177 FXUS64 KBMX 170615 AFDBMX Area Forecast Discussion National Weather Service Birmingham AL 1215 AM CST Sun Feb 17 2019 .UPDATE... For 06Z Aviation. && .SHORT TERM... Through Tonight. Surface cold front has shifted just south of the forecast area this afternoon, with sunny skies now across the south. Post frontal stratus has lingered near and north of the I-20 corridor. This has caused quite a range in temperatures today, with low 40s in the far northwest to mid 70s in the far southeast. For the rest of this afternoon and early evening, expect rain free conditions, but the clouds will remain across the north with near steady temps north of I-20. Low level winds will become southerly this evening in response to a weak surface low strengthening to our west, and the front to our south will begin to move northward as a warm front. After 9-10pm, temperatures should gradually increase overnight. Weak isentropic lift will bring a return of clouds to the entire area, and isolated showers across the north before midnight. Deep layer southwesterly flow increases as a upper level trough in the northern Plains. Rain coverage increases to our north and west, where a new cold front develops, and spreads into northern portions of Central Alabama before sunrise. With increasing shear and weak elevated cape, cannot rule out a few sustained updrafts and thunderstorms in the northwest, but strong or severe storms are not expected. 14 .LONG TERM... Sunday through Friday. Synopsis: Strong/moist southwest flow between a positively tilted trough over the southwestern CONUS and a strong 594 decameter subtropical ridge centered near the Bahamas will be the dominant weather feature for the extended period. As a front oscillates over the area, multiple rounds of rainfall are expected with flooding concerns by the middle of the week. Sunday/Sunday night: An upper low over the Siouxland region of the Plains will be in the process of shearing into an open wave on Sunday, while a strong (185kts at 250mb and 115 kts at 500mb) southwesterly mid and upper-level jet streak will extend from Texas to the Ohio Valley. At the surface, a warm front will be lifting northward through Central Alabama as a weak surface low moves northeastward into Kentucky and Tennessee, while a cold front moves eastward into Mississippi and West Alabama. A weak wedge will build into north Georgia in response to elongated high pressure over southern Canada. Southwest surface winds should help erode the wedge over all but our far northeastern areas. Moist isentropic lift and associated showers will be ongoing in our northern counties in the morning. Strengthening isentropic lift and frontogenetical forcing along with increasing moisture will result in a band of light to moderate rain developing across the northern two-thirds of the area by afternoon. Despite warm temperatures across our southern counties, little to no surface-based instability will be present. Additionally, 500mb height rises will be present, indicative of synoptic-scale subsidence. Therefore, severe storms are not expected, and there is only a small chance of a rumble of thunder with very weak elevated instability. Rain will continue into the evening hours, ending from northwest to southeast later in the night as the cold front pushes southeastward. Rainfall totals of up to an inch are possible. Monday/Monday night: Monday now looks to be a dry day outside of a few lingering morning showers in the far southern counties. Cool northerly post- frontal winds will be present with highs at or just below seasonal averages, as a 1040 mb high pressure builds into the Northern Plains. Southerly flow re-develops aloft after midnight Monday night, resulting in warm air advection and moist isentropic lift aloft over a cooler and drier air mass at the surface. Meanwhile easterly flow will develop at the surface as high pressure moves eastward to New England and a weak surface low begins to develop near the Texas Gulf Coast. Light rain will develop after midnight Monday night as the column begins to saturated. Tuesday through Thursday: Another shortwave trough will eject out over the Southern Plains from the southwestern CONUS trough Tuesday/Tuesday night. A wave of low pressure will lift northeastward into North Mississippi, preceded by an inverted trough. Southerly flow aloft will continue to strengthen as moisture increases. The strong isentropic lift will result in widespread moderate rainfall especially across the northern half of the area during the day on Tuesday. The surface low will try to pull the warm front northward from the coast, but will be inhibited at least