Area Forecast Discussion
Issued by NWS Northern Indiana
Issued by NWS Northern Indiana
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348 FXUS63 KIWX 171720 AFDIWX Area Forecast Discussion National Weather Service Northern Indiana 120 PM EDT Mon Jun 17 2024 .KEY MESSAGES... - There are nearly daily low chances for showers and thunderstorms this week, especially in the afternoon and evening hours. Expect coverage to be widely scattered. - Excessive Heat Warnings and Advisories are in effect due to an extended period of hot and humid conditions. Max heat indices this week will range from around 95 to 105, with the highest values in the warning area and in urban locations. Overnight lows in the 70s will provide little relief from the heat. High temperatures will be in the upper 80s and 90s. && .DISCUSSION... Issued at 522 AM EDT Mon Jun 17 2024 Bottom line up front: Dangerous heat and humidity is expected. Excessive Heat warnings and advisories are in effect. Heat indices through at least Friday look to be in the 95 to 100 degree range for most, with values of 100 to 105 looking more likely in the warning area (and we have higher confidence). The warning continues through Friday at 8 pm EDT, and the advisory will expire at 8 pm tonight. We will re-evaluate day by day for the advisories. Here is our local heat advisory/warning criteria for reference: Heat Advisory criteria is: "A heat index (HI) around 100 degrees or higher. Consider a warning if the maximum HI is going to be between 95 to 100 degrees for more than 4 consecutive days." Warning criteria is HI >105 and a minimum HI >75F for a 48 hour period (or if the advisory criteria continues for more than 4 consecutive days). This is because heat impacts are cumulative- particularly for more vulnerable urban populations. Why Warning vs Advisory? Given that we will have heat indices varying between 95 to 105 degrees Monday through Friday (5 consecutive days), with potential for Saturday to also have HI`s around 100 degrees, I thought it best to upgrade NW Ohio and portions of Northern Indiana to an excessive heat warning. It`s rare to get prolonged heat like this in June, and its the first of the year. I also have the greatest confidence in these locations (around 80% in us getting HI`s in the 100-105 degree range). Even if we don`t get true warning criteria in terms of specific heat index values and it lingers closer to 100 degrees, we`ll have possibly 6 consecutive days of advisory level heat (which verifies our local warning criteria). To make things worse, overnight temperatures largely stay in the upper 60s, low to mid 70s which will provide little to no relief from the heat. I decided on an advisory for the remainder of the area through this evening. I am confident we`ll see HI values around 100 degrees today. However, I am less confident through the remainder of the week how things will work out, particularly Tuesday when we have more potential for precipitation/cloud cover west and central (though pick your model-they are all over the place). I suspect that we will need additional advisories as we go into Wed-Fri, but my confidence wasn`t high enough to continue the advisory all the way through. It will be better to tackle the borderline areas day by day. Flies in the ointment: So what would make this not happen? There are nearly daily chances for showers and thunderstorms because we are in a hot, humid, and unstable atmosphere. Models, even the ECMWF which previously kept us mostly dry, has come to agree that the western edge of the ridge will continue to break down, which makes us susceptible to any little shortwave that moves through. We don`t have a firm handle on the exact placement of convection-as it`s largely impossible to narrow down due to the widely scattered nature of these patterns. We don`t expect any severe weather right now (no SPC Outlooks either), but I`d monitor for updates on that because it could quickly change (as we saw last night with the MCV overperforming all the way across Michiana). With any precipitation chances comes extra cloud cover-which could limit high temps. The other issue could be that models the past couple days have been overdoing the amount of moisture we`ll have. We`ve been mixing out in the afternoon a little better than what was depicted for the past couple of days in earlier forecasts. To account for this possibility in the upcoming week, when doing the forecast I made the assumption that the dewpoints would be closer to the mid-upper 60s to around 71. This might very well be too low, as some of the models have dewpoints as high as 72-75! If we get the higher dewpoints instead of mixing out, we may need to upgrade some of the advisories to warnings, and we`ll be more likely to see those 105 HI`s in the warning area (depending on storm/cloud coverage). If not, we should be in decent shape to verify the warning as long as we get to around a 95-100 HI each day. && .AVIATION /18Z TAFS THROUGH 18Z TUESDAY/... Issued at 120 PM EDT Mon Jun 17 2024 Expansion of a scattered cu field is readily apparent across much of Indiana as convective temperatures have been met in an unstable atmosphere. While VFR conditions are expected to dominate most, if not all, of the forecast there are a few challenges that need to be monitored that are not necessarily captured in the TAFs. Any subtle features passing through the area could easily pop a shower or thunderstorms at either site this afternoon and early evening. The greatest concern at this point is an area of persistent convection across east central IL, approaching KRZL. While trends suggest this may stay just west of KSBN, outflow boundaries from the convection could easily move east and provide lift for additional showers/storms. A few of the models actually key in on the KSBN area for one or more rounds of storms late afternoon into the early evening, but no consistent signals remain. For now have opted to go VCSH at both sites with timing and duration of any shower/storm difficult to capture at this point. While loss of heating will bring any precip chances to an end this evening, a weak disturbance, currently producing widely scattered showers across portions of Kentucky, will drift north late tonight into Tuesday morning. Think there will be at least an increase in mid/high clouds but some question as to chances for showers with this feature. Given the low confidence, have introduced a broken high cloud deck for the time being late tonight and defer to later forecasts for possible inclusion of showers/storms. && .IWX WATCHES/WARNINGS/ADVISORIES... IN...Heat Advisory until 8 PM EDT /7 PM CDT/ Tuesday for INZ005>008- 012>015-020-022-023-103-104-116-203-204-216. Excessive Heat Warning until 8 PM EDT Friday for INZ009-017- 018-024>027-032>034. OH...Excessive Heat Warning until 8 PM EDT Friday for OHZ001-002- 004-005-015-016-024-025. MI...Heat Advisory until 8 PM EDT Tuesday for MIZ078>081-177-277. Air Quality Alert until midnight EDT tonight for MIZ078-177- 277. MARINE...None. && $$ DISCUSSION...MCD AVIATION...Fisher