Plaincast

Messages

Areas of dense fog possible this morning

Hot and humid conditions expected Sunday into Monday

One or more thunderstorm complexes possible Sunday night and Monday night, with damaging winds and heavy rainfall the primary threats

Discussion

Central Indiana will be within a transition zone early this morning into the afternoon as an upper level trough exits eastward and a broad ridge builds in from the west. Initially, this will promote subsidence through the mid-levels, and in combination with prior day low level saturation will maintain a compressed saturated layer near the surface throughout the morning hours. This saturated layer will likely remain thin enough to avoid drizzle formation, but there may be enough latent heat fluxes over the Wabash valley for pockets of dense fog between 09-12Z this morning.

As stated, central Indiana will remain within this transition zone until the ridge axis nears overnight. This transition will consist of moderate WAA over the area throughout the day. WAA along with weak westerly surface winds should mix out the low level saturated layer during the morning hours, with the remainder of the day consisting of mostly upper level clouds within the strong jet stream and blow off from upstream convection. The resulting temperatures are likely to push into the upper 80s this afternoon, but likely will stay below 90 as the cloud cover dampens the typical daily temperature curve.

Forecast confidence is moderately high during the daytime today with one caveat, a current ongoing convective cluster over NW Iowa. The aforementioned transition zone will be narrow with a deep unstable airmass only a few hundred miles to the west of central Indiana. Although this area of convection is expected to pulsate throughout the night as it pushes to the E/SE, its resulting cold pool should remain consistent and will likely continue to initiate additional convective clusters over northern and central IL this morning.

The storm energy gradient is expected to remain west of the Indiana border as this cold pool nears from the west, likely shifting the overall storm motion southward and keeping thunderstorms to the west of our area. That said, there is a low chance that the cold pool and resulting convection remains strong enough to push into the less favorable atmosphere and is able to clip far western portions of central Indiana. If this does occur, severe wind gusts cannot be completely ruled out, and it would likely be between 14-17Z. As stated, this is a very low threat, but one that should be mentioned nonetheless. The other impact from this convective cold pool would be additional cloud cover and cooler air over the Wabash Valley, of which may lead to slightly cooler temperatures than points to the east.

The aforementioned deep instability will shift eastward with time as the ridge axis moves resulting in 3000+ MUCAPE late tonight through Monday night. Shear will also be plentiful as the jet stream remains aloft, with 35-50kt of effective shear. The combination of high instability and plentiful bulk shear will lead to a potent airmass, but we are still missing one key ingredient; a forcing mechanism. This is where most of the uncertainty resides as the source of initiation for these thunderstorms will be highly volatile and difficult to forecast with certainty more than 6-12 hours in advance with CAMs continuing to struggle to converge on a scenario. The other source of uncertainty will be storm motion. Unlike strongly forced events, storm motion will greatly depend on placement of any developing cold pools and its relation to the ongoing thermodynamic parameters. This could lead to a storm motion range from 320 degrees to potentially 20 degrees over the course of this event. The culmination of highly variable forcing mechanisms and a range in storm motions will lead to a wide envelope of scenarios over central Indiana for tonight through Monday; from little to no convection, to multiple potent convective complexes pushing into the area with damaging wind potential.

As far as chances for precipitation, they will initially remain below 40 percent for tonight, but will slowly increase throughout Monday as the number of convective complexes over the region is expected to increase and a greater source of forcing nears from the north is the form of a cold front. Greater coverage of thunderstorm complexes on Monday will also lead to linger outflow boundaries over the region providing additional sources of lift and therefor higher chances for rain. the front is expected to move through Monday night, ending rain chances late Monday night into Tuesday morning.