Area Forecast Discussion
Issued by NWS Indianapolis, IN
Issued by NWS Indianapolis, IN
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676 FXUS63 KIND 250519 AFDIND Area Forecast Discussion National Weather Service Indianapolis IN 119 AM EDT Tue Jun 25 2024 .KEY MESSAGES... - Thunderstorms possible Tuesday morning - More thunderstorms possible Tuesday PM into early Wednesday; severe storms possible with this round - Growing high heat threat for SW central Indiana tomorrow - A brief respite from heat and humidity late in the week && .FORECAST UPDATE... Upstream convective initiation along an advancing boundary continues to trend slower and further north, leading to greater uncertainty on when precipitation will reach central Indiana tomorrow. There are a few signals towards isolated to scattered convective development along a weak low level wave out ahead of the boundary late tonight into tomorrow morning. Any rain associated with this initial round will likely remain over northern portions of the region, with the bulk of the central Indiana thunderstorm threat now likely to be delayed til the early afternoon tomorrow. This delay in thunderstorm onset could cause a few forecast changes for tomorrow. First, later onset may allow for greater surface based instability to develop and therefor better chances for a damaging wind threat with DCAPE values higher. Second, the decreased cloud cover and lack of evaporative cooling could allow for temperatures over S/SW portions of central Indiana to push into the mid 90s. If temperatures do reach the mid 90s, dew point values over SW central Indiana in the low 70s will create dangerous heat conditions. Due to the uncertainty of precipitation onset, no headlines will be issued at this time. However, in subsequent updates, a close eye will be kept on this growing high heat concern. && .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 (06Z TAF Issuance)... Issued at 119 AM EDT Tue Jun 25 2024 Impacts: - Conditional chance of thunderstorms KLAF/KIND before 12Z - Additional thunderstorm chances at all sites this afternoon and evening - Wind gusts over 20kt mid morning through the afternoon Discussion: Low confidence forecast due to high uncertainty in convective development early this morning and its impact on convection later today. Current thunderstorms southwest of Chicago should move ESE and remain northeast of the TAF sites, but some additional development to the southwest in more unstable air remains possible. Outflows and cloud remnants from these storms may have an impact on additional thunderstorm development later today. Given the low confidence, continued with VC or PROB30 at all sites. Outside of convection, VFR conditions are expected. && .IND WATCHES/WARNINGS/ADVISORIES... None. && $$ UPDATE...Updike SHORT TERM...BRB LONG TERM...BRB AVIATION...50