Area Forecast Discussion
Issued by NWS Jackson, KY

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058 FXUS63 KJKL 171345 AFDJKL AREA FORECAST DISCUSSION National Weather Service Jackson KY 945 AM EDT Tue Oct 17 2017 .UPDATE...
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Issued at 945 AM EDT TUE OCT 17 2017 The GOES-16 visible sat still showing a good bit of fog and/or low stratus in the river valleys this morning. Therefore opted to keep this going for another hour or two in the SKY and WX grids. Otherwise only minor changes needed to deal with latest obs and trends. UPDATE Issued at 725 AM EDT TUE OCT 17 2017 Surface high pressure over the OH Valley remains in control of the weather and will lead to a milder day today as the airmass moderates. Valley fog, dense in some locations per observations and satellite, should lift and begin to dissipate between 8 AM and 10 AM. Hourly grids were freshened up based on recent observations and trends. This led to some adjustments to hourly temps and dewpoints and wx grids for fog initially.
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&& .SHORT TERM...(Today through Wednesday) Issued at 354 AM EDT TUE OCT 17 2017 Early this morning, a surface ridge of high pressure extends from the OH Valley and Appalachians to the Southern Plains. This high is expected to remain in control of the weather across the region through the period as it eventually becomes centered off the east of the area. Skies were clear overnight and valley fog has developed in many of the river valleys and along larger creeks and streams. The fog is evident on GOES 16 10.3-3.9 fog product and GOES 16 nighttime cloud microphysics. AWOS sites at BYL and I35 are both reporting dense fog at 7Z. As of 3 AM, valley temperatures were generally in the upper 30s with some low 40s on some ridgetops. Fog will continue to develop through the remainder of the night and some patchy frost may develop in valley locations that do not experience fog. Otherwise, the fog should begin to lift and dissipate around sunrise, before finally mixing out toward 10AM. Thereafter, sunny skies are expected with milder conditions expected today with surface high pressure in place and as a shortwave ridge moves across the area. Temperatures should average within 5 degrees of normal for this time of year. 1000 to 500 mb layer rh is progged by the models to drop into the 20 to 25 percent range. Mixing into this dry air aloft of which models have the driest of the air above 850 mb should lead to a bit lower dewpoints, especially across the east and southeast part of the area, compared to Monday with more mid 30s common for a few hours in the afternoon. When combined with the higher temperatures, rh will bottom out near 30 percent in several locations this afternoon with some locations bottoming out in the 25 to 30 percent range. The surface high will continue to dominate the weather tonight and into the day on Wednesday as noted earlier. A weak 500 mb shortwave will approach the area late tonight and then work into the area on Wednesday. This system will have very limited moisture and the only noticeable affect may be an increase in high clouds. Winds will also shift around to the southeast tonight and then the south on Wednesday as the center of the sfc high moves off to the east and northeast. Nocturnal inversion development should lead to a ridge/valley temperature split that is moderate in magnitude especially in the east. Valleys should reach the mid to upper 30s while ridgetops bottom out in the lower 40s. Went more toward the Coop MOS side of guidance for the valley low temps tonight. With the dewpoints expected to have dropped off a bit more today and high pressure remaining in place tonight, the potential will again be there for patchy frost in some of the deeper valley locations along with some fog as well. The frost potential will continue to be highlighted in the HWO. Only some passing high clouds are expected on Wednesday. Temperatures should moderate further with weak southerly flow in place as sfc high pressure centered to the east of the area continues to dominate. Temperatures should top out in the lower 70s. Even with some slight recovery in dewpoints as well with the southerly flow, rh will again bottom out near 30 percent if not below during the afternoon. .LONG TERM...(Wednesday night through Monday) Issued at 310 AM EDT TUE OCT 17 2017 The models are in fairly good agreement aloft for the bulk of the extended portion of the forecast. They all depict the weak shortwave moving through the Ohio Valley Wednesday night. This will temporarily suppress the heights through the Ohio Valley as it passes with some weak energy accompanying the broader wave. Heights will rebound quickly in its wake, though, as ridging will build north from the Central Gulf Coast up into Kentucky Thursday. This ridge then strengthens as it holds court over the Southeast part of the nation through the first half of the weekend. More model spread is found to the west at this time step with plenty of energy and troughing spilling through the Rockies and into the Plains. The stubborn ridge will hold over eastern Kentucky into Sunday even as a Full latitude trough heads toward the Mississippi River. At this point the model agreement breaks down with the 12z ECMWF curling the southern portion of its trough into a deepening closed low over eastern Texas while the GFS maintains a progressive trough into Kentucky Sunday night into Monday. Another difference will be the location of the core energy and troughiness from the models into Monday with the ECMWF concentrated with its Southern Plains closed low and the GFS keeping more to the north for the great Lakes. The fresh 00z version of the ECMWF does come closer to the GFS in the latter periods lending more confidence to their composite forecast. With the uncertainty late in the period will generally stick with a blended solution - not ready to throw in with any specific model solution. Sensible weather will feature sfc high pressure and a dry air mass leading to a continuation or renewal of the warm and dry weather that has characterized the start of autumn thus far. Even the mid level weakness at the start of the extended will be marked by little more than some extra high and mid level cloudiness. Given this pattern will focus mainly on hitting the ridge to valley temperature differences at night and the midday mix-down of drier air in the dewpoint grids. Will also allow for some showers moving into the area late Sunday into Monday as the heights will likely be falling with more energy intrusion regardless of which model ends up being more correct. Throughout the forecast made the typical terrain based adjustments to the grids most nights, aside from Sunday night into Monday as moisture will be higher with more uncertain conditions for an inversion. As for PoPs, basically kept them minimized through Sunday morning and then allowed them to move into the high chance category - in line with the 00z model consensus. && .AVIATION...(For the 12Z TAFS through 12Z Wednesday morning) ISSUED AT 744 AM EDT TUE OCT 17 2017 High pressure is in place across the OH Valley this morning. Valley fog has developed and has become dense, but not at any of the TAF sites. MVFR fog is currently observed at SYM and SME. The fog should mix out at both locations through 13Z. Otherwise, river valley fog will lift and mix out through about 14Z, with VFR conditions expected in all locations from then until at least 6Z. Additional valley fog should develop during the 6Z to 12Z and SME and SYM could experience some MVFR fog then though this should mainly just affect non TAF locations. Winds will be light and variable through the period with the high in place. && .JKL WATCHES/WARNINGS/ADVISORIES... NONE. && $$ UPDATE...DJ SHORT TERM...JP LONG TERM...GREIF AVIATION...JP is the U.S. government's official web portal to all federal, state and local government web resources and services.