744 FXUS64 KBMX 111547 AFDBMX Area Forecast Discussion National Weather Service Birmingham AL 947 AM CST Sat Jan 11 2020 .UPDATE... MESOSCALE UPDATE. && .SYNOPSIS... Aloft, a potent upper level trough extended from the High Plains south through Northern Texas. The trough is expected to attain a more negative tilt later today as it swings northeast over the Mid South region. To our southeast, an expansive deep-layer ridge was positioned southwest of Bermuda. Toward the surface, a cold front extended from Southeast Canada southwest through Michigan and further south through the Central Ohio River Valley into West Tennessee and through West-Central Mississippi down to the Gulf Coast. 05 && .SHORT TERM... /Updated at 0940 AM CST Sat Jan 11 2020/ Updated for 15Z Special BMX Sounding and analysis of recent mesoscale data. Watch Update... A second tornado watch is in effect until 4 pm CST for areas toughly along and west of Interstate 65 along with St. Clair County but not including Montgomery County at this time. The existing tornado watch for 5 northwest counties of Marion, Winston, Lamar, Fayette and Walker remains in effect until 1 pm CST. Current Situation... A line of strong to severe storms now extend from Northwest Alabama southwest to east of Jackson, MS. General shower activity was noted on radar across portions of East Mississippi and West Alabama. Mesoscale Model Initialization and Verification... Comparing RAP soundings to observations the T/Td fields appear to be running about 3 degrees too cold on temperature and around 1 degree too dry for dew points. Adjusting the point forecast soundings at MEI and CBM are indicating much higher instability values than depicted by the model, on the order of 30-50% higher surface-based instability values. This is being exhibited by recent deep convective development ahead of the line in South- Central Mississippi west of Meridian. Environmental Conditions... KBMX NEXRAD VAD VWP wind profiler data along with our latest 15Z BMX sounding both indicate a highly sheared environment with 0-1 km Storm relative helicity values from 250-350 m2/s2. Wind shear remains robust and is more than sufficient to support severe storms capable of producing high winds and embedded tornadoes in the line along with in any discrete convection that can develop ahead of the line. The convective line is being dynamically forced and will pose the broadest concern for high winds. Surface-based instability values are currently the limiting factor for supporting robust development ahead of the forced line of storms. Higher instability (Surface-Based Convective Available Potential Energy, SBCAPE) values around 500 J/kg per mesonanalysis data was over our far southwest counties and is expected to advance north and northeast with time through mid morning. As aforementioned, our analysis of data shows that these model-derived soundings are running slightly cooler and drier compared to observations and accounting for these issues results in SBCAPE values closer to the 700-1100 J/kg range. As the low-level instability continues to increase and further moistening aloft in the mid levels should help to further steepen lapse rates and reduce the weak capping in place over the area. So far the stronger isolated cells that have formed ahead of the line have been observed interacting with and being absorbed into the line and then posing a tornado threat. We will be watching the behavior of new deep convection west of Meridian very closely to see how it interacts with the line of storms. Special Soundings... We have completed the 12Z sounding and the 15Z sounding will be fully completed soon. We hope to launch an 18Z sounding if conditions permit locally. 05 Previous short-term discussion: /Updated at 0244 AM CST Sat Jan 11 2020/ Today through Sunday. Consensus of latest guidance clearly indicates that previous forecasts are on track with little changes needed other than minor timing changes. Confidence continues to increase that an intense QLCS producing widespread damaging winds with isolated gusts to 70 mph possible and embedded quickly developing tornadoes with a couple strong tornadoes possible. Extensive pre-frontal severe QLCS has developed as expected, extending from Illinois to the Texas Coastal Waters with confirmed reports of damaging winds, a recent TDS east of Shreveport, and a measured 70 mph wind gust near Beaumont TX. This QLCS will continue to maintain itself and/or intensify as it encounters a large area of 1500 J/kg of SBCAPE over Louisiana and Mississippi and the upper trough becomes more neutrally tilted. This potent shortwave trough will lift northeastward to the Mid-South by this evening, taking on a neutral tilt at mid-levels and a negative tilt at upper-levels. It will be accompanied by a 120kt mid-level speed max behind the QLCS, with 80-85kts of mid-level flow in closer proximity to the line, and a 70kt southerly 850mb LLJ. A current broad area of low pressure will consolidate into a 1000mb deepening surface low, and the pre-frontal QLCS will continue to race eastward across Alabama during the day today. Current light isentropic rain showers are not expected to have much of an impact on destabilization and high-res guidance is on board with temperatures rising into the low 70s today with some mid 70s south. This makes sense given strong southerly flow/warm advection, and temperatures are already 65-71 degrees as of 2 AM. These temperatures and mid 60s dew points will result in CAPE values of 500-1000 J/kg north of I-20 and 1000-1700 J/kg along and south of I-20. Backed surface winds and the strong LLJ will result in very strong 0-1 km SRH values of 400-600 m2/s2, with 65 kts of 0-6 km bulk shear oriented parallel to the QLCS, favoring a continued predominately linear convective mode. The HRRR does try to show the line breaking up into supercells which does not appear likely at this time given the shear vector orientation, though some high res guidance does try to veer shear vectors