Area Forecast Discussion (AFD)
Issued by Huntsville, AL (HUN)
000 FXUS64 KHUN 170442 AFDHUN Area Forecast Discussion National Weather Service Huntsville AL 1042 PM CST Sun Dec 16 2018 .UPDATE... For 06Z TAFS. && .NEAR TERM...(Tonight) Issued at 748 PM CST Sun Dec 16 2018 RGB imagery indicates valley fog developing in Franklin TN/Jackson AL with observations at KTHA, KBGF, KGAD and KJFX reporting fog developing in or near our forecast area. T-Td depressions are near or at zero, and fog tools all suggest at least areas of fog, some of it dense. Surface winds continue at 3-7 mph at a few spots, although many sheltered valleys have dropped to calm. We will continue to monitor during the next hour or so, but may need to issue a Dense Fog Advisory at some point tonight. Would like to see a bit more coverage to develop before issuing this evening. .SHORT TERM...(Monday through Wednesday night) Issued at 234 PM CST Sun Dec 16 2018 Monday and Tuesday will be near-repeats of each other, with slightly-above-seasonable temperatures topping out in the middle/upper 50s each day under a few clouds here/there. High pressure will build into the area at the sfc and aloft, with CAA the strongest on Monday night. As such, lows Tuesday morning will start out around the freezing mark, and will slowly rebound ahead of the next weathermaker set to affect the area as early as late in the day Wednesday, but more so for Thursday and Friday. An upper low will deepen over the Intermountain West during this time, and will become a cutoff low by Thursday into Friday over our area. While little in the way of cloud cover will affect the CWFA on Monday/Tuesday, strong southerly flow ahead of the deepening upper low will translate to an increase in cloud cover through the day Wednesday. While the bulk of the rainfall will hold off until between 06-12Z Thursday, some WAA showers may sneak into areas W of I-65 late Wednesday afternoon. Have kept this mention isolated with this forecast package, and coverage will be very spotty (at best) prior to 00Z. The real surge of moisture/showers arrives after 00Z as the better support/lift associated with this system nears the region closer to Thursday morning (mainly after 06-12Z Thu). Temperatures Wednesday night will be the warmest of the short-term period, only falling into the middle/upper 40s for much of the area. A few isolated/sheltered valleys in far NE AL/Srn Middle TN may bottom out in the lower 40s, but again, this is the mildest morning from now through Wednesday. The mild trend continues as the system nears for Thursday. More details on that below... .LONG TERM...(Wednesday night through Saturday) Issued at 234 PM CST Sun Dec 16 2018 Guidance continues to show a surface low developing over Arkansas and Missouri Thursday evening and overnight as an amplifying longwave trough axis stretches further south from the western Great Lakes region. A significant increase in moisture and isentropic lift (oriented in a southeast to northwest area of convergence from central/southern Alabama into northwestern Alabama/northern Mississippi) is shown by most models. Models show this surface low occluding over northeastern Arkansas/western Tennessee overnight Wednesday into Thursday morning. The combination of lift and a good moisture profile produce fairly widespread (50-60 percent coverage of rain) over most areas near and west of I-65. This higher coverage should shift east overnight into northeastern Alabama. Models do differ slightly concerning the track of the surface low with this system. GFS moves it more northeast, while ECMWF moves it directly east with time into northern Alabama. The former GFS solution could enable cold air aloft to be drawn into the surface/upper low after midnight. If this occurs, with the strong lift near the surface low and lagging upper low, could see some a mix of rain or snow develop during the morning hours on Friday. ECMWF looks a bit too warm though for snow on Friday with the more easterly movement of the low. Will have to watch this period, but even if snow occurred, right now the surface temperatures should not allow wintry precipitation to accumulate. Liquid precipitation totals in most guidance are around 1 inch, so some more quick rises on area streams are possible (especially where isolated heavier amounts of rainfall occur). Models linger some light rain or flurries in extreme northeastern Alabama and southern middle Tennessee Friday evening, but think this may be overdone. Expect clearing and strong cold air advection overnight Friday to drop temperatures into the upper 20s to lower 30s by daybreak of Saturday. High pressure moves into the area behind the departed system through the day on Saturday. Highs in the upper 40s to lower 50s return, with lows back in the lower to mid 30s. By Sunday evening, both synoptic models are showing precipitation pushing back into the area along an inverted trough axis from a developing surface low over the northwestern Gulf of Mexico. This may produce some wintry precipitation, but very uncertain and far in the forecast to say much more than that. && .AVIATION...(For the 06Z TAFS through 06Z Monday night) Issued at 1042 PM CST Sun Dec 16 2018 Areas of dense fog will continue to develop across north AL and southern TN through the morning hours. Fog should also settle in at KHSV and KMSL with LIFR conditions possible from 09-13Z with visbility of 1/2sm or less expected. The fog may be slow to dissipate Monday morning, so have kept MVFR visibility through 16Z. && .HUN WATCHES/WARNINGS/ADVISORIES... AL...NONE. TN...NONE. && $$ NEAR TERM...17 SHORT TERM...12 LONG TERM...KTW AVIATION...17 For more information please visit our website at weather.gov/huntsville.