Latest mid-range models forecasting INCREDIBLY unstable air this week

Wow. Not very often does the forecast make me say wow, but what's being predicted is just a few hairs shy of atmospheric zombie apocalypse in terms of instability. There remains several uncertainties, however, which I'll touch on below.

First, let's talk the greater continental U.S., starting with the Great Plains. Things are looking to get a bit wacky there this week, as confidence is increasing for what could be a major severe weather outbreak. The SPC has outlined a 30% severe risk for the south central plains on Thursday, and a 15% risk a slightly larger portion to the north and south. The wind profiles aren't exactly remarkable, but the pure amount of energy that may exist in the atmosphere could more than make up for it. We're talking 7000j/kg of SBCAPE in some areas. CAPE (convective available potential energy) consists of all the available energy that a thunderstorm can tap into. CAPE of 1000-2500j/kg can generate significant severe weather given strong wind profiles. When CAPE values exceed 4000, you're pretty much guaranteed severe weather even with weaker wind profiles should thunderstorms form. Normally I refer to CAPE as the "fuel tank" portion of the thunderstorm engine, but in this case it's more appropriate to compare it to explosive yield; 1000j/kg is like a firecracker. 2000j/kg is like a stick of dynamite. 3000j/kg is like a large block of C4. 4000j/kg is like an aircraft-delivered conventional explosive, or perhaps a large-yield guided missile. 

When you start getting above 6000 and 7000j/kg, you're dealing with nuclear yields. Storms with giant size hail and macrobursts - large downbursts encompassing several miles in diameter - are not uncommon with storms that form in such environments. I start thinking about a derecho risk when we start seeing values like this. The tornado risk associated with this system remains difficult to predict for now, but when you start seeing CAPE values like this, tornadoes are not uncommon. It remains to see how much low level helicity will be present. If the surface winds even get above 20mph, I think there'll be quite a few spin-ups around the country. 

NAM forecast SBCAPE values for Thursday evening.

GFS forecast SBCAPE values for Thursday evening.

While Michigan is not forecast to see the intergalactic levels of instability forecast for the plains by both the NAM and GFS, the NAM is predicting at least some crazy, low-orbit levels - 4000-5000j/kg in some areas of lower Michigan. Some of those I've spoke to are sitting on the fence about those levels, as the GFS is forecasting significantly less. While I admit the NAM's predictions don't have my full confidence, the latest run has predicted even more instability than the previous, and the key portion is temperature rather than dewpoint that is influencing the forecast difference. Part of the reason I'm starting to raise my left eyebrow a bit is that the GFS tends to low-ball Michigan on temperatures in the late summer because of exaggerating the influence of the lakes. While this is not late summer, it does have some characteristics. Widespread southwesterly winds should be prevailing over the next several days thanks to the redevelopment of both the Bermuda High and what will be a strengthening storm system to our northwest. This should funnel ample moisture into the state. Lake surface temperatures are quite high for this time of year thanks to a warm winter and very little ice cover. This slightly weakens the 'insulating' effect that the lakes have on our weather for the spring months. Most importantly, though, both the NAM and GFS are predicting dewpoints exceeding 70°. This is the clincher for me; the GFS is normally quite conservative about dewpoints in tandem with its usual slightly-lower temperature predictions. This time around, however, it's predicting temperatures in the 70s and dewpoints in the 70s - nearly 100% surface humidity. For this to be the case, we'd need to be in the midst of some serious rain and/or fog. Air would be condensing at a ridiculously low level, and while both models forecast early-day convection moving through, they have clouds breaking up by afternoon. I can't see how we won't reach 80 degrees on Thursday. 

If the temperatures get to this point, we're going to be dealing with a fairly unique situation - very strong to extreme pockets of instability in a weakly capped environment. The weak cap is important because it determines storm mode. When weak caps are present, they tend to keep a lid on convection. Then, somewhere, you'll get a region where temperatures overachieve - say pocket or a few pockets of 90° surface temps which can overcome the cap. This is a "loaded gun" scenario. The trigger is convergence, or lift - upward vertical velocity which pushes this extremely buoyant air skyward. Some moderate to strong areas of moisture convergence are forecast to work their way into Michigan by late afternoon. These can be outflow boundaries from western thunderstorms or lake breezes which act as miniature cold fronts. Then the gun goes off, and discrete supercells end up forming.

The words "discrete supercells" should be familiar to anyone who is familiar with tornadoes. However, I'm not quite ready to say there's going to be a tornado threat in Michigan on Thursday. The previous model run had some rather interesting wind profiles being forecast, especially around the eastern portion of the state. These are represented by the hodograph on the top right in the picture above. The wind speed is quite benign in terms of tornado potential - less than 10 knots of surface wind is being forecast, and you usually need to have 20 knots or more to get strong tornadoes. However, the "critical angle" is quite alarming. This angle represents the change in direction between the surface wind and the "storm motion vector". If you draw a line from the center of the graph toward the surface wind plot, then draw another toward the 500mb plot, then draw a line connecting the two, the angle between the surface vector and the connecting line is the critical angle. When this angle approaches 90° in either direction (plus or minus), you have right moving storms with very strong rotational properties necessary for significant tornadoes. Reed Timmer has a thing or two to say about the significance of critical angles. 

Now, the latest model run has backed off a bit from the image above in terms of wind direction, and again, surface wind speed, which is not forecast to be high, is just as important for the development of significant twisters. Nevertheless, should our observed soundings look anything like the one above, I will not be surprised if a tornado or two touches down on Thursday should storms initiate. Significantly large hail and damaging winds would be a sure thing in any storm that formed on Thursday should these levels of instability end up as high as what's being forecast. That's where I'm still on the fence, though; the timing of convection is anything but clear, and there has to be a trigger to break the cap. Therefore, I'm going to wait yet one more day before I make any changes to the outlook. The last thing I want to do is predict big storms, then have nothing happen. The potential is definitely there, though, and moreso than it has been all year.