570 FXUS64 KBMX 292153 AFDBMX Area Forecast Discussion National Weather Service Birmingham AL 353 PM CST Thu Nov 29 2018 .SHORT TERM... Through Tonight. Westerly flow aloft and southwesterly low level flow around surface high pressure centered to our southeast has resulted in an increase in clouds and light rain showers today. Most of this activity has been across the northern half of the area, further from the influence of the surface ridge. Tonight, a weakening surface low will move out of the Central Plains toward the Ohio River Valley, leaving a surface trough/convergent zone to our north. With increasing moisture and lift south of this zone, shower coverage will increase overnight, especially across the north and west. Temperatures will be noticeably warmer, with lows in the upper 40s east to upper 50s west. 14 .LONG TERM... Friday and Friday Night. Low level ridging is in place over the Florida Peninsula on Friday as a trough digs into the Rocky Mountains. A low level jet stretches through much of North MS, into the TN Valley and Eastern TN. The presences of this jet, in conjunction with isentropic lift, will allow for scattered to numerous rain showers to develop, mainly north of the I-85 corridor. The best chances will remain north of I- 59 early during the day on Friday. As the trough deepens into the Southern Rockies Friday afternoon, the pattern becomes more amplified, leading to temporary height rises over Central AL in response. This amplifying pattern lifts the low level jet northeastward, which could lead to a brief period of rain-free conditions sometime Friday evening, but confidence in an exact time of this happening is low at this time, so for now I've kept at least chance PoPs in the forecast through the evening hours Friday. At this same time, a surface low develops on the lee side of the Rockies and begins moving eastward through the Central Plains. This low will lift an effective warm front west to east across Central AL late Friday night into Saturday (after midnight Friday) leading to widespread rain with a few elevated thunderstorms. 25/Owen Saturday and Saturday Night. A potent upper level trough will become negatively tilted Friday night over north Texas and western Oklahoma then will swing east over the rest of Oklahoma along with into Kansas, Missouri and Arkansas by Saturday morning. The trough will rapidly move northeast into the Ohio River valley by Saturday evening with the upper low positioned over southern Iowa after sunset. Toward the surface, low pressure low will develop in response to the upper trough and strong forcing aloft across northwest Texas and western Oklahoma Friday night and will continue to deepen as it tracks northeast, becoming centered over eastern Kansas by Saturday morning and begins to occlude over southern Iowa by Early Sunday morning as a new low pressure area develops near Chicago. Most global models generate a convective complex near or offshore of the Louisiana coastline by 06z Saturday which moves northeast into Southern Alabama and the Western Florida Panhandle by 12z and propagates further northeast into the Central Florida Panhandle and into southwest Georgia by 18z. We will have the Low Level Jet (LLJ) present over our northwest counties as early as 09Z Saturday and exits by 15Z followed by another LLJ that overspreads our western and northwest counties by 21z and will move east across our north-central counties before exiting the state by 00z Sunday. Wind profiles including bulk shear and helicity values are more than supportive of rotating thunderstorms and poses a tornado threat. Hail may also be produced by the stronger storms Saturday afternoon as height falls begin to be realized with the approaching upper level trough. Surface-based instability remains in question with heavy coastal convection expected to our south along the Northern Gulf Coast through 15z and this will help limit low-level instability over our forecast area for Saturday morning and will confine the window for destabilization roughly from 18z through 00z for conditional convective development along and ahead of a surface trough that will move into the state by 00z Sunday and should exit the state before 12z Sunday. A substantial dry slot is expected to overspread much of our CWA by 21z and this will help suppress notable convective development for the remainder of the evening. Despite very favorable wind profiles, wind shear and helicity values, the tight time frame for low-level destabilization coupled with the lower sun angle along with reduced daytime hours coupled with expected cloud cover over our area and points south will hamper solar insolation and make low-level destabilization largely dependent upon low-level advection which will be aided by the Low Level Jet but confidence is low that sufficient instability will materialize to support more than the potential for a few strong to severe storms Saturday late morning through early evening. If coastal convection is not as robust as expected the severe weather risk would substantially increase across our forecast area while if coastal convection is more substantial and persists later the severe weather risk would further decrease across our forecast area. The feature to watch will be the timing and strength of the shortwave trough that lifts that coastal convective complex northeastward from the northern Gulf of Mexico into south Georgia by Saturday night. 05 Late Saturday Night through Wednesday. By late Saturday night, the surface low has moved into the Great Lakes region and progressive forcing has diminished, which substantially slows the forward speed of the main cold front as it approaches Central AL. Guidance hasn't handled this aspect of the system very well over the last few model runs, so the question will be how far southeastward does the cold front push before it stalls. I expect it at least push through a portion of the area, so I'm keeping rain chances high in the southern half of Central AL, and lowering PoPs generally north of I-20/22 on Sunday. This won't last long, as a shortwave impulse slides through the main upper level trough and develops a weak surface low in the Gulf States that lifts the cold front northward as an effective warm front Sunday night into Monday, leading to at least a chance of rain area-wide. By early Tuesday, the main 850mb trough axis has slid southeast of our area, which should push the cold front out of Central AL by Tuesday afternoon, ending rain chances from northwest to southeast. A cooler airmass dips into the Southeastern CONUS Wednesday into Thursday as low level ridging builds in ahead of another deepening trough in the Pacific Northwest. This should lead to clearer skies and cooler weather for Central AL as we get through the middle of next week. 25/Owen && .AVIATION... 18Z TAF Discussion. Scattered light showers/sprinkles will move across north Central Alabama through the afternoon. These should not cause aviation impacts at terminals. Expect MVFR cigs to spread from west to east across the area late this afternoon through the early evening. Rain showers will become more widespread overnight, reducing visibilities to MVFR, with cigs falling to 1000ft by Friday morning. Generally, winds will have a southerly component through this afternoon, varying from southwest to southeast at times, and cannot rule out a few gusts to 15-17kts. The one exception will be ANB that could maintain a more persistent northeast wind due to terrain. Overnight, winds will become consistently southerly, at 7-10kts through tomorrow morning. 14 && .FIRE WEATHER... Low level moisture will increase overnight with clouds deck at or below 1kft Friday morning. The main focus for rain showers through tomorrow afternoon is across the west and north. This activity will become widespread Friday night through Saturday. Cannot rule out thunderstorms on Saturday. No fire weather concerns are expected at this time. && .PRELIMINARY POINT TEMPS/POPS... Gadsden 50 62 54 65 58 / 60 70 70 100 50 Anniston 49 64 54 67 60 / 40 50 70 100 40 Birmingham 55 66 59 69 59 / 60 60 90 100 30 Tuscaloosa 57 70 61 73 58 / 60 60 90 90 10 Calera 53 66 58 69 61 / 40 50 90 100 20 Auburn 48 65 55 67 63 / 20 20 70 100 30 Montgomery 52 70 60 73 64 / 20 20 90 100 30 Troy 51 69 59 71 66 / 10 10 90 100 40 && .BMX WATCHES/WARNINGS/ADVISORIES/... None. && $$