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829 FXUS63 KLMK 211043 AFDLMK AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE LOUISVILLE KY 643 AM EDT Mon Apr 21 2014 ...Updated Aviation Discussion... .SHORT TERM (Now through Tuesday)... Issued at 305 AM EDT Mon Apr 21 2014 The synoptic pattern early this morning features a cutoff low just off the southeast coast. In its wake, an upper-level ridge has established itself over the Ohio Valley. However, an upper-level trough will approach and push through the region during the short term period, bringing chances for showers and isolated thunderstorms. Upper-level clouds continue to increase this morning out ahead of the aforementioned system. These clouds will continue to increase in coverage and thicken throughout the day. Despite the increasing cloud cover, still think there will be some sunshine from time to time today, especially across the eastern CWA. Additionally, mechanical mixing will be much better than yesterday, which should help to make up a bit for less sunshine. Therefore, will go with temperatures very similar to Sunday`s values, which puts highs in the upper 70s and lower 80s. The main forecast challenge will come with convection potential late this afternoon into tonight. Overall thinking is that this system may a bit of an underachiever due to a few factors. First, low-level moisture is lacking, and we once again have the potential to mix down drier air during peak heating today (especially east of I-65 where surface moisture advection will not be as robust). This will tend to limit overall instability late this afternoon. Additionally, a very dry layer exists between 600-800mb, which will be hard to overcome even with a slug of isentropic lift progged to slide across the region late this afternoon. Therefore, will continue with previous forecast thinking and keep low-end pops going along and west of I-65 by 00Z Tuesday. If an isolated shower or storm were to develop this afternoon in peak heating, it could be capable of some gusty winds given large surface dewpoint depressions and the potential for mid-level dry air entrainment. By tonight, synoptic forcing for ascent increases which will help pull deeper moisture into the region. This will enhance the precipitation coverage, but will still forecast pops in the 60-70% range. Given very weak convergence along the surface front and mediocre dynamics, can`t see going any higher with pops. The latest hi-res solutions support this thinking, showing a few gaps in coverage as the precipitation pushes through tonight. Given the region will be ahead of the surface front tonight, will go on the high end of guidance with lows, which puts them in the upper 50s and lower 60s. Precipitation will push to the southeast of the region by early Tuesday afternoon as the surface front slides across the region. Northwesterly winds behind the front will usher in a cooler and drier airmass as temperatures struggle in the 60s and lower 70s. .LONG TERM (Tuesday Night through Sunday)... Issued at 300 AM EDT Mon Apr 21 2014 Main forecast challenges in this period include just how cold it will get Tuesday night, and timing of precip associated with a late-week closed low in the Great Lakes. Respectable 1022mb surface high will nose southward from the Great Lakes, with the ridge axis near I-65 by 12Z Wednesday. Min temp forecast is fairly well in line with latest MOS guidance, with lows around 40 for most of the area, but plenty of areas in the Bluegrass region in the mid/upper 30s. Light NNE breeze staying up through the night would keep temps from bottoming out and mitigate any frost concerns. However, the GFS is fairly bullish with the ridging into the Ohio Valley, and if it verifies we could see quite a few of the sheltered valleys and typical cold spots flirting with the freezing mark. Next couple of forecast cycles will need a close look at the frost/freeze potential Wednesday morning, so stay tuned. Expect a fairly quick recovery in temps starting on Wednesday, going back above normal for Thursday and Friday. Next system is on tap for Thursday night, as a series of impulses will pinwheel around a "bowling ball" upper low making its way across the Great Lakes. Consensus seems to be building for the best rain chances to occur Thursday night as a negatively tilted impulse moves through, so will go with high-end chance/low-end likely POPs. Another impulse will reinforce the trofiness aloft Friday night and push a surface front through, but at this time it looks like that system will be moisture-starved. Below-normal temps are expected over the weekend, with rain chances re-entering the picture on Sunday ahead of an upper low developing over the Plains. `Tis the season for cutoff lows, and it looks like we will have the opportunity to become very familiar with this particular system as April comes to a close. That said, it is mostly beyond the scope of this forecast, and there are still details to be worked out yet. && .AVIATION (12Z TAF Update)... Issued at 640 AM EDT Mon Apr 21 2014 An area of low pressure will approach the Ohio Valley today. Out ahead of this feature, high and mid-level clouds will continue to increase through the day, but all cigs will remain VFR. Winds will increase out of the SW to around 10 knots today. By tonight, the system will spread showers and a few embedded thunderstorms across the region. Specific timing of these showers still is a bit uncertain, given coverage is only expected to be about 60%. Therefore, will continue with VCSH wording, although prevailing -SHRA will likely be needed at some point tonight. The moderate showers will be capable of dropping cigs/vsbys briefly to MVFR thresholds. In addition, guidance suggests the potential for some MVFR cigs toward the end of the TAF period and into the planning period for KSDF. These ceilings will largely hinge on precipitation coverage overnight, thus will leave cigs at VFR for now until trends can be better ascertained. && .LMK WATCHES/WARNINGS/ADVISORIES... KY...NONE. IN...NONE. && $$ Short Term........KJD Long Term.........RAS Aviation..........KJD