723 FXUS63 KFSD 132335 AFDFSD AREA FORECAST DISCUSSION National Weather Service Sioux Falls SD 635 PM CDT Mon May 13 2019 .SHORT TERM...(This Evening through Tuesday) Issued at 438 PM CDT Mon May 13 2019 Transition to a new season is underway at last! Temperatures have reached near to a shade above normal this afternoon with sunshine along with a decent amount of warm advection. A flat cumulus field remains largely east of Interstate 29, and a bit of mid-level clouds and perhaps a couple of sprinkles wandering into the mid-James River valley with increased 800-700 hPa warm advection. Flow aloft remains northwest, with next wave seen moving into northeast MT at mid-afternoon. Short term concerns largely with the timing of convection overnight, and potential for a few additional storms on Tuesday afternoon. Suggestion of increased degree of early evening convection moving into the west by most model solutions appears valid based on the initiation of new convection over last hour or two across northwest SD and southwest ND. ML instability is somewhat stronger out there at present from 500-1000 J/kg, but as storms traverse eastward, pretty much looking at a more elevated convective threat with perhaps 300-600 J/kg skinny CAPE profiles. Elevated shear is not terrible and could suggest a tendency to organize in a linear fashion for a time, but the likely outflow dominance with higher bases and dry layer below likely will limit coverage later evening moving eastward, before another upward trend in coverage with the low-level jet increase as precipitation shifts near/east of I-29. Could be some gusty winds from 35 to 50 mph with storms moving into areas west of the James River early in the evening. For this round, does not appear as if QPF values will be too high with progressive nature and smallish scale. Following precipitation, clearing skies with the surface gradient becoming a little flabby east/southeast could lead to some patchy fog late, mainly between I-29 and the James River. Some lingering showers and isolated storms very early morning across northwest Iowa, and perhaps a little fog early across portions of eastern SD. With advance of clearing behind wave, very easily could see stratus bloom between the back edge of higher clouds and the advancing pre-frontal troughing as diurnal cycle kicks in early in the morning with fairly good inversion and shallow moisture. Will gradually thin and advect clouds eastward through the day as the boundaries work eastward through the day. Additional convection along the trough/front does not appear to be a significant threat with deeper lift forcing lacking, and in fact larger-scale subsidence favored near the frontal boundary. Perhaps enough shallow forcing to get an isolated shower/storm or two mainly southwest MN into northwest IA from late afternoon through early evening, but would not bet the farm on it. Highs on Tuesday set to be quite warm, but wild card will be if and how much stratus holds on in pre- frontal areas. Have assumed a timely exit during late morning/midday to come up with highs mainly from the mid 70s to lower 80s. .LONG TERM...(Tuesday Night through Monday) Issued at 438 PM CDT Mon May 13 2019 Toward tail of the front in southwest SD/northwest NE, could see wave passing north of the area Tuesday night enough to bulge a few showers/isolated thunder into south central SD later at night. Otherwise, drying high pressure will nose in through the eastern Dakotas and Minnesota. The dry weather will continue through Wednesday, which should end up a fairly pleasant day as the surface ridge shifts east, as long as one does not mind an increase in southeast wind. There is high confidence moving ahead in the forecast in moving toward a more active pattern, as a broad ridge aloft gradually shifts eastward toward a more south-southwest flow aloft and approach of a strong trough by the weekend. However, confidence in forecast details is quite low as there are considerable difference in degree of amplification, timing of wave moving over and into the ridge as well as impact of convection within models on boundary location. The ECMWF with somewhat less wavelength and more amplitude to feature allows downstream wave amplification to steer convection along another frontal boundary pushing south Wednesday night to turn a bit further south and impact progress of a frontal boundary further southward heading into Thursday night. Depending on the frontal timing, very warm air ahead could lead to widespread 80s and a taste of more humid conditions. The GFS maintains elevated convection from Wed night north of the front and increases this again with slower progression on Thursday night, while the ECMWF would likely not be as warm, and more capped to convection as it lingers south of the area by Friday morning. Regardless of any low- level scenario, suggesting that capping could become a more significant feature to limit precip mid to late week, or at least shift the focus further north. Progressing through Friday, the ridge axis shifts east and a transition to stronger meridional flow aloft spreads into the area. The front will likely start the day south of the area, but how far northward the boundary moves during the day will suggest where the convective threat will develop. Warm temps aloft will be a challenge to getting anything to develop, and it could hold off until a wave in the southwest flow approaches during the evening. Low pressure and a dryline will be moving into central portions of NE by late day, and where it heats out most could be the flash point for convective development late day into the evening, with storms lifting quickly north-northeast. Last but not least, there will be potential for severe storms late week, especially Friday night and perhaps again on Saturday. The increase in severe threat by later Friday afternoon is a result of the increased meridional flow aloft, but largely unidirectional along the dry line. However, by evening the mode of convection could just as easily be warm frontal origin versus that initiating along the dry line across central NE/SD. Shear in this would be much greater and feature a decent hodograph area, even elevated. Multiple modes of severe weather would be possible, so those with outdoor plans Friday evening should be certain to keep an eye on how feature play out later this week. In terms of Saturday, there is little agreement on where closed wave lifts out into the plains, with GFS farther north and ECMWF pulling out through the central Plains. Either would potentially set up a chance for severe storms again on Saturday, dependent on destabilization and boundary location, with shear remaining plentiful. Another element to keep in mind is that the increase in moisture and likelihood of storms training along slow moving/waffling boundaries will bring areas of heavy rainfall. Potential exists for an average of 1 to 3 inches of rain over much of the area. Placement of the heaviest remains uncertain, but precipitation will be widespread with the very dynamic systems toward the weekend. && .AVIATION...(For the 00Z TAFS through 00Z Tuesday evening) Issued at 630 PM CDT Mon May 13 2019 Band of high based showers and thunderstorms will work through the region tonight. Storms should be elevated through 06Z to warrant VFR conditions, though some brief gusty winds are possible west of the James River. After 06Z, some lower MVFR ceilings may try to develop across northwest Iowa. Patchy fog may also be possible in the James River Valley towards sunrise. Cumulus cloud will develop on Tuesday, and be most numerous east of I-29. Could see a few scattered thunderstorms Tuesday afternoon across southwest Minnesota, but timing is ill defined at this point. && .FSD WATCHES/WARNINGS/ADVISORIES... SD...NONE. MN...NONE. IA...NONE. NE...NONE. && $$ SHORT TERM...Chapman LONG TERM...Chapman AVIATION...BT