277 FXUS64 KHUN 150819 AFDHUN Area Forecast Discussion National Weather Service Huntsville AL 319 AM CDT Mon Apr 15 2019 .NEAR TERM...(Today) Issued at 319 AM CDT Mon Apr 15 2019 Clear skies have returned to northern Alabama and southern middle Tennessee. However some winds between 5 and 10 mph are still occurring in northeastern Alabama. Elsewhere, winds are calm to around 5 mph (mostly east of the I-65 corridor). Dewpoints range from the mid 30s in southern middle Tennessee to around 41 degrees in most other locations with actual temperatures only 3 to 6 degrees warmer. With clear skies expected to continue temperatures should drop to or just above dewpoint values over the next few hours or by daybreak. As a result there may be some patchy fog towards daybreak, especially in northwestern Alabama between 5 and 8 AM. How far east this gets, is a bit more of a question, due to winds which may stay around 5 mph through 3 or 4 AM in northeastern Alabama. Some additional scattered cloud cover around 5000 feet in Arkansas may brush northwestern Alabama and southern middle Tennessee towards daybreak, but should have little effect on cooling. Patchy frost will be possible, especially in southern middle Tennessee where temperatures should fall into the 36 to 29 degree range by daybreak. Any patchy fog should not linger much past 8 AM. Then expect sunny skies and a cooler day than we have been experiencing the last several days. Despite a sunny day, highs should only reach the mid to upper 60s. .SHORT TERM...(Tonight through Wednesday night) Issued at 319 AM CDT Mon Apr 15 2019 The surface high is progged by most guidance to shift eastward enough Monday night to allow low level flow to return from the Gulf of Mexico. This should allow for much warmer conditions Tuesday night. Locations east of I-65 do not look to be as warm though, as the departing edge of the surface high keeps those areas cooler and drier. Further west, with sustained southerly to southwesterly return flow, lows should only drop into the mid 40s to lower 50s. Expect some on and off high cloudiness, but these clouds should be very high and thin in nature. A return to seasonal temperatures is expected by Tuesday into Wednesday, as the southerly/southwesterly flow increases in strength. This strengthening jet is expected develop ahead of a very strong storm system that moves from the southwestern U.S. into the Southern Plains during this period. The main effect will be much warmer and more moist conditions over the Tennessee Valley. In addition more prevalent opaque cloud cover is expected to develop on Wednesday/Wednesday night. Highs should climb back into the mid to upper 70s by Wednesday, despite the cloud cover. Low temperatures should climb into the mid to upper 50s by Wednesday morning and into the upper 50s to lower 60s by Thursday morning. Some showers ahead of this approaching storm system late Wednesday night are possible. An isolated thunderstorm cannot be ruled out given some elevated CAPE in forecast soundings, but they are not expected to be very strong. .LONG TERM...(Thursday through Sunday) Issued at 319 AM CDT Mon Apr 15 2019 An amplified upper trough will cross the Plains and move into the Mississippi Valley early Thursday morning, with the associated surface low moving into the western Great Lakes. A cold front will extend southwestward from the surface low from Minnesota through the Mid Mississippi Valley and into the ArkLaTex region at daybreak Thursday. Ahead of this system, southerly flow will quickly strengthen as the surface pressure gradient tightens and the TN Valley becomes situated between the approaching trough and a departing ridge. Even before the moisture return really begins in earnest, PW values will be above climatological norms and closer to the 75th percentile for late April. Given the amount of moisture, scattered showers and thunderstorms may develop over the TN Valley Thursday morning in response to isentropic lift as the WAA increases. The coverage isn't expected to be too high during the morning hours as the forcing remains well to our west. The system will advance eastward during the day, with the trough amplifying as it moves into the Mississippi Valley by the early evening hours. Meanwhile, the surface low will eject northeastward and into the Great Lakes. Given the continued amplification of the trough, a secondary surface low will likely develop along the cold front. Where exactly this occurs is still difficult to say, but latest guidance suggests this surface low will develop over the Mid South region. Ongoing showers and thunderstorms to our west will advance eastward, reaching northwest Alabama by the mid to late afternoon hours. There remains some timing differences among models, but the general agreement is high enough to warrant definite pops after 18Z Thursday. Widespread showers and thunderstorms will continue into the evening hours as the trough axis continues its eastward progression and shifts the cold front east of the Mississippi River. Forecast models are in fair agreement that the cold front will move east of the TN Valley between 06Z and 12Z Friday, with the bulk of showers and thunderstorms coming to an end. Looking at the severe threat, confidence remains low on how significant the threat will be on Thursday. It seems the main limiting factor will be very poor lapse rates between 700 mb and 500 mb during the main window of concern, likely due to the fact that the upper trough remains positively to neutrally tilted and well to our west for much of the afternoon. With that said, however, in the days leading up to the system, southerly flow will be fairly persistent and as it strengthens, dewpoints will climb into the lower 60s. Despite some shower activity in the morning, there should be enough breaks in the cloud cover and strong enough WAA for temps to climb into the mid 70s by the afternoon. This should provide sufficient low level lapse rates during the afternoon and early evening hours. But again, if the lower mid level lapse rates pan out, it will greatly limit the severe threat. So, I'm not overly confident that we will see the needed instability for an organized severe threat. With that said, however, the wind profiles are impressive as the low level jet intensifies as it crosses the Mississippi River. The GFS and ECWMF are in fairly good agreement that 850 mb winds increase as high as 60kts late Thursday afternoon as showers and thunderstorms move into the area. As this occurs, there may be a narrow window of steeper lapse rates as the upper trough tries to become negatively tilted and this would lend us some moderate instability. Certainly not a slam dunk scenario, however there is enough chance for severe weather that we will mention it in the HWO Thursday afternoon and evening. As stated above, the front will move east of the area by daybreak Friday, with the trough moving through the TN Valley during the day. Forecast models really diverge on their evolution of the trough, with the GFS closing a low over the lower Mississippi Valley and sending it through the TN Valley. The ECMWF on the other hand, keeps the closed low moving over the Ohio Valley. With either solution, drier air doesn't really make it into the TN Valley until Saturday, so we may see some isolated to scattered pockets of light rain. Temperatures will be quite cool on Friday, as CAA begins to take shape and cloud cover persists. This will keep highs from warming above the lower to mid 60s. Any lingering light rain will come to an end on Saturday as northerly flow filters in drier air in the wake of the departing low. Another cool day is expected on Saturday, as high temperatures only warm into the lower 60s. The axis of an upper ridge will be moving into the TN Valley on Sunday, while surface winds veer to the south. This will make for a beautiful day as highs warm into the lower to mid 70s under mostly clear skies. Rather cool nights are expected Friday night and Saturday night as temperatures fall into the lower 40s. && .AVIATION...(For the 06Z TAFS through 06Z Monday night) Issued at 1210 AM CDT Sun Apr 14 2019 Winds will continue to decrease as drier air filters in, ushering in clear skies tonight. Winds should remain elevated enough to keep more widespread fog chances at bay, but patchy fog is possible in the early morning hours. Confidence is too low at this time to include in the TAF. Northwest winds increase to near 10 kts tomorrow afternoon, then become light and from the southeast tomorrow evening. && .HUN WATCHES/WARNINGS/ADVISORIES... AL...NONE. TN...NONE. && $$ NEAR TERM...KTW SHORT TERM...KTW LONG TERM...73 AVIATION...30 For more information please visit our website at weather.gov/huntsville.