Area Forecast Discussion
Issued by NWS Jackson, KY

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000 FXUS63 KJKL 140249 AFDJKL AREA FORECAST DISCUSSION National Weather Service Jackson KY 1049 PM EDT Fri Oct 13 2017 .UPDATE...
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Issued at 1049 PM EDT FRI OCT 13 2017 Valley temperatures have fallen off quickly this evening under clear skies, but they are also reaching saturation, and satellite imagery shows fog developing. This will greatly slow the rate of fall overnight, and forecast morning lows have not been changed since the previous update. Models show some potential for a low cloud deck to develop over southern and southeastern portions of the forecast area toward dawn, but confidence in this is shaky. UPDATE Issued at 707 PM EDT FRI OCT 13 2017 Skies have cleared for the most part, and ridge/valley temperatures differences have already become apparent in the eastern portion of the area. Have updated the forecast to account for less sky cover this evening, and greater temperature variation. At this early stage, have not made any significant changes to overnight lows.
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&& .SHORT TERM...(This evening through Saturday night) Issued at 337 PM EDT FRI OCT 13 2017 High pressure is currently in control across the CWA and will remain so throughout the duration of the forecast period as it drifts slowly east of the area. A large mass of dry air is accompanying this system, resulting in mostly clear skies. As was the case last night, as temperatures fall overnight, a llvl inversion will set up and trap moisture near the surface. This will likely lead to good fog development across the CWA, especially in the valleys and near bodies of water. The one concern will be return southerly flow on the west side of the slowly departing high, which may impact the western CWA early in the morning and deter the fog. But confidence in this is low, as this wind shift/increase will likely take place during the day tomorrow instead and fog will be the result. Did not include any dense fog wording in the grids at this time since confidence on where and to what extent it would set up was somewhat low, though do expect there to be some localized dense fog across the CWA. Lows will generally be in the 50s, coolest in the valleys. Mostly clear skies will remain intact through the day tomorrow once fog lifts and clears out, save some diurnal CU that may develop in the afternoon. Under deeper southerly flow, afternoon temperatures are forecast to rise back to around 80 in many locations. Lowering heights are expected in the upper levels as we head into Saturday night as a shortwave continues to dig in the northern Plains/northern Mississippi Valley. At the surface, a low pressure system will be moving NE into the Upper Great Lakes, pulling a cold front eastward into KY. While the front will not reach us Saturday night, it will bring with it an increase in cloud cover as well as precip potential as we head into the day Sunday (start of mid term). This will deter from any fog development Saturday night and will also keep temps mild, generally in the low 60s. .LONG TERM...(Sunday through Friday) Issued at 250 PM EDT FRI OCT 13 2017 The focus of the Sunday through Friday period will be the passage of a cold front on Sunday and a period of more autumn like weather next week. Model trends point towards a quicker passage of the cold front on Sunday, with the front clearing the area by early Sunday evening. Instability on Sunday still appears limited, but there should be enough instability in a narrow ribbon in advance of the front to support at least the mention of a slight chance of thunder. We will continue with the forecast of a slight chance of thunder on Sunday which is in agreement with previous NDFD forecasts, and the latest SPC Day 3 Outlook which places our entire area in the general thunder outlook. Even if there end up being no thunderstorms on Sunday, which certainly is possible, it will still be rather breezy with some gusts to near 30 mph especially in the Bluegrass counties and other more open areas. A surface high will build into the area early next week in the wake of the cold front, and bring drier and cooler conditions. The superblend and model guidance support widespread mid to upper 30 degree minimum temperatures Tuesday morning and scattered mid and upper 30 degree temperatures on Wednesday morning. Use of the forecast builder generates areas of frost, especially on Tuesday morning. However, as is often the case, fog formation in river and stream valleys will inhibit frost formation. Despite that, some areas should see their first frost of the season. We will continue to highlight the frost possibility in the HWO. As the week progresses the surface high will drift east and upper level ridging with rising heights will bring a warming trend to the area. && .AVIATION...(For the 00Z TAFS through 00Z Saturday evening) ISSUED AT 906 PM EDT FRI OCT 13 2017 The period started with mainly clear skies and no visibility restrictions. Relative humidity during the afternoon and at the start of the 00z TAF period was lower than at the same time yesterday. This would argue for less in the way of fog tonight. There will also be strengthening warm air advection. While low level winds should not be strong enough to mix the valleys tonight, the ridges and some mid-slope locations will see a bit of stirring at times, which will also help to restrict the extent of fog. With that said, the normally colder and foggier valleys should still be IFR or worse by dawn, but TAF sites should not fair as bad. Restrictions are still forecast at KSME and KLOZ, and temporarily at KSYM. Models also suggest a low cloud deck trying to develop in the south and southeast portions of the forecast area toward dawn and lasting into the morning. Have accounted for this with MVFR ceilings in the forecast at KSME and KLOZ, but there is not good confidence in this aspect of the forecast. && .JKL WATCHES/WARNINGS/ADVISORIES... NONE. && $$ UPDATE...HAL SHORT TERM...JMW LONG TERM...SBH AVIATION...HAL

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