318 FXUS64 KBMX 172153 AFDBMX Area Forecast Discussion National Weather Service Birmingham AL 353 PM CST Thu Jan 17 2019 .UPDATE... Evening Update. && .SHORT TERM... Tonight through Saturday. Summary: Cloudy skies tonight with rain showers decreasing from northwest to southeast. Patchy fog developing through the night with low clouds persisting. Partly to mostly cloudy Friday with increasing clouds west late with a few showers northwest before midnight. Increasing chances for showers and thunderstorms through the day Saturday from northwest to southeast. Some strong to severe storms will be possible Saturday from late morning through sunset as a cold front moves from west to east across the forecast area with potential for damaging straight-line winds and a few isolated tornadoes possible. Gradual decreasing shower activity Saturday night. Details: Now through Friday: A shortwave trough is moving east into Georgia and an associated surface cold front was analyzed extending from Indiana southwest through Arkansas and into eastern Texas. To our west longwave ridging extended from central Texas northeast into western Montana while a negatively-tilted trough was positioned over the Pacific Northwest region into Nevada and Utah. As the shortwave trough quickly moves off the Carolina Coast early tomorrow morning the surface cold front weakens and stalls across our far northern counties tomorrow and remains across this area into early Saturday. Significant height falls through Friday morning over the central Rockies as the upper level trough ejects into eastern Montana and extends south through Wyoming and into Colorado. The ridge over the Plains deamplifies and is reduced to a shortwave as it progrades into the Mid South Region by midday Friday. The deepening trough will continue to sharpen as it moves into the western Plains, extending from South Dakota south into North Texas by late Friday afternoon. In response to the height falls a surface low is forecast to develop along the Lee of the Rockies in eastern Colorado early in the day Friday and will move southeast into western Oklahoma by Midday. Saturday: The upper trough will continue to gain amplitude while remaining neutral to slightly positively tilted as it approaches the Mid Mississippi River Valley early on Saturday. The trough will move into Mississippi Saturday afternoon and evening and will push across the area Saturday night. With the approach of the upper trough the stationary front across our northern counties will lift north as a warm front through the morning hours on Saturday as a surface low begins to occlude over Arkansas while an approaching cold front crosses the Mississippi River in the morning and reaches the Alabama/Mississippi State Line by midday and will continue east through the area Saturday afternoon and evening. The primary concern in the short-term period is the potential for strong to severe thunderstorms posing a damaging wind and isolated tornado risk. This risk will exist with highest potential generally near and south of the Interstate 20 corridor from late Saturday morning through sunset Saturday evening along and ahead of shower and thunderstorm activity with the eastward movement of a cold front. Uncertainty exists regarding available surface-based instability and has for the past few days, however, recent model trends have generally yielded some increase in instability values in this time frame over the past 48 hours. Abundant cloud cover with the potential for some pre-frontal shower activity will be factors working against sufficient instability while abundant speed shear with a defined Low Level Jet axis will favor strong to severe storm development. Warm air advection and positive moisture advection from the south is expected to contribute to near-surface destabilization through the day Saturday. Low-level directional wind shear profiles are marginal for tornado formation but if surface convergence toward the surface low increases and if sufficient instability can materialize with breaks in cloud cover and/or more robust advection this could support a more sufficient environment for isolated tornadoes. The convective mode of a line of storms will possess straight-line damaging wind potential as the vertical wind profile becomes more unidirectional by late afternoon and into early evening with best directional shear appearing to materialize early to mid afternoon. Some convective-allowing models depict a line of storms degrading into more cellular linear formation with time and this storm mode would have more potential for rotating storms possessing some tornado development potential. We hope to further refine the timing and location potential for strong to severe storms on Saturday in our further forecast updates. Saturday Night and Overnight: Cold air will rush into the area behind the cold front Saturday evening and if enough moisture remains generally after midnight Saturday night could end as snow flurries with highest potential generally north of a Carrollton to Gadsden line. Moisture will quickly depart to the east of the area after sunrise Sunday morning. 05 .LONG TERM... Sunday through Wednesday. Strong cold advection and steep low-level lapse rates may generate enough lift for sprinkles or flurries on Sunday morning, but no impacts are expected. Breezy conditions and temperatures struggling to rise out of the 30s across the North, will yield wind chills in the 20s on Sunday afternoon. A surface ridge should promote calming winds on Monday morning with temperatures falling into the low to mid 20s for many locations. After a cool and dry Monday, high pressure will shift eastward on Tuesday as the next upper-level trough moves across the Plains. Strong moisture return is indicated by model guidance for Tuesday evening through Wednesday, and rain chances should continue to increase as models converge on the timing of the cold front. 87/Grantham && .AVIATION... 18Z TAF Discussion. Light rain showers will continue to move west to east across the area through this evening. A heavier rain shower could bring vis down to IFR briefly, but generally MVFR conditions will dominate the aviation conditions through this afternoon. Cigs will lower below 1000ft this evening, and then to LIFR heights overnight. Rain showers will end from west to east after 18/05Z, with fog possible as winds diminish. Expect conditions to be slow to improve Friday morning, especially LIFR/IFR cigs potentially lasting through the morning hours. 14 && .FIRE WEATHER... Rain showers will persist this afternoon and through tonight. Showers will become confined to our far southeast counties Friday as we experience a brief break in precipitation before showers and thunderstorms arrive overnight Friday and through the Day Saturday. No fire weather concerns are expected at this time due to minimum relative humidity values remaining well above critical values. 05 && .PRELIMINARY POINT TEMPS/POPS... Gadsden 43 57 49 63 32 / 50 10 40 100 70 Anniston 45 61 51 64 33 / 50 10 40 100 70 Birmingham 46 60 53 66 33 / 50 10 50 100 50 Tuscaloosa 48 62 55 68 34 / 50 10 50 100 40 Calera 47 62 53 66 34 / 50 10 40 100 50 Auburn 48 61 53 65 37 / 50 20 30 90 80 Montgomery 50 66 54 69 38 / 50 10 30 100 60 Troy 52 65 55 69 39 / 40 20 30 90 70 && .BMX WATCHES/WARNINGS/ADVISORIES/... None. && $$