Area Forecast Discussion
Issued by NWS Jackson, KY

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013 FXUS63 KJKL 231450 AFDJKL AREA FORECAST DISCUSSION National Weather Service Jackson KY 950 AM EST Thu Feb 23 2017 .UPDATE...
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Issued at 950 AM EST THU FEB 23 2017 Made a quick update to bring hourly grids in line with observations. Temperatures are a bit slow out of the gate in our eastern valley locations this morning. Otherwise morning fog has been lifting and dissipating as expected. HRRR suggests that our slight pops should reach a bit further south but is initializing poorly based on our current radar mosaic. Consequently will hold off on any increases in PoPs until a more definitive trend shows up. Also updated the zone package to remove morning fog and refresh wording. UPDATE Issued at 640 AM EST THU FEB 23 2017 Opted to beef up SKY grids in the far NW portions the CWA given the latest IR SAT data. Otherwise more minor update to deal with latest obs and trends.
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&& .SHORT TERM...(Today through Friday) Issued at 400 AM EST THU FEB 23 2017 Morning surface analysis showing surface high pressure parked off the Carolina coast line. This is bringing some return flow to the region with winds out of the SE to SW for most. The valley sites have been able to decouple and the has lead to areas of dense fog especially in locations that saw rain yesterday and/or nearer bodies of water. Overall think the SPS will handle the situation, as we could see improvements as high clouds move into the region. WV loop does show a weak upper level wave moving east out of Missouri this morning with weak mid to upper level QV convergence seen in portions of southern Indiana and Northern Kentucky. This combined with mid level moisture being transported from the Baja is inducing a line of lower clouds to our NW, and this lines up well with some of the short term guidance data. That said did keep some isolated showers mainly north of the Mountain Parkway, but think the instability is overdone for this afternoon in much of the short term guidance. That said, did opt to remove thunder for the afternoon and this also aligns with the SPC thinking at this time. Clouds and potential for precip will keep temps in the low 70s for most which remains 20 degrees above normal for his time of year. All the focus will shift to the west, as deepening surface low and upper level system takes shape across the Central Plains by tonight. This dynamic system will begin to induce a LLJ out ahead of the main system, but the valleys will still have a chance to decouple. This could lead to some slight temp splits and perhaps some patchy fog. However do think as low level inversion mixes out the winds will begin to kick up early in the day Friday. Winds of 25 to 30 mph will be common in the afternoon across portions of the Bluegrass and Lake Cumberland regions. Right now model soundings suggest the driest air is not quite taped into, but this will have to be looked at for brief window of fire weather concerns Friday afternoon. Also this could lead to even higher afternoon temperatures with many locales looking at upper 70s and some even closing in on 80 degrees. These kind of temperatures would shatter the current record high temperatures for Friday of 71 at JKL and 70 at LOZ. Based on model trends think the Friday afternoon period will remain dry for most and therefore did slow POPs from what the model blend produced. .LONG TERM...(Friday night through Wednesday) Issued at 445 AM EST THU FEB 23 2017 Early in the period, a ridge is expected to extend north into the Carolinas while an upper level low is expected to be centered over the MS Valley region with an associated shortwave trough extending south through the MS Valley. From the Plains west to the Western Conus, a broad trough is anticipated. At the surface, an occluding area of low pressure will be nearing the western Great Lakes to start the period, while a cold front should extend south into the lower OH Valley and TN Valley from a triple point low over the Great Lakes region. A prefrontal trough could also be moving across the Commonwealth at that point. The cold front is expected to move across the area Friday night into early on Saturday as the sfc low tracks to the northern Great Lakes and into Ontario. Convection, a line or perhaps one or more broken lines of showers and thunderstorms should precede the front into the region late Friday evening or Friday night. This dynamic system will have a decent amount of moisture to work with a PW may approach 1 inch. Timing of the front is a limiting factor as instability should be limited to modest at best, but winds aloft and shear will be on the strong side with 700 mb winds near 60 KT. The shear is generally unidirectional as well so strong to damaging