Area Forecast Discussion
Issued by NWS Jackson, KY

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000 FXUS63 KJKL 221321 CCA AFDJKL AREA FORECAST DISCUSSION...CORRECTED National Weather Service Jackson KY 921 AM EDT Sat Jul 22 2017 .UPDATE... Issued at 750 AM EDT SAT JUL 22 2017 Did an update to the grids mainly to adjust the sky cover and tweak the T/Td grids per the latest obs and trends. These have been sent to the NDFD and web servers. && .SHORT TERM...
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(Today through Sunday) Issued at 415 AM EDT SAT JUL 22 2017 07z sfc analysis shows high pressure through the southeast portion of the area with a wavy front lying just northeast of Kentucky. Along this boundary a series of organized storms have been rolling east southeast through the Ohio Valley. The latest of these clusters are passing through the region with one to the northeast dissipating over central West Virginia with another still strong one progressing from northern Indiana across into northwest Ohio. On satellite, the cold and high cloud tops are evident with the Indiana storms (as the WV ones warm). These clouds are drifting southeast into Kentucky but thinning with time. The clouds are not preventing much of the radiative cooling, though high dewpoints are limiting the low temperature potential so that ridge to valley differences are running in the order of 5 degrees or less generally from 70 to 75 degrees. The high dewpoints - in the low 70s - will provide enough moisture to generate fog in the river valleys again tonight, though signs of it - including from GOES 16 nighttime microphysics composite channel - are so far quite limited. Meanwhile winds are light and variable to calm. The models are in fairly good agreement aloft with the longwave pattern through the region during the short term portion of the forecast. They all depict the southwestward retreat of the mid level ridge that has brought the heat and humidity these past several days as well as limiting the convection across the state. This ridge retreats in the face of wave after wave coming through in northwest flow just north of the JKL CWA. one of these waves is passing through West Virginia early this morning while the next will take a similar path across the area later today. A more potent one then digs southeast through the Upper Midwest/Great Lakes region and into the Ohio Valley for Sunday. This will bring plenty of energy and significant height falls to the region to close out the weekend. Given the model agreement will favor a general blend with a significant lean toward the higher resolution HRRR and NAM12 models for weather details and timing. Sensible weather will feature another broiling day for much of east Kentucky as temperatures climb again into the lower 90s with dewpoints in the lower to mid 70s. This will mean heat indices for the area topping out between 100 and 105 degrees - but not quite enough for heat advisories in the JKL CWA. We will, however, continue the SPS and HWO highlighting of the heat and humidity. The storm threat will be more significant then yesterday as the ingredients will be in place with good instability and flow aloft to support any storms that manage to develop. That will be the crux of the issue, though, as a small cap will be in place around 750mb and little in the way of triggers so long as outflow boundaries stay off to the north and east of the area - as suggested by the HRRR and NAM12. As such, have kept PoPs on the low end of guidance, more toward the MET rather than MAV, allowing the heat to be the main wx story. However, should storms to our north manage to send a significant outflow boundary this way more active weather could be in the offing with damaging winds the main threat and a secondary concern for hail and excessive rains. Will highlight this in the HWO and SPS. After another likely quiet night for most of the area, barring any MCS activity making it this far south, just as potent conditions, or greater, are expected for Sunday though with more support from falling heights and shortwave energy to better activate the heat and humidity driven instability - along with the sfc boundary likely settling through this part of the state. Fairly high PW air today and Sunday will also threaten excessive rains though the storms should be somewhat progressive given the winds aloft - training will be a concern, nevertheless. Temperatures will likely be lower for highs on Sunday given more clouds and storms around. Used the CONSShort as a starting point for all the grids with some adjustments to temperatures at night for ridge and valley differences along with a beef up in them today for highs - using persistence as a guide. Did tighten up the PoPs through the period trying to time the evolution of MCS and frontal activity - but uncertainty remains high throughout concerning the convectively driven elements of the forecast - which will also affect maximum temperatures and heat index values. .LONG TERM...(Sunday night through Friday) Issued at 345 AM EDT SAT JUL 22 2017 A shortwave trough is expected to work across the Great Lakes and Ohio Valley to start the period while upper level ridges remain in place centered east of Bermuda and over the Southern Plains to Four Corners regions. At the surface, a cold front associated with low pressure moving into the Northeast U.S. to start the period is expected to be gradually working south and southeast into and across the area early in the period, while high pressure is expected to be centered over the Upper MS Valley. The shortwave trough is expected to gradually rotate east and northeast into the Northeast US and across the Appalachians from Sunday night into Monday. Height rises should ensue from Monday night into midweek as the axis of the ridge works toward and then across the area, although as happened earlier this week, the center of the mid and upper level ridge should remain to the west of the area in the Plains states. Models diverge in the position and