Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 06/19/21

Area Forecast Discussion
National Weather Service Louisville KY
937 PM EDT Fri Jun 18 2021 .Mesoscale Update... Issued at 937 PM EDT Fri Jun 18 2021 An axis of 2000-3000 J/KG of CAPE still resides along and north of the I-64 corridor this evening, with explosive convection ongoing across south central IN into SW Ohio. Initial supercellular structures are transitioning to bowing segments as storms evolve. Of immediate concern is a cold pool dominated bowing segment coming out of the Cincinnati area and heading for our NE Bluegrass counties of Scott and Harrison. May have to warn on those if the damaging wind threat persists into our area. Of secondary concern is an isolated supercell currently over Brown, Bartholomew, and Jackson counties. This storm is heading southeast an an environment characterized by ~200 M2/S2 of effective SRH. LCL`s are a little high for a more concerning tornado threat, but something that will have to be monitored as it approaches Jefferson county Indiana over the next 1 to 2 hours. SPC went with a Severe Thunderstorm Watch until 07z for our northeast CWA counties. Most confident about the severe threat in our far north and east counties in the watch, with less confidence on southward extent into the overnight. Nevertheless, the right moving supercells and/or SE moving bowing segments could carry the severe threat a bit farther south than what models had shown, hence the extra buffer area. For what it is worth, a recent AMDAR sounding showed a strong cap at 800 mb over Louisville, so don`t think the threat will extend that far south, at least for the evening and first part of the overnight. Updated products already out. Will be monitoring. Previous Update... Issued at 650 PM EDT Fri Jun 18 2021 Latest mesoanalysis still shows a strongly unstable environment characterized by 3000+ J/KG of surface based CAPE along and north of I-64. However, latest AMDAR soundings all show a notable subsidence inversion around H8. This feature is expected to hold pretty steady through the evening hours, to the point that convection isn`t anticipated across our CWA. It would take a pretty notable trigger to overcome the capping, and given the lack of an outflow boundary or other mesoscale feature agree with the the hi res models that we`ll likely see a dry evening. Will continue to monitor just in case. After Midnight, there is enough data that suggests our NE CWA could catch the tail end of the convective complex that fires over central IN and translates southeastward. The placement of the Flash Flood Watch from day shift still looks good, but overall don`t have a lot of confidence in how much rain will actually fall out of this feature. If it stays progressive doubt we`ll see enough rainfall to get flash flooding, however if the southern end anchors against the deep layer flow and continues training occurs then numbers closer to the current WPC QPF may be realized. Will leave things as is at this time to see how things play out. Still think our best chance of any stronger or severe storms will arrive around or just after sunrise in our northern CWA when a convective complex drops in from the NE. Hi res models suggest that this will be a cold pool driven gusty line of storms by this point, with the main threats being widespread gusty winds/isolated damaging winds along with heavy rainfall and lightning. && .Short Term...(This evening through Saturday) Issued at 313 PM EDT Fri Jun 18 2021 Rich low-level moisture, including surface dew points in the low and mid 70s, is feeding northeastward ahead of a cold front stretching from Michigan to Kansas. This moisture is beneath an EML reaching from the central Plains to the Ohio Valley with local soundings showing large CAPE above a low level cap. Forcing ahead of the cold front and a 5H speed max over the upper Mississippi Valley have led to elevated thunderstorm development along a mesoscale boundary from northeast Illinois into northwest Indiana this afternoon. This activity is not the main show, but it is indicative of the primed atmosphere in this region for thunderstorm development. Tonight convection should increase in coverage and intensity, initially to our north, as the night goes on. Low-mid level flow will become more unidirectional with time, becoming general west-east after midnight. A potent low level jet will develop from northeast Missouri to northeast Indiana. This west-east jet should line up with the convection with thunderstorms heading east but backbuilding westward into the jet and diffluent 1000-500 thicknesses upstream. The corridor of maximum surface theta-e tonight will be from central Illinois to central Ohio, with a 850mb theta-e ridge axis over the northern half of the Ohio Valley in pool of high 850mb RH, further enhancing the idea that storms will, at least initially, move from west-to east from central Illinois through central Indiana to Ohio. 850-300mb MCS steering flow does start the evening in a WNW-ESE