Area Forecast Discussion (AFD)
Issued by Huntsville, AL (HUN)
699 FXUS64 KHUN 190542 AFDHUN Area Forecast Discussion National Weather Service Huntsville AL 1242 AM CDT Thu Jul 19 2018 .UPDATE... For 06Z TAFS. && .NEAR TERM...(Tonight) Issued at 848 PM CDT Wed Jul 18 2018 The little convection that did occur across the TN Valley this afternoon died very quickly with sunset. There has been some more persistent scattered activity focused to the west and south, associated with the first remnant cold front. A 2nd front is now draped from the Shoals to near Cullman. There is little air mass change with this front right away, but some spots in middle/east TN have seen dew points fall into the low-mid 60s. Our first window for showers/storms has closed, but rain chances are not over for the night. Several operational models continue to hint at MCS formation over the MS Valley later this evening, then push this into N MS and central AL. Model consensus is not great, so confidence is not high, but it is a feasible solution based upon conceptual models. The MCS track would be close enough to ramp PoPs back up (only to slight chance) towards daybreak across NW AL and Cullman County. The forecast already represents this well, and only minor adjustments were needed. With so much cloud cover across the area, fog should be a non-issue. The rest of the forecast is in good shape as well; minor tweaks were made to nudge dew points higher. .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 06Z TAFS through 06Z Thursday night) Issued at 1242 AM CDT Thu Jul 19 2018 No changes to previous aviation forecast reasoning. VFR conditions are expected thru 06Z/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...BCC SHORT TERM...17 LONG TERM...09 AVIATION...BCC For more information please visit our website at weather.gov/huntsville.