Area Forecast Discussion (AFD)
Issued by Huntsville, AL (HUN)
453 FXUS64 KHUN 220854 AFDHUN Area Forecast Discussion National Weather Service Huntsville AL 354 AM CDT Fri Jun 22 2018 .NEAR TERM...(Today) Issued at 354 AM CDT Fri Jun 22 2018 Light rain lingers over the area early this morning behind a decaying line of storms/MCS that is now situated west to east near Birmingham. The upper low is centered over St. Louis this morning with a broad surface low encompassing much of the OH river valley. A vort max around the base of the upper low helped to propel the line of storms into the area and is helping to push it south. Water vapor is showing drier air moving in behind this initial wave, which is also evidenced on forecast soundings. So we should be dry through 12z behind this rainfall. The upper low continues to move almost due east through 12z with the strong syntopic lift moving into NW AL between 12z-15z. At the surface, a frontal boundary will be snaked from the IN/OH boundary south through KY/TN and then southeast through MS/AR and be right at the NW AL border. Guidance is consistent in showing development along this front by 9-12z this morning in central AR. Guidance isn't sure if this development will move east and drop into our southwestern areas and then south through Cullman. OR...convection just blows up all along the front around 5z in NW AL, which seems most likely. So that line should move quickly to the SE and probably be out of the area by 18-19z. Even with that line, guidance is quickly recovering the atmosphere, since it is peak heating, and developing additional storms behind it. These move from NW Al around 21/22z through the SE areas by 2/3z tonight. (To note- each HRRR run is different in regards to these two rounds) In terms of the hazards/strengths of the storms, not sure how strong the morning round will be given how dry the atmosphere will be. These will probably just help saturate it for later. The core of the LLJ and upper jet streak start to move over at 12z but it will be a few hours later before the entire area is under the jet. It also depends on when the first line develops. If it's closer to 15z in the NW, instability/shear will already be increasing and a strong to severe threat would occur across the region, especially in the SE. If it's closer to 12z, the highest strong to severe threat would be in the SE as it gets there later in the morning. However, the second batch is what I would be most concerned about. These look to develop in a very supportive environment with sufficient lift, CAPE above 2500-3000 J/KG, 0-6KM shear around 40kts and even 0-1km shear around 20-25kts. Large hail would be the greatest threat with thick cape in the hail growth zone as well as reasonable WBZ/FZL heights. Damaging wind gusts would also be a threat just due to the amount of wind in the column and the shear although there isn't much of a mid level dry slot. In terms of tornadoes, a brief, isolated, weak tornado would be possible with the highest chances with anything that develops after 21z ish, and mostly in the west. This is when helicity would be maximized and LCL's would be lower (GFS maximizes everything slightly earlier). Even so, it's on the low end of climatological normals for tornadic events. Given how strong these storms could be overall, it wouldn't take much more than an interaction with an outflow boundary to produce a brief tornado. Confidence is low on this. The uncertainty with all of this is what happens with the morning convection. I have seen one too many times where models try to redevelop the afternoon batch but we become too stable from the morning batch. However, today, hires is in abnormally good agreement on that second batch. Quick mixing today and some breaks in the clouds should put highs in the lower to middle 80s. .SHORT TERM...(Tonight through Sunday) Issued at 354 AM CDT Fri Jun 22 2018 As mentioned above, the second batch of storms should be slowly shifting southeast and coming to an end after 2/3z. However, lift along the backside of the departing trough, lingering elevated instability combined with shear values still around 30kts and PW values near 2 inches, will keep scattered to isolated showers and thunderstorms around all night. Low temps right now are in the upper 60s to near 70 but may be slightly cooler if additional rain falls. Saturday is a bit tricky with GFS/ECMWF/NAM all consistently bringing a batch of heavy rain/storms west to east across the area by 18z. However, hires guidance (NSSL WRF, NMMB) has nothing except for a decaying MCS coming in around 21/22z. The upper trough will be much weaker and well into the NE at this time and a mid level ridge starts to build in from the SE. This looks to push the cold front from Friday back northward. This, combined with an upper vort max, will develop the showers/storms along it after 15-18z. Really wanted to go likely pops but in coordination with neighbors and because of uncertainty, stuck with a 50%. Although I would say chances are much higher than that for some thunderstorms at least by the afternoon. Given the soundings for Saturday, would think anything that does develop has the potential to be strong to severe. Hail would again be the main threat with potentially even more instability than Friday (temps will be warmer) and very similar shear values. The one thing lacking Saturday is the broad scale lift and the LLJ. PW values over 2 inches will mean storms will produce very heavy rainfall. If