065 FXUS63 KFSD 192247 AFDFSD AREA FORECAST DISCUSSION National Weather Service Sioux Falls SD 547 PM CDT Tue Oct 19 2021 .SHORT TERM...(This Evening through Wednesday) Issued at 220 PM CDT Tue Oct 19 2021 FORECAST HIGHLIGHTS: 1. One last warm day this week, as Fall is on our doorstep. 2. Thunderstorms, some severe, towards daybreak Wednesday. 3. Model trend upwards in QPF continues on Wednesday. 4. Much colder to end the week; cold continues into the weekend. 5. Still high uncertainty from Sunday onward. TODAY: Another warm and breezy day across the region as southerly winds are pushing temperatures towards the 70s once again. Fall is on the doorstep though, as a sharply defined frontal boundary is situated from near Oneill northward to Huron and then into Western Minnesota. Behind this boundary, temperatures are falling into the 40s, there is widespread low stratus, and wind gusts are over 30s knots at times. TONIGHT: The greatest concern overnight continues to be with the deepening Colorado Low that continues to wobble in model projections as it slides east. Latest NAM/RAP/HRRR/CMC all show a bit more deepening to the upper low as it is reaching the Tri- State area, which is taking place slight further east than 24 hours ago. The GFS is showing less change in the overall track than it did yesterday. Strong low-lvl moisture transport begins this evening, and once the nose of a 50 knot 500 mb jet and 100 knot upper jet arrives after midnight, we may begin to see a linear line of scattered showers and thunderstorms develop along the advancing 850 mb boundary. Initial instability may be on the lower side of guidance AOA 500-800 J/KG, however RAP/HRRR/NAM guidance shows MUCAPE peaking AOA 1200 J/KG once steeper mid-lvl lapse rates around 7.5-8 C/KM tied more closely to the upper circulation arrive between 12-16Z. The increasing upper flow will support sufficient shear for strong to isolated severe storms, some of which may exhibit supercellular characteristics. The greatest threat area will be along and south of I-90, with the focus for strong convection based on updraft helicity plots shifting into NW Iowa and adjacent areas of MN after daybreak and through mid- morning. Anticipate hail potential up to 1.25" to 1.5" at times especially in storms that show rotation. Given stability in the boundary layer, feel wind is a letter risk ATTM. Further west, believe it or not, but the strong dynamical cooling with the system could push thermal profiles slightly closer to the edge of wintry mix especially along and west of a line from Gregory to Chamberlain. Would not be surprised to see some sleet mixed in at times around daybreak, perhaps a few wet snowflakes in the higher terrain areas into mid-morning. WEDNESDAY: With the system deepening further east and slightly slower than previous guidance suggested, this places the area in a favorable position for a narrow TROWAL to rotate around the upper trough through the day on Wednesday. Some models focus this precipitation axis near the James River valley, others are near the I-29 corridor through the day. Have increased QPF and PoPs accordingly. Total QPF in this system over the larger area will likely average 0.25-1", but there are clear signals in model guidance that localized spots could see a quick 2" of rain by tomorrow evening. Temperatures will cool through the day, and with highs in the 40s and persistent rain/drizzle along with gusty winds, it will feel like a very raw Fall day. Used a non-diurnal trend in the afternoon. .LONG TERM...(Wednesday Night through Tuesday) Issued at 220 PM CDT Tue Oct 19 2021 THURSDAY-FRIDAY: Clouds gradually clear on Thursday and winds will weaken, but it will feel like a typical Fall day in the region. HIghs struggle to reach the 50s in some areas, and may only reach the lower 50s in others. Models have trended towards a bit more mid- lvl cloud cover Thursday night into Friday, and this may prevent a widespread frost or freeze now. Weak warm advection aloft thanks to a subtle shortwave moving through the NW flow may bring some sprinkles or increase mid-lvl clouds on Friday. Otherwise temperatures only warm slightly higher into the 50s and may reach 60 near the Missouri River. SATURDAY: Saturday continues to look like another very cool day in the region as we'll stay under the influence of high pressure moving into the Great Lakes. Cool easterly flow will hold temperatures into the 40s to low 50s east of Interstate 29, with some potential for temperatures rising into the middle 50s near the Missouri River further west. SUNDAY-TUESDAY: Medium range guidance remains only in slight agreement with the overall pattern to start next week. There still remains considerable spread in the ensembles regarding the depth and eastward extension of the upper trough over the western half of the CONUS by Sunday. Both deterministic ECMWF/GFS solutions do show increasing warm advection aloft likely leading to SHRA/TSRA risks Sunday into Monday. The GFS however is much wetter than any other solution due to it's higher amplitude trough moving into the Central Plains. Both ensembles support better focus of precipitation risks across the Lower MO river valley, with lower probabilities further north. That said, a 30 to 40 PoP seems reasonable given the pattern. Will not make any changes beyond Sunday due to model disagreement, but overall trends would support a brief warmup above normal. && .AVIATION...(For the 00Z TAFS through 00Z Wednesday evening) Issued at 543 PM CDT Tue Oct 19 2021 Showers and isolated thunderstorms will spread into the area late tonight through Wednesday, with ceilings and visibilities lowering into the MVFR/IFR range with the onset of precipitation. In addition, northerly winds will increase, gusting around 25 to 30 kts on Wednesday. && .FSD WATCHES/WARNINGS/ADVISORIES... SD...NONE. MN...NONE. IA...NONE. NE...NONE. && $$ SHORT TERM...Dux LONG TERM...Dux AVIATION...JM