.SHORT TERM (This Evening through Saturday)...
Issued at 146 PM EDT Fri Sep 20 2024

A broken line of convection continues to develop along a weak frontal boundary across the lower Great Lakes
To this point
development southwestward along the boundary back into northwest Indiana and northeast Illinois has been a struggle...with warm air aloft and a capping inversion between 850 and 700mb serving as deterrents. Despite clouds moving across the forecast area from earlier showers...18Z temperatures had warmed into the mid and upper 80s over much of the region.

It has been two weeks since there has been any sort of convective threat to talk about...and the aforementioned frontal boundary to our north will provide at least the opportunity for some locations across central Indiana to see some much needed rain later this afternoon into the evening. Remain a bit pessimistic on how extensive the convective coverage will look and will discuss this in greater detail below.

This Afternoon and Evening

Short range trends through the rest of the afternoon will be critical in determining how convection evolves into the forecast area through the evening. While the signal is there in most of the model suite for scattered convection later...the CAMs have not been overly helpful in the details so far today. While the presence of a narrow axis of deeper low level moisture and an overall increase in BL shear support convective development...the negative factors also present are likely the main reason for the struggles from the individual CAMs in addition to the mishandling of low level moisture
These negative factors

1) An abundance of dry air in the mid levels and a lingering cap in the 850-700mb layer 2) poor lapse rates 3) impacts from dry antecedent surface conditions

In a nutshell...still feel the overall model suite is overdoing convective coverage for late afternoon into the evening. The available instability and shear alone is suggestive of convective development
but anticipate that storms will be in a messy
chaotic multicellular mode that likely remain scattered at best due to the dry air aloft and the poor lapse rates. A rise in pops late this afternoon into the evening is warranted but have a hard time justifying much higher than 30-40 pops at this point and focused primarily across the northern half of the forecast area. Lightning and brief heavy rainfall will be the primary impacts to Friday evening activities but there remains the possibility for locally gusty winds with any collapsing cells.

Late Tonight through Saturday

The frontal boundary will migrate south tonight and eventually wash out across the southern part of the state by Saturday morning.
Cannot rule out a few showers accompanying the boundary south overnight but dry conditions are expected across the forecast area prior to daybreak Saturday. Model soundings are supportive of a sharp nocturnal inversion developing in the predawn hours and with light flow and the potential for an infusion of moisture from any rain later today in the near surface layer...the fog potential is elevated towards daybreak Saturday and focused especially across northern parts of the forecast area.

Any fog will burn off quickly Saturday morning with ridging aloft and a weak surface high reestablishing over the region. Expect yet another hot day with dry conditions and mostly sunny skies in tandem with light E/SE flow. Mid and high level cloud coverage may increase subtly late day Saturday...signaling a welcome transition to a more unsettled and cooler regime Sunday into the first half of next week.
More on this in the Long Term discussion below.

Temps...lows will slip down into the lower 60s over northern counties where a light northerly flow will develop in wake of the front. Further south expect mid and upper 60s. Low level thermals support upper 80s and lower 90s on Saturday but this will put a wrap on the recent stretch of hot weather with cooler temps to follow into early next week.

.LONG TERM (Saturday Night through Friday)...
Issued at 146 PM EDT Fri Sep 20 2024

Saturday Night Through Monday.

The pattern continues to trend more active for late this weekend into early next week as a frontal system associated with a broad trough across southern Canada pushes through the Midwest. Latest model runs and ensemble guidance suggests that the frontal passage is likely late Sunday into Sunday night with scattered to numerous showers out ahead of the front. A secondary wave just behind the main wave then looks to move through on Monday with the potential for heavier showers. The QPF for the initial event is likely a bit overdone with the very dry antecedent conditions but with saturation from the first event, some appreciable rainfall looks likely by Tuesday but there remains quite a bit of uncertainty as to where the higher axis of rain will fall.

There is high confidence in much cooler air behind the front with highs in the upper 70s to 80 on Monday followed by the low to mid 70s through the rest of the week.

Tuesday Through Friday.

Forecast confidence begins to fall off by Tuesday as models struggle on how quickly to push the Monday system through with at least a decent chance for wet conditions to continue into Tuesday before conditions begin to dry out for the middle of the week. There will remain some low chances with weak impulses within the northwesterly flow aloft, but the best forcing will be well gone by late Tuesday into Wednesday.

Focus then shifts to the low potential for more appreciable rainfall towards the end of the work week with models showing there may be a seasonably strong closed low to the west of Indiana which would advect significant amounts of Gulf moisture into the Ohio Valley.
This scenario looks unlikely based on how ensembles are handling the low, but there is a decent number of individual members that show the heavy rain outcome and will need to be monitored going forward.
Looking beyond Friday into the weekend, tropical moisture looks to continue to advect northward which could keep the active pattern going into the following week.

.AVIATION (18Z TAF Issuance)...
Issued at 1221 PM EDT Fri Sep 20 2024

Impacts:

- Isolated to scattered convection late afternoon and evening possibly impacting KIND and KLAF

- Fog developing late tonight through daybreak Saturday...especially at KLAF

Discussion:

Axis of mid and high level clouds diminishing as it drifts across central Indiana early this afternoon. Skies will quickly shift back to mostly clear for the afternoon before an increase in clouds coinciding with scattered convective development expands in from the northwest by late afternoon. Trends continue to support potential for convection to potentially impact KIND and KLAF in the 21Z to 01Z timeframe ahead of a weak frontal boundary. Confidence remains lower in any convection at KBMG and KHUF and will keep any precip mention out at those sites at this time. W/SW winds will generally be around 10kts through early evening but could see briefly gustier winds within any convection that passes over or near a terminal.

High pressure and dry air will reestablish overnight with light and variable flow likely to go near calm in the predawn hours. There is support for fog development at the outlying terminals and especially at KLAF that will likely last through daybreak. Expect mostly sunny skies Saturday with light southeast flow developing.

IND WATCHES/WARNINGS/ADVISORIES
None.