initially given the developing CAD wedge, and evaporative cooling effects of rain falling north of the warm front. High temperatures were lowered Tuesday given the easterly flow and evaporative cooling effects. Rainfall during the day does not look to be heavy enough to cause major flooding issues outside of poor drainage areas, but will cause soils to begin to saturate. Main forecast challenge will be Tuesday night into Wednesday. Drier mid-level air will temporarily move into most of the area except for the northwest/far west Tuesday night, decreasing rainfall rates over all but the northwest/far west. In the northwest/far west, PWATs and instability will be increasing. A band of heavy rainfall will set up in the vicinity of the surface low/surface trough track. Some model trends keep this just to the northwest of the forecast area Tuesday night, but it's still possible it could shift back to the southeast, especially if the effective warm front is not able to make it as far north due to rain falling north of it. Will keep in a low confidence flooding mention Tuesday night, mainly for the northwest. The cold front will slowly move into the western part of the state on Wednesday, with PWATs increasing to around 1.7 inches. Models indicate some differences with the speed of the front and where it will stall Wednesday night. This will be key to the flooding potential, with an increased flooding threat the further west it stalls. Also, some surface-based instability will develop, resulting in a potential marginal threat of severe weather. However, limiting factors will be uncertainty of where the warm front will be and if the atmosphere can destabilize given all the rainfall. Also, upper-level support will be very limited with only some weak glancing 500mb height falls. Therefore, confidence is too low to mention any severe threat in the HWO at this time given the dependence on mesoscale details. This does appear to be the main period to focus on for flooding potential, so will bump up the confidence slightly in the HWO for Wednesday. Limiting factors are the lack of uni-directional flow which will limit cell training except along the boundary, and lack of confidence in the position of the boundary. However the high PWATs are certainly something to watch. The flooding threat will probably linger into Thursday as the front stalls near the area. Sunday through Thursday rainfall totals from WPC have come down slightly, with 4-6 inches now forecast along and north of I-20. This is in line with the ensemble means. The higher values depicted by the deterministic models still remain possible, as this will depend on where any heavier bands set up. Areal flooding and river flooding look to be a good bet, with GEFS ensemble based river forecasts indicating the greatest risk of flooding being in the Tombigbee basin. The flash flooding threat remains more unclear, which will be closely monitored over the next couple days. 32/Davis && .AVIATION... 06Z TAF Discussion. Cigs are finally starting to lift as boundary layer winds have begun to increase and shift southeastward. The current regional radar indicates some light showers moving across the area. Included VCSH for all sites initially with prevailing showers beginning between 17-19Z. A few thunderstorms are possible this afternoon however, probabilities are too low to mention in the TAFs at this time. Have cigs lowering throughout the day from west to east. Cigs across the north will likely be below 1000 ft agl between 15-17Z with MVFR cigs still at KMGM and KTOI. Winds will begin veering to the southwest as a cold front approaches from the west. By the end of the period (roughly) the front should have passed through our northern sites. Winds there will have shifted to the west/northwest. 07 && .FIRE WEATHER... A brief period of rain free conditions this evening, but rain chances return tonight as a warm front lifts northward through the area, and a cold front slides eastward Sunday. Another break between weather systems is expected Monday, before a prolonged period of rainfall from Tuesday through Friday. Very wet conditions are expected, especially across the northern half of the area. There are no fire weather concerns at this time. && .PRELIMINARY POINT TEMPS/POPS... Gadsden 46 65 45 57 40 / 60 80 80 10 50 Anniston 49 67 48 60 43 / 50 80 80 10 50 Birmingham 47 68 46 59 43 / 50 80 80 10 50 Tuscaloosa 48 69 46 60 44 / 40 80 70 10 50 Calera 49 69 48 60 44 / 40 80 80 10 50 Auburn 54 71 53 64 49 / 20 50 80 20 40 Montgomery 56 76 54 65 50 / 10 40 80 20 40 Troy 57 77 56 66 52 / 10 30 60 30 30 && .BMX WATCHES/WARNINGS/ADVISORIES/... None. && $$