Area Forecast Discussion (AFD)
Issued by Huntsville, AL (HUN)
089 FXUS64 KHUN 231612 AAA AFDHUN Area Forecast Discussion...UPDATED National Weather Service Huntsville AL 1012 AM CST Fri Feb 23 2018 .NEAR TERM...(Rest of Today) Issued at 1012 AM CST Fri Feb 23 2018 Quiet weather still ongoing across the Tennessee Valley with high pressure still established to the southeast of the region and a stalled boundary off to the northwest. Continued SW flow off the Gulf of Mexico will raise/maintain dewpoints into the low to mid 60s. There will be some patchy low-level cloud cover (around 850 mb) and 3kft that will continue to filter into the region from the south today. However, drier air noted on model soundings should be enough to break up some/most of this by later this afternoon. As a result, temperatures should easily warm into the upper 70s to around 80 degrees once again. Have a forecast high of 80 degrees at bot KMSL and HSV, though this will remain below the record values of 83 degrees. Late in the afternoon, mesoscale models are hinting that a weak boundary (warm front) will pivot into far northwest Alabama around 00z. This would serve as a focus for a few isolated thunderstorms late in the day into the evening hours. Have updated the forecast to account for this possibility. .SHORT TERM...(Tonight through Sunday) Issued at 259 AM CST Fri Feb 23 2018 For tonight, a secondary warm front lifts northward through the area as the broad area of south-southwest low level flow increases to 30-35kt from the central Gulf Coast into the TN valley. This generates theta-e advection and moisture transport, and at least scattered elevated showers. Based on soundings, these showers would not be surface based, and instability aloft does not look very impressive at this point with little to no positive area on the forecast soundings. These showers will lift northeast of the TN River by Saturday morning. As the upper trough advances into the lower MO valley and upper Midwest, the broad LLJ will intensify to 40-55kt across the lower MS and TN valley during the day. The warm sector appears capped initially on Saturday morning. The GFS develops a skinny and tall CAPE of only 200-400 j/kg, while the NAM keeps the atmosphere rather capped. Will keep a chance northwest to slight chance southeast going, but have my doubts on the amount of thunderstorms we will experience until Saturday evening. Then it gets more interesting to our west near the MS River as a pre-frontal trough/line of thunderstorms should rapidly translate east-northeast into the mid South and lower TN valley region. The main limiting factor will be somewhat lower dew points that the models indicate could mix down and/or advect into the TN valley from the south-southeast during the day Saturday into the evening. This narrows the better CAPE values to our west. As such, pre-frontal supercells cannot be ruled out in our western counties during the evening hours, likely moving northeast from MS from the better instability. And deep layer shear/low level helicity profiles both support supercell structures. In a nutshell, I certainly agree with the SPC DY2 outlook with regard to placement of the SLGT risk for our area mainly along and W of I-65 and marginal risk further east. We will still forecast the line of thunderstorms moving east-southeast across the entire area with the weakening cold front. The question is timing. With the best upper trough/QG lifting into the eastern corn belt and western Great Lakes, the portion of the cold front in our area may tend to weaken and slow down as depicted by the NAM/ECWMF, while the GFS is a bit more progressive. Will go with a compromise, with showers exiting our eastern counties Sunday morning. .LONG TERM...(Sunday through Thursday) Issued at 259 AM CST Fri Feb 23 2018 The frontal boundary will exit our forecast area during the day on Sunday and get hung up in Central AL by Sunday night. With the frontal boundary still near by and a weak disturbance moving through ahead of a shortwave, precip chances linger in the forecast until Monday morning. Models differ on this however, as the ECMWF is more aggressive with the shortwave compared to the GFS, lifting the front northward a little bit and anticipates more rainfall. Took a compromise of the two for this forecast package. Drier conditions are expected to last into Tuesday with sfc high pressure across the region. We will feel a slight cool down behind the front, but temps will technically still be above normal. For Daytime highs will be in the mid/upper 60s on Monday and Tuesday. While overnight lows will be in the lower 40s on Monday, warming up into the lower 50s by Tuesday night. From here, models agree on the large scale pattern but not the timing of the next system. This is mainly due to the pace and evolution of the trough swinging out of the Southwest on Wed/Thurs. Sfc cyclogensis will develop Wed Night in the Southern Plains and lift into the Great Lakes by Thurs. Rain returns to the forecast on Tuesday night with the warm front. Plentiful moisture returns when we are in the warm sector, as PWATs reach over 1 inch. Widespread showers with a few thunderstorms are possible Wed and Wed night as the cold front approaches with a good amount of wind energy throughout the wind field. Kept closer to guidance and kept lingering showers in on Thurs. && .AVIATION...(For the 12Z TAFS through 12Z Saturday morning) Issued at 420 AM CST Fri Feb 23 2018 VFR flight weather conditions are expected through this evening. Southerly flow may gust around 16kt from midday into the afternoon before diminishing by evening. Ceilings of ~025agl will spread northward by 08Z with scattered showers possible. An isolated thunderstorm is possible, but have left out of the forecast due to its low probability. && .HUN WATCHES/WARNINGS/ADVISORIES... AL...NONE. TN...NONE. && $$ NEAR TERM...AMP.24 SHORT TERM...17 LONG TERM...JMS AVIATION...17 For more information please visit our website at weather.gov/huntsville.