we see the continued rain on Friday, Saturday could have some flash flooding potential but that depends on rainfall coverage until then. Most of Saturday night should be dry as the front pushes farther north into the southern middle TN counties. Another strong upper wave develops in the westerly flow aloft and will develop showers/storms along the frontal boundary Sunday morning. The GFS keeps this all in TN and is dry after 18z. NAM is similar, but keeps everything north out of our area. ECMWF keeps it raining in the TN counties all day. Will back off on POPs in the south for Sunday and keep high chance pops up north. Given ongoing rain/clouds, storms may not be *as* strong Sunday but the potential for a strong storm or two in TN still exists. Sunday will see the warmest temps with highs near 90. heat index values will be in the mid 90s to near 100. .LONG TERM...(Sunday night through Thursday) Issued at 354 AM CDT Fri Jun 22 2018 Most guidance continues to build an upper level ridge north into southern Tennessee on Sunday night. This may keep more widespread shower and thunderstorm activity north of the area and closer to Nashville and eastern Tennessee Sunday night. GFS is not as amplified with the ridge due to some dampening of it by some upper level energy and deeper moisture advecting southeast into its northern periphery. This would mean higher shower and thunderstorm chances in northern Alabama and southern middle Tennessee, if this forecast option is correct. For now though only kept 20 to 30 percent chance of precipitation in the forecast in Alabama. Further north in our counties in southern middle Tennessee, went with 40 percent chance. Overall another warm night expected with lows in the upper 60s in northeastern Alabama to lower 70s further west. Little shear is shown in guidance, but it will remain fairly unstable with SBCAPE values over 3000 J/KG. Thus, activity will be the typical summertime pulse type convection. With some dry air aloft and very moist low level moisture, some stronger storms capable of gusty winds around 50 mph, heavy rainfall, frequent lightning, and small hail will remain possible. By Monday, most models amplify the upper ridge quite significantly, pushing the northern edge of it into southern Indiana and Illinois. However, they still push varying amounts of upper level energy south or southeastward into the strengthening ridge. Most guidance show at least some widely scattered showers or storms developing during the day on Monday. GFS again is the most ambitious with more widespread QPF depicting stronger energy aloft making it into the upper ridge. At this point, going slightly above blend with 40 to 50 pop. Highs could be a tad cooler than Sunday, topping out in the 85 to 90 degree range mainly. The same threats remain in place on Monday as well, with the environmental conditions changing little. This activity should quickly wane with the loss of daytime heating, although GFS holds onto QPF through the entire night (due mainly to continued stronger upper level forcing shown in model). At this point, with such a strong upper ridge and little shear, leaning toward the lower pop (around 20 percent at best). Lows will not change much, but could be a tad cooler overall (mainly due to daytime mixing of dry air aloft lowering dewpoints). However, this may be a bit dependent on how quickly cloud cover scatters out or dissipates Monday night. Think upper 60s in the east and around 71 degrees in the west looks reasonable though. Despite a fairly amplified ridge over the area, models continue to show additional upper level energy sinking into the northeastern edge of the ridge on Thursday. Enough moisture is shown that isolated to widely scattered showers and storms could remain possible. A slight warming trend looks reasonable, despite scattered convection, as 925 mb temperatures warm due to the strengthening upper ridge. Again, not much change in thunderstorm threats are expected. By Thursday night into Friday, models are in even bigger disagreement on how strong the upper level ridge is and how far east the center of that ridge moves. The ECMWF shows the strong/drier ridge extending east into northern Alabama. However, GFS shows it much further west and a much stronger longwave trough axis over the area. At this point, keeping with the blend and a compromise between the two solutions keeping scattered showers and thunderstorms in the forecast. High temperatures are most uncertain during this timeframe. However, it should remain warm with highs in the upper 80s to lower 90s. && .AVIATION...(For the 06Z TAFS through 06Z Friday night) Issued at 1211 AM CDT Fri Jun 22 2018 Rain and embedded tsra will persist at the terminals thru 08Z, with brief periods of MVFR cigs/vsby in the heavier pcpn. However, as the most concentrated region of shra/tsra shifts south/east of the terminals, an IFR stratus deck is expected to develop rapidly in its wake. Cloud bases will likely remain in the 800-1500 FT range thru 14Z, before quickly lifting/scattering. Sct-nmrs tsra will redevelop by 16-18z and have included -TSRA at both locations. Storms should diminish by 21/22z with showers ending by 01z. With clearing skies some light fog will be psbl at both airports by end of the valid TAF period. && .HUN WATCHES/WARNINGS/ADVISORIES... AL...NONE. TN...NONE. && $$ NEAR TERM...LN SHORT TERM...LN LONG TERM...KTW AVIATION...70/DD For more information please visit our website at weather.gov/huntsville.