172 FXUS64 KBMX 022220 AFDBMX Area Forecast Discussion National Weather Service Birmingham AL 420 PM CST Sat Mar 2 2019 .SHORT TERM... Through Sunday Night. Although Central Alabama has been mostly dry this afternoon (after morning showers), mostly cloudy skies remain due to moist westerly flow aloft and low stratus caught under an morning inversion. A stalled front sits just to our southwest, with a 20+ degree temperature spread from northwest to southeast. Ahead of an approaching storm system, temperatures tonight will fall into the lower 40s northwest to upper 50s southeast. Overnight, a shortwave will eject into the southern Plains and approach the Lower Mississippi River Valley by sunrise. In response, a surface low will organize along the TX/LA Gulf coast and move northeastward. The stalled front to our south will lift northward, increasing the chance of severe storms across portions of Central Alabama tomorrow. The long term discussion issued this morning covers the overall set up for this event, and remains valid. It is included below. We did make updates to the expected threats for Central Alabama based on the latest model guidance and increased confidence. Widespread rainfall (with embedded thunder) is expected north of the warm front, which will limit the northward progress of any surface based instability. For that reason, we trimmed the northern extent of both the Marginal and Slight threat areas. Convection allowing models are in good agreement that the warm front will lift into at least the southern portion of the area, before instability is pinched off by the progressing cold front. This will coincide with a strong low level jet, and hodographs supportive of tornadoes in the southeastern portion of the forecast area. We have upgraded locations south of a line from southeastern Dallas County to Auburn to an Enhanced threat. Previous Long Term Discussion for Sunday and Sunday Night. Latest trends have shifted towards a more southward positioning of the surface low pressure system on Sunday. This is in response to indications of a less amplified shortwave trough embedded in antecedent zonal split-flow across the CONUS. Though this does narrow chances for severe weather across the north, there's still favorable warm sector conditions likely across the south/southeast Sunday afternoon as the trough ejects from the Southern Plains into the mid-Mississippi River Valley. Afternoon boundary layer destabilization south of a northward moving warm front suggests SBCAPE values as high as ~1,500-1,750 J/kg (given Tds low to upper 60s & ~6.0 C/km lapse rates). These higher values would need some clearing, thus will depend on where rain showers & thicker cloud cover are which will greatly impact insolation/low-level lapse rates. More likely values of ~500-1,250 J/kg, low-level shear (0-1 km SRH 300-400 m2/s2), & broad hodograph curvature suggest stronger storms/updrafts will carry a tornado threat during the afternoon/evening. Will need to watch any possible discrete cells & the developing multicell cluster/QLCS along the front as they interact with the tornadic warm sector. As the low pressure moves off to the northeast into Georgia ~4-6 PM, surface wind vectors will veer & help diminish low-level shear/helicity. Though this favors more unidirectional flow and a linear storm mode, westerly deep-layer shear vectors will still be oriented 30-45 degrees across the southeastward moving frontal boundary/QLCS. Thus, a low-end tornado & damaging wind threat exists until any ongoing discrete/QLCS thunderstorms completely exit the forecast area ~9 PM. There's still a lot of forecast uncertainty with this system as guidance progs have maintained a great deal of run-to-run inconsistency. For now will maintain the same advertised threats across the area but have trimmed back northward coverage. With rainfall totals forecast ~1.5-2.5", an areal flooding threat is also present & will favor poor-drainage/low-lying areas as expected. Flash flooding can't be completely ruled out though would be isolated given the transient nature of the system. This also takes into account already saturated soils across the area. Local streams & tributaries may also experience water level rises, but downstream rises on larger rivers should only experience delays in continued improvements. A much colder & drier airmass will move in behind the cold front Sunday night/into Monday. Stay tuned for future forecast updates for this severe weather threat. 14/40/Sizemore .LONG TERM... Monday through Saturday. Although we've experienced a nice taste of warm weather across the area, the cold Canadian airmass will stick around through mid-week & present cold/stable conditions as an anomalous longwave trough invades the eastern CONUS. Overnight lows will take a downward slide with low 20s/upper 30s Monday night, with widespread 20s (possibly upper 10s in some spots in the north) Tuesday night/into early Wednesday morning. Now would be the time to make plans/arrangements to protect any sensitive vegetation given upcoming sub-freezing temperatures. Once the large negative height anomaly & cold airmass moves off the East Coast by Thursday, a return to more zonal flow aloft should occur with associated airmass modification. This will favor warming temperatures & signal the transition to another storm system by late-week/into the weekend. 40/Sizemore && .AVIATION... 18Z TAF Discussion. A weak wave brought showers to the area this morning but has now moved to the east. Cannot rule out a couple showers across the south this afternoon, closer to the stalled front that sits just to our south, but chances are too low to include at any terminal. Under significant high clouds, a low cloud deck sits across much of the area. Can see some erosion occurring across the north, and expect the deck to scatter and lift slowly through the afternoon. A period of VFR cigs is expected across the north this afternoon and evening. At MGM and TOI, low level moisture will remain in place and expect at least a broken MVFR deck through the same time period. Overnight, low clouds will return ahead of an approaching cold front, with decks at 1500-2500ft. Winds will increase from the southeast, as rain spreads from west to east across northern terminals and cigs lower to around 1kft. Activity will arrive at at MGM and TOI just beyond this forecast period. 14 && .FIRE WEATHER... Dry conditions are expected this evening, but low clouds and patchy fog are possible. Widespread showers and storms will return tomorrow as a strong cold front moves through the region. Rain will spread into the far northwest before sunrise, with showers and storms possible through the day and early evening. There are no fire weather concerns expected at this time. && .PRELIMINARY POINT TEMPS/POPS... Gadsden 47 58 34 45 25 / 20 100 30 0 0 Anniston 49 64 36 48 26 / 20 100 30 0 0 Birmingham 50 60 35 47 27 / 20 100 20 0 0 Tuscaloosa 50 60 35 48 27 / 30 90 20 0 0 Calera 50 63 36 48 27 / 20 100 20 0 0 Auburn 55 70 40 53 32 / 20 100 60 0 10 Montgomery 57 74 42 55 34 / 20 100 60 0 10 Troy 58 74 43 55 35 / 20 100 60 0 10 && .BMX WATCHES/WARNINGS/ADVISORIES/... None. && $$