JUST coming to a boil will react violently. This is what happens when a low pressure system deepens to maturity, sometimes rapidly enough to be characterized as something known as bombogenesis, which is a relatively rare drop in pressure of 24 millibars over 24 hours. Even if it doesn't quite reach this criteria, explosive cyclonic deepening can push severe weather to extreme heights. You may have heard me talk about this before, as it was something I feared might happen back on June 22nd. It didn't quite turn out that way, yet the state of Michigan still saw quite a few tornadoes, one of which was strong. Now imagine if it had turned out that way.
When a storm takes on a negative tilt, one of the things that happens is that the cold front picks up like gangbusters and literally begins to pass the warm front near the center of the low. This is known as occlusion, and it helps to create very strong forcing for ascent, or lift, in front of the cold front, as it's wedged violently under the warm moist air ahead of it. In this type of environment, instability can be marginal, yet storms can still take off like rockets and drop violent tornadoes if moisture content is high enough. This is exactly what happened back on November the 17th of 2013. Very little atmospheric instability - 700-1000j/kg of MLCAPE in some places - was present ahead of the cold front. Normally, this would make for a "general thunder" day at best, or just a typical rainy fall day. However, the system which was moving this air was a very powerful winter-style storm, and when winter storms happen on warm days, bad things tend to occur to their south and east. Those "bad things" can be characterized by 136 tornadoes across six states, and 429 instances of wind damage, at least 19 of which were caused by hurricane force winds which exceeded 75mph. Pretty crazy, even for a Great Lakes November Gale.
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| November 17th 2013 300mb jet core as forecast by the NAM for roughly 1PM EST on that day |
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| August 19th 2015 300mb jet core as forecast by the GFS for roughly 2PM EDT this Wednesday |
Many if not most of the violent tornado outbreaks that have occurred in this country have happened due to negatively tilted troughs. November 17th was but one, and it's the one I tend to refer to the most as it was recent and in our neck of the woods. I do think there are lots of parallels between what the models are forecasting right now and that event. I don't think that it's quite there yet. A lot of other factors will need to become illuminated before anyone will be able to say that this event will be anything close to as extreme as that day was. With that in mind, if conditions do line up, it could wind up being worse. August is typically a much warmer month for the Great Lakes than November.
Things that could wind up making this go bust include, but are not limited to:
• Early day or previous day leftover convection sucking up energy before and/or tempering the atmosphere before the cold front arrives
• Overall lack of instability
• Timing of the cold front's arrival not coinciding with enough instability to generate storms
• A weakening of the storm system, causing the predicted negative tilt to vanish and a decrease in overall wind shear/horsepower potential, or the system not quite manifesting in the manner as is currently being predicted
Things that could make this go boom include, but are not limited to:
• A strong low level jet developing and sucking up ample moisture and warm air and drawing it into the Great Lakes
• Timing of the cold front's arrival coinciding with ample instability or - God forbid - peak heating
• Strengthening of the storm system into an even more powerful monster than what's currently benig predicted to form
I'm not going to lie. As of right now, this looks pretty bad. Michigan in particular may wind up receiving a bit more out of this than we did back in 2013, where the focus was Illinois and Indiana. The reason being is that the main jet streak is forecast to be a bit further north this time around. Upper level disturbance aside, the center of the surface low is also forecast to be further north, which is most concerning to me. The 12z GFS run puts it right in the "sweet spot" - roughly the same position that the surface low was in during the July 2nd 1997 Southeast Michigan Tornado Outbreak.
I'm anxiously anticipating getting data from the mid range and short term models on this for a clearer picture. Any categorical risk upgrades will be posted as they're made. For now, I'm maintaining the SPC's "slight risk" assessment.
I'm anxiously anticipating getting data from the mid range and short term models on this for a clearer picture. Any categorical risk upgrades will be posted as they're made. For now, I'm maintaining the SPC's "slight risk" assessment.
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