Area Forecast Discussion
Issued by NWS Indianapolis, IN

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113 FXUS63 KIND 242333 AFDIND Area Forecast Discussion National Weather Service Indianapolis IN 733 PM EDT Mon Jun 24 2024 .KEY MESSAGES... - Thunderstorms possible late tonight and Tuesday morning - Tuesday will be the warmest day of the week - More thunderstorms possible late Tuesday into early Wednesday; severe storms possible - A brief respite from heat and humidity late in the week && .SHORT TERM (This evening through Tuesday night)... Issued at 225 PM EDT Mon Jun 24 2024 A challenging convective forecast starts tonight and lasts through the short term period, with one or more MCSs or MCS remnants potentially impacting central Indiana. Mesoscale details will dictate timing and coverage of thunderstorms. See analysis below including potential scenarios. Models show moisture surging eastward into the area late tonight. NAM-based guidance appears a little overzealous with the magnitude of the ~850-mb moist layer/warm nose, hence the very strong buoyancy depicted. In reality, moisture advection should drive at least modest buoyancy such that convection would sustain, albeit probably in a weakened state. The upstream buoyancy magnitude later today is quite impressive and almost certainly to result in a cold pool- driven system. Shear magnitudes are marginal but sufficient. It would probably be the upshear propagation component along the western flank of the cold pool that would give central Indiana the best chance of being impacts late tonight through Tuesday morning. Uncertainties on how this evolves are linked to subtleties with the lower tropospheric thermodynamics, and cold pool depth/magnitude, primarily. Thus, forecast updates will be needed once upstream convection matures and the picture becomes more clear later this evening. Another note about convective potential tonight. There is a minor signal within high-resolution models of warm advection-driven convection occurring at the leading edge of aforementioned moisture surge. A strong capping elevated mixed layer will likely prevent deeper robust convection from occurring, but weaker convection rooted in the midlevels is possible during the predawn hours. Again, coverage and intensity with this should be limited, but it would be separate from the conditional remnant MCS mentioned above. There is a conditional chance of heat indices approaching Heat Advisory thresholds across southwest portions of central Indiana tomorrow afternoon. The marginality, along with uncertainties of remnant cold pool and midlevel clouds, and/or diurnal convective redevelopment preclude highlighting this concern at this time. By Tuesday afternoon, a couple scenarios are possible with regards to convection. 1. Diurnal redevelopment would favor any residual cold pool/differential mixing zones spawn from morning convection. These storms would occur in fairly weak shear but potentially strong buoyancy and could pose a damaging downburst threat. 2. Morning convection continues along residual cold pool flanks and may tend to propagate upshear, westward. This may be limited by warm capping elevated mixed layer. In a reasonable worst case scenario this could result in localized heavy rain and flooding. The next shortwave trough is still well west during the peak diurnal cycle, but should drive another MCS. It may be late in the evening or overnight before it reaches Indiana. We`ll still be on the southern periphery of stronger westerlies and higher shear values, but instability should be supportive of at least a low end severe threat with wind being the primary hazard. Details on this evolution and associated convective risks will be refined in subsequent forecasts. Quick disclaimer about use of CAMs in this pattern. Oftentimes, CAMs struggle with weakly forced but unstable environments. The details on the magnitude and footprint of the cold pool may not be modeled well, and this is a primary driver in MCS intensity and longevity, as well as forward propagating speed. Use individual CAM output carefully and consider the spectrum of outcome that the environment could support. && .LONG TERM (Wednesday through Monday)... Issued at 225 PM EDT Mon Jun 24 2024 Wednesday`s forecast is highly conditional on impacts of Tuesday night convection. The shortwave trough axis doesn`t cross until later in the day, but richer moisture and instability could be overturned by robust convection Tuesday night leaving relatively cool/stable conditions Wednesday. In this scenario, diurnal regeneration of convection would be to our south. Nevertheless, even in this scenario, robust convection may still be ongoing during the diurnal minimum, fed by a westerly low-level jet and moisture influx. We would have to watch this scenario closely during the morning as tendency for upshear propagation along a composite surface boundary within such a moisture rich environment could lead to localized flooding. Multi-model ensemble mean shows a dampening trend with prograding mid-upper ridge to our south for the latter half of the week. So, unlike this last recent trough/ridge progression, once cooler and drier cP air mass arrives late Wednesday, it should last into early Friday with below-climo temperatures along with less humidity. Temperatures will increase to near or just above late June climatology Friday afternoon into Saturday as southwesterly flow strengthens preceding the next shortwave trough. The window for rain chances with this next system is between Friday night and Saturday, with some timing differences in deterministic models and individual ensemble members. The pattern looks similar to the last couple of systems, however, with primarily frontal forcing amidst typical summertime instability south of stronger westerlies. One or more clusters of convection may impact central Indiana during the above mentioned time window. Another push of continental air is expected Sunday into early next week. During the week of July 1st, the pattern doesn`t change substantially. One or more progressive low-amplitude shortwaves will pass just to our north and we`ll be on the periphery of the ridge. Some indication within the ensemble mean data of a building ridge and more amplitude pattern during this period supporting above normal temperatures. && .AVIATION (00Z TAF Issuance)...
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Issued at 733 PM EDT Mon Jun 24 2024 Impacts: - Conditional chance of thunderstorms KLAF/KIND early Tuesday - Additional low thunderstorm chances at all sites Tuesday afternoon and evening Discussion: Shallow fog is not expected to redevelop tonight given stronger winds and mechanical mixing. VFR conditions should prevail outside of thunderstorms. Thunderstorm coverage and timing confidence is low. There is a low-mid range chance of thunderstorms approaching KLAF and KIND Tuesday morning. These storms may stay northeast of TAF sites and redevelopment may occur during the afternoon (end or just after TAF period) near the TAF sites. Expect amendments given the uncertainty that is typical with weakly forced convective events in the summer. For now, handled low potential with PROB30.
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&& .IND WATCHES/WARNINGS/ADVISORIES... None. && $$ SHORT TERM...BRB LONG TERM...BRB AVIATION...Updike