Area Forecast Discussion
Issued by NWS Indianapolis, IN
Issued by NWS Indianapolis, IN
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631 FXUS63 KIND 200208 AFDIND Area Forecast Discussion National Weather Service Indianapolis IN 1008 PM EDT Thu Sep 19 2024 .KEY MESSAGES... - Above normal temperatures continue into the weekend - Conditional risk for isolated showers/storms Friday evening - Cooler temperatures early next week with increasing rain chances && .FORECAST UPDATE... Issued at 951 PM EDT Thu Sep 19 2024 Forecast is in good shape this evening. Some scattered high clouds were moving across central Indiana. Light winds and mostly clear skies have allowed temperatures to cool quickly in favored areas once again. With thicker clouds not arriving until later tonight and mainly in the western forecast area, tweaked low temperatures down a bit in favored cold areas in the eastern half of the area. Otherwise just adjusted hourly forecasts to match current trends. Looking ahead to Friday morning, still looks like any lingering light showers/sprinkles from dying convection to the west will dissipate before moving into central Indiana. && .SHORT TERM (This evening through Friday)... Issued at 121 PM EDT Thu Sep 19 2024 We remain on the northeast periphery of subtropical ridging with above normal temperatures. The primary near-term focus has been adjusting relative humidity for fire weather interests. Although winds are too light for significant fire spread concern, antecedent dry conditions and drought have resulted in increasing initial attack-type fire activity, with fire magnitude requiring further assets unlikely. Lower tropospheric moisture model errors are common in these regimes where mixing processes are the primary driver for RH. Even the 5th percentile of blended model guidance is substantially more moist than a subset of high resolution models, which have been too aggressive with drying through the diurnal mixing process. So we`ve compromised with the official deterministic forecast by splitting the difference. This seems like the best decision given trends in ACARS vapor soundings of subtle lower tropospheric moisture advection to offset drying through the mixing process. We will continue to monitor observations through the rest of the day. A weak midlevel perturbation and associated warm advection will be enough for an increase in midlevel clouds and perhaps virga later tonight into early Friday. There is a conditional convective scenario late tomorrow as trailing weakly baroclinic surface boundary will progress into Indiana during the evening. A modest deeper plume of moisture will arrive but the details of this are where the greatest uncertainty lies regarding convective potential. The lower troposphere may not moisten sufficiently for true surface-based convection until after peak diurnal heating, and/or result in relatively high-based convection within narrow CAPE profiles. Whatever convection can develop at the end of the diurnal peak may be able to sustain into the evening as moisture advection increases. In a reasonable worst case scenario, richer moisture supports isolated to scattered convection along the front near the Illinois border, generally near and north of I-74, that would drift further east during the evening. Hodographs are elongated due to strong west- northwest flow aloft and would favor storm-scale organization and a marginal wind/hail threat. The most likely scenario is that convection will struggle to develop due to modest moisture at best. Regardless, the footprint of rainfall will be minimal and insignificant relative to the needs given the ongoing drought conditions. Slight enhancement of southwesterly lower tropospheric flow coupled with relatively strong mixing during the afternoon should again result in ~11-13 degree positive 2-m temperature anomalies Friday. Our normal high temperature is 78 degrees at IND. The record high for the 20th of September is 94 set in 1940. && .LONG TERM (Friday night through Thursday)... Issued at 121 PM EDT Thu Sep 19 2024 Prograding subtropical ridge continues to influence our weather through Saturday with ~10-12 degree positive 2-m temperature anomalies. Flow at the crest of the ridge will become increasingly perturbed the latter half of the weekend into early next week. This could couple with a more established subtropical moisture plume for some periodic rain potential during this period. Forcing is expected to be fairly weak and mostly driven by broad/weak moisture regime, so the multi-model mean rainfall totals are around 0.50-0.75 inches late Sunday through early Tuesday. In the current pattern, models have had a high bias, however. Temperatures are expected to be closer to climatological normals early next week as the ridge progresses and clouds and precipitation chances increase. From mid-week onward, there is growing spread among ensemble members on the synoptic-scale pattern. Clusters show fairly even distribution of model camps and little confidence on if the western ridge becomes more amplified or how progressive and deep the eastern trough is. Regardless, it does appear that this period of the long term forecast will feature generally dry conditions with mean northerly flow and temperatures near or just below mid-September climatology. Day 8-14: There is a trend in the medium-range multi-model ensemble mean for increasing high-latitude positive height anomalies. This should generally translate to near or just below normal precipitation for our region, along with above normal temperatures. && .AVIATION (06Z TAF Issuance)... Issued at 1008 PM EDT Thu Sep 19 2024 Impacts: - Isolated convection possible late Friday afternoon and evening - Winds 210-230 degrees with gusts less than 20 knots Discussion: Would not rule out some late afternoon and evening thunderstorms as a cold front moves in. However, chances are too low to put in the TAFs. Otherwise, good confidence in VFR flying conditions through the TAF period. && .IND WATCHES/WARNINGS/ADVISORIES... None. && $$ UPDATE...50 SHORT TERM...BRB LONG TERM...BRB AVIATION...MK