Area Forecast Discussion
Issued by NWS Indianapolis, IN
Issued by NWS Indianapolis, IN
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450 FXUS63 KIND 131845 AFDIND Area Forecast Discussion National Weather Service Indianapolis IN 245 PM EDT Sat Jul 13 2024 .KEY MESSAGES... - Periodic shower and thunderstorm chances through Wednesday, with one or more windows of severe threat possible, though uncertain - Hot and humid Sunday through Tuesday, possibly nearing Heat Advisory criteria, particularly Monday - Cooler and much less humid late week into next weekend && .SHORT TERM (This evening through Sunday)... Issued at 245 PM EDT Sat Jul 13 2024 Very warm afternoon ongoing as weak surface high pressure tracks through the region. A subtle upper feature passing just south of the forecast area is generating scattered convection near KEVV otherwise it was dry across the Hoosier state. 18Z temperatures were in the mid and upper 80s. Lot of uncertainty remains with respect to future progression of one of more convective clusters set to develop over the Upper Midwest and eastern Dakotas this evening then track quickly southeast overnight in the northwest flow regime aloft. Potential is there for convection in a weakened state to be approaching northern counties near or after daybreak Sunday but overall this remains a much lower confidence forecast for Sunday. This Afternoon and Tonight For the rest of the afternoon...despite the surface high...SBCAPE values in the 2000-2500 j/kg via the KIND ACARS sounding and mesoanalysis but overall low level moisture is lower than the previous couple afternoons. Within a low shear environment...cannot rule out a stray shower or storm through early evening but the lack of more appreciable moisture and forcing aloft will largely keep a cap on rain chances for the rest of the day. The best chance to see a shower may end up over far southern portions of the forecast area in closest proximity to the weakening upper wave and in an area with deeper moisture. Current satellite imagery shows a more enhanced cu field as well over the lower Wabash Valley and likely have a few small showers between Vincennes and Washington. Any cells will pulse in intensity and be short lived. Locally heavy rain and gusty winds would be primary impacts from isolated convection. Any isolated convection along with the diurnal cu will diminish prior to sunset leaving mostly clear skies through the evening. Dewpoints will begin to creep back up this evening as well making for an increasing muggy feel to the airmass overnight with light winds. Localized fog is a possibility towards daybreak Sunday but the potential for expanding mid and high level clouds associated with convection over the Great Lakes may largely mitigate fog concerns. Sunday As mentioned above...high uncertainty exists for Sunday due to the convective cluster(s) set to develop and dive southeast across the Upper Midwest overnight. Model trends today have slowed progression of the convection and focused it further north through daybreak Sunday keeping activity mainly north of a Chicago to Detroit line. Expect the leading edge of the cloud shield associated with the convection to expand into northern parts of the forecast area Sunday morning but remain skeptical in any organized convection making it as far south as north central Indiana. Of greater interest from a convective standpoint is the back edge of the MCS likely to be over Wisconsin in the morning. With strong surface heating taking place further south even with some amount of convective cloud debris through the morning...think there is a valid possibility of the southwest flank of the overnight MCS restrengthening by early afternoon and riding the instability and theta-e gradients which are poised to sit from northern Illinois southeast across northern Indiana and into northwest Ohio. If storms can redevelop a mature cold pool...could see convection drift into the northeast part of the forecast area through the afternoon carrying a damaging wind threat. Additional isolated convection would be possible further south where BL shear is weaker but within a highly unstable atmosphere. There still remains multiple moving parts to the Sunday forecast and the initiation and evolution of convection to our northwest tonight will be critical in determining rain chances...cloud coverage and subsequently temperatures. Temps...lows tonight will fall into the upper 60s and lower 70s from east to west. Even with morning clouds...should be able to get temperatures into the upper 80s and lower 90s by the afternoon. Any convection that can build south into the forecast area could impact that. With dewpoints back into the lower 70s over much of the area...max heat indices may rise as high as the upper 90s and lower 100s. While this will be oppressive and uncomfortable...these heat indices will remain below Heat Advisory criteria. Experimental NWS HeatRisk product suggests level 3 of 4/major heat health impacts possible across parts of the area Sunday...though it is important to remember that the product remains experimental and is simply one tool to quantify the threat for heat health-related