225 FXUS64 KBMX 171818 AFDBMX Area Forecast Discussion National Weather Service Birmingham AL 118 PM CDT Wed Apr 17 2019 .UPDATE... For 18Z Aviation. && .SHORT TERM... Today and tonight. Upper ridging will persist over Alabama this period as an upstream trof digs southward into the Southwest United States. There will be plenty of high clouds spilling over the ridge axis, so skies will be partly sunny for most locations today. Low level thickness values will warm slightly today, so highs today will be near or slightly warmer than Tuesday. Mild tonight with lows around 60 degrees for most locations. No rain expected. 58/rose .LONG TERM... Thursday/Thursday Night: Convection is expected to grow upscale into one or more MCSs over the Southern Plains late tonight. This activity will be near the Mississippi River early Thursday morning. It should weaken during the morning hours as it out-runs the better instability and LLJ, but may leave boundaries for later convection. Convection over MS should reintensify Thursday afternoon due to daytime heating and strong height falls/upper-level forcing associated with the approaching trough. Further east over Central AL, a few light showers are possible during the morning and early afternoon due to low-level isentropic lift and weak waves aloft, but the warm sector should be pretty much uncontaminated due to a capping inversion associated with a remnant EML. Mid-level clouds will temper daytime heating somewhat but most areas other than the far northwest will reach the upper 70s to low 80s. The cap may erode enough during the early to mid afternoon hours for a few storms to develop in our western counties well ahead of the line. Given strong bulk shear some of these could be strong with an isolated severe storm possible, but low-level SRH will be still be on the weaker side and LCLs will be a bit high during this time. After 4PM, a pre-frontal QLCS is expected to move into West Alabama aided by a sharp highly amplified trough aloft. There will be the potential for a few supercells to develop just ahead of the line as well. 60-70kts of 0-6km bulk shear with vectors oriented about 45 degrees relative to the line will allow for some supercell mesocyclones to be embedded in the line along with QLCS mesovortices. A strengthening LLJ will result in the threat for a few tornadoes during the late afternoon hours in West Alabama, as a secondary surface low tracking from northern MS to middle TN keeps surface winds backed, and the potential for damaging wind gusts. The low-level flow will be mainly parallel to the line, tempering the damaging wind potential somewhat except where bowing segments locally swing the line around more perpendicular to the flow, but there will be some DCAPE from lingering dry air. The one thing preventing a more significant event appears to be the moisture return in the wake of the recent frontal passage. Dew points remain in the 50s along the Alabama Gulf Coast and in the low 60s along the Louisiana Gulf Coast. Upper 60s dew points are located over the far western Gulf and Texas Gulf Coast. Low 60s dew points are expected across much of the area on Thursday, with 65 dew points confined to a narrow axis right along or just behind the line. Still, CAPE values around 1000-1500 J/kg around peak heating in our west-central and southwest counties warrants an upgrade to an enhanced risk in our forecast for those areas. Instability becomes more questionable as the line moves further to the east Thursday evening. Models have trended downward with CAPE values given the low 60s dew points as temperatures fall into the low 70s with the loss of daytime heating. The event will transition to more of a strongly forced high shear, low CAPE event, with CAPE values in the 200-700 J/kg range, weaker lapse rates, and saturating profiles. The QLCS will still certainly be capable of producing brief tornadoes and at least marginally severe winds, which certainly still warrants a slight risk in our forecast, but the expected impacts due to the limited instability do not warrant extending the enhanced risk to the I-65 corridor in our forecast at this time. Will have to continue to monitor dew points and instability potentially surging a bit in our southeast counties overnight, with a possible ramp-up in tornado potential, but models have trended downward with instability here as well, and it's uncertain if these higher dew points will be able to surge ahead of the line given its expected forward speed. So will also hold off on any increased risk in our southeast counties at this time. Expect storms to exit our southeast counties by 4 AM if not a bit sooner. As usual, don't get too caught up in colors on the map; all of Central Alabama needs to be weather aware Thursday night. Will continue to monitor instability trends and the degree of organization of the line as we get closer. Other threats to monitor include the potential for locally heavy rainfall. The progressive nature of the system should keep rainfall totals in the 1-2 inch range for most, but locally higher amounts will be possible given high PWATs and meridional upper- level flow. This will likely impact ongoing flooding along the Tombigbee River and cause minor flooding in poor drainage areas. Some isolated flash flooding may be possible given the rain last weekend, with creeks and streams now running above normal, but the potential for an organized flash flood threat appears low at this time. Gusty gradient winds are also possible just ahead of the QLCS given 40-50 kts of wind at 925mb, similar to last Saturday night, and a wind advisory may be needed in future updates. Friday through Tuesday: An upper low is expected to close off within the base of the trough over the Southern Appalachians, while a surface low develops over the Central Appalachians. Forecast sounding indicate deep saturated profiles Friday/Friday night as moisture wraps around the upper low, with steep lapse rates due to a -26C cold core at 500mb. This will be favorable for scattered to numerous showers and areas of drizzle Friday/Friday night. Strong cold advection with 850mb temperatures falling to around 0C will result in temperatures falling into the upper 40s to low 50s late Friday afternoon and Friday evening. Overall this will result in very un-springlike conditions for Good Friday. Cloudcover and some showers/drizzle linger into Saturday morning before strong height rises develop as the upper low drifts northeastward. Temperatures will begin to moderate but remain below seasonal averages. Warm advection will result in much warmer and pleasant conditions for Easter Sunday. Looking ahead to next week, the upper-level flow is expected to be weaker and much less amplified. Weak shortwaves may result in some rain chances, but confidence in timing is low given the weak features. 32/Davis && .AVIATION... 18Z TAF Discussion. VFR conds expected through most of the period, with high clouds streaming across the area. Winds today will be out of the south at 7- 11 kts. A storm system will approach the area Thursday. Ahead of this system, expect MVFR cigs to spread northward across western and central terminals early Thursday morning. Winds will increase from the south to southeast, at 10-12kts. Rain and storms will remain west of all terminals through at least 18/18Z. 14 && .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 80 59 77 53 56 / 0 10 30 100 60 Anniston 82 60 79 55 58 / 0 0 20 100 60 Birmingham 81 62 79 53 57 / 0 10 40 100 50 Tuscaloosa 82 62 79 52 60 / 0 10 90 100 40 Calera 82 61 79 53 58 / 0 10 40 100 50 Auburn 81 60 79 59 62 / 0 0 20 100 60 Montgomery 84 61 82 57 62 / 0 0 20 100 50 Troy 84 59 81 59 62 / 0 0 20 100 50 && .BMX WATCHES/WARNINGS/ADVISORIES/... None. && $$