629 FXUS64 KHUN 201851 AFDHUN Area Forecast Discussion National Weather Service Huntsville AL 151 PM CDT Thu Jun 20 2019 .NEAR TERM...(Tonight) Issued at 152 PM CDT Thu Jun 20 2019 Pre-frontal convection has begun to develop in southern middle TN. This is at the front edge of a ridge of higher SBCAPE of 2000-2500 j/kg in TN just ahead of a cool front. THe front was in southern KY into northwest TN. Latest HRRR runs have suggested that these pre- frontal storms will affect far northeast AL and southern TN through late this afternoon, while the cold frontal cells remain over northern and eastern TN. This as surface flow in northwest AL and southern TN becomes west-northwesterly and limits convergence. Subsidence behind the departing mid-upper level shortwave will also tend to limit further activity with the cold front this evening. The front will likely stall to our north this evening as a mid and upper level ridge axis amplifies quickly tonight over mid-lower MS valley. .SHORT TERM...(Friday through Saturday) Issued at 152 PM CDT Thu Jun 20 2019 The ridge axes will shift east into middle TN and AL on Friday as troughing advances into the Plains. This will create vast southerly low level jet through the southern Plains into the Ozarks meeting up with the warm front lifting back northeast into the corn belt. This sets up a favorable pattern for multiple MCSs over IA/IL that will track southeast through the OH valley as the low level jet veers to a southwest-west direction. The NAM is insistent on ridging/capping will hold over the middle TN valley on Friday keeping convection at bay. Forecast soundings Friday afternoon indicate potent instability (thanks to sfc temps in l90s and dew points in the l70s). The GFS is forecasting the MCS in the corn belt, but also a secondary MCS cluster over southeast MO into the mid South region by late morning into the afternoon. Steering flow would bring this into the central TN valley during the afternoon and evening. I'm not overly confident in this solution, but cannot ignore it completely. SPC has also included much of our area in a SLGT and MRGL risk area for Day 2. For now, will continue to include a mention of strong storms in case the forecast flip-flops tonight into Friday and the NAM solution appears more plausible. My confidence is greater on MCS activity ridge riding from the OH valley southeast and clipping our TN and northeast AL counties on Saturday. In either case, CAPE values will be sufficient to support strong to severe wind gusts and perhaps hail with these storm clusters if they develop. The other impact weather will be the growing heat/humidity. With temps rising into the l90s Friday into Saturday and dew points staying in the l70s, heat index values will reach around 100 degrees +/- both afternoons. .LONG TERM...(Saturday night through Wednesday) Issued at 152 PM CDT Thu Jun 20 2019 Strong upper ridging continues to be shown by most guidance on Sunday. Some upper level energy is shown pushing into the eastern edge of the upper ridge and into southern middle TN or northeastern Alabama during the afternoon hours. Both GFS and ECMWF develop some convection in these areas as a result. Instability ranging from 2000 to 3000 J/KG at least (some guidance as high as 4000 J/KG) is seen in most model output. Given forcing, isolated to scattered shower and thunderstorm activity will likely result. Couldn't rule out some strong thunderstorm activity. Main threats would be gusty winds around 50 mph and small hail. Despite convective activity, highs in the 90 to 95 range look reasonable. This should allow heat index values to climb to around 102 degrees. The synoptic pattern changes on Monday, as a frontal boundary moves into the area. Overall instability will be less, but still ample for strong thunderstorm development. There might be a better chance for an isolated severe thunderstorm due to increases in bulk shear ahead of the front. Cooler temperatures due to more widespread cloud cover and precipitation looks likely. This frontal boundary looks to stall somewhere over southern or central Alabama and Georgia into the middle of next week. By Thursday another weak front looks to move into the Tennessee Valley. This should keep at least scattered shower and thunderstorm activity in the forecast. This boundary also stalls and keeps much the same situation in place into next weekend. && .AVIATION...(For the 18Z TAFS through 18Z Friday afternoon) Issued at 1140 AM CDT Thu Jun 20 2019 Scattered to broken SC/CU (~030-040agl) will develop through mid afternoon along and ahead of a weak cold front. Isolated thunderstorms are possible for KHSV and KMSL, but the prob is too low to include at this time. Westerly flow of 10-15kt with gusts around 20kt will diminish rapidly around 00-01Z with a clearing sky behind the cold front. VFR conditions are expected through the rest of the period as well. && .HUN WATCHES/WARNINGS/ADVISORIES... AL...NONE. TN...NONE. && $$ NEAR TERM...17 SHORT TERM...17 LONG TERM...KTW AVIATION...17 For more information please visit our website at weather.gov/huntsville.