Area Forecast Discussion
Issued by NWS Huntsville, AL
Issued by NWS Huntsville, AL
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733 FXUS64 KHUN 230941 AFDHUN Area Forecast Discussion National Weather Service Huntsville AL 441 AM CDT Thu May 23 2024 ...New NEAR TERM, SHORT TERM, LONG TERM... .NEAR TERM... (Today) Issued at 441 AM CDT Thu May 23 2024 The convective cluster that produced primarily large hail and damaging winds in Texas and Louisiana last night has weakened significantly. It continues to push east well south of the area in the form of a shield of stratiform rain moving through east central MS. A shield of light rain extends northward from this convective system into central Alabama. Further north, mainly virga is occurring over portions of NW Alabama. Kept the previous thinking (20 PoP) that most of the precipitation on the northern edge system as it continues to move east will not make it into much of northern Alabama. The exception to this may be Cullman county, where brief light rain or a few sprinkles may occur towards daybreak. Most guidance depict the forcing producing this convective system to our south decaying and falling apart as it approaches eastern Mississippi and western Alabama around daybreak. Thus PoP was kept with a lowering trend towards 15 percent then. Shortly after daybreak, models show some additional development of disturbances aloft along the upper level trough axis that is stalled just to our northwest over western Tennessee into northeastern Texas. These move ENE along this trough axis today before maybe sinking southeast some this evening. This should concentrate the bulk of the precipitation today more over Tennessee and maybe portions of NW Alabama. Locally heavy rainfall would be most likely in those locations. The fairly fast mean 0-6 km flow combined with where the main axis these disturbances are traversing hints that any flooding potential may remain in Tennessee today. Lapse rates don`t look great in guidance, but shear, DCAPE, and instability do look like enough of a combination that a few severe storms could develop later this afternoon into the evening hours (again most likely in Tennessee). With a warm start (temperatures this morning in the upper 60s to lower 70s when you wake up, Despite some fairly widespread cloud cover this morning into the early afternoon hours, highs will likely still be able to climb into the lower to mid 80s in most locations near and south of the Tennessee River. If sunshine is more widespread than expected, highs could be a few degrees higher in the mid to upper 80s in those locations. && .SHORT TERM... (Tonight through Saturday night) Issued at 441 AM CDT Thu May 23 2024 With the loss of daytime heating, convection moving south from southern middle Tennessee should fall apart around midnight. But it might take that long given some of the remaining forcing moving south. A few of those storms could remain severe before dissipating. Main threat may shift more to large hail than damaging winds. We will have to watch for fog development overnight as winds are forecast to be very light. Persistent cloudy conditions though may keep fog from being very widespread or dense. A much stronger shortwave is progged by most guidance to develop over Arkansas and push ENE towards the area on late Thursday night into Friday. Very strong forcing at 700 mb is seen with this piece of energy aloft. As it moves east early Friday morning in to northwestern Alabama and western Tennessee, instability will be in place near and ahead of it (1000 J/kG to 2000 J/KG of MUCAPE) along with mid level lapse rates between 7.0 and 8.0 degrees/km. This instability only increases in guidance into the afternoon hours (2000 to 3000 J/KG). DCAPE is most models sounding is between 1000 and 1200 J/KG as this disturbance continues to move east into NW Alabama with ample shear to support organized severe thunderstorm development. Therefore, some storms could become severe during the late morning into the afternoon hours producing large hail or damaging winds. There is a very low chance of tornado or two given 0-3 km helicity values, but damaging wind and large hail are expected to be the main threats. Given the expected cloud cover and more widespread nature of showers and storms, high should be cooler only reaching the 79 to 85 degree range. Weaker shortwaves are shown in models lingering into the evening upstream of the area aloft. Shear weakens Friday night, but with decent elevated MUCAPE lingering (~1500 to 2000 J/KG) at least strong storms could linger into the nighttime hours. It will remain humid overnight with lows only dropping to between 65 and 70 degrees in most locations. During the morning hours on Saturday upper level ridging builds over the area behind the departed shortwave aloft well to our east over the Carolinas. This should help suppress lift and weaken upstream disturbances. Expect scattered showers and thunderstorms to continue, but coverage of showers and storms should be less as bulk shear remains weak (<25 knots). Not expecting severe storms until maybe in the afternoon, when another strong shortwave moves from Mississippi moves into the area. Shear will remain weak as this occurs, but instability will be between 2000 and 3000 J/KG and mid level lapse rates around 8 degrees/km. So damaging winds and large hail cannot be ruled out in the strongest storms. With the more scattered nature of the activity in the morning, more sunshine will likely be seen at times. So highs in the mid 80s seem reasonable. Lower chances (20 to 40%) of showers and storms are expected Saturday night, as the forcing moves east of the area into Georgia and the Carolinas. This should give us a break from the period of showers and storms overnight into Sunday. However, a potent storm system that develops in models late Saturday night into Sunday over the Upper Midwest moving into the western Great Lakes region will be the next impetus for additional showers and thunderstorm activity. This round looks to be more organized and severe. && .LONG TERM... (Sunday through Wednesday) Issued at 441 AM CDT Thu May 23 2024 We will likely get some additional cloudy conditions and maybe an isolated shower or thunderstorm with a pre-frontal trough axis ahead of this system on Sunday. However, overall should be a rain free day and hot with highs in the upper 80s to 90 degrees. This changes though as the main cold front pushes southeast Sunday night into the area. Very steep lapse rates, strong shear, high DCAPE values, and strong instability (3000 to 4000 J/KG) will provide the impetus for a MCS that develops along this front as it pushes into and through the area overnight into Monday morning. Saturated soils by then could produce a more enhanced threat for flash flooding or river flooding from this rainfall. Models differ on how quickly this system pushes through the area though, so activity cloud linger into the afternoon on Monday. However, the atmosphere will likely be worked over from earlier convection earlier in the day, so severe weather should not be much of an issue if that occurs. Finally northwest flow aloft and dry weather builds back into the area Tuesday into Wednesday. Highs will drop back into the upper 70s to lower 80s again thankfully with lows dropping back into the mid 50s to lower 60s. && .AVIATION... (06Z TAFS) Issued at 1153 PM CDT Wed May 22 2024 VFR conditions will persist at the HSV/MSL terminals overnight, featuring bkn-ovc layers of high-level clouds, a lgt SSE wind and perhaps a few sprinkles of rain. Models suggest that thunderstorm coverage will increase well to our NW in the 6-12Z timeframe, as an upper-lvl disturbance lifts northeastward and interacts with a subtle low-level convergence boundary. Based on latest high-res model data, it appears as if this activity may congeal into a small cluster of storms as it spreads southeastward late Thursday morning, and timing of PROB30 groups has been adjusted accordingly to reflect 17-21Z/MSL and 18-22Z/HSV. Additional storms will be possible into the late aftn/evening hours along a trailing outflow boundary that will likely be in the general vicinity of northern AL, but confidence is not high enough to include additional convection in the TAFs attm. Sfc winds will remain from the SSE at night and SSW during the day, with prevailing speeds of 5-10 kts. && .HUN WATCHES/WARNINGS/ADVISORIES... AL...None. TN...None. && $$ NEAR TERM...KTW SHORT TERM....KTW LONG TERM....KTW AVIATION...70