Area Forecast Discussion (AFD)
Issued by Huntsville, AL (HUN)
000 FXUS64 KHUN 212006 AFDHUN Area Forecast Discussion National Weather Service Huntsville AL 206 PM CST Thu Feb 21 2019 .NEAR TERM...(Tonight) Issued at 206 PM CST Thu Feb 21 2019 A potentially significant flash flood threat is evolving for tonight (and through Friday) for southern middle TN and portions of north Alabama. The next round of moderate to heavy rain is not lifting north-northeast as an 850 mb (elevated) warm front lifts back northward. The nocturnal LLJ tonight will increase intensity and coverage of this band of showers and thunderstorms as elevated instability increases. The mesoscale models and NAM all indicate a continually redeveloping solid band of training showers and thunderstorms through tonight into Friday from north central MS through the Shoals/NW Alabama into southern middle TN. Will elaborate more in the next section. With the threat of flash flooding increasing, and being at nighttime when the threat for motorists is higher, will transition to a Flash Flood Watch for southern middle TN and most of northwest/north central AL. Will keep the areal flood watch in effect in our southeast counties where existing flooding and lessor QPF is expected. Thanks for neighboring WFO coordination on this. .SHORT TERM...(Friday through Saturday) Issued at 206 PM CST Thu Feb 21 2019 Although WPC basin QPF is showing 1.5-2 inches through Friday night, convective rain rates may reach 0.5 to 1.0 inch per hour or better at times. This could cause rapidly rising water on already saturated and in some cases flooded soils. Thus, flash flooding will become a distinct hazard. By Friday evening, the 850 mb front and heavy rain area will shift northward, although the surface warm front will also be moving through. Low clouds, light rain and perhaps some drizzle may be possible Friday night until Saturday morning when deeper mixing should occur. On Saturday, a tight pressure gradient takes shape with a wide area of 50-60kt flow at 850 mb and 925 mb flow of 40-50kt. We may need to hoist a wind advisory, particularly with a higher risk of tree falls occurring due to the saturated soils. Depending on if the clouds can break at times Saturday, highs should still reach the 70s for most, although the CAD/wedge will limit clearing and keep temperatures/dew points much cooler in far northeast AL. Not much change regarding the threat of severe weather Saturday night. It appears that any pre-frontal convection may be possible after 22-23Z, with the QLCS pushing east with the front from approx 01-02Z thru 07-09Z. Forecast hodographs indicate a rather straight line and strongly sheared profile. 1km SRH values will still be sufficient for mesovortices and tornadoes along this line. If instability were to be higher, a threat of pre-frontal supercells would also grow, but confidence in this remains somewhat low. .LONG TERM...(Saturday night through Wednesday) Issued at 206 PM CST Thu Feb 21 2019 Gusty northwesterly winds between 15 and 20 mph with gusts up to 30 mph will hold on in the morning hours on Sunday, behind the departed front as it moves into eastern Georgia. Much cooler and drier air will move into the Tennessee Valley behind it. Despite abundant sunshine, highs should only climb into the 50s, with maybe a few lower 60s in southern Cullman county. Thankfully, a prolonged period of dry weather is expected, as a surface high remains in place over the southeast through the day on Tuesday at least. This should continue to keep the area dry and cooler through Tuesday with highs continuing to be in the 50s and lows in the 30s. Monday morning looks like the coldest period, before upper level flow becomes more zonal and the surface high weakens. From this point on in the extended, models diverge significantly. Models show upper level forcing associated with a weak longwave trough axis pushing east through zonal flow into the southeast and stalling midweek into the later portions of next week somewhere in the southeastern CONUS. GFS is much wetter and brings deeper moisture and more precipitation northward as far north as Tennessee and northern Alabama by Wednesday. However, ECMWF keeps this moisture and any precipitation in the Gulf of Mexico. So for now went with a blended approach to the forecast, introducing isolated chances of rain Tuesday night and keeping them in the forecast through Wednesday night. Daytime maximum temperatures will warm slightly during this period, but generally remain in the upper 50s to lower 60s. && .AVIATION...(For the 18Z TAFS through 18Z Friday afternoon) Issued at 1118 AM CST Thu Feb 21 2019 Moderate rain will spread into north AL over the next few hours. The rain will lower visibility to around 3-4SM (MVFR) to along with the already MVFR ceilings of 015-025agl. Tonight, showers and a few thunderstorms are expected to continue, mainly over northwest/north central AL. Ceilings will likely drop back below 010agl (IFR) with moderate to occasionally heavy rain expected. The greatest chance of thunderstorms with heavy rain will be at KMSL after 12Z (mainly 14-18Z with a TEMPO group), dropping visibility to 2sm or less. This is also possible at KHSV, so have included VCTS. && .HUN WATCHES/WARNINGS/ADVISORIES... AL...Flood Watch through late Friday night for ALZ007>010-016. Flash Flood Watch through late Friday night for ALZ001>006. TN...Flash Flood Watch through late Friday night for TNZ076-096-097. && $$ NEAR TERM...17 SHORT TERM...17 LONG TERM...KTW AVIATION...17 For more information please visit our website at weather.gov/huntsville.