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Area Forecast Discussion
435 FXUS64 KHUN 230245 AAA AFDHUN Area Forecast Discussion...UPDATED National Weather Service Huntsville AL 945 PM CDT FRI JUL 22 2016 .UPDATE... Kept sky cover generally cloudy through the overnight and maintained slight shower/thunderstorm chances. && .DISCUSSION... Shower activity that affected parts of the Tennessee Valley earlier today has either moved south of the region and/or faded with a loss of daytime heating. Despite a rather early start of shower and thunderstorm this afternoon afternoon, very warm to hot conditions were realized across the area. Both the Albertville and Muscle Shoals airports both exceeded 105 degree heat indexes during the peak heating earlier today, thanks to high enough humidity values. Strong upper level ridging sprawled across the Southern Plains and Rockies (in the 598 decameter height range at 500 mb) was sending a northerly flow over the region. Other former convection within this flow was headed toward the region. Given an already unstable lower atmosphere and weak upper support moving this way, decided to stay with lower end rain chances for the overnight. As is usually the case in the summer, numerical models have a tough time forecasting the if`s and when`s of convection...even in the short term. The HRRR was hinting at convection moving in to our NE areas late tonight; with the new NAM showing something similar --- then merging this activity with others from our NW before daybreak Saturday morning. That being said, most spots should remain dry. But those that receive showers will be affected by locally heavy downpours, gusty winds and frequent lightning. More clouds than sky is expected for the overnight. This should help temper fog formation for the most part before sunrise tomorrow. These clouds, especially if they last into Saturday morning will have an effect on high temperature potential later on. Right now, will keep the Heat Advisory going for tomorrow. But it could be removed later tonight if new guidance suggests cooler conditions to end the week. RSB && .AVIATION... (Issued 714 PM CDT FRI JUL 22 2016) For 00Z TAFs: Lingering ra will likely end at the MSL terminal within first 30 min of TAF period, and with no convection currently slated to impact HSV will mention no pcpn at onset of either TAF. Although several outflow bndrys remain in the area to initiate isold shra/tsra, expect conds to remain VFR this evening, with bkn/ovc as and cs decks, and lgt/vrbl winds. A stronger disturbance currently dropping swd thru the lower OH valley region is forecast to shift slowly sswwd across the area late tonight and tomorrow, resulting in a greater coverage of convection. This disturbance will likely lead to regeneration of shra/tsra to the nw of the area after midnight, with activity spreading/developing ssewd into the terminals just before or around sunrise -- and lingering thru first part of the morning. Tempos for tsra and vsby/cig reductions have been included during this period, but timing may need to be adjusted. Threat for storms tomorrow aftn will be dependent on coverage of morning activity, but we have included vcsh thru end of TAF period along with bkn-ovc cigs aob 8 kft. 70/DD && .PREV DISCUSSION... (Issued 254 PM CDT FRI JUL 22 2016) The 19z surface analysis indicated that a weak trough was moving southward out of western Kentucky and an area of showers/storms were developing along and ahead of the boundary. Will continue with at least slight chance to chance pops into the evening hours as the widely scattered/scattered storms are expected to further shift southeast into the forecast area. The short term models are in decent agreement involving the development of scattered showers/storms toward sunrise on Saturday and have bumped up pops slightly into the chance category especially after 10z Saturday. On Saturday, scattered showers/storms are expected to continue to develop across portions of the forecast area by the afternoon. Will continue with the heat advisory into Saturday as there is a level of uncertainty involving morning shower/storm development. There is also expected to be higher RH values present and warming occurring into Saturday afternoon, which will result in heat indices moving into criteria by Saturday afternoon. Will continue with chance pops for Sunday, due to the remnants of the old boundary being in place across the area, afternoon heating and high PWAT values contributing to the rain chances. By Monday, the focus will turn to a weak cold front that will be moving southeast into the Ohio valley region. Scattered showers/storms will develop ahead of the boundary across middle Tennessee and into north Alabama by the afternoon and will continue with higher chance pops. The GFS and ECMWF models indicate that the weakening boundary will slowly shift southward through middle Tennessee on Tuesday and will eventually dissipate by early Wednesday. The bulk of the shower/storm activity associated with the dying boundary should move northeast of the forecast area by Wednesday, with lesser chances of showers and storms possible. By late Wednesday/early Thursday, another mid level trough and weak frontal boundary will follow a southeast movement into the Ohio valley region. Scattered showers/storms will again be possible on Thursday and into Friday as the boundary sinks southward to near the forecast area into middle Tennessee. Will go with higher chance pops for Thursday and Friday and will keep temps close to GFS/ECMWF guidance through the extended forecast period. TT && .HUN Watches/Warnings/Advisories... AL...Heat Advisory until 7 PM CDT Saturday for ALZ001>008-016. TN...Heat Advisory until 7 PM CDT Saturday for TNZ096. && $$ For more information please visit our website at weather.gov/huntsville.