Area Forecast Discussion (AFD)
Issued by Huntsville, AL (HUN)
000 FXUS64 KHUN 211107 AFDHUN Area Forecast Discussion National Weather Service Huntsville AL 607 AM CDT Wed Aug 21 2019 .UPDATE... For 12Z TAFS. && .NEAR TERM...(Today) Issued at 252 AM CDT Wed Aug 21 2019 Satellite imagery depicts clearing skies over the TN Valley early this morning as the mid level convergence axis, which served as a focus for storm development yesterday, pushes southward. At 500 mb, the trough axis that was settled over the area for the past several days will finally push eastward as the shortwave over the Ohio Valley translates into the Northeast. Subsidence will increase over the TN Valley today, which should limit thunderstorm development later today. A weak surface convergence boundary will be draped through Middle Tennessee and may drift southward into our area this afternoon, serving as a potential area of thunderstorm development this afternoon and evening. Soundings indicate steep low level lapse rates around 7-7.5 C/km, which leads to the inverted-v signature indicative of a strong wind gust threat in any thunderstorms. Additionally, precipitable water values remain near 2" due to continued southerly flow transporting Gulf moisture into the region. Therefore, a heavy rainfall threat will be present within any thunderstorms, as well, which may lead to isolated wet microbursts. Overall, there will be a conditional threat for strong storms, because the likelihood of storm development is limited by the increasing subsidence; that said, any storms that do develop may produce strong winds and heavy rainfall. Otherwise, look for a relatively milder day with high temperatures in the low to mid 90s. Because of the increased mixing of drier air aloft, dew points will decrease once again into the upper 60s to low 70s, yielding heat index values between 100 and 105. Although below heat advisory criteria, this may still prove to be a dangerous heat day for sensitive groups (asthma, young children, elderly, etc.). .SHORT TERM...(Tonight through Friday) Issued at 252 AM CDT Wed Aug 21 2019 Thunderstorms should once again dissipate shortly after sunset with loss of daytime heating, although isolated thunderstorms may sustain for a few hours due to outflow boundary interaction. An additional shortwave trough at 500 mb will rotate into the Great Lakes region tonight into tomorrow, as a surface low deepens over the Northeast/Quebec. A cold front draped behind the surface low and extending through the Ohio Valley into the Central Plains will gradually sag southward through the midweek period. Moisture convergence on the south side of this frontal boundary will lead to increasing confidence and coverage of showers and thunderstorms each afternoon and evening for both Thursday and Friday. Some of these storms may be strong with gusty winds and heavy rainfall. Additional enhancement of storms is possible if any MCSs from the Central Plains or resultant outflow boundaries track into the TN Valley, but most of these should stay north of the region. Due to higher shower and thunderstorm coverage each afternoon, limited diurnal heating chances will limit high temperatures Thursday to the low 90s, and Friday will be even lower in the upper 80s. .LONG TERM...(Friday night through Tuesday) Issued at 252 AM CDT Wed Aug 21 2019 An active pattern will persist into the long term period, as a weak frontal boundary remains draped across the region. To start the period, a broad upper trough extending across much of the eastern CONUS will aid in the development of showers and thunderstorms along the boundary. Ongoing convection during the afternoon hours will diminish Friday evening, as a shortwave develops over the Southern Plains. This shortwave will amplify as it moves further east on Saturday and Saturday night, with flow aloft strengthening from the southwest. Meanwhile, the aforementioned frontal boundary will generally remain stalled across, or in the vicinity, of the TN Valley as the upper forcing continues to weaken. The shortwave over the Plains will further amplify as it passes through the Mississippi Valley on Sunday, and finally push through the local area Sunday night into Monday. This will keep scattered to numerous showers and thunderstorms in the forecast through much of the weekend, continuing through the first part of the upcoming workweek. As the trough passes east of the area on Tuesday, rain chances will begin to decrease, though scattered thunderstorms are still possible during the day. There are still some model discrepancies, so pinning down any dry periods during the long term is a bit difficult. However, at this point, the best chances for thunderstorms will be during the afternoon hours, as instability peaks. PW values will exceed 1.9 inches Saturday through Monday, which is near the climatological max for this time of year. So, heavy rainfall is likely over the weekend and we will have to watch for any flooding concerns. Temperatures will be on the "cool" side, compared to what we've experienced the last couple of days. As cloud cover, precip and lower upper heights linger of the region, temps will struggle to rise out of the mid 80s. Overnight lows will be relatively mild, with temps falling into the lower 70s. && .AVIATION...(For the 12Z TAFS through 12Z Thursday morning) Issued at 607 AM CDT Wed Aug 21 2019 Patchy fog/low stratus will continue in areas west and south of HSV and MSL, but are not expected to affect terminals this morning. VFR conditions will prevail through the TAF period. Scattered showers and thunderstorms will develop this afternoon and continue through the early evening. Confidence in placement and timing of storms remains too low to pinpoint specific timing with this TAF package. && .HUN WATCHES/WARNINGS/ADVISORIES... AL...NONE. TN...NONE. && $$ NEAR TERM...30 SHORT TERM...30 LONG TERM...73 AVIATION...30 For more information please visit our website at weather.gov/huntsville.