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Area Forecast Discussion
Issued by NWS Louisville, KY
Issued by NWS Louisville, KY
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294 FXUS63 KLMK 171447 AFDLMK Area Forecast Discussion National Weather Service Louisville KY 1047 AM EDT Sat Aug 17 2024 .KEY MESSAGES... * Strong to severe storms will be possible this afternoon and evening if morning convection doesn`t limit diurnal warming. The main threats are damaging winds and hail. * Scattered showers and storms will be possible Sunday afternoon, but severe weather is not expected. Lingering isolated showers will be possible on Monday, mainly east of US 127. * Dry weather is expected Tuesday through Friday. && .UPDATE... Issued at 1046 AM EDT Sat Aug 17 2024 So far this morning, isolated convection has moved across southern Indiana and into north central Kentucky, with latest radar returns showing this activity moving east of the I-75 corridor at this time. Earlier in the morning, outflow from these storms produced gravity waves which appeared to be trapped within the low-level stable layer, and areas of low clouds developed along these waves. By this time, a lot of that energy has mixed out as we have begun to destabilize; however, a lingering area of additional clouds is noted along the Ohio River, in addition to an unrelated area of low clouds across east central KY. Otherwise, mostly sunny skies are present across much of the area at this hour, allowing temperatures to quickly warm into the upper 70s and lower 80s. We should continue to warm into the upper 80s and lower 90s by this afternoon, with current satellite showing relatively little cloud cover that could stop the ascent in temperatures. As far as the shower and storm potential for this afternoon is concerned, preliminary 12Z guidance has trended a few hours later for convective initiation. While cameras from across the area do show nascent cu developing, SDF ACARS sounding in addition to 12Z BNA sounding shows elevated LFCs just above 850 mb, so it should take a while for things to destabilize enough to get additional rounds of convection. Additionally, faster mid-level winds will swing around the base of the trough later this afternoon, contributing additional shear and possibly a bit of vort advection which could add broad lifting. Finally, there is the sfc cold front which should begin to drop through the region later this evening. Any of the aforementioned features will assist in convective initiation, and it is probable that there will be multiple waves of showers and storms later today. Given latest guidance, would key on 18-20Z for afternoon convective initiation, with the main window for showers and storms expected between 20-03Z (4-11pm EDT). Have added PoPs across southern IN, though this would mainly be associated with the cold front later this afternoon. With poor mid-level lapse rates expected, and latest WV imagery showing quite a bit of mid-level dry air (higher DCAPE), it still looks like damaging winds would be the main severe threat, with hail being a secondary threat. Updated products with tweaks to afternoon and evening PoPs were sent a few moments ago. && .SHORT TERM /THROUGH TONIGHT/... Issued at 335 AM EDT Sat Aug 17 2024 The radar is quiet now, but another chance of thunderstorms is expected to return today. To our north, a stacked low pressure system sits over the Great Lakes, and as we head through the short- term this system will only slide from the western side of the Great Lakes to the eastern side. This will place the strongest winds on the southern side of the trough directly over southern Indiana and central Kentucky. Closer to the surface, the system`s cold front which is really weak and slow moving, making it hard to pinpoint its location at times, is approaching the CWA from its current location near the top of Illinois. The speed of this front and its passage is slower than what was expected a few days ago. It is now on track to get to southern Indiana this afternoon. Currently, ahead of the front, precipitable water values are near an inch, but with the approaching system increasing the pressure gradient over the Lower Ohio Valley, 10-20 mph southwest winds will lift precipitable water values to around 1.7" ahead of the front this morning. Dew points will be in the low 70s, so we have the moisture in place to develop some convection. Looking to our west, over southeast Missouri and central Illinois, there is some ongoing convection which is producing cloud debris, and some CAM model guidance has some more cells developing over southern Illinois over the next few hours. If these cells do develop they will produce more cloud cover and likely work east into the CWA. This convection could produce lightning and some gusts, but wouldn`t be severe. It could however produce enough cloud cover to limit the strength of any afternoon convection. . Most model guidance places the bulk of afternoon convection in the Bluegrass region, but this could also extend to near Louisville and south through the Lake Cumberland area. Hodographs show unidirectional marginal deep layer shear up to around 30 knots. The driving force for any convection today will be the instability, and this is where early convection and cloud debris could determine the chances for severe weather later today. If cloud cover helps to limit surface warming, MLCAPE values will likely be limited to around 1,500 J/kg, but if cloud cover is more limited, MLCAPE values could increase to around 2,000 J/kg, and where DCAPE values could approach 1,200 J/kg, wind gusts could be more of a concern. Some hail is also a possibility. As the front slowly works south through the CWA, winds will veer towards the west and west-northwest as they drop below 10 mph. Early in the night, lines of convection will likely move to the southeast as drier conditions follow from the northwest. Behind the front, an isolated shower can`t be ruled out, but most will remain dry. && .LONG TERM /SUNDAY THROUGH FRIDAY/... Issued at 335 AM EDT Sat Aug 17 2024 Renewed convection is expected on Sunday near the left exit region of an upper jet streak advancing from the Midwest to the lower Ohio Valley. Chances will be highest east of Interstate 65 closer to the axis of a 5H trough over the Appalachians. Mid-level lapse rates and instability will be weak, and deep layer shear will be low with a fairly uniform NW flow throughout the column. Nevertheless, seasonal atmospheric moisture and convective temperatures reached in the 80 to 85 degree range will lead to development of scattered popcorn showers and thunderstorms. No severe weather is expected. High temperatures will be in the lower and middle 80s. On Monday the upper trough over the Appalachians will shift only slightly eastward, with topographically assisted diurnal convective development expected over eastern Kentucky. A few showers and possibly a small low-topped thunderstorm or two could percolate as far west as the Interstate 75 corridor. Afternoon highs will only be in the upper 70s to around 80. The Tuesday through Friday period still looks dry as a large dome of high pressure slides from Ontario to the East Coast. This will initially provide southern Indiana and central Kentucky will cooler than normal weather, with highs Tuesday around 80 and lows Tuesday night in the 50s. Each subsequent day after Tuesday temperatures will moderate as the high moves by, and by Friday afternoon temperatures will be back in the middle and upper 80s with dew points in the 60s. && .AVIATION /12Z TAFS THROUGH 18Z SUNDAY/... Issued at 722 AM EDT Sat Aug 17 2024 As mentioned earlier convection is beginning to develop over southern Illinois and southwest Indiana. This activity is expected to move east towards HNB and SDF in the coming hours. Effects will be limited, but cloud cover and cooling effects from the showers/storms could help limit or delay afternoon thunderstorms at area TAF sites. Still believe LEX and RGA are the sites most likely to see thunderstorm activity during the afternoon hours, but these early showers could determine how much activity is seen later. && .LMK WATCHES/WARNINGS/ADVISORIES... KY...None. IN...None. && $$ UPDATE...CSG SHORT TERM...KDW LONG TERM...13 AVIATION...KDW