impacts. && .LONG TERM (Sunday night through Saturday)... Issued at 245 PM EDT Sat Jul 13 2024 Model guidance continues to advertise a fairly stark difference between the early part of the coming week and the latter part of the week into next weekend. Early in the week, a broad, seasonably strong upper level ridge centered over the western CONUS, combined with a subtropical ridge over the southeastern CONUS, will produce broad south/southwesterly low level flow, helping to maintain a hot and uncomfortably humid airmass over the region. While airmass convection early in the week is likely to be significantly inhibited by warm temperatures aloft (700 mb temps generally 10-13C), will need to carry some low PoPs in the north Sunday and Monday nights, respectively, as, given the position of central Indiana on the periphery of the larger ridges aloft, upstream organized "ridge-riding" convection may be able to get into the area in these periods, though uncertainty is relatively high given the significant dependence upon prior convection in such regimes. As we get into Tuesday and Wednesday, a shift will occur, with the weakening subtropical ridge helping to allow an anomalously strong (as low as the first percentile on GEFS model climatology) upper trough to dip into the Great Lakes region, driving the tail end of a cold front well south into the CONUS mid to late week. The addition of this frontal and upper level forcing will require higher thunderstorm chances, with likelies merited late in the day Tuesday into Tuesday night as the front itself pushes through the area. Some severe threat will be present at times, primarily in the form of damaging wind risk with any organized convection that can sneak into northern portions of the area late Sunday and late Monday nights, with perhaps a slightly more substantial severe threat Tuesday into Tuesday night as the front passes, given the presence of seasonably strong instability and adequate deep layer shear. SPC outlooks are reasonable, and experimental machine learning guidance has maintained fairly good signal for this threat as well for several days. Hydrologic concerns will require monitoring as well, particularly Tuesday into Tuesday night as the front gradually moves through the area. Antecedent conditions will be relatively wet given recent rainfall (and the potential for additional storms at times early in the week). Precipitable water values will again rise to 2+ inches, which is at or above climatological maximum, even given the fact that we are entering the yearly climatological peak in terms of PWAT. The front will be making steady but not quick progress through the area, and a significant depth of tropospheric flow will be mostly boundary-parallel, setting the stage for storm training risk. Post frontal passage, welcome relief is anticipated, as a significantly cooler and less humid airmass accompanies an unseasonably strong surface high pressure system (GEFS climatological 95-99th percentile for SLP and sub-10th percentile for PWAT). Dry and pleasant weather appears likely late in the work week into the coming weekend. Temperatures are likely to get into the 90s across all or parts of central Indiana Monday and Tuesday (especially Monday), though convection potential introduces some additional uncertainty in this regard. Low temperatures will generally be in the low to mid 70s. Experimental NWS HeatRisk product suggests level 3 of 4/major heat health impacts possible across the area Monday and perhaps in the south on Tuesday, though it is important to remember that the product remains experimental and is simply one tool to quantify the threat for heat health-related impacts - that said, if current forecasts were to verify, traditional heat index criteria will be approached Monday afternoon, with maximum apparent temperature values ranging from 100-105. Post-frontal passage, a drop of 10-20 degrees is expected for both highs and lows, with equivalent decline in dewpoint temperatures as well, making for unusually pleasant midsummer conditions late week into the weekend. && .AVIATION (18Z TAF Issuance)... Issued at 1217 PM EDT Sat Jul 13 2024 Impacts: - VFR through tonight - Low confidence in a few showers or storms approaching from the north Sunday morning Discussion: High pressure over the region will maintain dry conditions this afternoon with scattered diurnal cu and W/SW winds at 5-10kts. Cannot rule out a stray shower or storm late day into the evening but expected coverage does not merit any mention for the terminals. Tonight will be quiet over the region but high uncertainty lingers with respect to robust convective development across the upper Midwest overnight. The activity will grow upscale into a large cluster that will track southeast across the Great Lakes through daybreak Sunday. Recent model trends support convection remaining north of the terminals Sunday morning but mid and high level cloud debris from the convective cluster will likely spread south across central Indiana. Winds will become S/SW at 10-15kts by midday Sunday. && .IND WATCHES/WARNINGS/ADVISORIES...None. && $$ SHORT TERM...Ryan LONG TERM...Nield AVIATION...Ryan