Area Forecast Discussion (AFD)
Issued by Huntsville, AL (HUN)
000 FXUS64 KHUN 201220 AFDHUN Area Forecast Discussion National Weather Service Huntsville AL 620 AM CST Wed Feb 20 2019 .UPDATE... For 12Z TAFS. && .NEAR TERM...(Today) Issued at 335 AM CST Wed Feb 20 2019 There's a little bit of a break now in the rainfall/showers across most of the cntrl TN Valley this early Wed morning, after some very heavy rainfall/tstms impacted most of the area earlier this evening. Rainfall totals from yesterday afternoon/evening look to be in the 1-3 inch range, with some locally higher amounts. This brief reprieve in the moderate/heavy rainfall is only xpcted to last shortly past daybreak, as another wave of showers/tstms translates east across west TN/north MS. This activity looks to be developing/occurring along/just ahead of a cold front that is just now crossing the MS River. Latest model suites all have this sfc boundary moving into the mid TN Valley later today, with showers/embedded sct tstms continuing along the advancing front. Given the lack of any substantial buoyant energy/instability ahead of the front, most of the tstm activity will likely remain elevated, given the presence of a persistent low level weak inversion. However, a strengthening H85 jet out of the SW near 50kt may result in a few strong tstms capable of brief wind gusts near 40 MPH or higher. The more pressing issue will be additional rainfall totals later today with the oncoming moderate/heavy precip, which looks to be around 0.50-1.00 inch, with locally higher amounts in/near any of the stronger tstms. This additional rainfall coupled with the earlier precip will certainly maintain a continued risk for areal flooding, along with elevated river stages, in some locations across the area thru the afternoon hrs. As such, no change in the ongoing Flood Watch is needed at this point. Rainfall may begin to taper off some late this afternoon from the west, as the cold front continues to move east across the cntrl TN Valley. .SHORT TERM...(Tonight through Friday night) Issued at 335 AM CST Wed Feb 20 2019 The front will begin to exit NE AL to the east this evening, with rainfall continuing to gradually taper off. With some elevated CAPE lingering across the area, a few embedded tstms will remain possible this evening, before any convective activity tapers off with the onset of the overnight period. Rain chances though will continue into the morning hrs Thu, given an active SW flow pattern remaining in place over the region. Even with the front south of the area, rain chances will continue thru the day Thu, as an upper wave drifts north into the mid TN Valley. However, given the lack of more sig mid/upper forcing, rainfall amounts are generally xpcted to be rather light/less than a quarter inch into Thu evening. Rain chances will then increase going into early Fri, as an H85 front moves north into the gen area. Rain chances are a little more uncertain heading into the day Fri, as the stalled sfc front to the south begins to double back to the north as a warm front. However, pending how far north the warm front moves will likely determine the extent to which showers spread back into the area, with the latest model runs somewhat inconsistent with the position of the sfc boundary. However, with the passage of the H85 front to the north, some elevated instability looks to move back into the area and likewise may translate into a few embedded tstms within increasing showers thru the day Fri. This overall trend will continue into the evening hrs Fri, with rainfall/embedded tstms dependent on the location of the front. Additional rainfall amounts from tonight into Fri night look to be in the 1.00-1.50 inch range, with again locally higher amounts pending any convective development. The lack of more substantial rainfall totals thru Fri night may help to tempo alleviate much in the way of areal flooding. However, rivers stages that are in/near flood will likely remain fairly stagnant and likewise, the Flood Watch will be retained thru at least Fri. The watch may very well need to be extended into the weekend period, as another potent synoptic/storm system out of the srn Plains begins to lift into the Midwest states, with an attendant cold front stretching south into the extreme NW Gulf. .LONG TERM...