425 FXUS64 KHUN 280521 AFDHUN Area Forecast Discussion National Weather Service Huntsville AL 1121 PM CST Sun Jan 27 2019 .UPDATE... For 06Z TAFS. && .NEAR TERM...(Tonight) Issued at 740 PM CST Sun Jan 27 2019 Very quiet weather in the near-term as the Tennessee Valley remains under the influence of high pressure at the surface. With the weak trough that clipped the region earlier in day shifting well east of the area, zonal flow returns aloft. Thus, other than a few passing high clouds, a mostly clear sky will continue for the remainder of the overnight hours. After peaking in the mid 50s in most locations, temperatures have dropped 15-20 degrees into the upper 30s to lower 40s, thanks to the good radiational cooling setup. Dewpoints in the upper 20s to lower 30s will prevent them from falling more than an additional 5-6 degrees, however. This will result in most locations remaining at or just above the freezing mark by early Monday morning. All in all, the forecast remains on track and only a few tweaks were needed. .SHORT TERM...(Monday through Wednesday night) Issued at 233 PM CST Sun Jan 27 2019 An upper trough will dig into the Central Plains on Monday, with a surface low moving into the Great Lakes region. A cold front trailing south of this parent low will make its way eastward and arrive into the Mississippi River by sunset on Monday. Ahead of this, southwest flow through the vertical column will begin to filter moisture northward and into the TN Valley. Given ample sunshine and WAA, temperatures will feel quite warm, with highs in the mid to upper 50s during the day. A secondary piece of energy dropping SE out of the northern Rockies and into the ArkLaTex region will trigger pressure falls over southern MS/LA and a weak surface wave is progged to develop. This secondary wave will work in concert with the parent upper level trough to increase vertical motions over the area between 06-12z Tuesday and coincide with our best chances for snowfall. Thinking snow could start as early as 06z in NW AL and be covering much of the TN Valley by 09z. Then as the front and upper wave shift E/SE expect snow to end from NW to SE after 12z Tuesday. South to southwesterly winds ahead of this feature will allow for PWATs upwards of 0.75 inches to advect into the area. While not overly anomalous for this time of year, these values are on the higher side for a winter weather type of an event. As the forcing tied to the features mentions above overspread the area, expect a band of rain to develop as early as 9 PM along the MS River and W TN. Then as the Arctic front and secondary trough shift south this band of precip will move into NW AL. Forecast soundings suggest that enough of a warm layer will exist near the surface to keep this initial band of rain in liquid form. However, the strength of the Arctic front will send temps quickly below freezing early Tuesday morning and we can expect a rapid transition to all snow. Ensemble probability guidance has shown high confidence in receiving at least a quarter inch of QPF but low probability of anything more than 0.5 inches. So, have adjusted our QPF based on this thinking and values come in around 0.3 inches. Not all of this will be falling as snow and it's possible that up to 0.1 inch could fall as rain before the change over. Snow ratios will be close to 10 to 1 and that puts snowfall amounts on the low end at 1 inch and on the high end 2.5 to 3 inches. While the speed of the front might limit the overall length/time period for snow, time-height analysis from various data sets were all indicating a stronger vertical motions either in the dendritic snow growth zone or just below. This would suggest the potential for some brief heavier snowfall rates in any mesoscale banding that develops. Total snowfall plumes from various data sets were trending the mean snowfall towards the 1.5 to 2 inch range. Our current forecast fall inline with these values with some locations in southern middle TN receiving upwards of 2.5 to 3 inches. These amounts occurring either just before or possibly during the morning commute on Tuesday has prompted the need for a Winter Storm Watch to highlight the potential for travel impacts. Even though we could see some melting of snow initially, the fact that temps will remain below freezing until Wednesday afternoon, any melting that does occur should lead to refreezing and icy roads. The shot of Arctic air will be brief and temps only fall into the teens to lower 20s Tuesday night. High pressure will move across the Gulf on Wednesday and allow for a winds to switch to the SW for a brief period and highs Wednesday will make it into the low to mid 30s. A much stronger push of cold/dry air comes Wednesday evening with dewpoints dropping into the single digits and possibly below zero. High pressure moving into the OH Valley during the overnight period should allow for clear skies and light winds Thursday morning. This is a good setup for a strong radiational cooling night and models could be underdoing overnight lows on Thursday a tad. Knocked temps down with lows Thursday morning in the single digits in TN and lower teens for the rest of the area. However, as mentioned above we may need to nudge these values down further. Any light breezes we do end up having would result in Wind Chill values possibly dropping below zero. .LONG TERM...(Thursday through Saturday) Issued at 233 PM CST Sun Jan 27 2019 Some dangerously cold arctic air will be in place to start the day on Thursday morning, with wind chill values starting off in the 0 to 5 degree range across much of the Tennessee Valley. Thanks to some decent insolation and a southeasterly wind component, some warming will take place by the afternoon and allow temperatures to climb above the freezing mark and into the mid to upper 30s. The dry and mostly clear weather will be short-lived as a weak frontal boundary approaches the region from the Ohio Valley late Thursday night into Friday. There will be limited moisture available and fairly modest lift/forcing. However, think that we may see a window for a light mix of rain and snow Friday morning through early afternoon as this wave passes. Locations along and north of the Tennessee River will be more favored to experience precipitation. QPF is very light so would not expect there to be any impacts, and more than a few hundreths of liquid accumulation. Temperatures will also warm into the mid to upper 40s by Friday afternoon as this system pulls away from the area, and winds veer to the south. High pressure will begin to nose into the region Friday night into Saturday, promoting dry and fair weather for the first half of the weekend. Morning stratus will dissipate by early Saturday afternoon, resulting in a mostly sunny sky and our first chance for temperatures to warm above normal in six days, with highs expected to peak in mid to upper 50s. An area of low pressure will quickly shift northeast from South Texas and the lower Mississippi Valley late Saturday night into Sunday, bringing a potential for scattered to perhaps numerous light rain showers to close out the weekend. There is uncertainty on the timing of this system, so it's possible the onset of precipitation may be delayed later into Sunday depending on which solution verifies. && .AVIATION...(For the 06Z TAFS through 06Z Monday night) Issued at 1121 PM CST Sun Jan 27 2019 VFR conditions will prevail at each terminal overnight and into the day on Monday under a mostly clear sky. Clouds will increase by the late morning and afternoon ahead of the next frontal system. Winds will also increase and become gusty at times from the SW. A gradual lowering of 4-5 kft BKN to OVC ceilings will occur by Monday evening as the atmosphere moistens. Any precipitation should hold off until after 06z when a strong cold front will push into the region. && .HUN WATCHES/WARNINGS/ADVISORIES... AL...Winter Storm Watch from late Monday night through Tuesday morning for ALZ001>010-016. TN...Winter Storm Watch from late Monday night through Tuesday morning for TNZ076-096-097. && $$ NEAR TERM...AMP.24 SHORT TERM...Stumpf LONG TERM...AMP.24 AVIATION...AMP.24 For more information please visit our website at weather.gov/huntsville.