Area Forecast Discussion
Issued by NWS Indianapolis, IN
Issued by NWS Indianapolis, IN
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801 FXUS63 KIND 042243 AFDIND Area Forecast Discussion National Weather Service Indianapolis IN 643 PM EDT Thu Jul 4 2024 .KEY MESSAGES... - Scattered thunderstorms this evening, a few storms may pose a damaging wind threat in South Central Indiana - Drier overnight tonight with patchy fog - Dry conditions, less humid this weekend...Monday-Thursday expected to exhibit seasonable warmth/humidity with a few rain opportunities && .MESOSCALE UPDATE... Issued at 522 PM EDT Thu Jul 4 2024 Have seen a gradual uptick in isolated to scattered convection as subtle destabilization has occurred over the region and particularly across the southern half of the forecast area. A broad area of showers continues to pivot east across northern counties in an environment lacking substantial instability in the vicinity of a weak surface wave. Had a few reports of funnel clouds within this area of showers earlier over central and northeast Illinois but have not seen anything additional recently. While the potential for severe is still present for another 3 hours or so across south central Indiana...overall trends continue to hint at convection largely remaining below severe levels. While instability has struggled to increase...the presence of the surface wave and remnants of an MCV in the region complicates the convective potential somewhat as both features induce an infusion of low level directional shear that any storms could tap into. Current ACARS sounding at KIND shows this nicely with directional shear present from the surface through 850-900mb. There remains a zone from south of Terre Haute east through Rush County and points south where the combination of slightly higher instability on the northern flank of the gradient interacting with the increased low level directional shear could enable a few cells to briefly intensify to severe levels. Damaging winds with any embedded microburst would be the primary severe weather risk but the level of low level spin also is suggestive of a non-zero tornado threat. There are caveats which support the trend for convection to largely remain subsevere... mainly the presence of a subtle but lingering warm nose aloft and overall very poor mid level lapse rates. Along and north of I-70...convection will be weaker but still expect a steady increase in localized downpours over the next couple hours and perhaps gusty winds. Confidence continues to rise that most if not all of the convection shifting east of the forecast area near or just after 00Z as the surface wave and remnant MCV move out of the region. This will set the stage for dry but sultry conditions for outdoor activities during the second half of the evening. && .SHORT TERM (This evening through Friday)... Issued at 300 PM EDT Thu Jul 4 2024 Latest satellite and radar imagery show convection associated with this morning`s complex of showers and storms pushing off to the east with widespread cloud cover in its wake. Weak low pressure noted on surface observations continues to push eastward into Central Illinois and will work its way into North Central Indiana by this evening. Surface to 850mb thermal gradient exists just south of this low from around Kansas City through Southern Ohio acting as a quasi- warm frontal feature. However the low level environment on either side of the front is very warm and moist. ACARS soundings from IND and STL indicate stability along and north of this boundary with a developing "warm nose" around 850mb, keeping the lowest 1 km agl stable with a marginally unstable and drier environment above it. This warm nose will likely inhibit substantial convective development across Central and Northern Indiana over the next few hours. Thicker cloud cover along and north of the I-70 corridor may also further enhance the S-N thermal gradient across the area through the evening hours. Satellite imagery clearly indicates stability for the northern half of Indiana with a healthy cu field developing in Southern Illinois and Indiana as the environment destabilizes. KSDF ACARS soundings paints a very different picture of the environment along and south of the aforementioned boundary with little indication of an inversion and steep low level lapse rates with CAPE approaching 2500-3000 j/kg. This strong thermal and instability gradient across South Central and Southern Indiana may be the area to watch for convective development going into the evening hours. Southeast flow at the surface with westerly flow aloft, 40-60 kt bulk shear, 30-50kt 0-3km shear, and 100-200 m2/a2 effective helicity may be sufficient for organized convection to develop this evening with damaging winds, isolated tornadoes, and flash flooding as main threats. Confidence is low as guidance typically does not handle situations well that highly depend on micro to mesoscale features. Best threat for these storms looks to be along and south of the I-70 corridor; however if areas further north can destabilize over the next few hours, cannot rule out a stronger storm there as well. Central