Area Forecast Discussion (AFD)
Issued by Huntsville, AL (HUN)
428 FXUS64 KHUN 220302 AFDHUN Area Forecast Discussion National Weather Service Huntsville AL 1002 PM CDT Thu Jun 21 2018 .NEAR TERM...(Rest of tonight) Issued at 1002 PM CDT Thu Jun 21 2018 A loosely-organized arc of convection continues to translate east- southeastward across the region, driven by outflow from storms that developed earlier this afternoon across the mid-South region. High resolution model guidance suggests that the leading edge of the line of storms will remain intact as it advances further across the region this evening, and this seems entirely plausible based on the degree of synoptic scale ascent evident by 40 m 12-hr height falls and enhanced by increasingly diffluent flow aloft. Although individual storm cells will continue to weaken as they encounter an increasingly stable boundary layer across northeast AL, the threat for frequent cloud to ground lightning, locally heavy rainfall and brief wind gusts in the 40-50 MPH range will persist with the strongest storms. To the west of our CWFA, thunderstorms will continue to redevelop both along the westward trailing outflow boundary, and to the north of this boundary as the low-level jet strengthens atop a vast cold pool in the wake of the initial convective system. Deep-layer steering flow from the west will spread this activity eastward into the region late this evening and early Friday morning, and POPs have been adjusted upward region-wide to account for these factors. With time, we do anticipate the greatest coverage of storms to gradually shift south of the TN River, but this may not occur until closer to sunrise. If sufficient echo training develops later this evening, then a flash flood threat may materialize across portions of the region. .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 00Z TAFS through 00Z Friday evening) Issued at 704 PM CDT Thu Jun 21 2018 VFR conditions currently exist at the HSV/MSL terminals, with sct layers of cu/ac and moderate sw flow. However, a cluster of tsra is currently approaching the region from the NW, and this has warranted tempo groups for tsra btwn 02-05Z/MSL and 03-06Z/HSV. A few of the storms will be capable of producing frequent CG lightning strikes and wind gusts of 30-40 knots, which may prompt AWW throughout the evening. However, the main impacts will likely be brief periods of MVFR cigs/vsby in the heavier pcpn. The most concentrated region of shra/tsra should be south/east of the terminals by 06-07Z, with an IFR stratus deck expected to develop rapidly in its wake. Cloud bases will likely remain in the 800-1500 FT range thru 14Z, before quickly lifting/scattering with the onset of stronger southwest flow and boundary layer mixing. Sct-nmrs tsra will redevelop by 17Z, within a broad but ill-defined sfc trough axis. However, due less favorable support for storms aloft and a more scattered coverage anticipated, we will only include VCTS at this point. && .HUN WATCHES/WARNINGS/ADVISORIES... AL...NONE. TN...NONE. && $$ NEAR TERM...70/DD SHORT TERM...AMP.24 LONG TERM...12 AVIATION...70/DD For more information please visit our website at weather.gov/huntsville.