Area Forecast Discussion (AFD)
Issued by Huntsville, AL (HUN)
306 FXUS64 KHUN 141108 AAA AFDHUN Area Forecast Discussion...UPDATED National Weather Service Huntsville AL 508 AM CST Thu Dec 14 2017 .UPDATE... For 12Z TAFS. && .NEAR TERM...(Today through Friday night) Issued at 202 AM CST Thu Dec 14 2017 At 07Z, a cold front was situated from just northwest of London KY through Nashville to between Memphis TN and Jonesboro AR. Despite a decent surface pressure gradient, there is quite a large range in temperature across our forecast area this morning due to a few cold valleys decoupled from the boundary layer. Temperatures range from 25 at Valley Head up to 47 at Muscle Shoals. Satellite imagery indicates a clear sky over the TN valley at this hour. Behind the cold front, clouds are dropping southward through northern KY, southern IL into eastern and central MO. These are progged to make a bit more progress before isentropic downglide dissipates these before reaching southern middle TN or north AL as seen in NAM RH fields and HREF ceiling probability. So will go with a sunny forecast today with highs a few degrees cooler than Wednesday, with middle 40s to lower 50s. It will be a chilly night areawide with lows in the 20s except over the larger water bodies where readings stay near 30-32F. Flow should be just enough to prevent any fog development. A positively tilted trough/vorticity lobe will drop southeast from the eastern TN valley into TX on Friday. A relatively dry atmosphere should keep the sky mostly cloud free. A surface ridge will be building east across the southern U.S. into Friday night. After highs in the 40s, lows Friday night should once again drop into the 20s. .SHORT TERM...(Saturday and Saturday night) Issued at 202 AM CST Thu Dec 14 2017 Saturday should be a nice day as southerly flow advects 850 mb temperatures back into the 6-7C range. Clouds are unlikely to return before dusk. Highs should reach the lower 50s at most valley locations. As has been advertised in recent days, a quick moving shortwave in mid to upper levels will be ejecting northeastward out of TX Saturday night, reaching the MS river around 12Z Sunday. Good upper level divergence at 250mb across the mid South and TN valley will support good UVVs and induce a strong south-southwesterly low level jet of 50-60kt. Resulting isentropic ascent will be strong with good pressure advection indicated on the 295-305K levels. PWs will increase above 1.0 inches with very good MTVs also progged. So, despite low levels beginning rather dry with dew points in the 30s, will bring rainfall into most of the area after midnight Sunday morning. An elevated MCS (mostly showers) will develop and expand out of MS into north AL and southern TN at the nose of the strong low level jet and best isentropic ascent during the early morning hours of Sunday. Sounding profiles may support a few ice pellets mixing in over southern middle TN or northeast AL, but confidence is not high enough to include just yet. Have also left thunderstorms out for now due to positive Showalter Indices (SI), but we can't rule out some higher based thunderstorms rooted at or above 750 mb based on the steepening lapse rate profile. Will monitor future model runs as we move closer to the event. .LONG TERM...(Sunday through Wednesday) Issued at 202 AM CST Thu Dec 14 2017 In the beginning of this forecast period widespread rainfall associated with a shortwave trough axis moving within the subtropical jet. This rainfall will be occurring over the MS River Valley and gradually moving east into the western portions of the TN River Valley. In addition, a warm front should be moving north from the Gulf of Mexico towards the TN Valley on Sunday. This will result in temperatures rising after midnight and during the day on Sunday. Lows may actually have occurred around midnight on Sunday morning. Meanwhile, the kinematics accompanying the trough axis include a substantial increase in wind shear with low-level SRH values between 200-300 m2/s2 and curved hodographs/veering profile from the sfc to 850 mb. What this means is that despite a generally stable moist adiabatic environment, broad rotating and bowing segments will be possible especially as the warm front stalls over the TN Valley. In addition, daytime highs will be in the low 50s but there may be a north-south gradient in later forecasts as greater confidence is realized on warm front movement. As this trough moves off to the NE, the TN Valley (and much of the Southeast) will be under the influence of subtropical ridge centered just southeast of Florida over the Bahamas with strong SW flow aloft and southerly surface flow. The resulting warm air advection will bring some relief to the cold temperatures with daytime highs near 60 on Monday and overnight lows in the 40s. However, as an elongated trough moves within the subtropical jet over S Plains/MS River Valley, will likely see additional shower development within the SW flow (weak undulations within the SW flow). This may occur before the main trough and accompanying cold front arrives on Tuesday/Wednesday. There is typical model timing differences but given the trajectory of this front, temperatures should be only modestly affected and mostly the overnight lows once the front passes. Furthermore, will keep showers in the forecast for both Monday and Tuesday given the timing differences and potential pattern change. Speaking of the pattern change, it does appear that mean troughing/blocking pattern that had been in place for a while over the eastern half of the country will be replaced with ridge and trough. Left Wednesday dry and slightly cooler after the front passes sometime on Tuesday. && .AVIATION...(For the 12Z TAFS through 12Z Friday morning) Issued at 508 AM CST Thu Dec 14 2017 VFR flight weather conditions are forecast during the next 24 hours. A cold front will drop southeast through the area this morning bringing northwest-north winds of 5-10kt. The wind will turn more north-northeast this evening. && .HUN WATCHES/WARNINGS/ADVISORIES... AL...NONE. TN...NONE. && $$ NEAR TERM...17 SHORT TERM...17 LONG TERM...SL.77 AVIATION...17 For more information please visit our website at weather.gov/huntsville.