767 FXUS64 KHUN 222341 AAA AFDHUN Area Forecast Discussion...UPDATED National Weather Service Huntsville AL 541 PM CST Fri Feb 22 2019 .UPDATE... For 00Z TAFS. && .NEAR TERM...(Tonight) Issued at 258 PM CST Fri Feb 22 2019 Low level convergence has shifted north of the area, with the bulk of the precip falling to our west and north. A warm front was extending along the US80 corridor in southern Mississippi and southern Alabama, with isolated convection beginning to develop along this boundary. So far today, the area has received anywhere from a tenth of an inch of rain upwards to around three quarters of an inch. Thus, much of the river and stream flooding has had little opportunity to improve. With that said, there has been a decent break in the heavier rainfall, which has helped to alleviated some of the more widespread flooding. But, with already saturated soils and streams at their bankfull, any additional rainfall will quickly lead to additional flooding. The warm front to our south will slowly lift northward during the overnight hours, reaching the TN River by midnight. Meanwhile, a shortwave is expected to move through the area, with an increase in vorticity over the northwestern portion of the forecast. With this in mind, have maintained fairly high pops for the area, due to the warm front across our southern counties and vorticity max across our northern counties. Forecast soundings show steepening lapse rates aloft as the mid level shortwave moves through, which may lead to elevated convection late tonight. If this occurs, rainfall rates will be relatively high and accumulations may exceed the current forecast amounts of 1 inch. Confidence is somewhat low on this occurring, but there's enough of a threat to keep the Flash Flood Watch going through 6 am. Winds will shift to the south as the warm front progresses northward and WAA will become fairly strong. So many areas will only see a warming trend through the overnight hours, with minimum temps reached closer to midnight. .SHORT TERM...(Saturday through Sunday) Issued at 258 PM CST Fri Feb 22 2019 A warm front will push north of the TN Valley by daybreak on Saturday, with southerly flow strengthening at the surface. Meanwhile, a strong upper trough will eject east of the Rockies and move into the Central Plains. As the upper low deepens, surface cyclogensis will occur over the TX/OK Panhandles and trail a cold front southward. The upper low will pivot to the northeast during the day Saturday, sending the surface low into the Mid/Upper Mississippi Valley while the cold front swings into the ArkLaTex. Ahead of this system, the TN Valley will become well established in the warm sector, with scattered showers possible due to the strong WAA. One caveat to note is strong high pressure building along the lee side of the Appalachians which will build into our eastern counties during the morning hours. There is still some question as to how strong this high will be and what impact it'll have on the sensible weather pattern. Again, this will largely be for the extreme eastern portion of the area. But, the pressure gradient over the entire area will tighten quite a bit with the eastward movement of the low pressure to our west and the westward building high to our east. At this point, the warm front should be strong enough to overcome the CAD effects of the high, with much of the area warming into the mid 60s to lower 70s by the afternoon. Showers and thunderstorms developing along the cold front will push eastward, reaching the TN Valley by sunset on Saturday and then quickly tracking eastward, with much of the rain east of our area between midnight and 3 AM. There are some timing differences still among the various models, along with some slight discrepancies in the overall strength of the system. Regardless, most of the guidance is consistent on pushing the line of storms through the area between 5 PM and midnight, but the timing may need to be fine-tuned once the system really develops. In terms of the impacts tomorrow, well there are still some uncertainties to contend with. First will be if any isolated convection develops ahead of the main line of thunderstorms. At this point, models are still keeping a low level cap in place, with relatively weak lapse rates aloft. Mid level height falls are not overly impressive, as the subtropical ridge holds for a few hours during the day. With that said, if we realize any breaks in the cloud cover, we may see some isolated convection develop as diurnal heating increases and lends some surface based instability to tap into. This is a big if, and confidence on this occurring is extremely low. But, given the deepening upper low pivoting to our northwest, we will have enough wind shear for thunderstorms to become organized and possibly severe. The bigger impact during the day will be the gradient winds, which may reach upwards of 30 mph. This doesn't sound like much, but due to the extremely saturated soils, it won't take much for trees to fall. Now with the main line of storms, lapse rates will begin to steepen as the trough moves towards the area and we start to realize upper height falls. This will allow for destabilization to occur just as the sun begins to set. Meanwhile, the low level jet will strengthen just north of the area, with winds this far south reaching into the 55 to 60 knot range. These parameters will combine to help the eastward moving line of storms to maintain some strength as it enters northwest Alabama. The straight-line wind threat will still be main thing to watch, especially with those saturated soils. However, low level wind shear approaching the 30 to 35 kt range will keep the tornado threat over at least the western section of the area. But, this will be contingent on just how much surface based instability is realized. As it moves further east, the atmosphere may become too stable due to the loss of daytime heating and the low level cap not really eroding. So, the line of storms will start to become disorganized, though we can't rule out an isolated strong to severe thunderstorm over our eastern counties. And then there's the continued flooding threat. CAMs show mainly a thin line of storms moving quickly through the area, which will keep rainfall totals on the lower end. There may be enough break during the day to help areas recover some from the flooding experienced today. Due to this, and the uncertainty surrounding just how much rain will fall, will hold off on extending the Flash Flood Watch into Saturday but this may need to be re-evaluated tonight. The cold front will push east of the area late Saturday night, bringing a welcome end to the rainfall. Skies will clear rather quickly and northerly flow will bring in drier air across the region. Sunshine will be the main story on Sunday, which will be quite a change for the week we've had. Under the mostly clear skies, highs will warm into the upper 50s. .LONG TERM...(Sunday night through Thursday) Issued at 258 PM CST Fri Feb 22 2019 Dry conditions will persist into the new work week, as high pressure builds eastward. Zonal flow will keep a hold on the weather pattern aloft, and temperatures will generally remain mild. Highs will warm into the upper 50s to lower 60s through the week, with overnight lows in the 30s Sunday night and then only falling into the 40s Monday night and Tuesday. Forecast models diverge quite a bit on their solutions in the extended, with the GFS bringing another round of precip into the area on Thursday, while the ECMWF keeps things mostly dry. && .AVIATION...(For the 00Z TAFS through 00Z Saturday evening) Issued at 541 PM CST Fri Feb 22 2019 Low stratus has developed across the region in this very moist boundary layer, with IFR ceilings around 600-700 ft prevailing. Further reduction of cloud decks to LIFR levels will likely occur late this evening and overnight as a strong inversion sets in. A weak boundary lifting north into the region after midnight may trigger some scattered showers late tonight into early Saturday morning. Stronger SE/S winds will likely help improve ceilings back into the IFR range. Stronger, gusty winds will continue on Saturday ahead of a cold front, with IFR to low-end MVFR ceilings continuing through the period. && .HUN WATCHES/WARNINGS/ADVISORIES... AL...Flash Flood Watch through late tonight for ALZ001>010-016. TN...Flash Flood Watch through late tonight for TNZ076-096-097. && $$ NEAR TERM...73 SHORT TERM...73 LONG TERM...73 AVIATION...AMP.24 For more information please visit our website at weather.gov/huntsville.