Area Forecast Discussion (AFD)
Issued by Huntsville, AL (HUN)
699 FXUS64 KHUN 251520 AFDHUN Area Forecast Discussion National Weather Service Huntsville AL 920 AM CST Sun Feb 25 2018 .NEAR TERM...(Rest of Today) Issued at 920 AM CST Sun Feb 25 2018 The line of storms that moved through overnight is now well south of the area, along a prefrontal trough axis. The main cold front extends from the eastern Great Lakes, south-southwestward through Middle Tennessee and into Louisiana. This front has just passed east of the Muscle Shoals airport, where dewpoints have already drop around 5 degrees. Expect this front to continue its eastward progression through the day before it stalls to our south this afternoon. Ahead of the front, isolated to scattered light showers may continue, mainly east of I-65 through the remainder of the morning, though rainfall amounts aren't expected to amount to much. Additionally, extensive cloud cover will linger through the day and keep our temperatures from warming too much. Despite very weak cold air advection behind the front, temps will still only warm into the lower 60s. An upper level trough will move eastward through the day, with southwest flow aloft strengthening as it inches closer to the region. Despite a light northerly flow at the surface, plentiful moisture in the low and mid levels will persist. A wave will begin to move northeast late this afternoon, with light rain increasing from south to north, mainly south of the TN River during the daylight hours. .SHORT TERM...(Tonight through Tuesday) Issued at 423 AM CST Sun Feb 25 2018 A secondary mid-level vort max within the base of the broader longwave trough is predicted to shift eastward across the southern Rockies and into the adjacent high Plains today. Although this feature will become highly sheared as it translates into the mid-MS Valley tonight, weak synoptic scale ascent should promote development of a frontal wave of low pressure along the northwest Gulf coast by late this afternoon. As this wave lifts east-northeastward into AL/GA by 12Z, isentropic ascent above the shallow cool airmass at the surface will promote renewed development of light rain--which should impact the entire region overnight. The rain will end from west-to- east rather quickly Monday morning, with increasingly zonal flow aloft promoting clearing skies during the late morning/early afternoon, and allowing temps to climb well into the mid 60s. Conditions will be favorable for radiational cooling Monday night, as a surface ridge to our north spreads across the central Appalachians and winds become light/variable, and we have forecasted lows to drop into the u30s/l40s. Fairly widespread fog will also be possible late Monday evening/early Tuesday morning, although we have not included in the official forecast at this time. In spite of an increase in cloud cover on Tuesday, strengthening southeasterly return flow will force temps into the u60s/l70s during the afternoon. .LONG TERM...(Tuesday night through Saturday) Issued at 423 AM CST Sun Feb 25 2018 Extended range guidance suggests that deep-layer forcing for ascent will begin to increase once again by Tuesday night, downstream from another amplifying longwave trough over the western CONUS. Elevated warm/moist advection will begin to strengthen during the evening hours, but should peak early Wednesday morning when the low-level jet will be maximized across our region. This should favorably coincide with the northward movement of a warm front to bring widespread rainfall to the region overnight. Although forecast soundings suggests adequate elevated CAPE for embedded thunderstorms, it appears as if the main threat with this initial wave of precipitation will be locally heavy rainfall and flooding. Several episodes of moderate-locally heavy rainfall could potentially occur in the region from Wednesday through Wednesday night, as smaller scale disturbances eject northeastward out of the longwave trough migrating across the central Plains. The threat for heavy/flooding rainfall will also be enhanced by diffluent flow aloft, based on the predicted orientation of the trough and position of the subtropical ridge across the Bay of Campeche. Regardless of these factors and the potential for elevated convection within an environment characterized by PWATs of 1.4-1.6 inches, the threat for flooding will ultimately depend on where the warm front resides and it is too early to pinpoint location at this time. A deepening surface low across the MS Valley should send a fairly strong cold front southeastward on Thursday, bringing an end to widespread rainfall. However, clouds and a few showers will persist Thursday night into Friday as we will remain along the southern rim of a closed upper low shifting eastward across the lower Great Lakes. Northwest flow aloft will develop in the wake of this feature, allowing a cooler/drier Canadian airmass to settle into the TN Valley next weekend. && .AVIATION...(For the 12Z TAFS through 12Z Monday morning) Issued at 524 AM CST Sun Feb 25 2018 VFR conditions are prevailing across most of the forecast area as lingering rainfall from early cold front passage moves east--with the exception of northeast and central AL where MVFR conditions exist. In addition, an area of MVFR cigs exists over southern TN/NE MS. These cigs should gradually move east behind the area of rainfall currently over the TN Valley. Thus, have added a TEMPO group for the 12-14Z time frame for potential for MVFR cigs. There should be a break in precipitation between 14-02Z before the next round of rainfall arrives as a warm front shifts northward. VFR cigs should prevail after 02Z with light rain prevailing. && .HUN WATCHES/WARNINGS/ADVISORIES... AL...NONE. TN...NONE. && $$ NEAR TERM...73 SHORT TERM...70/DD LONG TERM...70/DD AVIATION...SL.77 For more information please visit our website at weather.gov/huntsville.