085 FXUS63 KLMK 210516 AFDLMK Area Forecast Discussion National Weather Service Louisville KY 116 AM EDT Tue May 21 2024 ...Updated Aviation Discussion... .KEY MESSAGES... * Dry weather and above normal temperatures continue on Tuesday. * Unsettled weather returns Wednesday, with on and off chances for showers and storms through the weekend. * Strong to severe storms possible Wednesday afternoon into Wednesday night, with an increased risk for localized flooding Thursday through Saturday. && .UPDATE... Issued at 854 PM EDT Mon May 20 2024 Another warm, dry evening with temperatures ranging from the mid 70s to lower 80s at this hour. Skies are partly to mostly cloudy, with an area of cirrus associated with an upper level shortwave trough sliding east. These clouds will move off to the east later tonight, leaving mainly clear skies for the early morning hours of Tuesday. No significant weather expected overnight with a very light southerly wind. && .SHORT TERM /THROUGH WEDNESDAY/... Issued at 327 PM EDT Mon May 20 2024 This afternoon, the surface high pressure axis is moving east of the Appalachian Mountains, allowing weak anticyclonic S/SW flow to emerge across central KY and southern IN. Another warm day is underway, as temperatures have already risen into the mid 80s in most locations, with highs expected to range from the mid 80s to around 90 later this afternoon. Reasonable amounts of low-level moisture have supported the development of scattered cumulus clouds, with development generally being suppressed by warm air around 700 mb. Localized convergence, possibly driven by terrain effects, has allowed for a few taller cu to develop into small showers over middle and eastern TN. While a very isolated shower cannot be ruled out across in the Lake Cumberland area this afternoon, most if not all should remain dry, and we'll continue with a dry forecast for now. Another dry night is expected tonight across the region, with the diurnal cumulus field expected to dissipate during the hours around sunset. While additional mid-level clouds are expected to move across southern IN during the evening and early overnight hours, there should still be enough clearing for reasonably good radiational cooling tonight. It is worth noting that some high resolution guidance (e.g., the 12Z HRRR and RRFS) do show a couple light showers developing late tonight on the leading edge of a 925 mb theta-E surge; however, would expect this to be overdone given how well-mixed the sfc-850 mb layer will be this afternoon and evening. Lows should be a few degrees warmer Tuesday morning compared to today, with almost all falling into the 60s. Tomorrow's setup looks pretty similar to today with another mid- level shortwave ejecting into the central Plains as ridging amplifies from the lower Mississippi Valley into the Tennessee and lower Ohio Valleys. Falling heights to the north and west of the region will increase wind speeds tomorrow, with 10-15 mph S/SW winds expected during the late morning and afternoon hours as the sfc pressure gradient strengthens. Once again, hi-res guidance tries to develop a diurnal cu field into a few showers and storms across the Bluegrass region during the afternoon hours; however, convection would be expected to struggle given poor mid-level lapse rates. At this time, will advertise a 10% chance for a shower or storm tomorrow afternoon, though almost all should remain dry. Late Tuesday night, ongoing convection across the mid-Mississippi Valley will begin to approach northwestern portions of the area as the mid- and upper-level shortwave and associated sfc cold front moves closer to the region. Convection will outrun the best instability and shear by sunrise Wednesday morning, with storms generally weakening as they approach southern IN. For most locations, Tuesday night will remain dry, with temperatures remaining mild in the presence of increasing clouds and southwesterly winds. && .LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/... Issued at 327 PM EDT Mon May 20 2024 Wednesday through Wednesday Night... A deep, nearly vertically stacked low pressure system will rotate from northern MN into southern ON Wednesday, with a trailing cold front sagging southeast into the Ohio Valley. A warm, moist airmass will be in place to the south of this boundary in southern IN and central KY, characterized by sfc dewpoints in the mid/upper 60s and PW values near 1.5 inches. HREF members indicate we could have weakening showers and perhaps a few storms ongoing at the start of the period Wednesday morning, but this activity would most likely continue to dissipate. A (relatively) much greater chance for renewed convection exists Wednesday afternoon into the overnight hours. Moderate destabilization appears likely by the afternoon with SBCAPE exceeding ~1000-1500 J/kg. 3-6 km lapse rates will approach 7 C/km over southern IN and northern KY. And with a moist environment and steep low-level lapse rates during peak heating, there is at least some risk for isolated damaging winds/wet microbursts. The strongest updrafts may produce small to marginally severe hail, but the hail potential will be kept in check by the moist profile and marginal (30-35 kt) deep-layer shear. Rain and thunderstorm chances will gradually ramp up Wednesday afternoon into the overnight hours. Exact timing/evolution remains somewhat unclear, but unsettled weather is expected along with warm, humid conditions. Highs will be in the 80s with lows in the mid/upper 60s. Thursday through Sunday... Unfortunately, it still looks like Wednesday's frontal boundary is likely to stall in the vicinity of the Lower Ohio Valley on Thursday as it parallels the W/SW flow aloft. Multiple mid-level shortwave disturbances, some convectively-enhanced, rippling through broad SW flow will keep a chance of showers and storms in the forecast Thursday into the weekend. Some stronger storms will be possible at times, but it is impossible to try to time individual waves of convection at this point. Mesoscale evolution each day will be dependent on the prior day's and night's activity. Moderate destabilization of a warm, moist airmass does look possible each day, but in general, deep-layer shear weakens late in the week. Depending on day-to-day evolution, think there will be an increasing risk for flooding Thursday into the weekend. Slower-moving storms and any cell training will boost the flash flood potential. Each day will not be a total washout, but there will be an increased risk for localized flooding. && .AVIATION /06Z TAFS THROUGH 12Z WEDNESDAY/... Issued at 116 AM EDT Tue May 21 2024 For the overnight period, VFR conditions are expected across the region. Scattered-broken cirrus will push through the region early this morning and then move east of the region by sunrise. Winds overnight will remain out of the south to south-southeast with speeds of less than 4 knots. For the daytime hours, the pressure gradient will strengthen a bit as a low pressure system moves from KS to MN. We'll see winds shift to the southwest with speeds of 8-10kts and gusts of 15-20kts at times during the afternoon hours but these winds will calm down toward sunset. Another diurnal Cu field is expected once again. && .LMK WATCHES/WARNINGS/ADVISORIES... KY...None. IN...None. && $$ UPDATE.......EBW SHORT TERM...CSG LONG TERM....EBW AVIATION.....MJ