Area Forecast Discussion
Issued by NWS Indianapolis, IN
Issued by NWS Indianapolis, IN
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646 FXUS63 KIND 040629 AFDIND Area Forecast Discussion National Weather Service Indianapolis IN 229 AM EDT Thu Jul 4 2024 .KEY MESSAGES... - Thunderstrom complex expected to move through from mid morning to early afternoon. Isolateds damaging winds possible - Expected lull in between rounds of storms during the mid afternoon and evening for 4th of July - Dry conditions and less humid late Friday through Sunday && .SHORT TERM (Today and tonight)... Issued at 227 AM EDT Thu Jul 4 2024 This Morning and Early Afternoon... 06Z IR satellite imagery is beginning to show the first glimpse of the development on a nocturnal MCS over eastern KS into western MO. This will continue to strengthen as it moves eastward supported by diabatic pressure drops and an anomalously moist surface layer. This MCS will progress relatively quickly, but will still take another 8- 10 hours before it reaches central Indiana tomorrow morning. Prior to its arrival, upper level cloud cover will help mitigate some diurnal cooling keeping the nocturnal inversion shallow. As this system approaches from the west, deepening low pressure in the MCSs wake will help increase southerly low level flow, destabilizing the shallow CIN quickly. This increase in low level flow (30-40kts of EFF Bulk Shear) along with elevated surface moisture (72-75 dew points) will also aid in convective organization, likely keeping the MCS fairly strong even in a non- climatologically prone time period. There is still some uncertainty on the specifics of where the MCS will pass through, but generally, the axis of precipitation should begin somewhere between Terre Haute and Evansville as it enters Indiana. As mentioned, the shear and elevated moisture should be able to keep the MCS organized, with any areas directly in its path under a possible severe threat (Terre Haute to Evansville and locations eastward). There is some potential for the severe threat to dive south along the LLJ axis, with central Indiana left in the weaker upshear component. If the MCS remains organized over central Indiana, the main threat will be damaging winds within wet microbursts, but an isolated QLCS tornado can not be ruled out due to the increased near surface southerly flow (0-1km SRH 80-120 m2/s2). Worst case scenario would be a more widespread wind threat along the UDCZ in the event surface temperatures and near surface lapse rates are able to increase rapidly prior to MCS arrival. Warm frontal like processes along the differential heating boundary with a modest SW LLJ should also allow for a greater precip shield north of the MCS, providing 0.5-1.0" of much need rain along and north of the I-70 corridor. The MCS should exit central Indiana by around 18Z +/- an hour. One non-precipitation item of note, temperatures pre-MCS arrival over far southern central Indiana may be able to get into the upper 80s late this morning into the early afternoon. If this does occur, our daytime high will occur earlier than typical. Late Afternoon through Evening... Following the MCS passage, local pressure gains should induce subtle ridging in the low to mid levels during the afternoon and evening hours ahead of an approaching low over the Upper Midwest. With this in mind, despite a frontal boundary near the Ohio River, mid level drying and subsidence may work to inhibit convective development during the latter half of the day tomorrow. Still, an isolated shower or thunderstorm cannot be ruled out, if an updraft is able to withstand the drying mid levels. Generally, skies should remain cloudy over much of central Indiana through the mid afternoon. As the mid levels begin to dry, a few pockets of sunshine are possible in the early evening hours. Given greater certainty of cloudy conditions, afternoon temperatures will be kept in the low 80s throughout central Indiana, but areas with greater evening sunshine may be able to reach the mid 80s. Tonight... After dusk, any remaining isolated showers and thunderstorm should begin to wain as diurnal cooling related CIN takes over. This should keep conditions fairly dry through the early overnight hours until a short wave and associated low approaches from the west. Another convective system is likely to develop in the warm sector of the short wave this evening over the Missouri Valley, of which will be the primary threat for rain over central Indiana late tonight. HREF members are still widely varied on strength and location of this MCS, leading to high uncertainty in the thunderstorm forecast for central Indiana as of this forecast update. && .LONG TERM (Friday through Wednesday)... Issued at 227 AM EDT Thu Jul 4 2024 On Friday, stronger synoptic-scale forcing arrives as closed midlevel low passes to our north and DCVA progresses across Indiana. Lower tropospheric warm/moist advection will occur early in the morning until the cold front arrives midday to early afternoon. A couple scenarios are possible. 1. the warm advection regime continues to feed convection that is ongoing to our south from the previous diurnal cycle, tempering new development further north at least somewhat. 2. an upstream convective cluster evolves and moves into Indiana during the morning and may result in greater rain coverage further north into central Indiana than scenario #1. Still, the northward extent will be limited somewhat by model indicated instability, which tapers with northward extent. Frontal convection during the early afternoon should be across the eastern portion of the area given currently modeled frontal timing. A post-MCS airmass would be more overturned and less conducive than a scenario where less convection is present during the morning. Refinements to the forecast will occur later today as the mesoscale details become more clear. Cooler (around 5F negative anomalies) and slightly less humid conditions are expected Saturday in the wake of the front. Midlevel height rises and strengthening southwesterly flow ahead of the next shortwave trough should bring temperatures back to near climatology on Sunday, but the subtropical moisture band isn`t particularly pronounced or anomalous. Forcing and moisture become sufficient for some precipitation by Monday, but even then the warm conveyor belt is somewhat fractured from deep subtropical moisture connection. It`s possible models are under-representing moisture given Beryl`s remnants at lower latitudes, augmenting trough strengthen and moisture which models can struggle with fully capturing latent heat release. So, if the forecast early next week trends, it may be toward a wetter scenario. Model spread grows a little into the middle part of next week, but the general pattern favors very modest air mass change in the wake of the shortwave trough. Deep moisture will be shunted away so rain chances will be low, even with one or more perturbations rotating through the broader trough. Temperatures should be near or just below early July climatology which is max 85 and min 66 at IND. && .AVIATION (06Z TAF Issuance)... Issued at 109 AM EDT Thu Jul 4 2024 Impacts: - Widespread rain and thunderstorms arrive after daybreak - Isolated convection this afternoon but confidence is low - MVFR ceilings possible withing rain/storms Discussion: Overnight will be quiet although some localized shallow ground fog is possible mainly at KBMG. Confidence is growing in a developing convective cluster late tonight in the mid Mississippi Valley that will track E/NE into the region after daybreak Thursday. Expect a 3- 4 hour window late this morning through the early afternoon with restrictions in visibility and ceilings as rain and storms impact the terminals. Weak ridging develops in the wake of the morning storms for Thursday afternoon which will limit more widespread convective redevelopment through the evening. Storms should remain isolated in coverage with the remnant boundary lingering over the area. Light and variable winds early Thursday will eventually become southwest up to 10kts in the afternoon. && .IND WATCHES/WARNINGS/ADVISORIES... None. && $$ SHORT TERM...Updike LONG TERM...BRB AVIATION...Updike