Area Forecast Discussion (AFD)
Issued by Huntsville, AL (HUN)
579 FXUS64 KHUN 121445 AFDHUN Area Forecast Discussion National Weather Service Huntsville AL 945 AM CDT Sun Aug 12 2018 .NEAR TERM...(Rest of Today) Issued at 945 AM CDT Sun Aug 12 2018 Mid/high level cloud cover continues to spread ewd into the cntrl TN Valley this late Sun morning along the srn base of an exiting upper trough pattern over the OH Valley/mid Atlantic states. Area radars have shown a little bit of light rainfall trying to develop over west cntrl MS, although this activity is having a little bit of trouble developing more to the east. Otherwise, the local area remains sandwiched between two weak frontal boundaries. The first is layered west to east from the Midwest states into the mid Atlantic while the second is located from NW MS into cntrl GA. Intermediate model guidance doesn't move the nrn boundary much while the srn front drifts a bit swd thru the day. In any case, there's not a real clear cut focus mech in place for potential showers/tstms later today, xcpt for perhaps along the wrn fringe of the srn boundary. This may at least warrant iso showers/tstms across much of north AL later today, or especially locations along/west of I-65. Outside of any rainfall, it should be a seasonably warm afternoon, as highs climb well into the lower 90s for most areas. .SHORT TERM...(Tonight through Tuesday night) Issued at 504 AM CDT Sun Aug 12 2018 The cutoff upper low is progged to remain nearly stationary across eastern OH tonight, with a secondary vort max rotating around this feature forecast to dig southeastward through middle TN and into the southern Appalachians. At the surface, the trough axis is expected to become oriented in a northwest-southeast fashion and eventually stall across the far southwestern corner of the CWFA. Although we will maintain a slight chance POP in this region throughout the night due to the proximity of the low-level boundary, a much drier airmass is expected to spread across the remainder of the region, with PWAT values falling to near or even slightly below 1 inch by 12Z Monday. Environmental conditions would normally favor the development of fog in this regime, but due to the magnitude of dry air advection, we have only included a patchy coverage at this point. Short term models suggest that the cutoff low to our northeast will begin to shift eastward across the central Appalachians and into the northern mid-Atlantic Monday/Monday night, as a high-amplitude mid- level ridge translates eastward across the region. We will maintain a very low POP across the southwestern portion of the forecast area to account for uncertainty regarding the precise location of the stalled surface wind shift axis. However, the majority of the region will remain dry and very warm with highs in the u80s-l90s. Fortunately, it appears that a deep mixing layer will develop on Monday afternoon, with falling dewpoints keeping Heat Index values in check. The mid-level ridge axis will cross the region on Tuesday, with flow aloft expected to back to west-southwest and increase on Tuesday night downstream from another trough lifting northeastward through the central Plains. Deep moisture will be advected northeastward into the region as this occurs, with PWATS rapidly increasing to 1.5-1.7 inches by Tuesday afternoon. We have indicated a low chance POP mainly for the northwestern half of the region on Tuesday/Tuesday night as a result of the moisture advection and related increase in instability. Highs will remain in the u80s-l90s on Tuesday, with mild lows in the l-m 60s early in the week expected to warm into the u60s-l70s by Wednesday morning. .LONG TERM...(Wednesday through Saturday) Issued at 504 AM CDT Sun Aug 12 2018 Guidance overall continues to hint towards delaying rainfall over Alabama a bit longer ahead of the approaching longwave trough axis over the western Great Lakes region southwest into Missouri on Wednesday. Mainly due to the strong upper level ridge in place over Alabama/Georgia into the northeastern CONUS. More persistent and thicker cloud cover may hold off until the afternoon hours. With 925 mb temperatures forecast to be 25 to 27 degrees (ECMWF as high as 27 to 29 degrees) and ample morning to early afternoon heating near and south of the Tennessee River, think that highs between 93 and 95 degrees looks possible in those areas. It may be even warmer if the ECMWF is right and the ridge holds the longwave trough axis further west into Wednesday afternoon across the entire area (maybe looking at highs in the lower to upper 90s areawide if it is right). Luckily it will not be extremely humid with dewpoints in the mid to upper 60s primarily. However, heat index values will still reach the 95 to 100 degree range most likely. It should be a bit cooler in southern middle Tennessee and near Lauderdale county in Alabama due to cloud cover, most likely upper 80s to around 91 degrees. Synoptic models seem to keep this trend into Wednesday night (the exception being the GFS, which brings higher pops into extreme northwestern Alabama by then). Given the strong ridge that the longwave trough has to break down, trending slower and closer to the ECMWF with pops. Kept most of the area free of shower or thunderstorm activity Wednesday night - the exception being northwestern Alabama with 20-30 percent pop. Thursday morning should be pretty warm with moisture pooling over the area. Kept lower 70s in the forecast, but would not be surprised if this ends up being closer to 71 to 76 degrees in many locations in northern Alabama and southern middle Tennessee. Most synoptic models finally break down the ridge over Alabama/Georgia Thursday morning as the longwave trough axis moves into eastern Arkansas/extreme southeastern Missouri. 500 mb winds and forcing picks up as a result. Bulk shear values will remain very low. However being on the right entrance region of a weak low level jet ahead of the longwave trough may help to provide better lift over the region. Thus have 30 to 60 pop (SE to NW) across the area. High temperatures should be a bit lower due to more prevalent cloud cover and rain chances, primarily in the upper 80s to around 90 degrees. PWATS are very high, around 1.9 inches, so heavy rainfall will be possible in stronger thunderstorm development. Some wind gusts to around 45 mph may occur with the strongest storms. As this boundary stalls somewhere across northern Alabama or slightly further north through Friday before sinking further south into central Alabama by late Friday night, high chance to more numerous showers and thunderstorms seem reasonable during this timeframe. PWATS will remain around 1.8 or 1.9 inches, so heavy rainfall in stronger storms and any training activity will remain possible. Cooler daytime highs (85 to 90 degrees) and quite humid conditions are expected. Lower instability values and a more saturated atmospheric profile should keep wind gusts a bit weaker with the strongest storms on Friday/Friday night. Models have quite different solutions synoptically Saturday through Sunday. At this point given model differences kept to blended guidance, keeping high chance to more numerous showers and thunderstorms and overall lower temperatures in the forecast through Sunday night. A bit more shear shows up in some models either Sunday or Monday with disturbances moving through the area. This could be a period of stronger thunderstorm activity, but pretty far out there in the forecast right now. && .AVIATION...(For the 12Z TAFS through 12Z Monday morning) Issued at 717 AM CDT Sun Aug 12 2018 VFR conditions will persist at the HSV/MSL terminals this forecast period. However, bkn-ovc altostratus/cirrostratus clouds generated by an upper air disturbance over the southern Plains will provide cigs AOA FL120 for much of the day as they continue to overspread the region from the west. A few light shra or pockets of lgt rain may occur in this regime later today, especially to the southwest of the terminals, but threat for impacts is too uncertain to include in the TAFs at this point. Sfc winds will veer to NNW late this aftn as a sfc trough shifts southward, and this should coincide with a more pronounced clearing trend. SKC conds are anticipated overnight, and may allow for the formation of patchy river valley fog as flow veers to the NNE and subsides. However, anticipated coverage of fog does not warrant inclusion of vsby reductions in the TAFs attm. && .HUN WATCHES/WARNINGS/ADVISORIES... AL...NONE. TN...NONE. && $$ NEAR TERM...09 SHORT TERM...70/DD LONG TERM...KTW AVIATION...70/DD For more information please visit our website at weather.gov/huntsville.