Area Forecast Discussion (AFD)
Issued by Huntsville, AL (HUN)
000 FXUS64 KHUN 210508 AFDHUN Area Forecast Discussion National Weather Service Huntsville AL 1108 PM CST Wed Feb 20 2019 .UPDATE... For 06Z TAFS. && .NEAR TERM...(Tonight) Issued at 916 PM CST Wed Feb 20 2019 The surface front that passed through the Tennessee Valley this afternoon has shifted south and east of the region, and precipitation has tapered off in its wake -- save for some isolated light showers and drizzle. The low clouds remain in place, however, with ceilings around 1500 feet or less. This low stratus and the moist boundary layer conditions will create patchy fog conditions across the region, with visibilities dropping as low as 1/2 mile or so in some areas. Will have to watch for the development of locally dense fog along the ridgetops of the Cumberland Plateau due to these potential low ceilings developing in these elevated areas. Temperatures have remained quite mild and in the low to mid 50s as of 03z. Will probably see some additional cooling overnight, but not much -- and would expect most areas to bottom out in the upper 40s by early Thursday morning. Most areas should remain dry, save for some occasional drizzle underneath the low cloud decks. Some minor tweaks made to the forecast, but otherwise everything remains on track. .SHORT TERM...(Thursday through Friday) Issued at 251 PM CST Wed Feb 20 2019 On Thursday, the 850 mb warm front and associated showers and possible thunderstorms to our south will lift rapidly north-northwest into northwest AL and TN. This may leave our southeast counties relatively rain-free through Friday, while areas from middle and southern middle TN through far northwest AL into northern MS stay under the gun for potentially excessive rainfall. Thus, agree with the WPC excessive rain outlooks for Day 2-3. The question is how much. Will stick with WPC guidance, but acknowledge that convective enhancement may yield higher totals. Will keep the entire Flood Watch in tact through Friday, but may be able to trim back some of the south and southeast counties in later updates given rain will be limited until this weekend in these areas. After lows in the 40s to around 50 again Friday morning, highs should jump into the 60s with the warm sector lifting northward. .LONG TERM...(Friday night through Tuesday) Issued at 251 PM CST Wed Feb 20 2019 Friday night, models continue to build upper level ridging east into the area. This should force strong upper level forcing along an axis stretching from eastern Arkansas into the Nashville and southeastern portions of Kentucky. This is good news, because it should give us a break from persistent and heaviest rainfall as this activity shifts northward and remains in northeastern Arkansas, northern Tennessee, and southeastern Kentucky (at least a good portion of northern Alabama). The exception to this will be the southern edge of some moderate rain along with a few thunderstorms which could linger in northwestern Alabama and southern middle Tennessee Friday night. At this point, the thunderstorm activity looks elevated with MUCAPE values only around 400 J/KG or less. So only expecting gusty winds and frequent lightning with any thunderstorms. Strengthening southerly flow originating from the Gulf of Mexico should advect much warmer and more moist air into portions of northern Alabama and southern middle Tennessee (especially in locations near and west of I-65) overnight Friday into Saturday morning, despite abundant cloud cover and scattered to more widespread precipitation. Lows in the upper 50s to lower 60s look reasonable. This activity should lift north and much of southern middle Tennessee on Saturday morning. However, a strong upper low and surface low moves from Kansas into Missouri on Saturday afternoon. Models quickly develop a line of strong convection over eastern Arkansas or Mississippi ahead of an associated dry line ahead of this storm system. This line of showers and thunderstorms push east into northwestern Alabama either in the late afternoon or evening hours. Models are in some disagreement with the timing of the arrival of this activity. ECMWF is a bit quicker with the arrival of this activity than NAM or GFS, which is a bit unusual. Even stronger southerly flow on Saturday and maybe some thinning of clouds, especially south of the Tennessee River, should allow high temperatures to climb into the lower to mid 70s near and west of I-65. Further east wedging effects of drier/cooler air should keep highs cooler in the upper 60s to around 70 degrees. Strong shear sets up ahead of this dryline on Saturday afternoon and evening. Models are showing between 200 and 600 J/KG of CAPE developing west of I-65. With wet-bulb zero heights dropping to around 10000 feet, some large hail seems possible. With strong 850 mb and 500 mb jets and plenty of helicity, models indicate that severe storms could produce damaging winds and some brief tornadoes as well. An even bigger impact might be additional flash flooding and flooding concerns on area rivers and streams. A wind advisory may be needed as mixing to 925 mb winds between 45 and 50 knots could allow sustained winds to reach that criteria. However, for now forecasting winds just below criteria with sustained winds between 15 and 20 mph and gusts up to 35 mph. Strong to severe thunderstorm parameters don't drop off much in the evening hours on Saturday, but should end as the front pushes through the area into Georgia after midnight. A much longer break from precipitation looks to setup finally Sunday into early next week, before a weaker front pushes through towards midweek, which may draw a few showers north into the area. Highs will feel much cooler Sunday into early next week, only reaching the upper 50s to around 60 degrees. A slower warming trend is expected towards the middle of next week. && .AVIATION...(For the 06Z TAFS through 06Z Thursday night) Issued at 1107 PM CST Wed Feb 20 2019 IFR to LIFR conditions will continue overnight and early this morning from low ceilings, and IFR to MVFR fog will develop continue at each terminal thanks to the very moist boundary layer conditions. Some gradual improvements will occur at both terminals after sunrise, but a weak front will generate some -SHRA/SHRA (and maintain sub-1000 ft decks) across the region from the late morning through most of the afternoon. This activity will taper off by this evening as the boundary lifts north, with scattered VCSH remaining through the period. && .HUN WATCHES/WARNINGS/ADVISORIES... AL...Flood Watch through Friday afternoon for ALZ001>010-016. TN...Flood Watch through Friday afternoon for TNZ076-096-097. && $$ NEAR TERM...AMP.24 SHORT TERM...17 LONG TERM...KTW AVIATION...AMP.24 For more information please visit our website at weather.gov/huntsville.