Area Forecast Discussion (AFD)
Issued by Huntsville, AL (HUN)
324 FXUS64 KHUN 150255 AFDHUN Area Forecast Discussion National Weather Service Huntsville AL 955 PM CDT Sat Jul 14 2018 .NEAR TERM...(Rest of tonight) Issued at 955 PM CDT Sat Jul 14 2018 Afternoon convection which lingered into the early evening hours across portions of north central AL has now dissipated, with no precipitation noted in the local CFWA at this time. However, the synoptic environment will continue to be dominated by weak low-level confluence between a broad surface trough extending from the Ozarks northeastward into the lower Great Lakes region and a surface high drifting eastward off the coast of NC/SC, and this could easily support redevelopment of convection early Sunday morning based on the moist/unstable airmass in place. Several higher resolution CAMS as well as deterministic guidance from most of the global models suggest that thunderstorms may redevelop between 09-12Z across northwest AL and drift east-southeastward, as the center of a weakening mid-level ridge shifts to our southwest. Thus, we have adjusted POP/weather grids to reflect a lack of precipitation through 09z, before increasing POPS in the hours prior to sunrise. Although any storms that develop should not be severe, they will be capable of producing frequent lightning and locally heavy downpours. Aside from the changes mentioned above, we have also included patchy fog for the entire region between 05-13Z, although this may ultimately end up being a mixture of haze/mist rather than pure fog. Most locations will only experience minor visibility reductions, but locations near large bodies of water and those that received significant rainfall earlier today may see visibility briefly drop into the 2-3 SM range. Other elements of the forecast appear to be on track, with lows expected to range from the lower 70s east to mid 70s elsewhere. .SHORT TERM...(Sunday through Monday Night) Issued at 216 PM CDT Sat Jul 14 2018 The upper-ridge will gradually sink south on Sunday into Central/South Alabama as a weak upper trough moves east from the Southern Plains into the Mid-Mississippi Valley. Despite this, the heat and humidity will remain the story on Sunday with Heat Advisory conditions possible again over Northwest Alabama -- and the rest of the area coming very close. Models have underdone dewpoint recoveries at night and the morning hours in this very moist environment. Reliable observations have shown readings in the mid 70s to even upper 70s at times for several days now, and am not expecting this to change tomorrow. Even with additional cloud cover and perhaps some more storm coverage, it's entirely possible another Heat Advisory may be needed again tomorrow for Northwest Alabama. With a weaker cap and less subsidence in place, storm coverage should be greater Sunday. Some breaks in the cloud cover during the morning hours should allow for some rapid destabilization by early afternoon and MLCAPE values up to 2500 J/Kg per the latest HiRes guidance. Thus, like today, a few strong, pulse storms will be possible again on Sunday. The aforementioned upper-trough will gradually move into the lower Ohio Valley Sunday night and eventually the Southern Appalachians on Monday. This feature will likely force a weak front into or close to the area sometime on Monday -- which would serve as a focus for additional showers and storms. Based on this, have tweaked PoPs up slightly for Monday to low-end likely values. A slightly cooling trend is also expected early next week due to this presence of this boundary and the expected convection. Therefore, despite the high RH values, it does not appear there would be a need for additional Heat Advisory products beyond Sunday. .LONG TERM...(Tuesday through Friday) Issued at 216 PM CDT Sat Jul 14 2018 A rather fluid and mildly unsettled pattern will dominate a few days of the long term forecast as we move into next weekend. At the start of the period, a weak cold front will be tracking south through Middle TN and will shift through the TN Valley during the day. While there is limited mid/upper level support for stronger, more organized convection on Tuesday, the the front itself will be able to tap into the low level moisture to produce some scattered afternoon storms, with more numerous showers and storms mainly south of the TN River. As the front sags south by Tues night/Wed morning, high pressure will building back over the region from the north and dry will advect across the region. While temps will only drop a couple degrees behind the front, dew points will be falling into the low 70s by Wednesday and mid 60s by Thursday. Given the "dry" air mass and weak subsidence inversion in place, rain chances will be limited across the region. Additionally, we'll see a temporary break from the 100 degree+ heat indices as values are only forecast to top out in mid/upper 90s on Wednesday/Thursday. By Friday, expect a slight surge in moisture as low level flow gains a more southerly component ahead of another weak front. Scattered showers and thunderstorm, with more humid conditions, return to the forecast. && .AVIATION...(For the 00Z TAFS through 00Z Sunday evening) Issued at 705 PM CDT Sat Jul 14 2018 VFR conditions are currently observed at the HSV/MSL terminals, with just a few lingering stratocu beneath sct-bkn high-lvl convective debris clouds. Although trends in IR satellite data indicate that the coverage of debris clouds will gradually increase this evening and overnight, light flow and abundant boundary layer moisture will contribute to the development of br/fg which could lead to MVFR vsby reductions btwn 05-12Z. Isolated-widely scattered shra and tsra will continue early this evening within a broad axis of low-level convergence draped across the TN Valley, with redevelopment of convection possible just before sunrise tomorrow. However, due to uncertainty regarding timing/coverage we will not mention this in the TAFs at this point. The threat for storms will increase more substantially by mid-late morning across NW AL, with convection expected to spread ewd thru the aftn. Due to higher confidence, a PROB30 group was included for each terminal. After assuming a SSE component before sunrise, sfc winds will veer to SW and increase to arnd 5 knots after 12Z tomorrow. && .HUN WATCHES/WARNINGS/ADVISORIES... AL...NONE. TN...NONE. && $$ NEAR TERM...70/DD SHORT TERM...AMP.24 LONG TERM...Barron AVIATION...70/DD For more information please visit our website at weather.gov/huntsville.