Area Forecast Discussion (AFD)
Issued by Huntsville, AL (HUN)
471 FXUS64 KHUN 251124 AFDHUN Area Forecast Discussion National Weather Service Huntsville AL 524 AM CST Sun Feb 25 2018 .UPDATE... For 12Z TAFS. && .NEAR TERM...(Today) Issued at 423 AM CST Sun Feb 25 2018 Within a broad longwave trough encompassing much of the Rockies and central Plains, a negatively-tilted shortwave disturbance will continue to lift northeastward over the upper MS Valley this morning, before closing off into a low over eastern Ontario later today. Models suggest that southwesterly flow aloft across the TN Valley will be maintained between this feature, and an elongated 500-mb ridge extending from the Bay of Campeche northeastward to the FL peninsula. At the surface, a cold front attached to the deepening surface low related to the northern stream trough will continue to decelerate as it pushes to the southeast of the CWFA later this morning. Although the threat for thunderstorms will diminish with the passage of the front, a 2-4 hour period of light-moderate rain will persist before the axis of postfrontal precip drifts further to the south. Although low-level clouds will tend to dissipate as the lingering rain ends, overcast decks of altostratus/cirrostratus will persist through the day with afternoon temps only warming a few degrees into the u50s/l60s. .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...70/DD SHORT TERM...70/DD LONG TERM...70/DD AVIATION...SL.77 For more information please visit our website at weather.gov/huntsville.