Area Forecast Discussion (AFD)
Issued by Huntsville, AL (HUN)
468 FXUS64 KHUN 150005 AFDHUN Area Forecast Discussion National Weather Service Huntsville AL 705 PM CDT Sat Jul 14 2018 .UPDATE... For 00Z TAFS. && .NEAR TERM...(Tonight) Issued at 216 PM CDT Sat Jul 14 2018 Another very hot and humid afternoon across the Tennessee Valley, with temperatures in the upper 80s to lower 90s as of 19z -- and dewpoints in the mid 70s. Thus, Heat Index values have climbed above 100 degrees across most of the region -- with portions of Northwest and North Central Alabama above 105 degrees. A Heat Advisory will remain in effect for the aforementioned areas through 6 PM CDT late this afternoon. Despite a ridge of high pressure sitting almost directly overhead, daytime heating and abundant moisture in the atmosphere has helped to develop scattered thunderstorms across the Tennessee Valley -- mainly west of interstate 65 so far. A current mesoanalysis indicates some fairly high CAPE values across the region (SBCAPE 3000-4000 J/Kg and MLCAPE values up 2000-2500 J/Kg. The result has been a few strong storms across the Shoals that are putting out a lot of lightning and some heavy rainfall. The higher instability will allow for some stronger updrafts and likely some gusty downdraft winds of 40-50 mph. The lack of any appreciable shear/flow will make this activity very pulse-like and slow moving. The storms will persist until early this evening when the atmosphere will stabilize with the loss of heating. With the waning convection, a mostly clear to partly cloudy sky is expected overnight as high pressure will remain over the region. As has been the case the past several nights, some patchy fog will develop late tonight into early Sunday morning due to the moist boundary layer. Some of this fog could be locally dense in areas that get rainfall this afternoon, and along rivers and lakes. .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...AMP.24 SHORT TERM...AMP.24 LONG TERM...Barron AVIATION...70/DD For more information please visit our website at weather.gov/huntsville.