orientation, but overnight becomes more west-east. 0-6km bulk shear values increase to 50kt after midnight from central Illinois into central Indiana. This, plus the above, suggest a large bow echo MCS may develop and sweep from west to east (SPC SREF derecho composite is progged to be around 2). Shear and instability decrease to the south, however a strong cold pool and sharp outflow boundaries can cause this type of system to develop farther southward than the data might initially suggest, so storms are still in the forecast tonight for southern Indiana and north central Kentucky, with the best chance of the strongest storms likely between midnight and dawn. Freezing heights suggest small hail will be possible (marginally severe in the strongest updrafts). Sounding progs show that the storms may be elevated, but only slightly, so we still can`t rule out some stronger gusts making it down to the surface. Another issue to consider is heavy rain. The main corridor of heavy rain will, again, be to our north...especially if backbuilding does develop. Surface dew points will be in the 70-75 degree range and precipitable water values will be over two inches. So, after coordinating with IND and ILN, will issue a Flash Flood Watch for areas from Scottsburg to New Castle. Tomorrow the synoptic surface boundary will stay well to our north, while the tropical system moving northward through Mississippi will stay well to our south. The models don`t print out much convection here between these two systems. But, with a warm and slightly unstable atmosphere in place and possible outflow or differential heating boundaries from early morning convection and clouds, will continue with a low PoP. It will be another hot and muggy day with highs in the upper 80s to lower 90s and dew points of 65-70. .Long Term...(Saturday night through Friday) Issued at 257 PM EDT Fri Jun 18 2021 By Saturday night, rain showers are expected to have come to an end. Most model guidance has the quasi-stationary front to our north, over northern Indiana, keeping precipitation farther north, while the low pressure system coming out of the Gulf of Mexico keeps its associated moisture to our south. The trend is drying the CWA out through Monday. High temperatures on Sunday are expected around 90, a few degrees cooler on the eastern side of the CWA. On Monday, a deep surface low, moving east-northeast through the Great Lakes, will pull a trailing cold front through southern Indiana and central Kentucky. With the front stretching southwest to northeast as it passes through the CWA, The northwestern side of the CWA will experience precipitation earlier in the day, probably during the late morning hours. Showers will expand south and eastward in the following hours. If this timing holds, expecting a line of stronger storms and heavier precipitation to form in the late afternoon to evening hours. This line would continue to the east until out of the CWA by early Monday night. Looking at the severe weather potential, model soundings show very strong instability (over 3,000 J/kg) in the southern half of the CWA, but deep layer shear (0-6 km) is around 30 knots. DCAPE values reach around 850 J/kg, and there is some strong low level storm relative helicity at times. These values don`t guarantee severe weather, but dew points in the 70s and precipitable water values over 2" in unstable environments with some forcing do cause heavy rainfall. Expect gusty winds, lightning, and heavy rainfall with the line. As the event nears, timing and details will come into better focus. This is a strong June cold front. Monday`s high temperatures around 90 degrees will be reduced to the 70s on Tuesday as northerly winds keep temperatures suppressed, but on Tuesday night, high pressure quickly moves past the region. Return flow will slowly lift temperatures into the mid 80s by Friday. By Thursday or Friday, another shortwave pushing along upper flow will bring the next round of precipitation to the area. Guidance still hasn`t found agreement, so kept the blend in the forecast. && .Aviation...(00Z TAF Issuance) Issued at 737 PM EDT Fri Jun 18 2021 IMPACTS: Showers and thunderstorms possible Saturday morning. DISCUSSION: A W-E oriented boundary across central IL/IN will be the focus of convection this evening with a line a showers and storms expected to move south through central KY Saturday morning. Potential for heavy rain and strong convective winds exists with this convection that could reduce VIS to MVFR and possibly to IFR in the heavier storms. Latest guidance suggests the best time frame for storms will be from roughly 19/12Z through 19/15Z affecting HNB, SDF, and LEX. Another round of showers and thunderstorms is possible after 20/00Z. CONFIDENCE: Medium confidence in timing of thunderstorms. && .LMK WATCHES/WARNINGS/ADVISORIES... IN...Flash Flood Watch through Saturday morning for INZ078-079. KY...Flash Flood Watch through Saturday morning for KYZ032-033. && $$ Mesoscale...BJS Short Term...13 Long Term...KDW Aviation...CG