Area Forecast Discussion
Issued by NWS Jackson, KY

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000 FXUS63 KJKL 292039 AFDJKL AREA FORECAST DISCUSSION National Weather Service Jackson KY 439 PM EDT Wed Mar 29 2017 .SHORT TERM...
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(This evening through Thursday night) Issued at 428 PM EDT WED MAR 29 2017 As of mid to late afternoon, an upper level ridge extended from the Caribbean across Florida and into the Southeast, OH Valley, and on to the southern Great Lakes. Meanwhile, an upper level low was moving across the Plains and was centered over south central KS as this time. At the surface, low pressure was centered over southeastern KS with a cold front south into East TX and a warm front extending east near the AR and MO border then southeast into GA. Meanwhile, surface high pressure was centered over northern Ontario with ridging south into the OH Valley and Appalachians. Persistent low level clouds have gradually mixed out through the day with just some cumulus remaining across most of the area although some of the low stratocu lingers northeast of the area over HTS. The axis of the mid and upper level ridge will move east of the area this evening and tonight with the axis of the surface ridge also moving east of the area. At the same time, the upper level low will meander into eastern KS late tonight and then to the mid MS Valley by Thursday evening. The Surface low is expected to reach IL on Thursday evening with the warm front lifting into the area late tonight and across the area on Thursday. This should set the stage for some isolated to perhaps scattered convection on Thursday, mainly afternoon and early evening. As the surface low approaches the southern Great Lakes at the end of the period and cold front tracks across the area late in the period and the upper level low moves into IL and IN, more organized convection is anticipated. With high pressure in place, and a few hours of decent net radiational for the first half of the night before increasing clouds associated isentropic lift ahead of the warm front and approaching upper level low and trough, eastern valley locations should experience temperatures dropping off through the 50s and into the 40s within a couple hours of sunset. Some valley fog should develop at least in the river and larger valleys and could become dense in spots, but confidence in this is low since mid and high clouds are expected to increase late and the pressure gradient will also be increasing. Trends for this possibility will continue to be monitored. With the increasing pressure gradient and winds aloft overnight, winds should increase a bit late tonight in the more open terrain locations of the far west or northwest as well as the higher elevations generally 2500 feet and above where gusts of 25 to 30 mph are possible late. The pressure gradient will increase across the entire area with mid and upper level moisture increasing a bit more quickly than at the lower levels. Winds will have a downslope component for much of the day especially in the southeast and this should limit chances for isolated to scattered convection there. Warm advection will also be occurring especially as the region gets into the warm sector from midday into the afternoon and temperatures could approach 80 in the normally warmer locations. This may be most likely for valley locations immediately downwind of Pine and Black Mountains. Also due to this downsloping have undercut the blend forecast dewpoints a bit for the southeastern counties during the day on Thursday. The best forcing as well as instability with CAPES possibly nearing 1000 J/KG in the southwest and 0 to 6km shear reaching the 45 and 55KT range will be favorable for at least some strong to possibly severe convection during Thursday evening into Thursday night. Some isolated to scattered convection will be possible in the evening, but a line or lines of thunderstorms in advance of the cold front and 500 mb trough axis is expected to move into the area. This convection will be arriving as the boundary later begins to stabilize, but shear and CAPE should be substantial enough for the threat of damaging winds and hail possibly exceeding 1 inch in diameter and maybe a stray tornado, particularly western locations closer to the sfc and upper low track and slightly more favorable arrival time after diurnal heating and expected higher dewpoints. Chances for severe are obviously much more favorable to the west and especially southwest of the area closer to the I 65 corridor and west. Obviously there remains uncertainty in the degree of instability when this line or line segments would arrive due to timing differences. In addition to the chance for wind and hail, locally heavy rain cannot be completely ruled out. These threats continue to be highlighted in the HWO and Weatherstory, etc. .LONG TERM...(Friday through Wednesday) Issued at 400 PM EDT WED MAR 29 2017 Models appear to be in good agreement with mid/upper level features until the very end of the extended window. Overall pattern is quite progressive as well. Cutoff low will be pulling off to the east at a rapid pace Friday. Shortwave ridging rides across the region through the weekend. A second cutoff low then transits the Tennessee Valley for the start of the new week before sliding out into the Atlantic by late Tuesday. Final wave of low pressure pushes into the Mississipi Valley and lower Ohio Valley region by the end of the forecast period. At the surface, a frontal boundary will be all but through eastern Kentucky by Friday morning. Wrap around moisture from the exiting system will allow for some lingering instability showers Friday which will taper off to very light rain and/or drizzle by Friday night. We do lose dendritic support which would suggest drizzle through this time frame. But boundary layer moisture is so deep that any lingering precipitation may effectively seem more like very light rain. Thus left any mention of drizzle out for this cycle. The remainder of the weekend looks dry with seasonably warm temperatures. Rain overspreads the area again from the west-southwest late Sunday night and Monday as the second storm system of the extended period pushes through the Commonwealth. Some marginal, mainly elevated instability will keep the threat of thunder in the forecast Monday into Monday night. Decent southeast winds associated with this second system may eat away at potential rainfall totals given downsloping effects. Consequently model QPF may be a bit overdone across our southeast. Once again wrap around moisture from this system may keep a chance of showers going into mid week, at least until another system manages to work its way into the Ohio Valley at the very end of the period, or possibly just beyond bringing another round of rain showers and thunderstorms to the area.
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&& .AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon) ISSUED AT 215 PM EDT WED MAR 29 2017 Low level moisture continues to mix out with high pressure in place. This is leading to improvement from MVFR to VFR from south to north. SME and LOZ are already VFR and JKL, SJS, and SYM should improve to VFR during the first 2 to 3 hours of the period. VFR should then persist through the end of the period. An increase in high and then mid level clouds should occur 0Z to 12Z, with some low clouds and cumulus development possible late in the period. A stray shower or even a thunderstorm cannot be completely ruled out late, but better chances will come after the period. && .JKL WATCHES/WARNINGS/ADVISORIES...NONE. && $$ SHORT TERM...JP LONG TERM...KAS/RAY AVIATION...JP

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