171 FXUS64 KBMX 021813 AFDBMX Area Forecast Discussion National Weather Service Birmingham AL 1212 PM CST Sat Mar 2 2019 .UPDATE... For 18Z Aviation. && .SHORT TERM... Today and tonight. A cold front that pushed slowly southward through Alabama on Friday recently passed Troy and most observing sites are now reporting light north to northwest winds. Patchy light showers will continue to track eastward across the area through the early morning hours due to a weak upper level disturbance. Drier mid level air will advect into the area this afternoon behind the exiting short wave trof. There may be a few lingering showers south of I-20 this afternoon, but most areas will be rain free. Skies will remain cloudy today and tonight as a cool and moist air mass gets trapped under an inversion around 850mb. Rain free conditions will continue into the evening hours, but showers and possibly a few thunderstorms will rapidly spread into west Alabama after midnight ahead of a short wave trof. 58/Rose .LONG TERM... Sunday and Sunday Night. * Slight Risk for severe weather Sunday afternoon and evening. 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. 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... A weak upper-level disturbance developed rain showers across the area overnight and into this morning. Rainfall amounts will be less than one tenth inch. In addition to low clouds overnight, patchy fog is also possible before sunrise Saturday near the stalled front. A few showers are possible today across the south, but most of the area will be dry through tonight. Widespread showers and storms will return for Sunday and Sunday evening as a strong cold front moves through the region. There are no fire weather concerns expected at this time. && .PRELIMINARY POINT TEMPS/POPS... Gadsden 60 47 60 34 45 / 10 40 100 40 0 Anniston 62 49 66 36 48 / 20 30 100 60 0 Birmingham 61 49 63 35 47 / 30 50 100 40 0 Tuscaloosa 62 51 65 35 48 / 30 60 100 30 0 Calera 62 51 66 36 48 / 30 40 100 50 0 Auburn 67 54 68 40 53 / 30 20 100 90 0 Montgomery 69 57 73 42 55 / 30 20 100 80 0 Troy 71 58 76 43 55 / 30 20 90 90 0 && .BMX WATCHES/WARNINGS/ADVISORIES/... None. && $$