034 FXUS63 KFSD 191717 AFDFSD Area Forecast Discussion National Weather Service Sioux Falls SD 1217 PM CDT Thu Jun 19 2025 .KEY MESSAGES... - Isolated storms are possible mainly late evening and overnight. If storms develop, they could be strong to severe. Better chances in southwest MN and northwest IA near and east of the Buffalo Ridge. - The end of the week heat wave remains on track. Dangerous heat is likely Friday through Sunday with heat indices spanning from the upper 90s to the 100s during this period of time. Please make sure to stay hydrated and limit prolonged outdoor activities where possible. - Confidence is beginning to increase in heavy rainfall potential returning to the region next week. Details are still uncertain so continue to monitor the latest forecast for the most up to date information. && .DISCUSSION... Issued at 402 AM CDT Thu Jun 19 2025 Quiet conditions begin the day today with light and variable winds and mainly clear skies. Winds will begin to strengthen a bit as mixing increases this morning. However, winds will further strengthen this afternoon out of the south/southwest with gusts up to 25-30 mph as mid level winds begin to increase aloft. 850 mb temperatures will warm to the low to mid 20s which will result in above average high temperatures up to the upper 80s to mid 90s. The warmest temperatures will occur along and west of the James River. While forcing for ascent will be negligible through the afternoon hours, BUFKIT soundings suggest that locations west of the James River may reach convective temperatures which could result in an isolated storm forming. While the probability of this is low (~20% chance), still can`t rule it out completely despite a weaker cap in place. Should a storm form, it could be strong to severe given that ample buoyancy will be in place with CAPE values up to 1,000 to 1,500 J/kg and deep layer shear will have a magnitude of about 40 to 45 knots. Storm chances could increase this evening and night as the low level jet (LLJ) ramps up. However, the best chances for storms looks to be north and east of the forecast area where the nose of the LLJ will lie. Still have left chance PoPs in place in the event that the LLJ is able to get a storm to develop within the warm advective regime despite lacking moisture in and around 850 mb. Overnight temperatures will fall to the 60s and 70s. The anticipated heat wave begins on Friday as the thermal ridge begins to push into the Northern Plains from the front range of the Rockies. 850 mb temperatures will warm to the mid 20s to up to about 30C aloft. Mixing these temperatures to the surface will result in hot high temperatures into the 90s to 100s, warmest along the Missouri River Valley and along and west of the James River. At the same time, dew points will moisten up to the 60s and 70s. This will result in heat indices reaching up to 100 to about 110F Friday afternoon. Southerly winds will again be breezy across the area with gusts up to about 35 mph expected which could help temper the heat and humidity just a bit. A large cap will be in place on Friday which will keep any rain and storm chances at bay. Any chance for storms will come Friday night but it will be well north of the forecast area where the best forcing for ascent will lie. With high dew points in place, overnight low temperatures will only fall to the 70s. The hottest day of this heat wave will come on Saturday as the thermal ridge advects right over the forecast area. 850 mb temperatures will further warm to the upper 20s to low 30s aloft. In fact, per the NAEFS ensemble these 850 mb temperatures will place in the 99.5th percentile of climatology. 700 mb temperatures are more impressive as they are placed in the top of all climatology. All this to say, Saturday will be a hot, humid, and breezy day with highs warming to the mid 90s and 100s with heat indices into the 100s across the forecast area. Some break in the heat is expected on Sunday as a cold front will be pushing through the area. Still, high temperatures will warm to the 90s to low 100s with heat indices up to 100s. Though the warmest heat indices will reside east of the James River due to the previously mentioned cold front. Have extended the Extreme Heat Watch into Sunday for locations along and east of the James River and left it in place for the entire area for Friday and Saturday. With the front in the area and the northern stream upper level jet dropping down to the Northern Plains, showers and storms may return to the area Sunday evening and night. While some new record high temperatures could be set during the heat wave, record warm low temperatures have the highest probability of being broken. Although the heat will be done for next week. Confidence is increasing in rain chances sticking around for a large part of next week. With the previously mentioned cold front dropping south of the forecast area, high temperatures will drop to near seasonable levels in the 70s and 80s. At the same time, the forecast area will reside in the right entrance region of an the northern stream upper level jet. While the surface boundary looks to be south of the forecast area on Monday and Tuesday, the elevated boundary will still remain over the forecast area. With mean flow parallel to the boundary, weak shortwave troughs rotating through the upper level flow, and precipitable water (PWAT) values up to the 90th percentile of climatology per the NAEFS ensemble, soaking rain will be possible. Especially as pattern recognition comes into play given how this pattern looks to play out as a Maddox Frontal Pattern. This is a known pattern for heavy rainfall and even flooding. The ensembles support this potential as they show increasing probabilities for exceeding a half an inch of rain each day Monday through Thursday next week. Severe weather could accompany the rainfall in this pattern but at this time, Wednesday and Thursday look to have the best chances for seeing strong to severe storms as medium range guidance is in decent agreement in the boundary returning northwards, bringing greater instability to the area. Starting to think that this pattern could end up being a longer duration, lower/weaker rainfall rate event though details can certainly change as this event draws closer. Please continue to monitor the latest forecast for the most up to date information. && .AVIATION /18Z TAFS THROUGH 18Z FRIDAY/... Issued at 1214 PM CDT Thu Jun 19 2025 VFR through the period. The main concern will be isolated strong thunderstorm potential in parts of southwest MN and northwest IA late evening into the overnight. Also of concern will be LLWS across southeast SD, northeast NE, southwest MN and northwest IA late tonight into Friday morning. && .FSD WATCHES/WARNINGS/ADVISORIES... SD...Extreme Heat Watch from Friday morning through Sunday evening for SDZ039-040-054>056-060>062-065>071. Extreme Heat Watch from Friday morning through Saturday evening for SDZ038-050-052-053-057>059-063-064. MN...Extreme Heat Watch from Friday morning through Sunday evening for MNZ071-072-080-081-089-090-097-098. IA...Extreme Heat Watch from Friday morning through Sunday evening for IAZ001>003-012>014-020>022-031-032. NE...Extreme Heat Watch from Friday morning through Sunday evening for NEZ013-014. && $$ DISCUSSION...Meyers AVIATION...08