Area Forecast Discussion (AFD)
Issued by Huntsville, AL (HUN)
162 FXUS64 KHUN 201135 AFDHUN Area Forecast Discussion National Weather Service Huntsville AL 635 AM CDT Fri Oct 20 2017 .UPDATE... For 12Z TAFS. && .NEAR TERM...(Today) Issued at 338 AM CDT Fri Oct 20 2017 A powerful system that came ashore the Alaska Panhandle and British Columbia coast yesterday has resulted in troughing digging along the western CONUS. This system was being pushed eastward by a strong zonal flow over the North Pacific, with winds greater than 150kt in parts of 200-300mb region between 40 and 50N. A deepening trough to our west usually means amplifying ridging to the east - which is the case this go around. This will translate to warmer than normal temperatures to end the week. The trough advancing eastward along with a cold front and possible developing low over the southern CONUS, will bring unsettled weather to the Tennessee Valley starting Sunday. In the short term, surface high pressure was in place just to our ENE over the southern Appalachians, resulting in light winds locally. The light winds, mostly clear skies and close temperature/ dewpoint convergence has led to patchy fog development across parts of the Valley. Per a night micro-physics view from experimental GOES- 16, this fog as has been the case the last few nights has been confined to sheltered valleys and near large bodies of water. Any fog should dissipate shortly after sunrise, with another nice day expected weather-wise. Highs should warm into the upper 70s, with 80-81 in the warmest spots. Normal high temperatures for today are around 73 degrees. .SHORT TERM...(Tonight through Sunday) Issued at 338 AM CDT Fri Oct 20 2017 A not as clear night, as additional moisture filtering across the region brings few to scattered clouds. The clouds and a slight uptick in winds (from light/variable to SE 2-4kt) should be enough to keep fog formation minimized. Lows should be warmer, into the lower 50s - with seasonable normal lows around 49 degrees. The last day of the week will be warm and dry, with highs warming into the upper 70s to lower 80s in a few more spots. Warmer highs approaching the mid 80s is possible in a few spots if cloud cover holds off more than expected. In the meanwhile, the system to our west will edge closer, with more clouds especially during Saturday night. Even warmer with lows only in the mid 50s east to lower 60s west. Winds will also increase into the 5-15 mph range. Have kept a chance of showers and thunderstorms in on Sunday, especially in the afternoon as showers preceding the cold front make their way to the forecast area. Only "general" storms intensity wise are expected, with minimal instability/shear values needed for more substantial convection. More clouds and showers should keep high temperatures on Sunday held into the mid/upper 70s. .LONG TERM...(Sunday night through Thursday) Issued at 338 AM CDT Fri Oct 20 2017 Cold front should be just W of the cntrl TN Valley by the start of the extended forecast period, as an upper ridge axis slowly makes its way E of the mid/srn Atlantic seaboard. The moisture column should be rapidly increasing Sun evening as the T/Td spread diminishes, given an almost stacked S/SSW flow thru the air mass layer. Upper trough pattern is still xpcted to move out of the Plains states and into the ern Conus Sun night/early Mon, helping the attendant front push into the SE states. Clusters of showers, along with embedded tstms, should be developing along/just ahead of the advancing front, with most of the precip spreading into the mid TN Valley from the W starting Sun night. PW content looks to climb as high as 1.75 inches by early Mon morning, and then perhaps continue climbing closer to 2 inches by the middle of the day, in closer proximity to the frontal passage. This certainly suggests the potential for locally heavy rainfall, within total rainfall amounts around 2-3 inches. Buoyancy/shear continue to look modest at best, with SBCAPE values perhaps reaching 1500 J/kg Sun night into Mon and Bulk Shear numbers hovering just below 40KT. This may translate into a couple of strong tstms, but the prob remains fairly low attm. Going with a slightly faster 00Z ECMWF solution, the cold front is pretty much E of the local area by Mon evening, with just a trailing band of showers extending along the passing upper trough axis. The blended guidance seems to support this thinking as well, with rain chances quickly ending/tapering off from the W early Tue, as a reinforcing front sweeps ewd thru the region. A cooler/drier thermal advection stream, evidenced by thickness values diving swd well into the cntrl Gulf region, should then spread well into the region by the middle of the week, with overall temp trends falling around 10-15F. && .AVIATION...(For the 12Z TAFS through 12Z Saturday morning) Issued at 635 AM CDT Fri Oct 20 2017 Scattered low clouds in the vicinity of KMSL could cause brief LIFR <500 ft AGL ceilings for the start of the TAF. Those clouds should scatter out shortly after sunrise this morning. VFR weather otherwise is expected to continue over the KHSV terminal. Light/variable winds this morning will become SE-S generally less than 5kt this afternoon. Winds again should become light/variable after dusk. A presence of more clouds and a slightly higher wind late in the TAF should help minimize fog formation before daybreak Sat AM. && .HUN WATCHES/WARNINGS/ADVISORIES... AL...NONE. TN...NONE. && $$ NEAR TERM...RSB SHORT TERM...RSB LONG TERM...09 AVIATION...RSB For more information please visit our website at weather.gov/huntsville.