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Area Forecast Discussion
Issued by NWS Jackson, KY
Issued by NWS Jackson, KY
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870 FXUS63 KJKL 032324 AFDJKL AREA FORECAST DISCUSSION National Weather Service Jackson KY 724 PM EDT Wed Jul 3 2024 .KEY MESSAGES... - Sultry weather continues through Friday. Afternoon heat indices may reach near 100 in some locations. - A potential for rain arrives tonight and lasts at least into Friday night. A few thunderstorms on Thursday and Friday could produce locally heavy rainfall and strong wind gusts. - A cold front finally pushes through as we move into the weekend, with dry and somewhat cooler and less humid weather arriving. && .UPDATE... Issued at 711 PM EDT WED JUL 3 2024 The early evening update raises PoPs along Interstate 64 and points north in accordance with more CAMS indicating convection impacting this area this evening into the overnight. Most of these CAMS indicate that the current activity will move into northern parts of the forecast area later this evening, generally weakening as they do so especially daytime heating diminishes. Other changes were minimal and were to blend in latest observed temperatures with the forecast for the next several hours. && .SHORT TERM...(This evening through Thursday night) Issued at 430 PM EDT WED JUL 3 2024 Late this afternoon, an upper level ridge of high pressure was centered across the southeastern Conus and extended into the Northeast Conus and west to TX. Meanwhile an upper level low was centered over northern Canada with a broad trough to the south and southwest into the Upper MS Valley to Four Corners vicinity and another upper level ridge was in place over the eastern Pacific. At the surface, a frontal zone extended from Quebec to the eastern Great Lakes to mid MS Valleys to the southern Plains with a ridge of sfc high pressure extending from the Southeast Conus to off the eastern seaboard. Southerly flow continues to advect a warm and moist airmass into the region with dewpoints in the mid 60s in the higher terrain along the VA border and then upper 60s to low 70s elsewhere. Meanwhile, PW continues to increase with about 1.5 inches in Pike County to near or in excess of 1.8 inches in the west. Ahead of the boundary, convection has developed from OH southwest into portions of IN and western KY. An isolated shower has recently developed to the west of the Lake Cumberland area. Thus far, cirrus has been thick enough for temperatures in the central and southern portions of the area from reaching 90 or higher for the most part. This evening and tonight, the axis of upper level ridging is expected to shift east and south with the ridge weakening a bit. At the same time, a gradual trend of height falls is expected over the Lower OH Valley region and the Commonwealth as the rather broad area of upper level trough begins to expand east into the Great Lakes and MS Valley. One or more weak shortwaves or impulses may track into or near the OH Valley late tonight. PW should increase tonight, especially across the northern two thirds of the region nearer to the sfc cold front as the boundary sags toward the OH River. PW may remain near 1.7 to 1.8 in the southeast, but climb to 2 to 2.25 inches further north and west. This would bring PW values well above the 90th percentile and as high as the 95th to 99th percentile. Convection ahead of the boundary may spread southeast and into the area during the evening or possibly an outflow from that activity may arrive as well. This and the subtle height falls could lead to isolated to scattered activity mainly in the evening and then later at night. The highest probability for any storms is north of the Mountain Parkway. With the boundary becoming nearly parallel to the upper flow and the high PW values, some spotty very heavy downpours may occur. Thursday, the boundary should remain nearly stationary near the OH River before gradually becoming a bit diffuse while a shortwave trough/impulse moves across the OH Valley region on Thursday. On Thursday night, another stronger shortwave to the south of an upper low moving from the Plains and upper midwest to near the western Great Lakes approaches the OH Valley preceded by a cold front. PW during this time should climb to around or in excess of 2 inches areawide or well above the 90the percentile. With the boundary to the north and generally parallel to the upper flow, the threat of some training cannot be ruled out. Although mid level level lapse rates will be meager, steep low level lapse rates on the order of at least 7 to 8C/km may develop and MLCAPE may reach at least 1000 to 2000 J/kg in the afternoon to early evening if not a bit higher near 2500 J/kg in the southwest where less heating may contribute to temps near 90 with mid 70s dewpoints expected. Bulk shear will be a bit limited though possibly peaking near 20KT in the south and approach 30KT in the north. This might allow for some multicells or line segments to form if cold pools were able to amalgamate and given the high moisture content in the column a microburst or two cannot be ruled out. Some strong to locally damaging wind gusts will be a threat with convection for the Independence Day holiday along with torrential downpours. Coverage of convection may peak during the afternoon to early evening