Area Forecast Discussion (AFD)
Issued by Huntsville, AL (HUN)
000 FXUS64 KHUN 211130 AFDHUN Area Forecast Discussion National Weather Service Huntsville AL 630 AM CDT Sun Oct 21 2018 .UPDATE... For 12Z TAFS. && .NEAR TERM...(Today) Issued at 500 AM CDT Sun Oct 21 2018 An amplifying mid-level longwave trough will continue to spread eastward across New England and the mid-Atlantic region this morning, before lifting northeastward and offshore later today. Strong deep- layer north-northwesterly flow across the local forecast area will begin to subside and back to the west-northwest by late this afternoon as this occurs, and a low amplitude ridge translates eastward across the southern Plains. At the surface, a rather strong cold front continues to accelerate southward and should be approaching the north central Gulf coast by sunrise. The pressure gradient to the south of a Canadian high settling into the mid-MS Valley has been sufficient to maintain gusty north-northeasterly low-level flow for much of the night, with temperatures expected to fall into the m-u 30s for most of the region by 12Z, given the magnitude of CAA. In fact temps may still reach 32F for a couple of hours on either side of sunrise in portions of northern Lincoln/Moore and Franklin Counties in TN, but will not last long enough to warrant a Freeze Warning. Although the surface ridge is expected to shift eastward across KY/TN today and reduce the local pressure gradient, at least weak CAA will persist this morning, and with a model consensus of 850-mb temps in the 5-10C range most locations will struggle to achieve max temps in the m-u 50s. .SHORT TERM...(Tonight through Tuesday night) Issued at 500 AM CDT Sun Oct 21 2018 It appears as if mid and high-level flow will continue to transition to more of a zonal orientation tonight and tomorrow, to the north of a subtropical ridge retrograding across the southern Gulf of Mexico. Models still suggest that the passage of at least a couple of weak disturbances embedded in this regime will yield a gradual increase in cirroform cloud cover, which could begin as early as this evening and continue through Monday. At this time, the high-level cloudiness is not expected to be widespread or dense enough to have a significant impact on low temperatures tonight, in an ordinarily favorable pattern for radiational cooling. However, we have increased temps just a bit above previous forecasts to account for this possibility. With this in mind, we no longer have any locations with temps reaching 32F, and the probabilities for a widespread frost have been reduced, but are still greatest across portions of northwest AL and southern middle TN. Conditions will support the initial development of frost in the 4-6Z timeframe, with frost becoming most widespread between 9-13Z. Light low-level southerly flow will return to the region on Monday, as the surface ridge shifts further eastward, and this will promote afternoon temps in the mid 60s for most of the area. Another strong norther stream vort max is predicted to dig southeastward into the Great Lakes region on Monday night/Tuesday morning, before lifting northeastward into coastal New England Tuesday afternoon/night. This will force winds aloft to veer to the northwest once again, with a weaker/secondary cold front in the wake of the vort max expected to enter the TN Valley late Tuesday afternoon or Tuesday evening. With the exception of a potential interruption from mid and high-level clouds, conditions will be favorable for radiational cooling once again on Monday night, with lows expected to drop into the lower 40s. In spite of the passage of the reinforcing cold front, stronger mixing and weak thermal advection should result in even warmer lows on Tuesday morning. Highs will rebound into the m60s-around 70 for most of the area on Tuesday. .LONG TERM...(Wednesday through Saturday) Issued at 500 AM CDT Sun Oct 21 2018 On Wednesday, most models continue to show general upper level ridging building over the southeastern CONUS (centered over Missouri and Arkansas). Two systems will impact locations to the south and west of the Tennessee Valley into the afternoon hours. One is a surface low in the northwestern Gulf of Mexico and a warm front that extends eastward from this feature. At the same time, a longwave trough axis is shown pushing into the western edge of the ridge axis. There won't be many sensible weather impacts from either of these systems through Wednesday night, except maybe some increasing cloud cover. Afternoon highs on Wednesday should be near or below normal, given forecast 925 mb temperatures, likely in the mid to upper 60s at least. Developing mostly cloudy conditions toward midnight into the daybreak hours on Thursday should keep overnight lows warmer (in the mid to upper 40s) than the previous few days. But no precipitation is expected, as the surface low pushes due east and keeps this activity along the Gulf Coast and into southern Alabama primarily. During the day on Tuesday, models start to diverge. Some models move this surface low further northeast, while models such as the GFS keep it moving almost due east still. If models showing the further northeast movement of this low verify, isolated to scattered shower chances are possible. At this point SuperBlend is sticking closer to this solution. Therefore, kept isolated to widely scattered precipitation chances in the forecast on Thursday. Abundant cloud cover definitely looks like the more certain outcome. This will likely lower afternoon highs into the upper 50s to lower 60s (maybe even cooler than that). The discrepancies with the surface low track continue and are even more significant Thursday night. ECMWF would be much wetter in locations east of I-65. At this point, given such discrepancies, kept to middle ground with scattered chances of precipitation. Additional upper level energy and cold air aloft swings southeast via the main jetstream into the Midwestern states on Friday. Although the bulk of the precipitation from the Gulf of Mexico system should move east of northern Alabama by Friday, lingering clouds and possibly some drizzle or very light rain looks possible east of I-65. With zonal flow, high temperatures will likely remain in the upper 50s to lower 60s. As we move into next weekend, models diverge again concerning how quickly and where this upper level energy and better moisture convergence actually goes. However, kept isolated to scattered showers in the forecast given the strong upper level forcing shown by models pushing into the area. It should be slightly warmer though over the weekend with highs back in the lower to mid 60s. && .AVIATION...(For the 12Z TAFS through 12Z Monday morning) Issued at 630 AM CDT Sun Oct 21 2018 VFR/SKC conds will exist at both terminals today, with NE flow on the order of 10G18 knots expected to subside throughout the afternoon hours as a surface ridge migrates eastward across KY/TN. The local pressure gradient will relax early this evening, with winds becoming lgt/vrbl overnight, as scattered high-level clouds begin to overspread the region from the west. && .HUN WATCHES/WARNINGS/ADVISORIES... AL...NONE. TN...NONE. && $$ NEAR TERM...70/DD SHORT TERM...70/DD LONG TERM...KTW AVIATION...70/DD For more information please visit our website at weather.gov/huntsville.