Area Forecast Discussion (AFD)
Issued by Huntsville, AL (HUN)
000 FXUS64 KHUN 132352 AFDHUN Area Forecast Discussion National Weather Service Huntsville AL 552 PM CST Thu Dec 13 2018 .UPDATE... For 00Z TAFS. && .NEAR TERM...(Tonight) Issued at 313 PM CST Thu Dec 13 2018 Southeasterly winds and fairly strong warm air advection through the day aided by peaks of sunshine, have pushed temperatures into the lower 60s in portions of northwestern Alabama. Likely will be as warm as it will get this afternoon, with thicker cloud cover building back into the area from the south over the next few hours. Low/Mid level moisture will quickly build north as well into the evening hours, but dry air (dewpoints around 30 to 40 degrees) east of I-65 will need to be modified significantly before rainfall aloft can make it to the ground. More moist air west of I-65 should be able to modify more quickly as the southerly/southeasterly low level jet (around 925 mb) increases to between 30 and 40 knots tonight. This should allow isolated to scattered rain to push into northwestern Alabama early this evening and become widespread in coverage after 10 PM or so. Further east, expect scattered rainfall to develop around midnight and become widespread overnight. This increasing moisture/lift is all courtesy of stronger isentropic lift generated north of a warm front extending from central Arkansas southeast into extreme eastern portions of Lousiana. Lows should be tempered by cloud cover across the area, but temperatures should fall a bit late through the early evening hours into the mid 40s to around 50 degrees overnight east of I-65 (where thicker cloud cover may take a bit longer to develop). Rainfall this evening should be generally light, but up to a quarter looks possible west of I-65. The best lift looks like it will shift quickly into northeastern Alabama around and after midnight. .SHORT TERM...(Friday through Saturday) Issued at 313 PM CST Thu Dec 13 2018 This will likely produce heavier rainfall east of I-65 as the warm front moves into those locations before pushing into Georgia/eastern Tennessee just after daybreak on Friday. A strong tongue of warm air advection in the boundary layer is shown along and behind that warm front Friday morning. This could allow temperatures to climb into the mid 60s, but at this time only including highs in the lower 60s in the forecast. This may need to be raised in future updates. Instability is not shown in guidance until Friday afternoon, as the warm air advection continues to bring more moist and possibly slightly unstable air into the area. However, there is a large degree of uncertainty still in forecast data whether this will materialize. Bulk shear values remain very strong during the afternoon/early evening hours on Friday. If surface based instability can develop (GFS is more strongly hinting at that), some strong to severe storms would be capable of producing damaging winds as the surface low and phased upper low pushes east into northwestern Alabama. Either way, elevated instability and low wet-bulb zero heights around 7000 feet set the stage for hail up to the size of quarters as this occurs in the mid afternoon hours. This threat could linger into the early evening hours, as the surface and phasing upper low move through northern Alabama. After midnight, rainfall will continue through Saturday on the back side of the surface/upper low. In all, through Saturday, between 1 and 1.5 inches of rainfall looks possible. This could cause some quick rises on area river/streams, but is not expected to produce widespread flooding conditions. Cooler air will move in behind the front/surface low late Friday night into Saturday. Cloud cover and precipitation also help to keep highs from climbing above the lower 50s. Cold air advection Saturday night should drop lows into the lower 40s in most locations. .LONG TERM...(Saturday night through Wednesday) Issued at 313 PM CST Thu Dec 13 2018 Models suggest that strong north-northwest flow aloft will be in place over the TN Valley on Sunday, in the wake of an upper low shifting east-northeastward across the southern Appalachians. As the deep-layer cyclone advances further eastward and a shortwave ridge aloft translates across the region, winds throughout the vertical column will subside even further during the period Sunday night- Tuesday, before assuming a more pronounced southwesterly component on Tuesday night and Wednesday. At the surface, we anticipate any lingering light rain across northeastern AL/southern TN to end during the morning hours on Sunday, with the southeastward advection of drier air and lack of large scale forcing for ascent resulting in dry conditions through at least the first half of the work week. Low stratus clouds in the wake of the departing upper low should persist well into the late afternoon hours on Sunday before clearing, which along with northwest winds/weak CAA will make for a cool day as temps should only reach the lower 50s. Due to the predominately north Pacific origin of the incoming airmass and based on expectations of mostly clear skies, highs on Monday-Wednesday should easily reach the m-u 50s, with lows cooling into the 30-35 degree range each morning. && .AVIATION...(For the 00Z TAFS through 00Z Friday evening) Issued at 552 PM CST Thu Dec 13 2018 Conditions will start out as VFR however as rainfall approaches, CIGs will lower into MVFR by 01-02Z. Coverage of rain will increase around midnight and drop CIGS into low-end MVFR around 1,500ft with IFR CIGs possible. A dry slot and lighter winds are expected early Friday morning which will diminish rain at HSV but light rain could linger across MSL into the afternoon. To account for uncertainty, included a PROB30 at MSL 15-20Z. CIGs do not seem to improve by the end of the TAF period. && .HUN WATCHES/WARNINGS/ADVISORIES... AL...NONE. TN...NONE. && $$ NEAR TERM...KTW SHORT TERM...KTW LONG TERM...70/DD AVIATION...JMS For more information please visit our website at weather.gov/huntsville.