Area Forecast Discussion (AFD)
Issued by Huntsville, AL (HUN)
000 FXUS64 KHUN 151240 AFDHUN Area Forecast Discussion National Weather Service Huntsville AL 640 AM CST Thu Nov 15 2018 .UPDATE... For 12Z TAFS. && .NEAR TERM...(Today) Issued at 530 AM CST Thu Nov 15 2018 A closed mid/upper-level low currently situated across southeastern MO will continue to deepen and lift slowly northeastward into the lower OH Valley this morning, before accelerating eastward across KY later this afternoon ahead of a northern stream trough digging into the upper MS Valley. As this occurs, deep layer flow will veer to the west-northwest during the late morning/early afternoon hours, inducing stronger isentropic descent and ending the threat for measurable precipitation across our CWFA. However, prior to the onset of stronger subsidence, large scale ascent related to the upper low will be sufficient to warrant a slight-low chance of light precipitation, mainly for the northeastern half of the forecast area where the depth of the low-level saturated layer will be a bit greater. This precipitation will initially come in the form of light freezing rain for portions of southern middle TN and Jackson county, AL, but should transition to light rain for all but the highest elevations during the late morning hours as temperatures rise very slowly above freezing. Further to the southwest of the region where measurable precipitation will be possible after 12Z, forecast soundings depict a fairly shallow saturated layer just above the surface, and this implies that only a few flurries of snow will be possible, mainly east of a line from Florence to Decatur and Guntersville. The threat for lingering light rain and snow flurries across northeast AL and southern TN will slowly diminish this afternoon. However, low stratus clouds will be slower to clear the region, and should result in a very small diurnal temperature range, with most locations only reaching the lower-mid 30s for max temps. A few upper 30 degree readings are possible across the southwestern counties, if sufficient scattering of the low cloud deck can occur later this afternoon. .SHORT TERM...(Tonight through Saturday night) Issued at 530 AM CST Thu Nov 15 2018 Models suggest that west-northwest flow aloft will continue from tonight-Friday morning, in the wake of the departing trough, but should back to the west and subside later in the day as a mid-level vort max digs southeastward across the northern high Plains. Deep- layer flow will remain unamplified, but should back further to the west-southwest and increase once again during the period from Friday night-Saturday night as another/stronger vort max drops southeastward through the northern/central Rockies. At the surface, a broad ridge of high pressure will remain centered along the Gulf coast, providing light/variable flow at night and light west-southwesterly flow during the day when boundary layer mixing will be deeper. Although periods of high-level clouds will be possible in the flow aloft, abundant sunshine both Friday and Saturday will lead to warmer afternoon temps in the mid 50s Friday and lower 60s Saturday. Conditions will be quite favorable for radiational cooling tonight, with dewpoints in the l-m 20s, and this should lead to lows in the 25-30 degree range for most locations. Given very rich soil moisture, we may also see development of freezing fog across portions of the region early tomorrow morning. Depending on the coverage of high- level clouds at night, radiational cooling processes could be quite efficient on both Friday and Saturday nights as well, along with an attendant threat for fog or freezing fog. However, due to gradual modification of the airmass, we anticipate slightly warmer low temps both mornings. .LONG TERM...(Sunday through Wednesday) Issued at 530 AM CST Thu Nov 15 2018 Models continue to show this fairly weak and dry cold front northwest of the Tennessee Valley region on Sunday. ECMWF is little more aggressive with its movement eastward than the GFS and retains deeper moisture and thus some light showers along it. This should make for a pleasant day on Sunday, while just a bit cooler (highs in the upper 50s to lower 60s). GFS has a hard time retaining this moisture as the front pushes further southeast into northern Alabama towards daybreak on Monday into the afternoon hours. Given that the forcing is not very strong very dry air is in place ahead of the front, think GFS might be on the right track. But for now kept a compromise between the two by including some isolated showers and significant cloud cover across northern Alabama Sunday morning (mainly north of a line from Cullman to Scottsboro communities) through noon. Most guidance weakens and moves this front into north central Alabama Sunday afternoon through Monday, before stalling it. As a result, some mixing in the boundary layer near the surface looks possible Sunday night and should keep winds from decoupling (near 5 or 6 mph) through Monday night. Some lingering scattered cloud cover and these winds should keep fog at bay around daybreak on Tuesday. Some guidance hints at lows dropping into the upper 20s to lower 30s near and north of the TN/AL border by daybreak on Tuesday, but this cold air advection may be too aggressive given the stalled nature of the front. For now kept a bit warmer and closer to blends with temperatures dropping into the lower to mid 30s instead. Models show very little evidence of the front holding together at the surface on Tuesday. However, aloft there is still a weak signal of the boundary aloft through central/southern Alabama. This should keep at least some scattered cloud cover over most of northern Alabama through much of the day. Slightly cooler daytime highs in the lower to mid 50s will be the main change into Tuesday with a tad more cold air advection behind the front. High pressure builds further south on Tuesday night as a more zonal flow aloft develops. Overall, this will mean little difference in weather conditions. Another cool night (but not unseasonably so) is expected with lows dropping into the mid 30s. Dry and near normal temperatures will continue on Wednesday with highs in the mid to upper 50s and sunny skies. By Wednesday evening, stronger shortwave energy aloft develops over Texas/Arkansas. This forcing pushes east approaching Mississippi and northwestern Alabama by Thursday morning. At this point, models keep the atmosphere very dry near this feature. Also, ridging tends to keep this energy moving northeast into the lower Great Lakes or Ohio Valley due to the blocking area of high pressure over the Appalachians. So only introduced isolated rain shower chances just into northwestern Alabama as a result. && .AVIATION...(For the 12Z TAFS through 12Z Friday morning) Issued at 640 AM CST Thu Nov 15 2018 Satellite imagery and surface observations confirm that an overcast deck of low stratus clouds will persist across the region for much of the day, as an upper low moves northeastward across KY. MVFR cigs in the 1500-2500 ft range are expected to continue thru 21Z/MSL and 23Z/HSV, with a brief period of IFR conditions also possible between 12-16Z. Although a few pocket of light snow flurries may impact the HSV terminal early in the period, they should result in no significant impact to airport operations and the probability of occurrence is too low to mention in the TAF. Stratus will begin to lift/scatter from SW-to-NE late this afternoon, and we have indicated mostly clear skies after 16/02-04Z. Due to recent rainfall and abundant soil moisture, patchy BR will be possible at both airports near the end of the TAF period. Sfc winds will veer from WSW to WNW and remain at speeds arnd 10 knots today, before becoming lgt/vrbl overnight. && .HUN WATCHES/WARNINGS/ADVISORIES... AL...NONE. TN...NONE. && $$ NEAR TERM...70/DD SHORT TERM...70/DD LONG TERM...KTW AVIATION...70/DD For more information please visit our website at weather.gov/huntsville.