Area Forecast Discussion (AFD)
Issued by Huntsville, AL (HUN)
131 FXUS64 KHUN 182313 AFDHUN Area Forecast Discussion National Weather Service Huntsville AL 613 PM CDT Wed Jul 18 2018 .UPDATE... For 00Z TAFS. && .NEAR TERM...(Tonight and Thursday) Issued at 225 PM CDT Wed Jul 18 2018 The "cold" front has dropped south into southern middle TN and will continue to make slow progress southwestward this evening. Dew points will lower into the 60s as a result, for all but western portions of northwest AL. This will also be an area where a slight chance of thunderstorms will be maintained this evening and again late tonight and early Thursday morning as residual MCS activity forming over the mid South may enter these locations. Otherwise, a brief but welcome drop in dew points and mugginess should be felt later tonight and early Thursday morning. .SHORT TERM...(Thursday night through Friday night) Issued at 225 PM CDT Wed Jul 18 2018 As usual with NW flow in mid summer, the models are giving varied solutions synoptically and convectively induced. In general, a rather potent northwest flow shortwave will be dropping southeast through the eastern corn belt and mid MS valley Thursday afternoon and evening. This will induce a return to southerly flow across the MS into the lower TN valley. Clusters of thunderstorms, possibly of the HP supercell variety due to the increasing bulk shear values, will develop along the wave Thursday afternoon and evening across the mid MS valley. These will likely feed off increasing low and mid level wind fields. NW flow will enhance sr-inflow for these storms with lengthening curved hodographs and deep layer bulk shear values of 40-60kt. So, one round of thunderstorms may arrive late Thursday night into Friday morning. There are indications that an MCS could propagate southwestward into the inflow and theta-e axis over the lower MS valley. Then, a second round of convection and possible supercells/MCS may develop Friday afternoon into Friday night as the next shortwave drops into the base of the deepening trough. At this point, will favor higher (likely) PoPs northeast and lower (chance) PoPs southwest into Friday Night. Given the amount of bulk shear and sr-helicity present in the forecast hodographs, severe HP storms may be possible both Friday into Friday night. .LONG TERM...(Friday night through Tuesday) Issued at 225 PM CDT Wed Jul 18 2018 Latter half of the global models continue to indicate a fairly active wx pattern in place across the cntrl TN Valley heading into this part of the forecast package, as a potent upper level low/trough axis drops into the Midwest/OH Valley states. Dynamic forcing at the base of this trough pattern is xpcted to be quite strong, with mid- level flow around 50kt. With an active nwly flow regime in place, some of the models continue to hint at a series of MCSs affecting the area beginning early Sat, and continuing into the afternoon. With SBCAPE values increasing into the 3-4K J/kg range, some of the tstms Sat could be strong/marginally svr capable of producing damaging wind gusts. This could especially be the case within embedded bowing clusters/segments, if these MCSs do in fact impact the mid TN Valley on Sat and actually provide enough time between complexes for the air mass to recover. Even with the rainfall/subsequent cloud cover, temps may still be able to climb well into the upper 80s/near 90F, mainly during breaks in the convection or perhaps in locations which do not receive any rainfall. Overall conditions should then improve heading into the evening hrs Sat night, as a cold front associated with an area of low pressure to the north moves into the region. Rain chances may linger into Sun, as the front continues to gradually move ewd, before showers/tstms come to an end by Sun evening, with the trough axis translating into the Appalachians and weakening. While the passage of the front may not offer much in the way of cooler air, with highs Sun/Mon still xpcted to predom reach the upper 80s for most areas, some drier air will briefly filter into the region from the north for a few days. Rain chances though will work their way back into the forecast near the end of the period, as a subtle increase in moisture over the cntrl TN Valley results in the prob for iso showers/tstms on Tue. && .AVIATION...(For the 00Z TAFS through 00Z Thursday evening) Issued at 613 PM CDT Wed Jul 18 2018 VFR conditions are expected thru 00Z/20 as slightly drier air filters into the area. Some guidance suggests a chance of fog at KMSL, but this will be discounted for now due to thick cirrus clouds in place. && .HUN WATCHES/WARNINGS/ADVISORIES... AL...NONE. TN...NONE. && $$ NEAR TERM...17 SHORT TERM...17 LONG TERM...09 AVIATION...BCC For more information please visit our website at weather.gov/huntsville.