Weekend severe weather chances? Very difficult to tell...

The final runs of the day have come out for most of the short, mid, and long range forecast models (or the first runs of tomorrow, if you want to get technical).  All of the models seem to indicate that extraordinary instability will be present in our state this coming weekend into Monday, but I'm just not sure if I'm feeling it on this one.

The Storm Prediction Center's standard long range "we dunno" outlook pretty much sums up my opinion this time around.

The big reason is the most important factor to the development of severe thunderstorms - timing. Quite a few models are indicating that several storm systems will be marching across the northern US/southern Canadian provinces, but when it comes to Michigan, the times that they will begin to affect the weather here appear to be a bit disjointed.  Wind shear should be plentiful with these systems, but they may move through the state when there's little instability to be had.  Right now, rain looks likely on Thursday, and wind shear will be plentiful, but there doesn't appear to be any indication of instability at all.  The best chance of severe thunderstorms appears to be Friday, when the really unstable air moves in, but this time shear might be a bit less, or the system might move through before daytime heating can weaken a capping inversion enough to initiate thunderstorms.

So far, there are some indications thunderstorms will initiate Friday evening, and there will be plenty of instability for them to use as fuel if they do.  Below is a Skew-T diagram representing conditions predicted by the latest run of the NAM (00z 07/15).

This confusing looking graph is basically a plot of atmospheric conditions as air rises.  The red line represents the temperature of the air at that point in the atmosphere.  The green line represents its dewpoint.  The blue line represents a parcel of air which starts at the surface, and its temperature as it rises.  Instability can is represented by just how far to the right of the red line the blue line is. Wind speed and direction at different points in the atmosphere are represented by the barbs on the right.

A couple things stand out in looking at this graph.  The aitmosphere is forecast to be VERY unstable; that rising parcel of air is going to be quite a bit warmer than the surrounding air it encounters as it rises, meaning it should rise uninhibited into the upper troposphere, while its surrounding air tries to sink downward.  This is how thunderstorms form, and combined with the actions of convergence (lift) and divergence (drop) initiated by fronts, it causes the main phenomenons (updrafts and downdrafts) that make them dangerous. The air surrounding a surface based parcel at the "mixed layer" where all the action takes place (between 850 and 750mb, or around 5,000 to 10,000 feet) will decrease in temperature very rapidly as the parcel rises. The difference between temperature at one point of the atmosphere vs. another is called a lapse rate, and these steep lapse rates at this layer usually lead to explosive thunderstorm growth and powerful updrafts that can generate large hail. Winds appear to be forecast to be mainly unidirectional (blowing one direction), but the winds are blowing at a fairly steady rate, which does increase in speed with height.  This allows thunderstorm updrafts to be at different locations than thunderstorm downdrafts.  Without increasing winds, downdrafts can crash down and choke off an updraft, leading to thunderstorm decay and dissipation. With these winds, thunderstorms can last longer, and grow more intense over time.

Keeping all this in mind, I'd say that, given the data, if thunderstorms do form on Friday, the main storm mode would likely be multicell clusters/lines with possible bowing segments, and a main hazard of damaging wind. Large hail would also be possible given the huge drop in temperatures at the mixed layer.  It's still very difficult to say if they will occur, but the conditions do seem right at this point, especially given how high dewpoints at the surface are forecast to rise.  Beyond Friday, there should be another chance of storms on Sunday night and possibly Monday, though it's just too early to tell on either scenario.

More model data will be needed before I'll get a clearer picture as to what's going to happen with the weather this weekend.  There's also the chances of storms forming to our west and influencing the weather prior to Friday. This is definitely one of the more difficult to predict scenarios I've encountered this year.