555 FXUS63 KFSD 131045 CCA AFDFSD Area Forecast Discussion National Weather Service Sioux Falls SD 520 AM CDT Wed Mar 13 2019 .SHORT TERM...(Today and Tonight) Issued at 519 AM CDT Wed Mar 13 2019 Upper-level low pressure now starting to congeal across the southern Rockies, and a likely historic development of surface low pressure is soon to commence in the lee of the Rockies with lifting of the negative tilt wave toward the central Plains. Impressive northward surge of elevated instability through the Plains with widespread showers and thunderstorms across NE and IA at 09z, and these will continue to spread northward ahead of the system today. Warmer and more moist air also working northward near the surface. The latter is creating the initial concern of the day - dense fog. Really about as widespread dense fog as have seen in quite a while, as higher moisture content of southeasterly flow continues to flow across the impressive snowpack. It is likely that fog will be a part of things until winds come up with the system later tonight/Thursday morning. However, there should be some improvement in visibility once precipitation spreads northward into the area during the morning. Have maintained the current advisory this morning, and added in a couple of counties to the western fringe where mass has advected overnight. Back to the elephant in the room. A colossal winter storm will develop over the next 12-24 hours, moving from eastern CO this morning to northern Iowa/southern MN by midday Tuesday. The initial push of precipitation will be almost exclusively rain through the day today, with a glaring exception being a small area with surface temps below freezing at onset of precipitation from K9V9 to KHON. Should be noted that the window for freezing precipitation would seem to be fairly short given tendency for temps to warm to near or just above freezing as clouds lower, and with additional boost from precipitation warming from aloft. Could be a few areas toward K9V9 to KHON that see enough of a period to get a tenth or so accumulation of freezing rain, but this will also succumb to the warming expected today as readings climb into the mid to upper 30s. The precipitation in warm advection wing will spread north and wrap northwest across the area today, as trowal becomes established around the system. There is enough elevated instability that a few rumbles of thunder are likely, even through some areas north of I-90. Behind this band, it appears a fairly formidable dry slot will push northward, diminishing precipitation from the south during the afternoon and early evening, and forcing the initial banding to wrap even more westward within deformation zone north of the ejecting upper wave. During the later evening, the cold front aloft will begin to surge northward toward the lower Missouri River valley, increasing frontogenetic forcing through a fairly deep layer. Would expect to see another resurgence in precipitation into southeast SD/northeast NE/northwest IA, which will likely focus by very late night and early morning. The cooling will begin to collapse fairly quickly eastward by late night and Thursday morning, which will become problematic on several fronts. Timing of this cooling is difficult with impacts from the warmer air aloft wrapping north, but strong dynamic cooling battling quite effectively. When the transition does occur, it will do so quickly and lead to a rapid degradation of conditions. This band will be a bit more persistent into the afternoon as it draws eastward with upper wave, and have raised snowfall chances east through the afternoon, especially early. The other feature of the system will be brutally strong winds, with many locations exhibiting from 45 to 50 knots of wind within the mixed layer behind the surface low/inverted trough. In summary: Winter Weather highlights- A threat for areas of freezing rain this morning, but looks like a short window before temps warm above freezing. Considered backing off the winter storm warning in the south central for 12 hours, as much of the day will be warm enough for just rain. Removed more marginal threat for icing within the winter weather advisory, but allowed area to continue starting this evening. Further east, did not resolve the watch at this point late tonight, as could end up with need for blizzard highlight at some point, and will have another group of solutions to more effectively determine the location of the secondary snow band. Since there has been an eastward shift in models, most notably with the 06z NAM and several ensemble members, have expanded a winter storm watch further southeast by a couple of counties. As wind will quickly become brutally strong as cooling forces transition, have mentioned in the watch area the chance for blizzard conditions. Could end up being more snowfall along the band from Yankton to Sioux Falls to Marshall if transition is a bit quicker on Thursday morning, within zone of strong 925-750 hPa frontogenesis resulting as trowal shifts from warm frontal to cold frontal structure. Hydrology - Still looks good for a widespread rainfall of 1 to 2 inches across the area, with a considerable amount of water within the remaining snow cover. All this spells issues with both ponding of water due to rainfall rate, and amounts of precipitation expected. Will need to monitor for ice jams on rivers and streams. Winds - Likelihood of wind gusts late Wednesday night into Thursday from 45 to 55 mph, perhaps a shade higher. ANY falling snow which occurs during this window will be of great concern for potential blizzard conditions. .LONG TERM...(Thursday night through Tuesday) Issued at 519 AM CDT Wed Mar 13 2019 Very little consideration to forecast beyond the storm. Temps look to remain generally below normal until at least sometime next week under impacts of northwest flow. && .AVIATION...(For the 06Z TAFS through 06Z Wednesday night) Issued at 1226 AM CDT Wed Mar 13 2019 Very intense low pressure system expected to impact the region through the forecast period leading to low level wind shear. Widespread dense fog is expected to continue along and east of the James this morning. As precipitation moves into the region, visibilities may temporarily improve, but expect visiblit to fall back down when precipitation lightens up. Do not expect sigificant improvements till northwest winds develop on the backside of the system. && .FSD WATCHES/WARNINGS/ADVISORIES... SD...Flood Watch through Thursday evening for SDZ062>071. Dense Fog Advisory until 1 PM CDT this afternoon for SDZ038>040- 053>056-058>062-064>071. Winter Storm Watch from late tonight through Thursday evening for SDZ039-040-054-055-059>061-065-068. Winter Storm Watch from Thursday morning through Thursday evening for SDZ056-062-066-067-069>071. Winter Weather Advisory from 10 PM this evening to 7 PM CDT Thursday for SDZ053-058-064. Flood Watch from 10 AM CDT this morning through Thursday evening for SDZ038>040-050-052>061. Winter Storm Warning until 7 PM CDT Thursday for SDZ038-050-052- 057-063. MN...Flood Watch through Thursday evening for MNZ089-090-098. Dense Fog Advisory until 1 PM CDT this afternoon for MNZ071-072- 080-081-089-090-097-098. Winter Storm Watch from Thursday morning through Thursday evening for MNZ071-072-080-089-097-098. Flood Watch from 10 AM CDT this morning through Thursday evening for MNZ071-072-080-081-097. IA...Flood Watch through Thursday evening for IAZ001>003-012>014- 020>022-031-032. Dense Fog Advisory until 1 PM CDT this afternoon for IAZ001>003- 012>014-020>022-031-032. Winter Storm Watch from Thursday morning through Thursday evening for IAZ001-012. NE...Flood Watch through Thursday evening for NEZ013-014. Dense Fog Advisory until 1 PM CDT this afternoon for NEZ013-014. Winter Storm Watch from Thursday morning through Thursday evening for NEZ013. && $$ SHORT TERM...Chapman LONG TERM...Chapman AVIATION...BT