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Area Forecast Discussion
Issued by NWS Indianapolis, IN
Issued by NWS Indianapolis, IN
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887 FXUS63 KIND 151807 AFDIND Area Forecast Discussion National Weather Service Indianapolis IN 207 PM EDT Mon Jul 15 2024 .KEY MESSAGES... - Thunderstorms with damaging wind potential late this evening and tonight - Flash flooding possible across portions of the area tonight through tomorrow - Pleasant weather with cooler temperatures and lower humidity Thursday into the weekend && .SHORT TERM (This evening through Tuesday)... Issued at 207 PM EDT Mon Jul 15 2024 An active convective period continues with challenges regarding thunderstorm timing and intensity, inherent to uncertainty with mesoscale details. It appears that thunderstorm formation this afternoon will be limited or non-existent across central Indiana, and a round of thunderstorms will arrive late this evening with damaging wind potential into the night. We are also concerned about flash flooding potential tonight through part of tomorrow. See meteorological analysis for more details. On the synoptic scale, we remain on the southern periphery of midlevel westerly flow. Perturbations within the flow are likely significantly augmented by diurnal convective blow-ups. Modeling of this process requires mesoscale precision in the initial conditions that isn`t always possible, and can spiral into chaos quickly. This is the reason for the diversity we`re seeing among the CAMs with regards to placement and timing of convection. It is important not to focus on any one specific model in this pattern. Our three main concerns as far as weather impacts are: 1. Dangerous heat 2. Severe thunderstorms with damaging wind 3. Flash flooding In regards to the near-term heat, through early evening temperatures and dew points will be high enough to meet traditional Heat Advisory criteria (105F heat index) across portions of central Indiana. In the latest ACARS vapor sounding and short term model guidance, it appears that moisture depth is sufficient to preclude mixing of drier air and meaningful fall dew points through the diurnal peak. Southwesterly breeze and some cloud cover from diurnal cumulus are components that will keep the Wet Bulb Globe Temperature risk category contained slightly, though still "high". Thunderstorm development today should be isolated at most given residual subsidence-enhanced capping. Evidence of this can be seen in laminar flat look to cumulus field in visible satellite imagery all the way back into Illinois. Water vapor channel imagery shows core of the subsidence moving east early this afternoon. It`s possible the stronger capping may exit before peak diurnal period sufficient for a few convective cells, especially near any residual mesoscale boundaries, but this is low probability. Upstream over Iowa and closer to the influence of a weak midlevel impulse, diurnal convection will develop later this afternoon. Given the magnitude of instability, thunderstorms should become quite robust. This will occur at the southern periphery of stronger westerlies aloft and sufficient shear values for a forward- propagating MCS to evolve. The downstream environment would favor approximately 30-40 mph east-southeasterly system motion. Moderate shear would probably support trailing stratiform rain quickly evolving and enhancing the cold pool, perhaps more than modeled. Thus, confidence in an MCS impacting a good portion of central Indiana late this evening and overnight is fairly high. Wind damage is the primary threat and there is at least a low chance of significant wind damage, especially earlier in the night before it begins to outpace the better parameter space. Northwest portions of central Indiana, including Newport, Lafayette, Crawfordsville, and Kokomo have the highest potential for wind damage, tapering somewhat with southeastward extent into the Indy metro and eastward later in the night. Flash flood concerns today and tonight are multi-faceted. Given recent rainfall (1-2 inches and locally higher over the past 72 hours) even a quick-moving forward-propagating MCS with limited duration of heavy rates could have some flash flood concern. These systems tend to be quite efficient rainfall producers in a short amount of time, and could easily exceed latest RFC 1-hour flash flood guidance of around 1.50 inches across the northern third of central Indiana. This would yield marginal/low-end flooding or flash flooding, most likely. Of greater concern is a relatively low probability scenario later tonight on the western flank of MCS cold pool, and how it interacts with the low level jet. It appears back-building and training are possible across a focused area that is difficult to pin down at this time. Warm cloud depth is seasonally large with model indications of rich moisture in this layer. In fact, overall precipitation efficiency may be enhanced by an east-west band of mid-upper level moisture. Uncertainties remain on where a mesoscale convective flash flooding scenario could potentially evolve, but the potential magnitude despite spatial coverage combined with aforementioned more widespread low-end threat justifies a Flood Watch. Uncertainties grow further tomorrow on the severe and flooding potential. This will largely depend on where the remnant MCS cold pool resides during the afternoon once diurnal destabilization occurs. The synoptic front will still be lagging to the northwest so another active convective day seems likely, largely driven by mesoscale details that we can`t resolve with precision at this time. There may be an ongoing flood/flash flood threat early with residual convection, given indications of modest instability remaining and the aforementioned reasons for potentially locally heavy rainfall amounts. In addition to flooding, moderate instability developing later in the day but with marginal deep layer shear should mean sporadic damaging wind potential from loosely organized multicell clusters is the main severe threat. Again, this is conditional on mesoscale details, as one plausible scenario would hold this threat primarily to our south. There is some potential for dangerous heat across far southern portions of our forecast area tomorrow. We will not issue a Heat Advisory at this time due to uncertainties on how far south the cold pool from overnight convection will be, residual midlevel clouds, and potential ongoing convection or development before peak heating. Will reevaluate tonight and early tomorrow based on trends with convection. && .LONG TERM (Tuesday night through Monday)... Issued at 207 PM EDT Mon Jul 15 2024 Wednesday, it appears that the synoptic front will be across our south or just south of the area. We`ll continue low probabilities but confine them a little further south than the previous forecast. Drier/cooler continental air mass will lag some until the amplifying synoptic trough axis moves through late Wednesday. The air mass change will be most noticeable Thursday continuing into the weekend before gradually modifying Sunday into early next week. Moisture returns then, but the synoptic flow pattern is rather quiescent. Organized precipitation appears unlikely but we may enter a scenario next week with diurnal isolated to scattered convection daily. After a period of 5-10F negative temperature anomalies relative to mid- July climo, temperatures should return to near climo next week. For the day 8-14 day period there`s not a strong signal of above or below average precipitation or temperatures at this time. && .AVIATION (18Z TAF Issuance)... Issued at 1247 PM EDT Mon Jul 15 2024 Impacts: - Southwest winds gusting to 20-25kts this afternoon - Restrictions possible within convection late this evening through early Tuesday - MVFR ceilings possible Tuesday morning - Southerly wind gusts to around 20kts Tuesday Discussion: Pockets of VFR cu with mid level clouds lingered over parts of central Indiana early this afternoon with gusty southwest flow. A mix of sun and clouds will persist through the afternoon with occasional gusts up to 20-25kts before diminishing this evening. Confidence continues to grow in a substantial convective cluster developing over the upper Mississippi Valley early this evening and tracking E/SE overnight. While the details are not fully in focus yet...these storms are likely to impact the terminals from north to south overnight with strong winds from variable directions and torrential rainfall that will produce restrictions through early Tuesday. High uncertainty for Tuesday morning as convection may be ongoing in parts of the area. Model soundings support potential for MVFR ceilings to develop through the morning with wind gusts resuming from the south. && .IND WATCHES/WARNINGS/ADVISORIES... Heat Advisory until 8 PM EDT this evening for INZ021-028>031- 035>049-051>057-060>065-067>072. && $$ SHORT TERM...BRB/Ryan LONG TERM...BRB AVIATION...Ryan