a bit by afternoon so some embedded supercells are certainly possible especially the further south you go. CAMs do still indicate some pre-line convection developing across our south-central and southeast counties later this afternoon, which could pose a supercell tornado threat, but still indicate the line will be the main show. The strong 0-1km SRH will be very favorable for QLCS tornadoes (where the line can take on more of a negative tilt and become orthogonal to the 0-3km shear vector) and tornadoes with embedded supercells in the line, both of which could be strong. Damaging winds are also likely with downward momentum transfer and some pockets of DCAPE, especially in portions of the line that can orient more perpendicular to the strong LLJ where local wind gusts of 70 mph will be possible. Regarding timing, did bump the start time up to 9 AM mainly for the far northwest counties based on the current line orientation and radar extrapolation, but most areas will see the threat hold off until after 11 AM. Also changed the end time to 7 PM as the line should be in Georgia at that time. Some argument could be made for upgrading the risk to moderate, but don't want to get too caught up in risk levels as everyone should take this seriously regardless. Severe thunderstorm warnings should also be treated seriously given the strong wind potential and potential for tornadoes to spin up with little lead time, and especially mobile home residents and those with many trees near their home should consider moving to a safe place even for severe thunderstorm warnings. Brief heavy downpours along the line may cause minor flooding of poor drainage areas, but short duration of heavy rain at any one location should prevent much of a flooding threat. Have already seen a couple 35 mph gradient wind gusts in our area, with several sites recording 35-45 mph gusts across Mississippi and Louisiana so the wind advisory remains on track. Dry and cooler (but still above normal) conditions should move in behind the front tonight. The front will stall from south Georgia to the central Gulf on Sunday as it becomes parallel to the southwest flow aloft and encounters a stout subtropical ridge. Another negative tilt short wave will lift from the Central Plains to the Midwest, with the only local impact being that it will cause the front to begin to lift northwestward as a warm front. The 925-850mb warm front will lift northwestward ahead of the surface warm front, and associated over-running isentropic lift will result in the potential for a few showers and perhaps an isolated thunderstorm in the southeast by Sunday afternoon, though better rain chances hold off until Sunday night. 32/Davis .LONG TERM... /Updated at 0244 AM CST Sat Jan 11 2020/ Sunday through Friday. The frontal boundary will stall just to our south by Sunday morning and then begin to lift back to the north by the afternoon. This boundary will remain draped across the region through Wednesday as there really is not a big push to get it out. Will need to keep an eye on rainfall totals through the period as we could definitely see rises in our river basins. Finally a cold front and short wave trough will move through Wednesday night and get the system to finally exit stage right. This pattern really looks more like a late spring one, with enough instability for thunderstorm development especially during the afternoon hours, somewhat enhanced deep layer shear but weaker low- level shear, and enhanced mid-level lapse rates but a lack of a synoptic focus. So there will be some potential for stronger storms with gusty winds/hail and possibly marginal severe threats, though confidence this far out is low. Two possible periods to watch would be late Monday afternoon/Monday night, and then ahead of a frontal passage Wednesday when shear increases. Some locally heavy rainfall would be possible as well. There still some uncertainties with the models so will continue to mention some small rain chances through Friday. Another strong storm system will work into the region next Saturday and will need to monitor for any potential severe weather with it. 16 && .AVIATION... 12Z TAF Discussion. A severe squall line currently located over Mississippi will impact the terminals later today. Ahead of the line, MVFR cigs are occurring with winds gusting to 30-40 kts at times with low-level wind shear also present due to a strong low level jet. This line may reach TCL as early as 11AM and will impact the rest of the terminals through 7 PM, bringing temporary low visibilities, damaging winds, and the potential for tornadoes. Subsidence and a dry slot should allow cigs to rise to VFR behind the line, but wraparound moisture will bring in additional low clouds tonight. 32/Davis && .FIRE WEATHER... A strong storm system will impact the area through this evening with widespread rainfall expected. Look for strong winds on Saturday as well with gusts up to 40 mph. Dry conditions return overnight, but rain chances return in the southeast Sunday afternoon with high rain chances through at least mid-week. There are no fire weather concerns at this time. && .PRELIMINARY POINT TEMPS/POPS... Gadsden 73 44 65 46 67 / 100 40 10 40 60 Anniston 74 46 69 50 69 / 100 40 10 50 70 Birmingham 72 44 67 49 67 / 100 20 10 40 60 Tuscaloosa 75 44 67 47 67 / 100 10 10 40 60 Calera 72 45 66 49 67 / 100 20 10 50 70 Auburn 71 48 68 57 70 / 100 60 30 70 70 Montgomery 76 47 70 57 72 / 100 40 20 70 70 Troy 76 50 72 59 73 / 100 50 40 70 60 && .BMX WATCHES/WARNINGS/ADVISORIES/... Wind Advisory until 9 PM CST this evening for the following counties: Autauga...Barbour...Bibb...Blount...Bullock... Calhoun...Chambers...Cherokee...Chilton...Clay...Cleburne... Coosa...Dallas...Elmore...Etowah...Fayette...Greene...Hale... Jefferson...Lamar...Lee...Lowndes...Macon...Marengo...Marion... Montgomery...Perry...Pickens...Pike...Randolph...Russell... Shelby...St. Clair...Sumter...Talladega...Tallapoosa... Tuscaloosa...Walker...Winston. && $$