566 FXUS63 KFSD 091802 AFDFSD Area Forecast Discussion National Weather Service Sioux Falls SD 102 PM CDT Tue Oct 9 2018 .UPDATE... Issued at 1258 PM CDT Tue Oct 9 2018 Models continue to show cooler air pushing into central parts of South Dakota overnight. With precipitation continuing, and cooling profiles, snow looks likely during the morning hours on Wednesday. Ground temperatures are still warm, so accumulations may be short- lived; however, given the early-season accumulating snow and potential for slick roads, have decided to issue a Winter Weather Advisory for a handful of counties in the central part of SD. Flurries are possible Wednesday morning anywhere west of I-29 and north of I-90, but accumulating snow currently looks to remain confined to the higher elevation areas of central SD west of the James River. && .SHORT TERM...(Today through Wednesday) Issued at 432 AM CDT Tue Oct 9 2018 There is about as meridional a large scale pattern as you would expect to see this morning with a trough from the Canadian Rockies all the way into central Mexico. There is a distinct northern stream portion dipping down into northern MT, with a wave lifting through the big bend area of TX. This is the wave that will result in the more significant weather this afternoon through Wednesday as it lifts and phases in with the northern stream wave over the Dakotas. Starting out this morning, we have lost for a short time the lift forcing at the top of the moist layer, with most areas seeing drizzle drop off overnight. Will begin to see a little reversal in this over the next few hours as flow begins to back a bit toward a cross frontal direction, and also with development of showers across southern NE working toward the lower Missouri River corridor after daybreak, supported in a vast array of hi-res solutions. Coverage of the warm advection wing of precipitation ahead of northward pushing wave should continue to increase into the afternoon and the evening, spreading west/northwest toward the mid-level frontal boundary in central SD. Eventually, heading into the evening will find a punch of dry air in the mid levels working northward through the eastern 2/3 of the area which should diminish precipitation from south to north and push concentrated activity back into the trough axis west of the James River. Short term concern 1 - Flood Watch: While rain is expected, especially later this morning through early evening, the amount of rainfall continues to trend down in the overall sense across the area, as well as focus more toward central Iowa. Therefore, have dropped a large portion of the Flood Watch, leaving only a small four-county area in southeast portions of northwest Iowa where could yet see locally up to a couple inches of rainfall through the evening. Ironically, could easily end up with some slightly higher amounts back toward where the mid-level frontal boundary resides through the afternoon and evening toward/west of the James River valley, and where the more persistent deformation precipitation will remain into early Wednesday. Short term concern 2 - Snow, yes snow: As a result of an increased consensus of phasing the trough features tonight and early Wednesday over the Dakotas (GFS and most of the GEFS continues to be the holdout) the forecast has taken a 180 degree turn today, or at least a 90 degree turn in an unseasonable (some might think unreasonable) direction. By later night, a strong frontogenetic axis will have organized through central SD with jet punching northward into southwest MN. Cold air will continue to fall into the area at low levels as a cyclone wraps up across IA. Thermodynamic profiles quickly cool after midnight west of the James in support of snow (with a minor issue on saturation depth to cool enough temps for ice for a few hours). Eventually, southeast collapse of the thermal ribbon will increase saturation depth and force a quick change to snow or mix into the James valley toward morning, with the transition working eastward during the morning, mainly through the northern CWA where precipitation chances with northward shifting negative tilt trough will pull through midday. Most solutions try to keep in some near surface warm layer and limit snow accumulation, but with some shallow instability in the column could see some enhanced snow rates later tonight into early morning to overcome this, and feel too warm of ground in models likely hampering the cooling of this layer. Could see some 1-3 inch accumulations from the mid James River valley toward south central SD, most significant amounts concentrated at higher elevations due to marginal temperatures. There is considerable uncertainty being so close to the thermal division between rain and snow, with some SREF members showing as much as 4-6 inches, and others very little. Likely up to around an inch of snow for some of the pothole highlands between KBKX and KHON as well on Wednesday morning. If any snow was to occur toward KFSD, would likely only be a couple of flurries during the morning hours on Wednesday. Another thing about tonight through Wednesday will be the strong winds as cold advection and the 30-40 knots of flow in the 925-850 hPa layer work to surface. Likely enough mixing to get some 30-35 mph gusts, mainly during the afternoon and early evening. .LONG TERM...(Wednesday Night through Monday) Issued at 432 AM CDT Tue Oct 9 2018 Temps Wednesday night will be quite cool, and this is in the face of considerable cloudiness rotating around the exiting low and continuation of at least 10 to 20 mph northwest winds. Could see a bit more clearing as well as wind decrease toward south central SD, but with either end up with some mid 20s where these conditions align, or some upper 20s to around 30 with a decent enough northwest wind. Either should be enough to end the growing season most areas. Plenty of stratocumulus will rotate through areas near/east of I-29 on Thursday with coldest air aloft cycling through with residual moisture at low-levels. Would not be shocked if a few flurries were seen around southwest MN. Surface ridge moves over on Thursday night, and will be below freezing most areas, with exception the far south where clouds increasing ahead of another strong northern stream wave. GFS appears to struggle again with pulling a wave farther northward ahead of this feature, spreading a band of light precipitation as far north as I-90 later Thursday night and early Friday. Other solutions continue a more dry route, perhaps glancing the Missouri River corridor. Preference to the latter, but cool enough that some mix or change to snow still in play. Weekend remains somewhat uncertain on the larger scale, with interaction between stronger northern stream and waves (including remnants of Sergio) trying to slide northeast ahead of each digging system. For now, have not made any alterations to the forecast beyond Friday which indicate a chance for precipitation mainly east of I-29 Saturday afternoon with the reinforcing northern stream wave, then a glancing chance for light precipitation northeast Nebraska into northwest Iowa Sat night into Sunday. Temps overall remain cool through the period with perhaps some readings into the lower 50s at best for Saturday. But fear not, the pattern looks to be changing to bring warmer and dry conditions out beyond the period! && .AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon) Issued at 1258 PM CDT Tue Oct 9 2018 Rain and low ceilings are likely through the afternoon and evening for the region. A few embedded thunderstorms could also affect sites in northwest IA, including KSUX. After midnight, locations east of I-29 will see rain decrease in coverage and intensity. Meanwhile, west of the James River valley, sites such a KHON and K9V9 will see rain change over to snow. Light snow accumulations on paved surfaces are possible in these locations. Snow looks to decrease in intensity and move north of the area by the afternoon hours Wednesday. && .FSD WATCHES/WARNINGS/ADVISORIES... SD...Winter Weather Advisory from 1 AM to 1 PM CDT Wednesday for SDZ050-057. Winter Weather Advisory from 1 AM to 4 PM CDT Wednesday for SDZ038-052. MN...None. IA...Flood Watch through late tonight for IAZ014-021-022-032. NE...None. && $$ UPDATE...VandenBoogart SHORT TERM...Chapman LONG TERM...Chapman AVIATION...VandenBoogart