per recent CAMS and then decrease in coverage as the impulse departs during the evening, though confidence in this timing and coverage scenario is not high. In between shortwaves a lull in coverage in the late evening into the overnight may occur with a possible uptick in coverage again toward dawn on Friday ahead of the cold front and next shortwave. Current dewpoints and guidance suggest most locations will not fall below 69 or 70 tonight with coalfield ridgetops and more open terrain areas likely not falling below the mid 70s unless wetbulbing form convection occurs there. Otherwise, more in the way of cloud cover is expected in the north on Thursday nearer to the boundary with mid 80s north and upper 80s to low 90s south for highs. Dewpoints should be well into the 70s with heat indices peaking in the low 90s north and mid 90s to around 100 south. .LONG TERM...(Friday through Wednesday) Issued at 500 PM EDT WED JUL 3 2024 The long-term starts off an an active note as anomalously deep but dampening upper level trough (for July) passes from the Midwest on early Friday morning on into Quebec Saturday. This system will push a cold front across eastern Kentucky between 0 and 12z Saturday. The front itself won`t bring much of a temperature change, rather it will bring noticeably lower humidity levels (mid 70s dew points vs. low to mid 60s dew points). In the face of dwindling upper level support and a persistent upper level ridge over the Southeast US and Western Atlantic, the frontal boundary eventually hangs up along the eastern slopes of the Appalachians by early Sunday. This will allow for an area of surface high pressure to dominate eastern Kentucky from late Saturday through Sunday. Meanwhile, additional upper level energy will drop out of northwest Canada over the weekend and reinforce troughing over the central CONUS by Monday, though that trough will likewise propagate northeastward and dampen by middle of the week. As flow turns southerly over the Ohio Valley ahead this next trough, the remnant boundary to our south and east will be pulled back north, allowing higher humidity levels to bleed back into eastern Kentucky on Monday. As that trough pivots through the Ohio Valley later Tuesday and Wednesday, it will sweep another cold front through our area. Ahead of the first cold front on Friday, the ingredients might come together for another day of numerous thunderstorms. Increasing flow aloft is forecast to generate 25 to 35 knots of effective shear. However, the evolution of overnight convection upstream could have an impact on how much instability is realized. If convection or debris cloud cover arrives early in the day, then the severe threat would be minimized due to relatively meager instability. Conversely, several hours of intense surface heating would be supportive of more widespread strong to severe convection. The RAP13 suggests 1,000 to 2,500 J/kg of MLCAPE which combined with the anticipated shear would be more than sufficient for organized multicell clusters and even a line segments. Strong to damaging winds appear to be the primary severe weather threat. Anomalously high PWATs and warm rain processes will support torrential downpours; however, relatively dry antecedent soil moisture, low stream flows and progressive cell motions should all minimize the flooding concern. Aside from the severe weather and heavy rain threat, strong surface heating, assuming that upstream convection does not adversely impact sky cover, would support highs in the mid and upper 80s on Friday and dew points in the 70s would buoy heat indices well into the 90s to near 100F. The shower and thunderstorm threat diminishes from northwest to southeast on Friday night as the cold front passes, though a shower could linger into the day on Saturday in those counties adjacent to the Virginia border. Behind the front, a noticeably less humid air mass will move in on Saturday while temperatures drop back a few degrees to mainly the middle 80s under mostly sunny skies. The fair weather continues Saturday night and Sunday with cool low temperatures in the upper 50s to mid 60s and highs in the mid to upper 80s. Looking ahead to new work week, shower and thunderstorm chances return on Monday as the humidity returns and lingers until Wednesday when the next cold front passes. The hottest temperatures of the period are forecast on Monday with highs in the upper 80s to lower 90s for most. The heat backs off into the the mid 80s for highs by Wednesday behind the second cold front. && .AVIATION...(For the 00Z TAFS through 00Z Thursday evening) ISSUED AT 724 PM EDT WED JUL 3 2024 VFR conditions are expected to largely prevail through the period with cumulus and mid level clouds at times. However, an approaching front and then a passing disturbance should combine to lead to isolated to possibly scattered convection at times this evening through the rest of the TAF period, with the greater coverage of convection being most probable between 14z and 21z Thursday. Within any thunderstorms sub-VFR conditions can be expected, with some potential for strong gusty winds with the stronger storms. Winds will mainly be from the southwest at around 10KT or less during the period outside of thunderstorms. && .JKL WATCHES/WARNINGS/ADVISORIES... NONE. && $$ UPDATE...CMC SHORT TERM...JP LONG TERM...GEERTSON AVIATION...CMC