Area Forecast Discussion (AFD)
Issued by Huntsville, AL (HUN)
000 FXUS64 KHUN 211705 AAC AFDHUN Area Forecast Discussion...UPDATED National Weather Service Huntsville AL 1205 PM CDT Thu Jun 21 2018 .UPDATE... For 18Z TAFS. && .NEAR TERM...(Rest of Today) Issued at 857 PM CDT Thu Jun 21 2018 An upper-level shortwave and associated LLJ is pushing east into the Tennessee Valley this morning. A plume of very moist, tropical moisture from the Mexican Rivera and South Texas continues to advect into the region along the northern edge of a broad subtropical ridge displaced to the SE over the Florida Panhandle. The combination of this lift and very deep moisture (PWATS as high as 2.2 inches!) has generated broad area of moderate to locally heavy rain showers and scattered thunderstorms across Northern and Central Alabama. This activity will remain tied to the LLJ and will continue to overspread the region through 18z, before tapering off from west to east, thereafter. The best instability currently is displaced to the south of the region, generally, so have adjusted Wx grids to favor rain showers as the predominant precip mode, with scattered storms embedded within this activity. Models continue to hint at the development of some instability (CAPE values as high as 1500-2000 J/Kg) late this afternoon and evening. That will be a big if, should the cooler, cloudy regime be the rule through most of the day. However, if some breaks can develop late in the day, there may be a brief window for a couple of strong storms, should something close to the aforementioned thermodynamic environment be realized. The dense cloud cover and rain-cooled air will limit temperatures and readings will likely run around 5 to perhaps 10 degrees below normal, peaking in the low to mid 80s across much of the region. Regardless, with dewpoints in the low to mid 70s, it will feel very humid outside today and it won't take much to generate shower and/or thunderstorm activity. .SHORT TERM...(Tonight through Saturday Night) Issued at 330 AM CDT Thu Jun 21 2018 Hires guidance gives us possibly a small break from 22-00z today before a MCS/line of storms starts to develop near Memphis around that time. Not completely sure on what will happen once it gets here as it all depends on how unstable we are. If we get pretty worked over today, the line probably will have a hard time holding together, and if it does, it won't be strong. Having said that, the NAM holds onto instability near 1000 J/KG of CAPE all night with the GFS/RAP bottoming out to near zero after 06z. Shear stays strong (40kts of 0-6km shear out of the west) through the night so the line/MCS will be sustained by that and the base of the upper trough pushing into the region. For now, will go with a 50 POP overnight to account for uncertainties regarding this line and the fact that guidance is showing scattered showers/storms all night. It also looks like we get dry slotted of sorts tonight with drying occurring at the 700-500mb level and farther aloft, even a little at the 850-700mb level. This drops PW values to 1.5". What happens Friday is anyone's guess at this point as model consistency is slim to none. To start, the upper low will be over the IN/IL border with a weak surface low beneath it. The cold front could be near the AR/MS border by 12z Friday and into AL by 00z Saturday. This front, combined with strong lift along the base of the trough, and another 30-40kt LLJ will provide sufficient support for thunderstorms, even strong to severe thunderstorm development. Even with the overnight drying and only a slight increase in PW values Friday afternoon, we will saturate a bit more, with afternoon dew points in the upper 60s. The dry air early in the day may also help us mix out the clouds and destabilize us. The less aggressive GFS still has CAPE around 1800 J/KG at its max Fri afternoon with 0-6km shear around 40 kts and 0-1km shear at 15-20kts. Decent lapse rates and WBZ value around 10kft could again result in marginally severe hail. But with the LLJ/shear that we have and some mid level drying, would expect damaging winds to be the greatest threat. SPC in their slight risk mentions a non zero threat of a weak tornado. We can't completely rule that out with helicity around 150 m2/s2 and the shear/instability/lift combo. Although LCL's are a bit high if we see the true heating/afternoon mixing the models are suggesting. (The NAM is highest on ALL these parameters so chances increase if the NAM is realized). There are always caveats! The dry air, for one, could make us too dry to initiate many thunderstorms. Also, if the overnight convection is more widespread, we may not clear out/mix out enough to realize any of the parameters mentioned above. The NSSL WRF and 3KM NAM for example, are almost bone dry for us on Friday, just a few isolated storms. WPC seems to be going towards a drier solution as well with their QPF forecast. Would think given the synoptic set up, we will get strong to severe storms Friday, even if they are slightly more isolated in nature. Highs should