Area Forecast Discussion
Issued by NWS Jackson, KY

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000 FXUS63 KJKL 280053 AFDJKL AREA FORECAST DISCUSSION National Weather Service Jackson KY 753 PM EST Mon Feb 27 2017 .UPDATE... Issued at 753 PM EST MON FEB 27 2017 Area of clearing is passing across the area, more significant clearing than what we had advertised. Models have trended in kind with less cloud cover through the overnight period. Accordingly made adjustments to sky cover and lowered temps a bit, especially in our eastern valleys. Increasing gradient wind with return flow and expected return of cloud cover in our southwest during the pre dawn time frame may be enough to keep our western valleys from decoupling as much and thus help keep temperatures up a bit more. However, am concerned that lower adjustments to overnight may not be enough overall, especially if clouds do not return in our southwest. Will reevaluate and update as necessary. With cooler temperatures and a stronger nocturnal inversion have lowered winds in our more sheltered valleys as well. This will also set up the potential of some valley fog as well. The forecast package has been updated, including grids and zones. && .SHORT TERM...(This evening through Tuesday night) Issued at 417 PM EST MON FEB 27 2017 Showers have exited eastern Kentucky this afternoon as a surface trough slides across the Appalachians. Abundant cloud cover has only allowed temperatures to warm into the low-mid 50s, although a few late day breaks may still allow a few locales to warm into the upper 50s. May see a minor ridge/valley temperature split materialize this evening as 20-25 degree dewpoint depressions remain in place. Have seen these dwindle over the past couple of hours as passage of the aforementioned surface trough has allowed winds to veer more south to slightly southwesterly, negating the downslope effect. Warm air/moist advection will materialize later this evening and tonight as a surface low ejects out of the central Plains into the Midwest, subsequently lifting a warm front through the Tennessee Valley and eastern Kentucky Tuesday morning. This will take place downstream of an upper level trough moving across the Intermountain West, increasing 0-6 km shear to on the order of 50 knots. Following early morning showers moving in across the Bluegrass and Lake Cumberland regions, showers and thunderstorms will increase across the remainder of eastern Kentucky throughout the late morning and afternoon. Instability does look marginal as temperatures warm into the mid 60s to near 70, with surface-based CAPE values perhaps approaching 500 J/kg as a stubborn elevated mixed layer remains in place, stemming from earlier day warm air advection aloft. Regardless, enough low level veering, leading to 0-1 km storm-relative helicity values of 250-400 m2/s2 as winds remain backed near the warm front, will exist for updrafts to remain discrete, given they can grow tall enough to become organized throughout a significant depth in a highly sheared environment. Although any stronger storms are expected to be isolated in nature, gusty southwest winds will become moreso underneath any storms that develop. Isolated tornadoes can also not be ruled out given rotating updrafts and sufficiently low LCLs as PWATS of near 1.2 inches advect in. Winds aloft will increase further Tuesday night as the upper trough draws nearer. While an overall lull in shower/thunderstorm activity is expected late Tuesday afternoon/evening, gusty winds will continue through the night. Higher ridges may gust to upwards of 35 mph or greater as deep layer shear further intensifies. Shower and thunderstorm chances will increase later in the night toward dawn Wednesday ahead of the approaching cold front, but more widespread storms should hold off until after sunup. Any storms that do develop will have the potential to produce severe wind gusts as higher momentum air aloft will not have far to mix down. Additionally, some drying aloft may lead to an increased threat for hail through the night. Will also have to monitor the potential for isolated flooding given the degree of moisture in place, but the overall progressive nature of this system should keep widespread flooding a low risk. .LONG TERM...(Wednesday through Monday) Issued at 309 PM EST MON FEB 27 2017 The primary forecast issue in the extended period will be the potential for strong to severe thunderstorms on Wednesday. A fairly well defined cold front is forecast to move across the area on Wednesday, which will bring widespread showers and some thunderstorms to eastern Kentucky. The best potential for strong to severe thunderstorms will likely be between 18 and 23Z on Wednesday, when the model data is suggesting the best instability will combine with the lift along the front the strong low and midlevel wind shear. The primary threat with the strongest storms will be damaging wind gusts. The rain should taper off quickly as the front moves east of the area early Wednesday night, with the last rain forecast to exit the area by 6Z. We should see a brief break from the precipitation Wednesday night and Thursday, as a ridge of high pressure sets up just to our south. The models are still suggesting that weak area of low pressure aloft will move across the Ohio Valley region Thursday night into Friday morning. The temperatures during that time would allow for rain and snow showers to occur, should the model data be correct. Since the models have been forecasting the passing upper low toward the end of the week fairly consistently, decided to keep precipitation in the forecast for the northern half of the forecast area for Thursday night into early Friday. The system will be starved for moisture, so only slight chance to chances PoPs were used once again. Any snow accumulation that would occur would likely be confined to our far northwestern counties, and would be a tenth of an inch or less on grassy areas. Once this fast moving systems moves past us, the weather should be dry Friday through Sunday. A bit better developed weather system is being forecast to affect the area Sunday night into Monday. However, the models are not in the best agreement to end the period, so only slight chance to chances PoPs were used in the forecast for Sunday night into Monday morning. The blended model PoPs seemed way overdone, so the lower PoPs were used to account for the model uncertainty. Temperatures during the period will be above normal yet again, with daily highs varying from the 40s to the 60s across the area. Nightly lows will be cold during the first half of the period, with minimum values in the 20s and 30s expected. Low temperatures Saturday night are forecast to fall into the upper 30s and lower 40s, with minimum values expected to only fall into the mid to upper 40s Sunday night. && .AVIATION...(For the 00Z TAFS through 00Z Tuesday evening) ISSUED AT 753 PM EST MON FEB 27 2017 Skies are clearing across the area this evening. This presents some problems, especially with respect to the potential of fog through the overnight. Models are a bit slow picking up on the clearing and consequently may not be picking up on the potential for valley fog either. Models do suggest a return of mainly mid level cloud cover through the pre dawn hours at our southwestern terminals. In addition, southerly gradient winds are expected to increase some through the overnight which should help keep our western valleys from decoupling. Concern is that these may not materialize, leading to a stronger potential for some early morning fog Tuesday. With these recent and quickly changing trends will evaluate further and update the forecast as necessary. Winds will continue to increase Tuesday ahead of a disturbance passing through the region. This feature will generate rain showers which will approach from the southwest by Tuesday morning. Ceilings will lower in response, eventually dropping into MVFR territory. In general model guidance is suggesting only an isolated to low end chance type threat for thunder Tuesday. In addition, am not overly impressed with surface based instability due to lack of heating though there is some elevated instability showing up in some of the forecast soundings. For now decided to leave out any mention of thunder in the vicinity of terminals and instead may reintroduce the threat of thunder as a general cloud element with future TAF issuances. && .JKL WATCHES/WARNINGS/ADVISORIES...NONE. && $$ UPDATE...RAY SHORT TERM...GUSEMAN LONG TERM...AR AVIATION...RAY

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