Area Forecast Discussion
Issued by NWS Jackson, KY

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000 FXUS63 KJKL 221148 AAA AFDJKL AREA FORECAST DISCUSSION...UPDATED National Weather Service Jackson KY 748 AM EDT Wed Mar 22 2017 .UPDATE... Issued at 743 AM EDT WED MAR 22 2017 Hourly grids have been updated to account for the cold air advection and drier dewpoints advecting into the area on north to northeast flow between high pressure and the cold front sagging across the Gulf Coast states. Winds have been a bit gusty at times, but as winds aloft slacken through the morning sustained winds and gusts should slacken. Low humidities are expected this afternoon as temperatures warm and dewpoints drop due to advection and mixing. && .SHORT TERM...(Today through Thursday) Issued at 400 AM EDT WED MAR 22 2017 Early this morning, the axis of shortwave trough is moving across Ontario to the eastern Great Lakes to Appalachians. Meanwhile, the axis of an upper level ridge extended from northern Mexico north across the high plains. At the surface, a cold front continues to push further south across the Gulf states in response to the upper level trough. Meanwhile, surface high pressure centered over MN and far west Ontario is building across the Great Lakes and OH Valley region. As the axis of the mid and upper level ridge moves east near the start of the period, mid and upper level heights will begin to rise and center of surface high pressure will reach the central Great Lakes by evening. As this trough departs clouds should thin with any cu and ac becoming more thin through the day and dissipating. At the same time, a very dry airmass, especially aloft will move into the area with 1000 to 500 mb mean layer rh progged to be 20 percent or less this afternoon. PW is also expected to plummet to well below average for this time in March, below 0.20 inches this afternoon. All of these and some mixing into this very dry subsidence layer point toward dewpoints reaching the lower MET MOS numbers as compared to model blends for mid to late afternoon and yielding rh briefly reaching the 18 to 26 percent range area wide. The dry airmass should also warm despite cold air advection and reach the mid 40s far north to low to mid 50s south. The dry airmass will remain in place tonight as the axis of the upper level ridge approaches and the center of the surface high reaches upstate NY by dawn. There will be some increase in clouds initially high and then low mainly after midnight. However, with the dry airmass and a slackening pressure gradient, deeper valley locations should decouple around or shortly after sunset and broader valleys a few hours later. The airmass will be driest and clouds will arrive later generally east of I 75 and the latest in the far northeast. COOP MOS guidance and dewpoints support typically colder spots such as Sandy Hook and the Paintsville KY Mesonet site reaching the lower 20s with temperatures for most of the area east of I 75 bottoming out in the 20. Since the dewpoints will be low and dewpoint depressions expected to not close in on the 0-2F range, frost formation will be minimal. Most ridgetop and southwest locations should experience temperatures reaching around 30 degrees. With the increase in high clouds and eventually mid clouds between 2 AM and 8 AM from southwest to northeast confidence in hourly temperatures later at night is on the low side. Some valley locations may actually warm during that period as net radiational cooling decreases. However, a freeze warning will almost certainly be needed for some portion of the area for tonight given the advanced nature of some vegetation due to the warm February. Warm air advection which will already have begun tonight will increase substantially on Friday with 850 mb temps expected to rise to around 5C by late afternoon. There will be some mid and high clouds from time to time and probably some cu around 850 mb or just below as upper levels and then mid and lower levels begin to moisten with the flow from the surface 850 mb becoming more southerly. A downslope component will linger, however, in the far east and dewpoints will be slower to increase nearer to the VA and WV borders. Temperatures will warm to above normal levels on Wednesday and should reach the upper 50s in the far north to the mid 60s in the south. This will begin a period of above normal temperatures that should linger well into the long term period and beyond. .LONG TERM...(Thursday night through Tuesday) Issued at 352 AM EDT WED MAR 22 2017 Above normal temperatures and multiple rain chances will be the story for the long term period. An area of surface high pressure will be parked of the Mid Atlantic Coast to begin the period and heights will be on the rise as upper level ridge builds east. This will combine with lifting warm front to bring above average temperatures to the region. Right now looks like this warm front will be dry here in the eastern side of the state Thursday night. All eyes then turn to the Rockies for already developed upper level closed low and nearly vertically stacked system. This will system will eject east and then northeast into the Mississippi River Valley by Saturday. Given the models are backing off on speed with this did opt to back off on POPs for Saturday. Overall think the best chances will occur Saturday night into Sunday based on the latest guidance. Keep some slight thunder in the grids with some instability Saturday afternoon through Sunday. Overall this system becomes less significant, as the surface low weakens as it moves NE into the Great Lakes. The ridging behind this will be brief, as another wave ejects east out of the Rockies into the Mississippi River Valley Monday. Given timing uncertainty with this will keep some POPs in the grids on Monday, but could see a small window of drying before precipitation chances rise again Late Monday into Tuesday. && .AVIATION...(For the 12Z TAFS through 12Z Thursday morning) ISSUED AT 748 AM EDT WED MAR 22 2017 VFR will prevail through the period with high pressure dominating. A few mid and high clouds are possible initially, but most of the 12Z to 6Z period should be relatively cloud free with a dry airmass expected through the entire column. However, moisture will increase toward the later end of the period at the high and mid levels and an increase in mid and high clouds is anticipated late. the area. Winds will average out of the north to northeast near 10 KT initially with higher gusts, then during the 15Z to 20Z period slacken to 5 to 10KT. With the loss of daytime heating, winds will diminish to 5KT or less in most locations during the 22Z to 3Z period. && .JKL WATCHES/WARNINGS/ADVISORIES... Freeze Warning until 10 AM EDT this morning for KYZ044-050>052. && $$ UPDATE...JP SHORT TERM...JP LONG TERM...DJ AVIATION...JP is the U.S. government's official web portal to all federal, state and local government web resources and services.