Area Forecast Discussion (AFD)
Issued by Huntsville, AL (HUN)
062 FXUS64 KHUN 151057 AFDHUN Area Forecast Discussion National Weather Service Huntsville AL 557 AM CDT Wed Aug 15 2018 .UPDATE... For 12Z TAFS. && .NEAR TERM...(Today) Issued at 420 AM CDT Wed Aug 15 2018 The storm system currently centered over western Kentucky and southern Illinois/Indiana is close by and producing scattered to more numerous showers and thunderstorms in those areas. Further south some isolated to widely scattered showers can be seen as far south as Murfreesboro, TN and in extreme northwestern Mississippi. Most model guidance moves the upper level forcing with this storm system northeast with time today. The movement to the northeast should keep precipitation north of the area, but some isolated showers or thunderstorms could come as close as McNairy or Maury counties in Tennessee. Some higher dewpoints are already pooling ahead of this system though over northern Alabama and southern middle Tennessee, with dewpoints west of I-65 around 70 degrees. This and abundant mid/high cloudiness should help keep lows a bit warmer west of I-65 in the 70 to 73 degree range. near and east of I-65, though drier air is still in place and this should allow lows to drop into the 63 to 68 degree range in most of those locations, especially with more prevalent and thicker cloud cover holding off until closer to daybreak in those areas. Dewpoint depressions are currently zero or almost zero east of I-65 in northern Alabama. Thus, keeping areas of fog in the forecast, which could become dense (lowering visibilities to around 1/4 of a mile at times through daybreak) in some areas of the Paint Rock Valley, near Lewis Smith Lake and in the river valleys of DeKalb county. Cloud cover associated with activity skirting the area through portions of the afternoon hours, will keep high temperatures a bit lower in southern middle Tennessee, mainly the upper 80s to lower 90s. Futher south impacts and the extent of thicker cloud cover is more uncertain. Given the very warm 925 mb temperatures forecast, think highs could be a degree above guidance near and south of the Tennessee River. For now, frecasting highs to reach the 93 to 96 degree range in most of these areas. The exception of course will be the higher elevations of northeastern Alabama, where highs will likely be in the lower 90s. Not expecting any precipitation at this time. As the main storm system moves only slightly east during the day today, additional energy aloft develops on the southern edge of the longwave trough axis associated with this system over Arkansas. What happens with this energy will be the next puzzle of the forecast. .SHORT TERM...(Tonight through Friday) Issued at 420 AM CDT Wed Aug 15 2018 As the main storm system weakens and meanders around the central Great Lakes region Wednesday night, this energy moves closer to northwestern Alabama. The main impact this will have is to continue to increase moisture advection and surface dewpoints over the area. This should keep lows warmer in the lower 70s at least. However, forcing still remains north and west of the region through daybreak on Thursday. During the day on Friday, moisture and instability will couple with some weak lift ahead of the feature as it moves east towards Memphis to produce 20-30 percent chances of of showers or thunderstorms. With several models showing SBCAPE values between 2000 and 3500 J/KG west of I-65 and in southern middle Tennessee, cannot rule out a marginally severe storm or two. However, most storms should just be strong and very pulsy in nature, given the weak bulk shear with this feature. Models forecast stronger upper level energy to move southeast from the western Great Lakes area courtesy of the main jet stream and into the briefly weakening longwave trough axis over the central Great Lakes region Thursday night. This should amplify the longwave trough axis and storm system over the central Great Lakes region. The end result of this will be higher rain chances overnight (30-60 percent). This trend should continue into Friday and Friday night, as the stronger forcing and slightly stronger shear closer to the main longwave trough axis associated with the storm system moves closer to the region. Will have to watch this period for stronger thunderstorm activity given higher shear, but a more saturated atmospheric column might minimize this threat. .LONG TERM...(Friday night through Tuesday) Issued at 420 AM CDT Wed Aug 15 2018 A wet period already on-going at the end of the workweek will continue into the weekend, as upgliding lower level moisture and approach of an upper level system bring clouds, and widespread showers and thunderstorms to the Tennessee Valley. The 305K region indicated continuing moisture upglide in the 900-750mb level, keeping clouds and convection. The GFS depicted a series of upper level systems traversing the region during that time frame, providing additional upper support. The ECMWF was similar but displaced a bit further to the north with its depiction of upper level support. Model mass precipitation fields are suggestive that much of the area should receive measurable rainfall in the Saturday/Saturday night time frame. Some of the storms during Saturday could become strong, with gusty winds the main threat. Map view and soundings do indicate a rather unstable atmosphere with surface based CAPES greater than 2000 both Sat/Sun. With the showers, high temperatures to end the week should be in the mid/upper 80s, then rise into the upper 80s on Sunday. Given that upper support should be displaced further to the north Sunday, decreased rain chances into the high "chance" range. Do not have very high confidence in the forecast after the weekend. A summary for first couple of day of a new work week will be unsettled weather, with scattered showers/thunderstorms in the period. More clouds than sun should keep highs into the mid/upper 80s, and lows in the upper 60s to around 70. The models were hinting at a frontal boundary moving across the region in the middle of next week. If this occurs, lower rain chances and slightly cooler conditions could be realized the latter half of next week. && .AVIATION...(For the 12Z TAFS through 12Z Thursday morning) Issued at 555 AM CDT Wed Aug 15 2018 VFR conditions are expected throughout the TAF period. && .HUN WATCHES/WARNINGS/ADVISORIES... AL...NONE. TN...NONE. && $$ NEAR TERM...KTW SHORT TERM...KTW LONG TERM...RSB AVIATION...KTW For more information please visit our website at weather.gov/huntsville.