winds will be possible as any thunderstorms should be able to mix down higher momentum from aloft. SPC has placed the entire region in a slight risk of severe thunderstorms for Day 2 with the exception of the far southeast which is in a marginal risk. The HWO has been updated to highlight the threat although there is uncertainty as to how quickly convection might arrive into the region or perhaps develop and how far east the strong to severe threat would last as locations further east may decouple in the evening and these areas would also be farther front the best forcing. Most if not all locations should experience measurable rainfall on the order of two tenths of an inch, but with convection, amounts should be variable. Meanwhile, the coldest air will lag behind closer to the axis of the 500 mb trough, and a secondary cold front or trough should usher in the coldest of the air from about midday Saturday through Saturday evening. Although a dry slot should have brought a decreasing chance for showers by dawn on Saturday, the chance of showers will persist as cold air advection and solar insolation steepens low level lapse rates. This will lead to a continued chance for showers during the day on Saturday, but as drier air moves into the area with high pressure building in at the surface and aloft, this threat should diminish by around sunset on Saturday. Highs on Saturday should occur in the morning or by midday, with falling temperatures by mid to late afternoon. 850 mb temperatures should bottom out around -8C to -10C near 6Z or about 1 am on Sunday. Surface high pressure will build into the area on Saturday night and with some clearing and the colder airmass in place, overnight lows will fall into the 20s. These readings are not atypical for later part of February, but are colder than what has been experienced recently. By late Saturday night and Sunday the flow aloft will be generally westerly and the airmass will moderate on Sunday. Near normal highs are expected on Sunday. Uncertainty then grows by Sunday night through the middle of next week. The 0Z GFS is a bit quicker dropping a shortwave south along the west coast of the CONUS over the weekend and then across the Four Corners region and into the Plains by Sunday night. The GFS runs have generally been more robust with this shortwave as well and have a stronger and more northern surface low track. Meanwhile, the ECMWF is generally slower and weaker. For now have followed the Superblend for timing of pops and temperatures to start the week. If precipitation were to arrive as quickly as the GFS and Superblend pops would suggest on Monday, there could be some precipitation type issues at onset, especially if eastern valley locations can decouple and drop below freezing on Sunday night. The temperature profile is marginal at best for rain versus snow on forecast soundings then as well. At this time, we have continued with a chance of rain or snow showers for late on Sunday night into very early on Monday with the uncertainty. Further warm air advection should change any linger precipitation to rain on Monday morning. The region will remain very mild through early next week with the axis of the 500 mb trough approaching by midweek. Models vary from run to run and model to model, but the general consensus at this time is that an area of low pressure should track northwest of the area around or just before midweek with a cold front approaching in advance of the mid and upper level trough that should approach the MS Valley near the end of the period. This pattern will lead to continued unsettled weather with the best chances for precipitation near the end of the period. Given the mild pattern, thunder cannot be ruled out Tuesday night into Wednesday as well and models do have some limited instability during that period as well. && .AVIATION...(For the 12Z TAFS through 12Z Friday morning) ISSUED AT 622 AM EST THU FEB 23 2017 Clear skies overnight has kept the valleys in the fog and this has affected LOZ/SME at times through the night. This will lift by around 9 AM for most. A weak upper level disturbance is moving east and this is aiding in developing an area of stratus across the Bluegrass to begin the period. This will move SE through the day, but lower ceilings are expected to remain across the north. Most of the short term guidance including the CONSSHORT are too low with CIGs through the period. That said, did keep the lower MVFR CIGS at SYM and keep other sites VFR. Winds will remain light through the period, but 850MB jet does ramp up toward dawn Friday across the Bluegrass and Lake Cumberland region. Given the low level inversion in place some LLWS is possible at SME/LOZ early before they mix out. && .JKL WATCHES/WARNINGS/ADVISORIES...NONE. && $$ UPDATE...RAY SHORT TERM...DJ LONG TERM...JP AVIATION...DJ

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