101 FXUS63 KLMK 272259 AFDLMK Area Forecast Discussion National Weather Service Louisville KY 659 PM EDT Sat Mar 27 2021 ...Updated Aviation and Mesoscale Discussion... .Mesoscale Update... Issued at 655 PM EDT Sat Mar 27 2021 Monitoring two areas of convection at this hour that will have impacts for the overnight period. The first is along an old stationary front/outflow boundary that stretches from northeastern TX into northern TN. The second is along a cold front currently stretching from central IL into southeastern OK. Showers and storms along the stationary front/outflow boundary have already drifted into southern KY at this hour. A quick look at the environment/mesoanalysis reveals that instability is at a relative minimum along and east of I-65, while it increases steadily as you head west. This is largely due in part to weaker mid level lapse rates from clouds/convective debris earlier in the day that drifted into central/eastern Kentucky. Storms that have also pushed east of the I-65 corridor in southern Kentucky have also tended to weaken and fall apart, which makes sense given the lack of 'fuel' storms have to work with to the east. Over the next several hours, we will see showers and storms increase in coverage as they fire off along the stationary front/outflow boundary to our south. Latest high-res model runs have shown a slight northern trend with the heaviest QPF axis, and now have it crossing into portions of south-central and eastern Kentucky. The flash flood watch was expanded earlier in the day to account for this potential, and so far looks good. Will monitor for any westward expansion if trends continue, though think it's current placement will handle the bulk of things. The severe weather threat remains complicated overnight. As the cold front approaches from the west, we could see instability slightly increase across the region as a ~50kt LLJ noses into western and central Kentucky and increases low level moisture/Td's. But, it's possible that convection to our south in TN/AR/AL could disrupt some of that moisture transport northward and hamper storms currently along the cold front. Some of the latest HRRR runs show this scenario playing out with the convection along the cold front weakening as it pushes into southern IN and central KY due to very weak instability. Perhaps a more concerning signal to watch will be near the Lake Cumberland region, where an organized cluster or line of storms coming out of TN with access to limited instability but a lot of shear could result in a low-end wind and tornado threat. In summary, think the best chances for flooding and severe weather will be across southern KY tonight into early tomorrow morning, though portions of central KY and southern IN could see some isolated strong to severe storms as well. Most activity looks to push east of the region by sunrise. && .Short Term...(This evening through Sunday) Issued at 250 PM EDT Sat Mar 27 2021 ...Slight Risk for Severe Storms Tonight... Low pressure is analyzed over eastern Iowa this afternoon, and a warm front remains draped across the Tennessee Valley. Persistent, moderate isentropic lift and moisture transport across this boundary has fueled numerous showers and storms across TN today. The persistent convection has produced 0.50-1.00 inches of rain across southern KY, including some areas with relatively low FFG (<2" in 3 hours). Given the arrival of record PW values tonight and the likelihood of additional convection, will expand the Flash Flood Watch slightly. The sprawling convection across TN certainly seems to be impeding destabilization to the north over central KY. Occasional precipitation, plenty of clouds, and rain-cooled outflow pushing north will likely continue to affect the northward extent of appreciable CAPE this afternoon and evening. HRRR and other hi-res models keep convection to a minimum over central KY through sunset. Tonight, a positively-tiled upper level trough swings over the Upper Midwest and mid-Mississippi Valley. This finally helps induce a stronger low-level response by 06-12z Sunday with sfc cyclogenesis over Michigan. The southwesterly LLJ steadily increases in intensity this evening and overnight ahead of the approaching trough. Currently have 30 kts at 850 mb over Mississippi and southwest TN, but values of 40-50 kts will develop over the area by 03-06z tonight. Between 00-03z this evening, the best chance for scattered storms is in southern KY within the warm conveyor belt. Around and after midnight, convection will increase in coverage as the cold front begins to approach from the west. A narrow plume of MLCAPE >= 500 J/kg seems probable ahead of the cold front, but the degree of destabilization remains the biggest limiting factor in the overall severe threat. A high shear, low CAPE environment will likely emerge ahead of a quasi-linear convective system with damaging winds as the primary threat. There is also a risk for QLCS tornadoes, which are typically very quick to develop and as quick to dissipate. The main time frame of concern is roughly 11 pm - 6 am EDT, but instability may already be nil by 6 pm (depending on prior development and evolution upstream). Still like the Slight Risk from SPC, and it's more of a Marginal threat from Lexington northward in the Bluegrass region where moisture return and destabilization are both weaker. Showers continue over the eastern forecast area Sunday morning, with dry conditions expected area-wide in the afternoon. Sunday looks windy and cool with temperatures generally in the 50s. .Long Term...(Sunday night through Saturday) Issued at 235 PM EDT Sat Mar 27 2021 To start the extended period, expect to find upper level trough exiting off to the east as upper level ridging moves across the central CONUS. At the surface, area of high pressure will drift across the OH Valley Monday and move just off the East Coast by Tuesday morning. Mostly dry weather will prevail Monday and most of Tuesday. High temperatures on Monday will remain in the upper 50s to lower 60s and then warm into the upper 60s and lower 70s for Tuesday as we move into deeper southerly flow on the backside of the high. By Tuesday evening, a large upper trough will be progressing across the central CONUS with an associated surface low rotating northward into the Hudson Bay. Expect trailing N-S oriented cold front to push through the OH Valley Tuesday night into Wednesday, bringing increasing chances for precipitation. Compared to tonight's system, the influx of moisture ahead of the front Tuesday night is expected to be weaker, with PWATs currently progged to top out in the 1.0 - 1.2 inch range. GEFS and EPS members suggest QPF values ranging from T - 1.0 inch across the area with mean values in the 0.2 - 0.4 inch range which corresponds well with WPC QPF. In the wake of the cold front, surface high pressure will build in from the northwest and dominate our weather through the end of the work week. WPC Cluster Analysis shows good agreement between models on this with all four clusters suggesting dry conditions for Thursday and Friday. Temperatures will remain cooler on Thursday under steady northerly flow and CAA, with highs topping out in the 40s. Additionally, low temperatures Thursday and Friday morning are forecast to drop into the 20s and may see some frost. Temperatures will then gradually moderate into the weekend with highs reaching into the 60s again by Saturday. && .Aviation...(00Z TAF Issuance) Issued at 658 PM EDT Sat Mar 27 2021 A cold front will move through the region overnight, and showers/storms are expected to form ahead of it. Cigs/Vis will likely drop into the MVFR range with isolated instances of LIFR/IFR in heavier showers/storms. Gusty, variable winds are also possible within the vicinity of thunderstorms. The precipitation should push east of the region after sunrise, but MVFR cigs will persist into the early afternoon hours before finally beginning to lift and break up. && .LMK WATCHES/WARNINGS/ADVISORIES... IN...None. KY...Flash Flood Watch through Sunday morning for KYZ073>078-081-082. && $$ Mesoscale...DM Short Term...EBW Long Term...JML Aviation...DM