magnitude of the ridge during the second half of the week with the GFS stronger and further west with the center and axis of this ridge by Thu into Fri. This leads to some differences in the upper level flow becoming more northwesterly with weaker or convectively driven shortwaves moving around the ridge through the north- central U.S. and into the Great Lakes/Oh valley, with some mesoscale convective systems possible initially north of East KY through midweek. Also, downstream of the ridging, an upper level low and stronger shortwave trough as well as associated sfc low pressure system will move across the Canadian Prairies and across James Bay and into northern Quebec by the end of the period. At the surface, high pressure should build into the area behind the front Sunday night into Monday and usher in a drier and colder airmass and some relief from the recent heat. This high, however, should move east of the area by Wed into Wed night, with an increase in humidity from Thursday into Friday. As has been the case with extended models for the past few days, uncertainty remains in timing of a cold front that will begin to shear out during the second half of the week. This front should drop south of the Great Lakes and to the OH River or even south across the area before starting to stall. Behind the front, temperatures will return to near normal with highs holding in the 80s and lows dropping below 70 in all areas. Scattered to numerous showers and thunderstorms are expected ahead of the front early in the period, on Sunday evening. With the ridge building in or in place across the area, chances for convection should be generally diurnally driven and more or less isolated in nature from Monday into Wednesday. Tuesday and Wednesday should be end up dry in most locations, but with the center of the ridge west of the area, the cap could be broken in a few locations each day. More organized convection will be possible during the second half of the week as the upper level flow becomes more northwesterly and a stronger shortwave trough and cold front approaches and moves into the Ohio Valley.
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&& .AVIATION...(For the 12Z TAFS through 12Z Sunday morning) ISSUED AT 755 AM EDT SAT JUL 22 2017 Mainly VFR conditions will prevail through the period, as we remain under the waning influence of an upper level ridge for the better part of another 24 hours. Isolated to scattered convection will be possible for mainly northeast parts of eastern Kentucky during the afternoon hours, but confidence - based on the latest HRRR - is still too low to include VCTS at any site today or tonight. Light and variable winds will increase to between 5 and 10 kts out of the west southwest for this afternoon then diminish in the evening. &&A shortwave trough is expected to work across the Great Lakes and Ohio Valley to start the period while upper level ridges remain in place centered east of Bermuda and over the Southern Plains to Four Corners regions. At the surface, a cold front associated with low pressure moving into the Northeast U.S. to start the period is expected to be gradually working south and southeast into and across the area early in the period, while high pressure is expected to be centered over the Upper MS Valley. The shortwave trough is expected to gradually rotate east and northeast into the Northeast US and across the Appalachians from Sunday night into Monday. Height rises should ensue from Monday night into midweek as the axis of the ridge works toward and then across the area, although as happened earlier this week, the center of the mid and upper level ridge should remain to the west of the area in the Plains states. Models diverge in the position and magnitude of the ridge during the second half of the week with the GFS stronger and further west with the center and axis of this ridge by Thu into Fri. This leads to some differences in the upper level flow becoming more northwesterly with weaker or convectively driven shortwaves moving around the ridge through the north- central U.S. and into the Great Lakes/Oh valley, with some mesoscale convective systems possible initially north of East KY through midweek. Also, downstream of the ridging, an upper level low and stronger shortwave trough as well as associated sfc low pressure system will move across the Canadian Prairies and across James Bay and into northern Quebec by the end of the period. At the surface, high pressure should build into the area behind the front Sunday night into Monday and usher in a drier and colder airmass and some relief from the recent heat. This high, however, should move east of the area by Wed into Wed night, with an increase in humidity from Thursday into Friday. As has been the case with extended models for the past few days, uncertainty remains in timing of a cold front that will begin to shear out during the second half of the week. This front should drop south of the Great Lakes and to the OH River or even south across the area before starting to stall. Behind the front, temperatures will return to near normal with highs holding in the 80s and lows dropping below 70 in all areas. Scattered to numerous showers and thunderstorms are expected ahead of the front early in the period, on Sunday evening. With the ridge building in or in place across the area, chances for convection should be generally diurnally driven and more or less isolated in nature from Monday into Wednesday. Tuesday and Wednesday should be end up dry in most locations, but with the center of the ridge west of the area, the cap could be broken in a few locations each day. More organized convection will be possible during the second half of the week as the upper level flow becomes more northwesterly and a stronger shortwave trough and cold front approaches and moves into the Ohio Valley. .JKL WATCHES/WARNINGS/ADVISORIES... NONE. && $$ UPDATE...GREIF SHORT TERM...GREIF LONG TERM...RAY/JP AVIATION...GREIF

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