Area Forecast Discussion
Issued by NWS Louisville, KY
Issued by NWS Louisville, KY
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458 FXUS63 KLMK 271042 AFDLMK Area Forecast Discussion National Weather Service Louisville KY 642 AM EDT Fri Sep 27 2024 .KEY MESSAGES... * Widespread moderate to heavy rain and windy conditions today as the remnants of Helene move through the Lower Ohio Valley. * Potential for 3 to 5 inches of rain through early Saturday morning. Minor river flooding may develop in the lower Green River. * Wind gusts of 45 to 55 mph are possible this afternoon with higher gusts possible over eastern Kentucky and southern Indiana. && .SHORT TERM /THROUGH SATURDAY/... Issued at 350 AM EDT Fri Sep 27 2024 Synopsis...TC Helen has moved inland and it is currently moving to the northeast across the Southeast US. Binary interaction with the broader cut-off upper low located over the western section of the Tennessee Valley will drag the decaying tropical system counter- clockwise over the region and towards the Lower Ohio Valley. The rapid translational speed of the low pressure center and its potential track will allow the strong dynamics of the right semicircle to transition across the forecast area today, enhancing moisture advection and windy conditions. Today...Persistent moisture influx from TC Helen rainbands stretching along the retrograding frontal boundary is supporting widespread moderate rainfall this morning and will most likely continue to do so the rest of the period. Although PW values will surge above 2 inches and depth of warm cloud layer will exceed 10kft, latest HREF solution (27/0Z) shows heavy rainfall evenly distributed across the region and very low probability of 1-hr QPF greater than 1 inch at any given moment. As a result, still expecting minor flooding issues, especially in low-laying areas and poorly-drained areas with slight risk of flash flooding. IN addition, Flood Watch still in effect until Saturday morning with slight spatial modification as another tier of counties was added to the north after collaboration with surrounding offices. As for river flooding, the points along the Green River Basin (Rochester and Woodbury Upper) have around a 20 percent chance of reaching minor flood stage on Saturday. Biggest change with this forecast update is the issuance of a High Wind Warning (HWW) for portions of the Bluegrass, northern Kentucky and southern Indiana. Latest model runs (ECMWF, GFS, HREF) have indicated greater than 60 percent of attaining wind gusts of 58 mph or more late this morning and particularly during the afternoon. ECWMF and HREF have these kind of winds extending farther south while the strongest winds in the GFS ensemble member runs are more constrained to the north of the CWA. The difference can be probably explained by differences in the low pressure track and how the associated wind field reacts to the binary interaction. That being said, model data and further collaboration with surrounding offices support issuing a HWW from late this morning and until late around 28/0Z. Last but not least, the forecast took into account the presence of moist adiabatic lapse rates and persistent cloud cover, but seems to be the case that 50-knot low level momentum could mix down during the afternoon with a drying trend noticed in the PW and cloud fields in both the ECMWF and HREF models. Tonight...It will remain rainy and windy but strongest wind gusts will fall below High Wind Warning and even Wind Advisory criteria. && .LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/... Issued at 350 AM EDT Fri Sep 27 2024 Saturday - Tuesday... The remnants of Helene are expected to wobble across the Ohio River Valley through the weekend, gradually weakening as it does so. As a result, we`ll keep periods of rain showers (categorical pops) in the forecast Sat/Sun, and then taper to lingering chances on Monday, and a slight chance east of I-65 by Tuesday. Saturday will carry an additional .5" to 1.25" of rainfall, with Sunday amounts dropping to between a tenth and a quarter inch. Negligible amounts less than a tenth of an inch are then expected for spotty areas Monday and especially Tuesday. Only real concern for any lingering or additional flooding concerns will be on Saturday for locations that get another inch or so. FFG values and overall saturated grounds likely won`t want to handle much more rainfall by then. Much will depend how much falls on Friday, and how well we handle that rainfall to determine whether any lingering flood threat carries into Saturday. May have to contend with a river basin or two getting into action or minor flood should the overall rainfall amounts of widespread 4-5" verify for an event total. If so, The Green River basin would be the most likely spot for some river points to get into action or Minor Flood. Small diurnal ranges due to heavy cloud cover and periodic precipitation will also be a continuing theme through the weekend with highs on Saturday only around 70. Sunday highs are slightly milder in the low and mid 70s, and then mid to upper 70s by Monday. Lows in the low and mid 60s expected each night. Tuesday Night - Thursday... Surface high pressure and dry zonal flow aloft take hold through mid week. May even start to get some SE CONUS upper ridging building in toward late week. As a result, expect a dry stretch of weather with highs back up into the upper 70s and around 80 degrees. && .AVIATION /12Z TAFS THROUGH 18Z SATURDAY/... Issued at 639 AM EDT Fri Sep 27 2024 Impacts/Confidence: - High confidence in persistent IFR ceilings and visibilities throughout the period. - High confidence in gusty winds today with 35+ knots gusts this afternoon at LEX, RGA, SDF, and HNB. Discussion...Persistent rain from the periphery of TC Helen has moved in from the south bringing with it IFR CIGS and decreasing VIS. Current forecast reasoning is that IFR ceilings will predominate for most, if not all, the TAF period while IFR/MVFR visibility will eventually set in as the heavier shower activity impacts the terminals. Surface observations have show winds rapidly increasing this morning (especially at LEX). Peak wind gusts will occur late this morning and during the afternoon. Strongest wind gusts (above 40+ knots) are likely along the northern terminals (including LEX and SDF) with widespread values between 25 to 35 knots elsewhere. By tonight, winds will diminish in response to substantial weakening of the remnants of Helen but still sporadic gusts of 20 to 30 knots are possible before midnight. In addition, winds will shift from the east to the south given the low pressure track across southern Kentucky. && .LMK WATCHES/WARNINGS/ADVISORIES... KY...High Wind Warning from 10 AM EDT /9 AM CDT/ this morning to 8 PM EDT /7 PM CDT/ this evening for KYZ029>043-045>049- 054>057-065>067-077-078. Flood Watch through Saturday morning for KYZ023>029-038>041- 045>049-053>057-061>067-070>078-081-082. Wind Advisory until 2 AM EDT /1 AM CDT/ Saturday for KYZ023>028-053-061>064-070>076-081-082. IN...High Wind Warning from 10 AM this morning to 8 PM EDT this evening for INZ076>079-083-084-090>092. Flood Watch through Saturday morning for INZ089. Wind Advisory until 1 AM CDT Saturday for INZ089. && $$ SHORT TERM...ALL LONG TERM...BJS AVIATION...ALL