(Saturday through Tuesday) Issued at 335 AM CST Wed Feb 20 2019 An upper low will dig into the Southern Plains early Saturday morning, with an upper ridge remaining anchored over the central Atlantic. At the surface, cyclogensis will occur near the TX/OK Panhandles in response to the deepening upper low, while high pressure persists along the Gulf. Given this set-up, deep southerly flow will quickly filter in a more unstable airmass northward and into the TN Valley. The area will see no shortage of moisture well ahead of this, given the prolonged period of rainfall leading into Saturday and this moisture will linger through the day. As the upper trough moves eastward, the surface low will take a more northeastward trajectory, reaching the Mid Mississippi Valley Saturday afternoon. A cold front will trail south of this low, with showers and thunderstorms moving eastward ahead of the cold front. Forecast models are in fair agreement with the overall timing of this system, though there are some minor discrepancies with regard to the strength and placement. Based on the latest forecast, the cold front will move through the TN Valley late Saturday night, with showers and thunderstorms moving through ahead of the front during the afternoon and evening hours. Ahead of the precip, strong WAA will allow highs to warm into the lower 70s, despite mostly cloudy skies during the day. In terms of impacts with this system, it will certainly be something to watch closely over the next couple of days. The trough is expected to take on a negative tilt as it crosses the Plains and moves into the Mississippi Valley. Meanwhile, the surface low is expected to deepen quite a bit as it races into the Ohio Valley Saturday afternoon/evening. All of this will give way to fairly strong wind shear which will be enhanced by the low level jet reaching upwards of 60 kts over the TN Valley. The limiting factor is going to be the more northward track of the low, but if it does deepen as much as models are indicating, there will be enough wind shear over the TN Valley to at least help generate organized thunderstorms. In terms of instability, the southerly flow will send dewpoints into the upper 50s to lower 60s by Saturday afternoon. However, mid level lapse rates may struggle to steepen as the trough moves into the Great Lakes keeping the heights falls minimum as far south as the TN Valley. This will limit the surface based instability despite those warmer dewpoints forecast to filter into the region. If we are able to see more sunshine and warm a bit more than forecast, low level lapse rates will be able to steepen and allow us to tap into some surface based instability. With all of that said, there looks to be a window during the late afternoon through early evening hours when the ingredients will line up and lead to a chance for organized thunderstorms, a few of which could become severe. This is still a lower confidence forecast due the reasons above, however the strength of the system may be enough to overcome the limitations. If the trough and surface low move a little further south, a greater chance for severe thunderstorms would be possible. Given the quick movement of the expected precip, rain accumulations will be less than an inch. This may keep the river flooding ongoing for a little bit longer, however flash flooding doesn't appear likely outside of any intense rainfall rates. The cold front will pass through the TN Valley between midnight and 6 am on Sunday, with high pressure building in its wake. Dry conditions will persist through Monday, as the high slides east across the Ohio Valley. Temperatures will be fairly mild for late February, with highs in the upper 50s to lower 60s and lows in the upper 30s to lower 40s. Winds will shift to the south late Monday night, with a return of moisture filtering into the region. Several shortwaves may move through the region in a zonal flow pattern aloft and combine with the increase in moisture to produce isolated to scattered showers on Tuesday. Rainfall amounts will generally remain light. Temperatures on Tuesday will be similar to Monday, with highs warming into the upper 50s to lower 60s. && .AVIATION...(For the 12Z TAFS through 12Z Thursday morning) Issued at 620 AM CST Wed Feb 20 2019 IFR cigs/conds are in place at the two main airports this early Wed morning, as a broken line of shra/embedded tsra begins to move into NW AL. This precip is ahead of a cold front that is now beginning to cross into west TN/north MS. This line of shra/tsra will continue to move ewd thru the area this morning, with vrb winds 30kt gusting near 40kt, coupled with reduced vis, in/near some of the stronger tsra. Little change in IFR cigs is xpcted thru most of the day, even with the passage of the shra/tsra to the east. Sustained sfc winds out of the SSE near 20kt with gusts 30kt will begin to turn more WSW later today with the frontal passage. Cigs may then lift more into the 1-2K ft range heading into the overnight hrs, as low level flow diminishes and turns more north. && .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...09 SHORT TERM...09 LONG TERM...73 AVIATION...09 For more information please visit our website at weather.gov/huntsville.