Area Forecast Discussion (AFD)
Issued by Huntsville, AL (HUN)
697 FXUS64 KHUN 212008 AFDHUN Area Forecast Discussion National Weather Service Huntsville AL 308 PM CDT Thu Jun 21 2018 .NEAR TERM...(Tonight) Issued at 308 PM CDT Thu Jun 21 2018 An upper-low over Missouri will continue to deepen as it tracks into the Ohio Valley this evening and overnight. Ahead of this feature a shortwave trough/cold front as triggered a growing line of strong to severe thunderstorms over the Mississippi Delta region of eastern Arkansas. This line of storms will continue to push ESE/SE across Western Tennessee and Northern Mississippi late this afternoon and potentially into Northwest Alabama by 00-01z. Current observations suggest the thermodynamic environment is less favorable across Northern Alabama, with CAPE values around 1000-1500 J/kg (as opposed to 2500-3000 J/kg further to the NW). Thus, am expecting this activity to weaken by the time these storms make it into the Tennessee Valley (especially as the atmosphere stabilizes with the loss of solar heating). However, cannot rule out a localized strong to perhaps marginally severe storm across NW Alabama late this evening -- mainly from gusty, damaging outflow winds. As alluded to, this activity will continue to weaken as it moves across the region between 03-06z, with generally scattered general thunderstorms prevailing through the remainder of the night. The high RH values and PWATs will make for heavy downpours in any of the more robust activity that remains. The cloud cover and precipitation will keep low temperatures on the mild side, in the lower 70s. Due think that some persistent SW winds and these clouds will limit fog potential overnight despite the moist boundary layer conditions. However, some low ceilings from stratus are certainly possible, especially early Friday morning. .SHORT TERM...(Friday through Saturday) Issued at 308 PM CDT Thu Jun 21 2018 Attention quickly turns to Friday as the upper-low swings across the Ohio Valley, dragging a cold front into the Tennessee Valley and Central Alabama. Clouds will continue to linger across the region Friday morning, but a plume of drier air aloft, ahead of the actual shortwave/cold front help to scour some them out -- resulting in some partial clearing and opportunities for heating and destabilization from late morning through the afternoon hours. Model soundings suggest between 2000-3000 J/kg of CAPE is possible by 18-21z Friday. Steep low/mid level lapse rates and LI values around -4C to -7C were also progged per the NAM/GFS model soundings by the afternoon as well. This will set the stage for the possibility of robust convection from early Friday afternoon through Friday evening. Additionally, a LLJ will begin to develop and lift NE into Northern Alabama by 18z, helping to provided some added lift and 30-40 kts of 0-3/0-6 km wind shear. Thus, organized convection should quickly initiate along/near the frontal boundary, as this additional forcing kicks in. Given the aformentioned parameters in place, the threat for strong to severe thunderstorms will exist across the region, mainly in the form of damaging winds and large hail (thanks to the combination of CAPE and Melting Levels around 13 kft). While isolated tornadoes will not be likely, one cannot totally be ruled out given the added wind shear with this system. Given that the profiles suggest this wind shear will mostly be unidirectional, the tornado threat will mainly be confined to areas where surface winds can locally back and/or and mesoscale boundaries. The greatest window for strong/severe storms will be around 3 to 9 PM CDT. Thereafter, this activity will wane by late Friday evening as the boundary sags south. Still, do have a mention of scattered general thunderstorms in the forecast through the overnight hours. With the boundary nearby for Saturday and models showing the potential for good destabilization again, think that scattered strong to severe storms will be possible again on Saturday -- through coverage will be lower due to the weaker shear and the lack of forcing as the upper trough lifts NE into the Appalachian Mountains. .LONG TERM...(Saturday night through Wednesday) Issued at 308 PM CDT Thu Jun 21 2018 The extended period begins Sunday morning with ongoing showers and storms in the vicinity, thanks to an H5 disturbance moving W to E in relatively zonal flow aloft. Additional showers and storms will develop through the day along a stalled frontal boundary to the N, sprawled W to E across TN. As such, PoPs were oriented higher to the N and lower to the S, with the boundary expected to sag S overnight into Monday. Storm threats Sunday/Sunday evening include gusty/damaging winds (with microbursts possible) thanks to MUCAPEs around 3000 J/kg and DCAPEs approaching 1500 J/kg. Showers and storms will affect the area off/on Monday as upper level ridging begins to build in aloft, eventually ending convective chances by Monday night/Tuesday. This brief dry period will linger for much of midweek next week as the upper level ridge becomes entrenched over much of TX and the SW States. We'll be on the Ern periphery of the ridge aloft, with sfc high pressure anchored well N/E of our local CWFA over the SErn Great Lakes region. This will set up Erly flow at the sfc, and NW flow aloft. Models are trying to spark convection beneath the ridge by Wednesday/Thursday, but am not going much above low-end chance PoPs for that time period just yet. It all depends on how far E the upper ridge extends, governing mid/upper level subsidence in place. At the sfc, there will be moisture slowly returning to the area to end the week as sfc high pressure off the E Coast provides SErly flow around the Wrn periphery of the sfc high. That could easily spark isolated/scattered storms, but am not too confident in much more than that this far out. Overall in the extended, daytime highs will top out in the upper 80s/lower 90s, with morning lows remaining rather soupy, starting out in the 69-74F range. By the weekend, the upper ridge begins to retrograde Wwrd and break down over our area, putting the CWFA back into NW flow aloft. As disturbances ride up/over the ridge, this should allow MCSs to march SE into the area, providing showers and storms once again as we head into the last weekend of June. Somewhat lower temperatures would be expected during this time with the additional cloud cover likely in place, but will wait and see on the finer details coming to light in the coming days. && .AVIATION...(For the 18Z TAFS through 18Z Friday afternoon) Issued at 1205 PM CDT Thu Jun 21 2018 VFR conditions will be the predominant flight category at each terminal this afternoon, with only light, isolated showers creating VCSH at both sites. Ceilings/vis may drop this evening at HSV and MSL from a cluster of storms progged to drop SE thru the region between 00-06z. Thereafter, dry conditions, but low MVFR ceilings around 1-2kft are expected to prevail. Clouds will gradually lift to VFR levels after sunrise. More robust convection is expected after 18z on Friday, beyond the scope of this TAF period (and was not mentioned). && .HUN WATCHES/WARNINGS/ADVISORIES... AL...NONE. TN...NONE. && $$ NEAR TERM...AMP.24 SHORT TERM...AMP.24 LONG TERM...12 AVIATION...AMP.24 For more information please visit our website at weather.gov/huntsville.