Area Forecast Discussion (AFD)
Issued by Huntsville, AL (HUN)
425 FXUS64 KHUN 211115 AFDHUN Area Forecast Discussion National Weather Service Huntsville AL 615 AM CDT Wed Mar 21 2018 .UPDATE... For 12Z TAFS. && .NEAR TERM...(Today) Issued at 340 AM CDT Wed Mar 21 2018 Synoptically this morning an upper low is centered over the Ohio Valley with a weak surface low stacked below it. The lingering low level moisture behind this low is keeping a stratus deck over areas around Huntsville and east. This deck has been slowly eroding from the west as the upper low moves off to the east and the drier air moves in. With plenty of dry air and mid level subsidence, skies should be mostly sunny by the end of the afternoon. However, that doesn't mean warm! One more shortwave along the base of the upper trough will move in this afternoon bringing a reinforcing shot of cold air and shifting the mostly westerly winds to northwest. This will keep highs in the lower 50s to upper 40s in the northeast areas. This shortwave will be enhanced by an upper level jet streak with 30-40kts at the low levels. With sufficient mixing in the dry lower levels, we should be able to tap into some of this wind. Sustained winds this afternoon could be around 20 MPH with gusts up to 30 MPH possible. .SHORT TERM...(Tonight through Friday Night) Issued at 340 AM CDT Wed Mar 21 2018 Winds should diminish fairly quickly tonight as we cool off after sunset and surface high pressure starts to build in from the west. Although we won't see calm winds tonight, the combination of clear skies, winds less than 5 mph and dew points dropping into the mid 20s, will allow low temps to drop to below freezing, upper 20s to around 30. Therefore, and in coordination with the neighbors, will issue a freeze warning from 7z/2am to 13z/8am for the entire area. Some areas may not see a full three hours below freezing but it is widespread enough that the freeze warning is warranted. With the high pressure in place Thursday and a developing mid level ridge pressing in from the west, we should see sunny skies and highs in the mid to upper 50s with winds less than 10 mph. A mid and upper level ridge strengthens significantly over the intermountain west Thursday night as a low deepens over the northeast. This will create a pretty strong upper level jet oriented more north to south stretching from MN down through GA. This lift combined with a slight increase in mid level moisture has allowed guidance to show some light QPF tracking into our area between 6-12z Friday morning. Guidance has backed off in the last two days with this, leaving only the NAM with any QPF. Will pull back the pops from the last forecast and leave it only in northeastern areas since the mid level lift isn't too strong and there really isn't much moisture. Lows will be warming into the upper 30s/lower 40s so it should be all rain and maybe a few snow flurries in S. middle TN, if anything. Guidance is also coming in drier for Friday and Friday night likely due to the lack of any appreciable lift and only a mid level layer of moisture, so just enough for a mostly cloud day. Removed any pops from Friday but will leave them in for Friday night as a surface low starts to develop over KS/MO. This will start to extend a frontal boundary to our area by 9-12z Friday night/Saturday morning. With lows Friday night in the 40s, it will be all rain. Highs on Friday will warm into the lower 60s. .LONG TERM...(Saturday through Tuesday) Issued at 340 AM CDT Wed Mar 21 2018 Most guidance moves the surface low from Missouri southeast into northwestern Tennessee on Saturday. A boundary extends southwest from the surface low into northeastern Texas. Good shear and low level helicity will be in place over the area ahead of this boundary, as the surface lows swings towards northwestern Tennessee. However, very meager moisture advection occurs on Saturday and a strong temperature inversion is shown by models around 700 mb. These two factors will likely keep shower activity very light and keep any isolated thunderstorms from developing. As the surface low continues to slide southeast Saturday night, the models try to break the cap. They do not however really do this, but come close. If models do break it though, there may be just enough moisture and good enough low level lapse rates for a few strong storms capable of producing some small hail, but for now left out of the forecast given the strong cap and limited moisture expected. High chances of showers look like a good bet though Saturday and Saturday night. It should be fairly warm over the weekend with highs in the 60s to lower 70s and lows in the upper 40s to mid 50s. Models diverge quite a bit by Sunday into Sunday night concerning how far south the cold front associated with the departed surface low moves. Regardless though Sunday and Monday will be cooler. GFS does not move it to the florida panhandle (like the ECMWF), but moves in into southern/central AL, before lifting it further north late Monday night as a warm front. If models are correct it should be a bit more unstable as this warm front moves northeast through the area Sunday night into Monday. Overall forecast atmospheric soundings do not look very impressive. Some rumbles of thunder and brief heavy rainfall may be possible with these storms, but not much else. Mainly scattered showers and few thunderstorms (none too strong) look possible Sunday night into Monday. The next surface low and associated storm system develops over the Rockies/Oklahoma on Sunday night into Monday morning. However, once the warm front pushes north of the area by Tuesday evening, a drier atmospheric profile returns, as upper level ridging builds into Mississippi and Alabama. This strengthens on Tuesday, keeping a strengthening storm system over Arkansas and Texas. This gives us a break from precipitation Monday night through Tuesday. This period could be a bit warmer than models are currently indicating, based on ridging over the area. Overall models are similar, but do hold off higher rain and possibly thunderstorms chances until Wednesday afternoon evening of next week, given the strong upper level ridge the system is trying to move into. && .AVIATION...(For the 12Z TAFS through 12Z Thursday morning) Issued at 615 AM CDT Wed Mar 21 2018 Clouds are lingering near KHSV as an upper system lifts east but should stay few or sct for the rest of the morning. Another batch of clouds are moving in from the northwest and may impact KMSL, maybe KHSV, in the next few hours. Although they may go bkn or ovc, heights will stay VFR. Both terminals should be seeing mostly clear skies by the afternoon. Winds will also be gusty today out of the northwest. They should increase quickly this morning with gusts around 30kts possible by this afternoon. They will diminish with sunset this evening. && .HUN WATCHES/WARNINGS/ADVISORIES... AL...Freeze Warning from 2 AM to 8 AM CDT Thursday for ALZ001>010-016. TN...Freeze Warning from 2 AM to 8 AM CDT Thursday for TNZ076-096-097. && $$ NEAR TERM...LN SHORT TERM...LN LONG TERM...KTW AVIATION...LN For more information please visit our website at weather.gov/huntsville.