Area Forecast Discussion
Issued by NWS Jackson, KY

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088 FXUS63 KJKL 161418 AFDJKL AREA FORECAST DISCUSSION National Weather Service Jackson KY 1018 AM EDT Fri Mar 16 2018 .UPDATE...
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Issued at 1018 AM EDT FRI MAR 16 2018 Have updated NDFD for latest observational trends. Main change was to speed up the rate of dewpoint fall as drier air advects in from the northeast. UPDATE Issued at 715 AM EDT FRI MAR 16 2018 The winds have started to stir up from the north to northeast at JKL and other locations around the CWA and as a result helped to mix out much of the ridge to valley distinctions as well as advecting in colder air from the northeast. Have updated the near term grids to account for this as well as to tweak sky cover for the next few hours. These grids have been sent to the NDFD and web servers.
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&& .SHORT TERM...(Today through Saturday) Issued at 455 AM EDT FRI MAR 16 2018 08z sfc analysis shows high pressure over the Upper Midwest and nosing into northeast Kentucky. At the same time, low pressure is coming out of the Southern Plains and is starting to bolster a boundary lying across southern parts of the area with increasing clouds. Before they fill in very much, clear skies (and light to calm winds) have allowed a fairly large ridge to valley temperature split to develop, primarily in the east, ranging from the lower 40s on the hills to the upper 20s in the sheltered low spots. To the west, conditions are more uniform due to better mixing most of the night. Meanwhile, dewpoints are generally in the low to mid 20s throughout the area. The models are in better agreement aloft for the short term portion of the forecast. They all depict a flattening ridge slipping over and through Kentucky today in the face of a dampening mid level closed low spreading east along the Ohio River later tonight and into Saturday. The energy associated with this will start to impact the area after midnight with several separate impulses passing through eastern Kentucky during the day Saturday. The NAM is slightly stronger with these than the other models and was preferred. In general, a model blend was favored on account of the improving agreement, with a lean toward the higher resolution NAM12 and HRRR in the near term. Sensible weather will feature a cooler day than yesterday as the high pressure exerts its fleeting influence over the region. Meanwhile, a boundary to the southwest will become more of a warm front, one with with increasing and lowering clouds, that will start to lift northeast by the end of the afternoon. This will become more activated this evening with showers and possible thunderstorms expected to move through eastern Kentucky ahead of the potent sfc low cutting east-southeast through the northern parts of the state overnight into Saturday morning. Luckily, the front will bring enough warmer air aloft and at the sfc for the JKL CWA to prevent a concern for freezing rain or sleet at its onset. The low`s movement will keep much of eastern Kentucky in the system`s warm sector with at least some instability building along with a good wind profile for storm development. In fact, some locations in the Cumberland Valley could reach 70 degrees (while upper 40s for highs are found near and north of Interstate 64 where the warm front will stall out). Accordingly, SPC has placed much of the area in an outlook for a marginal threat of severe weather on Saturday with large hail and potentially damaging wind gusts the main threats. Have included thunder in the wx grids for this and highlighted the severe potential in the HWO. This core of the sfc low will pass through eastern Kentucky by 00z Sunday with cooler conditions to follow. Used the CONSShort and ShortBlend to kick start the grids with some significant temperature and dewpoint adjustments in the near term per the current conditions reflecting a strong ridge to valley difference for the eastern valleys. As for PoP, did bring some small chances in a tad quicker this evening and into the overnight as the sfc low gets closer. .LONG TERM...(Saturday night through Thursday) Issued at 515 AM EDT FRI MAR 16 2018 A bit of an active upper level pattern expected through the extended, as multiple shortwaves move across the region. The first of these will occur through the night Saturday, with ridging returns for the day Sunday. This ridging will quickly become dampened as a closed upper level low quickly gains strength and pushes into the southern Plains by Monday morning. From here it will change paths, quickly shifting eastward and across the state of Kentucky Monday night. While it is expected to exit NE of the state Tuesday afternoon, it will merge with another closed low system and result in overarching troughing in place across the region through Thursday night. Rising heights will be the story for Friday as both models agree on an area of ridging moving eastward into the state. As for sensible weather...With each passing upper level disturbance, expect impacts in our weather. Therefore, will say the extended portion of the forecast does look generally wet with varying temperatures and weather types. Saturday night will feature an exiting low pressure system and a shift to northerly winds. Ongoing rain chances at 0Z Sunday will start to exit to the east and dry out from the west during the overnight, with all rain chances exiting the CWA by 12Z. Light N to NE flow and high pressure will remain in control for the day Sunday, with abundant sunshine allowing temps to rise back into the 50s across the CWA. The high pressure system will then dissipate as it quickly shifts eastward Sunday night ahead of an approaching low pressure system over the southern Plains, correlating to the above mentioned closed upper level low in the same location. As the upper level closed low shifts eastward into the day Monday and traverses Kentucky into Monday night, so the surface system will follow. According to the latest GFS, the surface low is expected to track along the southern half of Kentucky starting between 0 and 6Z Tuesday and continuing into eastern Kentucky by 18Z Tuesday before getting hung up in the mountains by 0Z Wednesday. It should finally start shifting away from the state by Wednesday afternoon. Precip associated with this system will begin to affect the southern CWA late Sunday night, expanding across the entire CWA by Monday and Monday night. Likely precip chances will then remain in the forecast for much of the CWA through the day Tuesday and into the day Wednesday before the system shifts eastward Wednesday afternoon. Pops will be slow to taper off in the far eastern portion of the state with upslope flow aiding in moisture release. Chance pops won`t fully taper off here until Thursday afternoon. As for precip types during this period... There was some discussion of possible thunder Monday afternoon with the approaching low and increasing precip chances. However, looking at soundings, would agree with previous day shift`s decision to remove thunder from the CWA. Lapse rates and shear are good in the low levels in the southern half of the CWA where surface southerly flow is in place, but a mid level inversion and lack of instability (CAPE and LIs) are both factors that lowers confidence in its occurrence. In addition, soundings don`t look very saturated, and forecasted afternoon QPF is under 0.1 inches, so there may not be enough rainfall/precip to instigate lightning. After Monday, forecast looks more certain for only rain. However, as the bulk of precip starts to move eastward Tuesday night into Wednesday, colder NW flow could result in overnight temperatures in the low to mid 30s in some locations (mainly north). This may enable some snow to mix in with rainfall, though the ground should remain too warm during the short period of time it could mix in to produce any accumulations. As such, took out snowfall accumulations that the forecast builder had added in. Temperatures will be even colder Wednesday night into Thursday morning under continued NW flow (upper 20s to around 30), so there will be a better chance of any lingering moisture changing over to snow this night/early morning. However, QPF will be quite light by this point, so not expecting significant accumulations or impacts this far out in the forecast. && .AVIATION...(For the 12Z TAFS through 12Z Saturday morning) ISSUED AT 750 AM EDT FRI MAR 16 2018 VFR conditions will continue to be the rule through the bulk of the forecast period as some mid level clouds build to the south with a developing warm front later this morning. These clouds will thicken and lower in time as the boundary lifts northeast through eastern Kentucky. Along this front a few showers will be possible just after midnight - increasing in coverage through the night and into Saturday morning. Winds generally will be light and variable through the TAF period. && .JKL WATCHES/WARNINGS/ADVISORIES... NONE. && $$ UPDATE...SBH SHORT TERM...GREIF LONG TERM...JMW AVIATION...GREIF is the U.S. government's official web portal to all federal, state and local government web resources and services.