861 FXUS64 KBMX 162304 AFDBMX Area Forecast Discussion National Weather Service Birmingham AL 604 PM CDT Tue Apr 16 2019 .UPDATE... For 00Z Aviation. && .SHORT TERM... Tonight through Wednesday afternoon. Fair weather will continue through the short term period as high pressure shifts eastward tonight and on Wednesday in advance of an upper-level trough. A ridge axis should extend far enough west tonight to promote calm or light winds, and temperatures should fall into the upper 40s to mid 50s for most locations. Warm advection will continue on Wednesday as southerly flow strengthens around the western side of the low-level ridge. Aside from some upper 70s in the North, widespread lower 80s are expected for afternoon highs. 87/Grantham .LONG TERM... Wednesday/Wednesday night: Southwest flow develops aloft Wednesday as ridging moves eastward ahead of troughing over the Plains. Mid and high level clouds will be increasing but low-levels will remain very dry. Some virga is possible late Wednesday afternoon and Wednesday night due to a weak shortwave aloft and mid-level isentropic lift. A few sprinkles cannot be ruled out, but do not expect any measurable precipitation. Low-level southerly flow between dual low pressure centers over the Midwest and Southern Plains and high pressure over the Atlantic will result in warmer temperatures Wednesday afternoon, and especially milder overnight lows Wednesday night. Thursday/Thursday night: The closed low embedded in the southern stream trough over the Southern Plains will open up into a wave Wednesday night as it phases with a northern stream trough. A jet streak and cold air advection on the back side of the trough will result in further trough amplification, with a highly amplified sharp (narrow width) neutrally tilted trough developing by Thursday night from the Great Lakes to the Gulf Coast. This will be a very dynamic system with 110kts of meridional upper-level flow developing, 75kts at 500mb, 65 kts at 700mb, and 50-60kts at 850mb. At the surface, low pressure centered over the Great Lakes associated with the northern stream trough will be the main feature with a cold front extending southward. But the amplifying trough interacting with the cold front along with potential convective processes will result in a potential secondary surface low developing. This will be an important feature for the evolution of the severe threat. The ECMWF is most concerning with a 998mb low tracking along the Highway 278 corridor but other guidance is weaker or further north with the track of this secondary surface low. Convection over the Southern Plains on Wednesday will grow upscale Wednesday night. Given later timing of storm initiation in the Plains, forcing and the LLJ remaining west of the area, and dry air over Central Alabama, MCS activity should remain off to our west Wednesday night, leaving the warm sector fairly uncontaminated. A remnant EML will keep the forecast area capped through mid-afternoon Thursday before forcing begins to erode the cap. The MCS activity Wednesday night will probably leave a boundary over western MS by Thursday morning. Convection over MS should continue/re-intensify during the day on Thursday, with widespread convection moving into our far western counties during the mid to late afternoon and sweeping across the area Thursday night due to strong forcing for ascent/height falls from the trough and the right entrance region of an upper-level jet streak. LCLs may be a bit high initially during the afternoon but will lower significantly during the late afternoon/early evening. Mid and upper-level clouds will limit the warming potential somewhat Thursday afternoon, though the strong southerly flow will probably allow temperatures to warm a couple degrees more than what raw model guidance shows. Therefore CAPE values may be a bit underdone in the guidance but in general will be around 1000-1500 J/kg in West Alabama Thursday afternoon. Models appear to be dropping CAPE values too quickly during the early evening hours as southerly flow should partially offset nocturnal cooling. Somewhat narrow moisture axis and strong forcing/meridional flow may limit the potential for more discrete development ahead of the line, but do expect supercells embedded in the line of convection rather than just a QLCS due to 50-70kts of 0-6 km bulk shear and the orientation of the shear vectors relative to the line, somewhat similar to Saturday night. 200-300 m2/s2 of 0-1km SRH and backed surface winds will result in a main threat of tornadoes within the line with some damaging winds also possible. The limiting factor at least during the late afternoon and early evening is that moisture return is not as impressive as the last event with dew points running in the low to mid 60s, probably due to the recent frontal passage in the Gulf, which will limit instability. Mid-level lapse rates are also forecast to weaken during the late afternoon and evening due to cooling at 700mb and warming at 500mb, and saturating profiles. Of concern though, is model indications of a surge of mid to upper 60s dew points and CAPE values of 1000 J/kg after 10PM into our southeast counties, which would maintain the tornado threat through the overnight hours in our eastern and especially southeast counties as the upper-level dynamics increase. Overall the best instability looks to be across our far western and southern counties where the greatest threat may end up being, but all 39 counties do have a risk of severe storms. Uncertainty over the track/intensity of the secondary surface low and the associated impacts on dew points/instability/SRH precludes an upgrade to an enhanced risk for our southern and/or far western counties at this time. Will continue to monitor the mesoscale trends as we get within range of the CAMs. Timing appears to be from approximately 3PM in the far west, through 6 AM Friday in the far southeast, which will be refined in future updates. The secondary surface low will also have an impact on the potential for strong gradient winds ahead of the storms. The progressive nature of the system should limit the overall flash flooding potential, but the surge of high PWATs and strong upper-level forcing combined with last weekend's rainfall could result in some localized flooding issues. Friday through Monday: Confidence in the PoP and temperature forecast for Friday is low, as the GFS closes off an upper low over the Southern Appalachians quicker than the ECMWF, resulting in more rain wrapping around a developing low pressure system with very cold air at 500mb and cooler surface temperatures. Have chances PoPs and highs in the low 60s in the forecast for now, but would need to increase PoPs and lower highs into the 40s/50s if the GFS verified for Friday. Otherwise, expect mainly dry weather for Easter weekend, with temperatures moderating by Easter Sunday. Dry conditions should continue for Monday as well. 32/Davis && .AVIATION... 00Z TAF Discussion. A VFR forecast is expected for the next 24 hours. We have surface ridging in place over the SE U.S. currently in control over the weather pattern. Winds should go light and variable to calm across the terminals under the ridge. Winds are expected at generally 5 to 10 KTS turning to the SE to S during the day Wednesday as the ridge weakens and pushes east. The only clouds expected will be that of some cirrus moving from generally W to E in the upper flow around the Gulf of Mexico upper ridge. 08 && .FIRE WEATHER... Rain free conditions are expected through Thursday morning. The next rain chance should come on Thursday afternoon and Thursday night as a strong storm system moves through. Relative humidity percentages will fall into the 30s on Wednesday afternoon, and critical fire conditions are not expected. && .PRELIMINARY POINT TEMPS/POPS... Gadsden 49 80 58 78 55 / 0 0 10 20 100 Anniston 50 81 59 79 56 / 0 0 10 20 100 Birmingham 55 81 62 78 55 / 0 0 10 40 100 Tuscaloosa 53 81 61 78 53 / 0 0 10 70 100 Calera 53 81 61 79 55 / 0 0 10 40 100 Auburn 53 80 59 79 58 / 0 0 10 10 100 Montgomery 52 83 60 81 57 / 0 0 10 20 100 Troy 52 83 60 81 58 / 0 0 0 20 100 && .BMX WATCHES/WARNINGS/ADVISORIES/... None. && $$ 87/32/08