Area Forecast Discussion
Issued by NWS Jackson, KY

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000 FXUS63 KJKL 231212 AAA AFDJKL AREA FORECAST DISCUSSION...UPDATED National Weather Service Jackson KY 812 AM EDT Mon Apr 23 2018 .UPDATE...
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Issued at 802 EDT MON APR 23 2018 The band of showers over the southern counties and northeast TN seems to be holding together quite well despite southeast low level flow. Cloud bases are on the high side, but rain is measuring. With that in mind upper pops a bit over the southeast counties as this lifts through to the 70 to 90 percent range. Otherwise, some slight adjustments were made to hourly grids based on observations trends with not other changes. Recent HRRR runs continue to lift the initial band to the north and northeast with potential redevelopment late in the day in the southwest with some heating. A rumble or two near the TN border cannot be ruled out.
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&& .SHORT TERM...(Today through Tuesday) Issued at 445 AM EDT MON APR 23 2018 The axis of mid and upper level ridging will continue to depart to the east and northeast of the area today with sfc high pressure also moving further northeast of the area. This will allow the mid and upper level low currently centered near the Memphis TN vicinity to meander into middle TN this evening and then reach the Southern Appalachians by the end of the period. Moisture has already been increasing across the area, especially aloft. However, the flow from around 800 mb to the surface is downslope which is slowing down increases in dewpoints and rh at low levels. Despite this, showers or light rain has been falling across the southwest part of the area overnight from a mid cloud deck in a band east of the center of the mid and upper level system. Today, the model consensus is for a band or bands of showers to continue moving north and east across the area through mid afternoon. Toward midday activity will likely become more isolated to scattered in nature over southwestern counties. At the same time, temperatures aloft should begin to cool. Thus with daytime heating especially in the southwest part of the area additional activity is expected develop by late afternoon or early evening while a drying trend over much of the northern and eastern sections from mid afternoon into the evening. Enough instability may be present in fact for some isolated thunder to extend into the Lake Cumberland region and possibly as far east as Whitely and Bell counties or approximately where the SPC general thunder outlook extends. Momentum transfer in bufkit from the NAM has gusts into the 20 to 25 knot range from late this morning into this afternoon for many area with up to 30kt possible, especially behind this band where some limited solar insolation may add to the mixing. Gusts on top of Black and Pine Mountains, the Log Mountains of Bell County and the Flatwoods area of Pike County may reach as high around 35 kt from late morning into the afternoon. The southeast downslope component of the flow should lead to a rain shadow downwind of Pike Mountain across portions of the southeastern counties and downward adjustments were made to the initialized superblend qpf in those areas to account for this. The downslope flow should also lead to locally warmer max T in valley locations such as Harlan and Cumberland, Whitesburg and Cumberland northeast to Dorton and Elkhorn City where upper 60s to near 70 are anticipated. Locations closer to the center of the upper level system and less influenced by downslope flow especially from I 75 west should experience more persistent showers through the evening into the over night hours while some southeastern locations may be rainfree for much of the night. A qpf trend which should begin during today should continue into tonight with qpf twice or more times greater in some of the southwest and western sections compared to the southeast. Chances for showers should increase into central portions of the area late Monday night as a band of showers is expected per model consensus. This band of showers should continue progressing north and Northeast across the area into Tuesday afternoon. The coldest temperatures over the next couple of days at 850 mb should be during Tuesday morning into Tuesday afternoon. Steepening low level lapse rates may lead to diurnal uptick in the rain rates from the showers on Tuesday in the initial band with an isolated thunderstorm or two also possible. Some redevelopment of showers or even a thunderstorm or two may develop closer to the center of the mid and upper level system on Tuesday afternoon. Diurnal ranges will be much more limited than recent days through the near term period with highs generally in the low to mid 60s and lows tonight generally in the low to mid 60s. These readings will be below normal for highs which are around 70 for this time of year while lows will be a few degrees above normal as compared to normal lows for this time of year in the mid and upper 40s. .LONG TERM...(Tuesday night through Sunday) Issued at 405 AM EDT MON APR 23 2018 A 500mb low will be collocated with a surface low pressure system across Kentucky starting out at 0Z Wednesday. This will continue to shift eastward out of the state throughout Tuesday night and into the day Wednesday, with pops/rain chances expected to be on a decreasing trend during this time. Upslope flow will keep some chance pops in place through the day Wednesday, especially across the SE, but additional QPF will be under a tenth of an inch through 0Z Thursday. Models then lose considerable agreement by 0Z Thursday, with the NAM and GFS planting a developing closed low over the south/central Plains, and the ECMWF showing a strong closed low over the northern Ohio Valley and southern Great Lakes region with a shortwave trough over the central/southern Plains instead. The ECMWF shows this closed low shifting SE across Ohio Wednesday night and into the day Thursday, while both the GFS and ECMWF show the other shortwave/low system tracking just south of Kentucky into the deep south during this time. Surprisingly, which ever model pans out, neither actually pulls precip from either system into the state, so will keep Thursday dry in the forecast. Models then come into much better agreement for another system which will affect the region to end out the workweek. A 500mb shortwave will follow deep troughing across the region, deepening the trough as the axis moves through Kentucky Friday night into Saturday morning. At the surface a cold front is expected to shift from west to east across the state during the day Friday and into Friday night, bringing with it a swath of chance pops. Instability is still lacking, so will not include any mention of thunder at this time. In fact soundings are actually quite dry, so not expecting much QPF from this either. Even after tweaking QPF up slightly from the Superblend, still only have well under a tenth of an inch for the duration of the system. During much of the extended period, temperatures will actually remain relatively uniform, with highs in the low to mid 60s and lows in the 40s. However, if clearing occurs overnight Friday night and Saturday night (coupled with light winds), some sheltered valleys may fall into the upper 30s. Ridging will slowly take hold throughout the day Sunday and continue into Monday. This will allow temperatures to climb into the low 70s both days. && .AVIATION...
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(For the 12Z TAFS through 12Z Tuesday morning) ISSUED AT 800 AM EDT MON APR 23 2018 An upper level low will meander across TN through the period and reach near the northern portions of the Cumberland Plateau in TN by the end of the period. For much of the period the bulk of the region will maintain a downslope component of the flow with lower levels slow to saturate. Thus mainly low and mid level clouds will predominate through 3Z before the lower levels saturate in the southwest as the center of the mid and upper level low nears. This should lead to MVFR reaching near I 75 by the end of the period with IFR possible near Lake Cumberland. Some brief MVFR vis is possible in showers or any thunderstorms across western and northwestern sections of the area through 0Z. Otherwise, vis may fall as low as MVFR to IFR prevailing in the southwest after 6Z. Winds should continue or increase to near or in excess of 10KT at the TAF sites early in the period period with some gusts over 20kt. Winds should generally slacken around or after 0Z.
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&& .JKL WATCHES/WARNINGS/ADVISORIES... NONE. && $$ UPDATE...JP SHORT TERM...JP LONG TERM...JMW AVIATION...JP

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