Area Forecast Discussion
Issued by NWS Northern Indiana
Issued by NWS Northern Indiana
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170 FXUS63 KIWX 150120 AFDIWX Area Forecast Discussion National Weather Service Northern Indiana 920 PM EDT Sun Jul 14 2024 .KEY MESSAGES... - There are periodic chances for showers and storms (20 to 60%) tonight through Tuesday night, with one or more rounds of strong to severe storms possible during this timeframe. - Best chances for severe weather will be on Monday late afternoon into the evening, when there is an Enhanced Risk (level 3 of 5) for severe weather. Damaging winds 60-75+ mph and a few tornadoes possible. - Alongside the severe threat, flooding is also possible in any storms, especially in areas that remain saturated from the remnants of Beryl. - Hot and humid Monday, depending on storm coverage, timing, and cloud cover. Afternoon heat indices mid 90s to near 100 degrees. - Cooler, drier weather is anticipated midweek through next weekend. && .UPDATE... Issued at 919 PM EDT Sun Jul 14 2024 Previous forecast trends were maintained, with only small update item of note to slightly increase PoPs across northwest half of the area late evening into the overnight. Compact convectively enhanced vortex continues to track east across northwest/north central Illinois this evening with ring of convection initiating over the past few hours from eastern Iowa into southern Wisconsin and northern Illinois. Background synoptic shear remains marginal, but RAP analysis suggests a small pocket of 35-40 knot effective shear associated with this small scale vort max. Despite this, convection has been rather loosely organized up to this point with several storm mergers/interactions across NW Illinois and some periodic signs of some mid level rotation due to these mergers. Still expecting these storms to congeal into a more linear feature late this evening/early overnight as they track east. Upstream 00Z RAOBs from KILX/KDVN indicates very strong instability with 3000-3500 MLCAPEs. Earlier stabilizing effects from northern Indiana early day MCS will be eroded by westerly advection of this better upstream instability axis along with eastward progression of some better mid level lapse rates. Some low level CIN is expected to develop this evening locally for mixed layer parcels however, so some question as to the vigor of this convection as it tracks east into a slightly less unstable environment locally. There could be some tendency for more vigorous storms to develop upwind across central/east central IL as northern portion of the storms across southern Great Lakes become more outflow dominant. Some isolated severe threat still appears to be intact west of US 31 locally in the 05Z-08Z period with a period of potential gusts into the 40 to 60 mph range. Hail threat should be tempered with eastward extent as storms congeal later this evening. Locally heavy rainfall will be some concern across the west tonight, especially considering recent heavy rainfall over the past week. && .DISCUSSION... Issued at 231 PM EDT Sun Jul 14 2024 Multiple rounds of storms are expected over the next 48 hours, with one or more rounds being strong to severe. Our CWA is on the periphery of a building ridge in the Plains, and with northwest steering flow, this puts the Upper Midwest and Great Lakes regions in a very favorable zone for storms. Several strong shortwaves that drop into the Midwest in the coming days may help to sustain several rounds of MCSs. SPC has us in a Day 1 Slight Risk (level 2 of 5), a Day 2 Enhanced Risk (level 3 of 5), and a Day 3 Marginal Risk (level 1 of 5). The next 48 hours come with a lot of uncertainty in the coverage, timing, and number of rounds of storms. Alongside the severe threat, flooding is also possible with any of the MCSs. Many locations, especially in the Wabash and St Joseph river basins, have had 5 to 8 inches of rain this week. With saturated ground and elevated rivers, it won`t take much for these areas to experience more flooding. High PWATS of and the potential for storms to backbuild along outflow boundaries has me concerned about flooding potential in addition to the multi-day severe threat. As the initial round of storms today has now exited the area, dry conditions will return for the next several hours before yet another chance for storms. Hi-res models have yet to come to a strong consensus, but it is looking more plausible (medium confidence) that an MCS will develop over eastern IA/northwest IL late this evening and move eastward into our CWA between 10 PM to 12 AM EDT tonight. With ample MLCAPE 2000-3000 J/kg and DCAPE 1000+ J/kg, damaging winds would be the main hazard overnight. The greatest confidence in severe weather is along and west of US 31, as the MCS should weaken as it pushes eastward overnight. There is increasing confidence in severe weather Monday late afternoon and into the evening. SPC has introduced 30% hatched wind probabilities into the Day 2 Outlook, prompting the need for a Enhanced Risk (level 3 of 5) upgrade across northern Indiana. Another progressive MCS/bow-echo could develop upstream of a much more favorable environment and ride along the highest theta-e axis through southern WI, northern IL, and into northern IN. Strong buoyancy, increasing vertical shear, and favorable thermodynamics (dewpoints low 70s, highs upper 80s to low 90s, MLCAPE of 4000 to 5000 J/kg) could certainly yield a high-end impact damaging wind event. A derecho is possible with organized damaging winds 60-75+ mph and a few tornadoes possible. Heat and humidity build on Monday, with highs in the low 90s and heat indices potentially in the upper 90s to low 100s. Heat Advisory headline could be needed. Uncertainty in storm coverage, timing, and lingering cloud cover could all result in temperatures being lower than what is currently forecasted. An upper level trough will pivot through the Great Lakes on Tuesday, resulting in chances for storms ahead of and along the front. A continental, cooler airmass from Canada will sink south midweek, resulting in seasonable highs in the 70s, a stark decrease in humidity, and dry conditions. && .AVIATION /00Z TAFS THROUGH 00Z TUESDAY/... Issued at 755 PM EDT Sun Jul 14 2024 Temporary stabilizing effects from earlier convective complex have allowed for mainly dry conditions through early this evening. However, stronger westerly upstream flow will overspread northern Indiana over the next several hours in advance of next upstream mesoscale convective vortex across NW Illinois. This will allow for recovery in instability via advection. Convection has already initiated with this feature with moderate to strong instability across these areas. This showers and storms should encounter some instability gradient as they reach into northwest Indiana, but localized stronger shear/forcing with the small scale vort max should allow for another round of thunderstorms at the terminals in the 04Z-08Z timeframe. Strongest gusts to around 40 knots expected at KSBN with these potential storms with brief IFR vsbys possible. These showers and storms should wane late tonight, with next upstream possible convectively enhanced short wave arriving toward end of this valid period with more thunderstorm chances. Given a good deal of uncertainty at that forecast distance in this pattern, will omit mention in TAFs for now. && .IWX WATCHES/WARNINGS/ADVISORIES... IN...Flood Watch through late Monday night for INZ012>015-020-022- 103-104-203-204. OH...None. MI...Flood Watch through late Monday night for MIZ078-177-277. Air Quality Alert until midnight EDT tonight for MIZ177-277. MARINE...None. && $$ UPDATE...Marsili DISCUSSION...Johnson AVIATION...Marsili