be able to reach into the mid 80s with lows in the upper 60s/lower 70s. The convection should mostly push to the south by Friday night along outflow boundaries. The low will be moving out to the east as we enter a northwest flow regime behind it. With lingering moisture and instability, will carry chance pops through the night. Another shortwave approaches on Saturday and it looks like we will either get another MCS in from the NW in the afternoon or just a batch of showers/storms. Although instability will be impressive, shear is back around 20kts with DCAPE/WBZ being unimpressive. Will still see heavy rainfall with high PW values and a strong storm is possible. With less rain/clouds, highs should be in the mid to upper 80s. Much drier air moves in Saturday night and it could be mostly dry. To keep in line with my neighbors, have kept an isolated to scattered POP in. Another wave/MCS will be moving into S. MO and move across TN through 12z so we may start to see some of that by 10-12z. .LONG TERM...(Sunday through Wednesday) Issued at 330 AM CDT Thu Jun 21 2018 Models weaken the surface low as it moves east with slight upper ridging building into the region by Sunday. Another longwave trough axis (and associated frontal boundary) swings east-southeast into northern Tennessee Sunday afternoon. Good forcing and moisture advection ahead of this feature should allow scattered to more numerous showers and thunderstorms to develop. The highest precipitation chances look to be near and north of the AL/TN border, due to zonal flow in place aloft. With decent 0-6 km shear values between 30 and 45 knots shown by most guidance and SBCAPE values over 3000 J/KG, could see some strong to severe thunderstorm activity, mostly in the TN counties. The main threats look like damaging winds and frequent lightning right now. Not sure if this stronger activity and high precipitation chances will make it very far south into northern Alabama at this point. Thus, have a decent gradient in high temperatures from north to south with mid to upper 80s north of the TN/AL border and lower 90s further south. Very high PWATS (around 1.9 inches) will be in place, so heavy rainfall will also be a threat, especially if training occurs in southern middle Tennessee. It is hard to say exactly where this frontal boundary ends up Sunday night. However, models hint it may still linger somewhere between northeastern Alabama and the Birmingham area by daybreak. Thus left mainly scattered shower and thunderstorm chances in the forecast through Sunday night. Expect a very warm night as moisture convergence should pool higher dewpoints towards daybreak through northern Alabama with lows only dropping into the 70 to 73 degree range in many locations. Yet another piece of energy swings southeast on the southwestern edge of another upper low swinging from eastern Canada into Maine on Monday. Most models show at least some of this energy associated with a weaker frontal boundary swinging into the area Monday afternoon. At this point shear looks a good bit weaker. However, with a very moist low levels, good instability, and good low level lapse rates, stronger microbursts look possible with pulse thunderstorm activity. Gusty winds of at least 50 mph look possible given DCAPE values and instability. Also, will need to watch for training storms again for any heavy rainfall potential with PWATS remaining around 1.7 inches. Temperatures look to remain in the upper 80s to around 90 degrees with expected precipitation and cloud cover. As a stronger upper level ridge builds into the Tennessee Valley Monday night through Tuesday night, it looks like we may receive a break from scattered to more widespread shower and thunderstorm chances. Cannot rule out isolated to widely scattered activity though. With a bit more instability and dry air aloft, stronger storms producing gustier winds, small hail, and frequent lightning will remain possible though. High temperatures should climb a bit into the low 90s in most locations, except in higher elevations of northeastern Alabama and southern middle Tennessee. Precipitation chances could increase again by the end of the week, as additional pieces of energy affect the area. && .AVIATION...(For the 18Z TAFS through 18Z Friday afternoon) Issued at 1205 PM CDT Thu Jun 21 2018 VFR conditions will be the predominant flight category at each terminal this afternoon, with only light, isolated showers creating VCSH at both sites. Ceilings/vis may drop this evening at HSV and MSL from a cluster of storms progged to drop SE thru the region between 00-06z. Thereafter, dry conditions, but low MVFR ceilings around 1-2kft are expected to prevail. Clouds will gradually lift to VFR levels after sunrise. More robust convection is expected after 18z on Friday, beyond the scope of this TAF period (and was not mentioned). && .HUN WATCHES/WARNINGS/ADVISORIES... AL...NONE. TN...NONE. && $$ NEAR TERM...AMP.24 SHORT TERM...LN LONG TERM...KTW AVIATION...AMP.24 For more information please visit our website at weather.gov/huntsville.