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SEVERE THUNDERSTORM WATCH for Much of Central Indiana UNTIL 9PM

Thunderstorms this afternoon and evening with severe weather likely. Damaging winds are the primary hazard. Secondary hazards of large hail, tornadoes, and flooding also possible.

Rain free Tuesday-Thursday amid near to slightly below normal temperatures/humidity. with northerly breezes Tuesday, Wednesday

Discussion

Satellite and radar imagery shows a large complex of showers and thunderstorms over the Chicago area as of 2pm. This cluster of storms is likely going to impact our area to some extent this afternoon and evening. At the surface, a distinct axis of greater instability (3000-4000 J/Kg MLCAPE) extends NW to SE through most of central Indiana. Surface winds are generally southwesterly to the west of this boundary. Along it, light and variable conditions are noted. To the east, a southeasterly wind is observed.

Aloft, strong southeasterly shear between 45-55kt extends over most of Illinois, Indiana and Ohio. This mean shear vector is almost exactly parallel to the instability axis. Further to the north, a potent mid-level vort max is shown which was the catalyst to ongoing convection. Now that storms have initiated, the immediate mesoscale concern is how storms evolve coming into our forecast area in the next few hours.

One scenario is that the storms develop a strong cold pool and begin propagating southeastward along the instability axis. This is plausible since the shear vector points southeastward along this axis, and cold pool shear balance will be easier to achieve. In this scenario, severe winds will be the primary hazard. However, embedded supercells, as have been seen over Chicago this afternoon, would be capable of all hazards including large hail and tornadoes. Additionally, a QLCS tornado threat may develop if the low-level jet ramps up significantly this evening.

A second scenario is similar to the one above, but the low-level jet ramps up a bit earlier and storms propagate more southward owing to greater upshear growth. This would send the storm cluster over our western forecast area with similar hazards to scenario one. All in all, the main difference is timing and influence from the low-level jet. Guidance shows the low-level jet increasing substantially around 02z to between 30-40kt at 925mb. However, the storms may already be through our area by that point. Renewed development on the western edge could occur if a residual cold pool lingers over the area. Flooding could occur if storms begin redeveloping over the same areas as the initial round of activity.

After tonight, the vort max and associated surface low depart eastward with a cold front sweeping through by dawn. Northern winds usher in cooler and drier air with temperatures falling towards normal for the remainder of the week. Another vort max may arrive this weekend, embedded within weak westerlies aloft, bringing precipitation chances back by Friday. Given the weak flow and likewise weak vort max it would seem prudent to keep precipitation chances capped at chance (40 percent). Model uncertainty is high, and such weak/subtle features can be hard to pin point this far out.