Area Forecast Discussion (AFD)
Issued by Huntsville, AL (HUN)
855 FXUS64 KHUN 211704 AFDHUN Area Forecast Discussion National Weather Service Huntsville AL 1204 PM CDT Fri Sep 21 2018 .UPDATE... For 18Z TAFS. && .NEAR TERM...(Rest of Today) Issued at 950 AM CDT Fri Sep 21 2018 An area of moderate to heavy showers continue to develop from northeast of Lester in Limestone county, AL into the Fayetteville, TN area. Further south of this activity isolated to widely scattered showers are starting to develop at this time. All of this activity is developing along a convergence axis that is being strengthened a bit by an area of low pressure moving west into the eastern Gulf of Mexico that extends northward into the area. Most guidance moves this area of low pressure only slightly west during the day. Models differ slightly on how where this convergence boundary moves during the afternoon today. However, the new 12Z NAM seem more scattered to more numerous convection near and east of I-65 as this boundary shifts slightly east during the day. Increased cloud cover and pops the remainder of the morning to reflect this. During the mid afternoon hours, it hints at more concentrated activity along this boundary. Based on satellite, plenty of sunshine/heating is occurring near and east of I-65. Based on area morning soundings and SPC analysis, it looks as though thunderstorms will be very hard to come by this morning due to lower instability parameters and a cap aloft. However, as we get more daytime heating, at least isolated thunderstorms look possible this afternoon. In addition, models hint at some scattered activity ahead of the approaching cold front well to our northwest pushing into northwestern Alabama. Kept 40 pop in late this afternoon to account for this. Overall, lowered high temperatures near and east of I-65 into the mid to upper 80s in most locations due to more cloud cover/higher pops. Further west, think more heating in the afternoon due to lower pops/less cloud cover will allow highs to climb into the lower 90s, despite some cloud cover this morning. .SHORT TERM...(Tonight through Sunday) Issued at 202 AM CDT Fri Sep 21 2018 The upper ridge position will remain anchored across north GA into central and northeast AL through tonight into Saturday. Tonight, clusters of showers and thunderstorms will continue within the moist south- southwest flow at 850 mb across middle TN southwest into northern MS, clipping far northwest AL. As the shallow surface boundary shifts southeast into middle TN, far northwest AL into northern MS, there will indeed be a sharp gradient in the potential for showers and thunderstorms. Locally heavy rainfall may clip far northwest AL during this period if the boundary and convergence does indeed slip southeast into this area. This may be somewhat short-lived as the models suggest the front and convergence will lift back northwest on Sunday. This will set up a good heavy rain pattern for areas from the Red River valley through AR, northwest TN and KY as numerous shortwave impulses propagate out of the southern Plains into the H valley.. For our areas, some or most of our central and southeast counties may remain dry both Saturday night into Sunday as the influence of the ridge and somewhat drier low to mid level flow will take place. .LONG TERM...(Sunday night through Thursday) Issued at 202 AM CDT Fri Sep 21 2018 A somewhat active pattern looks to continue through much of the next work week, as an upper trough begins to move eastward and into the Plains. This will allow for southwest flow in the mid to upper levels to strengthen, with a steady stream of moisture advecting northward into the TN Valley. Additionally, weaknesses within the upper flow pattern will generate at least some lift over the area, and keep the chance for showers and thunderstorms in the forecast for Monday and Tuesday. Meanwhile, the upper trough is expected to weaken a bit as it moves into the Mississippi Valley, though a cold front will continue to push eastward with the parent low over the northern Ohio Valley. Model discrepancies remain fairly high with regard to the system, as the latest forecast runs are a bit different than those yesterday. The weakening upper trough will be slower to move eastward Tuesday, as the strong subtropical ridge over the Atlantic holds. The surface low will continue its momentum eastward late Tuesday night, causing the cold front to become more east to west oriented as the southern extent of the front loses the upper forcing with flow aloft becoming more zonal over the southern CONUS. While the GFS continues to show an initial shortwave moving through and generating widespread precip on Wednesday, both the GFS and the ECMWF stall the front just north of the area. Models inconsistencies further increase after the end of the period, as the GFS shows a stronger closed low pushing eastward through the Great Lakes which would finally bring the front through the area on Friday. Meanwhile, the ECMWF has the upper low much further north, with zonal flow persisting over much of the eastern CONUS and the stalled front remaining north of the area. Given so much uncertainty, and the likelihood that model solutions will continue to differ for at least the next day or so, will follow closely to the blended guidance. This will make the highest chance for showers and thunderstorms on Wednesday, with the initial shortwave, with continued rain chances through the end of the week. Temperatures through the period will be closer to seasonal norms, as the upper ridge moves east and upper height falls allow for some cooling. Additionally, cloud cover will be fairly high and will combine with at least scattered thunderstorms to further help temps remain in the lower to mid 80s on Monday and Tuesday. The rest of the week will be highly dependent on just how quickly that front moves through, though an even greater coverage of cloud cover and precip will maintain highs in the upper 70s to lower 80s through the end of the period. Overnight lows will generally remain in the mid to upper 60s through the period. && .AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon) Issued at 1200 PM CDT Fri Sep 21 2018 Expect by 18Z or shortly after, cigs will be at times around or below 5000 feet through 22Z at both terminals, especially at KHSV. Most guidance depicts KHSV having a better chance of showers or thunderstorms directly affecting its terminal during this period due to better forcing near and east of I-65. Thus, included a tempo group for -TSRA and cigs around that level. Could see some MVFR or lower vsbys or cigs at times with this activity. At KMSL, not as certain we will see a -TSRA directly affect its terminal, so only included a tempo for broken cigs around 5000 feet. For now, left out -TSRA this evening ahead of an front that edges closer. Included restrictions to visibility into MVFR realm, due to fog between 22/08Z and 22/12Z overnight. Should see lower cloud cover redevelop after 22/13Z, but not sure about precpitation, so left any out through the remainder of the TAF. && .HUN WATCHES/WARNINGS/ADVISORIES... AL...NONE. TN...NONE. && $$ NEAR TERM...KTW SHORT TERM...17 LONG TERM...73 AVIATION...KTW For more information please visit our website at weather.gov/huntsville.