and North Central Indiana will likely remain on the stable side of the boundary, with isolated, non- severe convection possible, but not likely through this evening. Tonight... After dusk, any remaining isolated showers and thunderstorm across Central Indiana should begin to wain as the atmosphere further stabilizes. This should keep conditions fairly dry through the early overnight hours. Guidance does show additional convection developing along the boundary south of the region; however most of that precipitation will likely also remain along and south of the Ohio River. Central Indiana remains in a weird location, north of the main boundary, but ahead of an approaching low and secondary front to the north and west. Lower confidence exists in the northward extent of convection by the early morning hours tomorrow. Lately guidance has been way overdoing overnight convection and brining it in too fast. So will go with consistency and keep the overnight hours drier. Although with such a saturated airmass and a low level inversion keeping moisture trapped at the surface, would not be surprised to see patch fog again tonight and towards sunrise tomorrow. Added patchy fog across the whole region to account for this chance with the Wabash Valley and lower lying areas and valleys having the best chance for fog development. .Tomorrow... There could be some showers and maybe a thunderstorm ahead of the approaching low and associated "cold" front tomorrow morning. HREF members and RAP guidance shows isolated showers and storms at best during the mid to late morning hours with more widespread convection developing east of the region later in the day. Expect a drying trend through the afternoon hours across Central Indiana with peaks of sun likely and temperatures rising into the low to mid 80s. && .LONG TERM (Friday night through Thursday)... Issued at 300 PM EDT Thu Jul 4 2024 Friday Night through Sunday... Broad yet weak surface high pressure will cross from the central Plains to the Mid-Atlantic this weekend ahead of a slowly- approaching, pronounced mid-level long wave trough over much of the central CONUS. Dry conditions will therefore return to central Indiana as precipitable water values aloft drop by almost two-thirds below today`s near-record levels. West-northwesterly light to moderate breezes Saturday will bring the coolest day of the next week, when widespread upper 70s are expected north of the I-74 corridor, perhaps at the cost of some lingering morning cloudiness. Low to mid-60s minimums will be the rule for both Friday and Saturday nights amid noticeably lower dewpoints. Expect more typical July conditions for Sunday as modest southerly flow returns mid- to upper 80s and seasonably moderate humidity. Monday through Thursday... The next work week should return a touch more warmth and humidity to continue the overall seasonable conditions from Sunday. The central US upper trough will continue to approach during the early week while weakening and probably focusing more of a weak SW flow into Indiana. THis combination of WAA ahead of lower heights should make for a very warm day Monday ahead of milder readings through the mid- week. Probably a few to at times scattered showers around the realm during Monday-Tuesday while the trough is approaching, especially Tuesday if a southern vort could phase into the trough axis while crossing the Midwest. Rainfall amounts would likely be lighter and not contribute to further drought improvement for most locations, but isolated t-storms/downpours would be possible. A trend to mainly dry conditions for the Wednesday-Thursday timeframe assuming the trough pulls across the Midwest on schedule, with isolated afternoon showers possible. Since Indianapolis` last 90F+ on June 22, IND has only reported one day above 87F (88F, July 3)...and the highest temp currently forecasted for the next 7 days is 88F on Monday, 7/8. && .AVIATION (00Z TAF Issuance)... Issued at 643 PM EDT Thu Jul 4 2024 Impacts: - Scattered convection diminishing this evening - Potential for patchy fog and MVFR to IFR conditions late tonight into Friday morning - Chance for scattered showers through mid afternoon Friday - W/NW winds gusting to 20-25kts Friday afternoon Discussion: Ongoing scattered convection early this evening is already moving east of the terminals with any lingering light showers behind it also diminishing. The rest of tonight will be quiet with winds dropping off to light N/NW then going variable to near calm in the predawn hours. With the rain earlier today...fog is an increasing threat late tonight with model soundings also supporting the potential for lower MVFR stratus to expand into the region under a shallow inversion. May even briefly see IFR conditions near or after daybreak Friday morning. High uncertainty on additional rain chances for the first half of Friday ahead of a cold front and will only highlight VCSH mention at all terminals. After 18-19Z...all threats for rain ends with winds shifting to W/NW and becoming gusty into Friday evening. && .IND WATCHES/WARNINGS/ADVISORIES...None. && $$ SHORT TERM...CM LONG TERM...AGM AVIATION...Ryan