


Area Forecast Discussion
Issued by NWS Sioux Falls, SD
Issued by NWS Sioux Falls, SD
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051 FXUS63 KFSD 181123 AFDFSD Area Forecast Discussion National Weather Service Sioux Falls SD 623 AM CDT Wed Jun 18 2025 .KEY MESSAGES... - Patchy fog is possible across the area this morning. Should any fog develop, it will burn off during the morning hours. - Another day of afternoon showers and storms is expected. Severe storms are not expected but a highly isolated heavy rainfall/flash flooding risk is possible. - Severe storms may be possible Thursday night though details are uncertain regarding the location of potential storms. - Confidence continues to increase in a heat wave coming for the end of this week. "Heat Risk" and "WBGT" categories rise into the "High" category Friday through Sunday. All outdoor events and activities should begin preparing now as headlines appear likely. && .DISCUSSION... Issued at 338 AM CDT Wed Jun 18 2025 Quiet conditions begin the day early this morning. This could change over the next few hours though as clear skies and light winds could allow for radiational fog to develop before sunrise. Will monitor this potential for the rest of the night. Should any fog develop, it will burn off during the morning hours. Today will be a rinse and repeat of what we have seen over the past few days. Diurnally driven showers and storms are expected to develop this afternoon before weakening during the evening hours. Instability values remain more on the monitor side with magnitudes less than 1,000 J/kg. Shear remains weak so severe storms are not expected. However, with tall skinny CAPE profiles in place, the potential exists for highly isolated heavy rainfall/flash flooding to take place. This is due to mean flow remaining at or below 20 knots which will keep storm motions slow. Outside of rain chances, today`s high temperatures will be a bit on the cool side as 850 mb temperatures only warm to the mid teens aloft. Mixing these temperatures to the surface will result in high temperatures into the upper 70s to low 80s. Low temperatures will fall to the upper 50s to low 60s overnight. Warm air advection (WAA) begins to strengthen on Thursday as upper level flow takes on more of a westerly component aloft. This will push 850 mb temperatures up into the 20s aloft. Mixing these temperatures to the surface will result in hot high temperatures from the 80s to low to mid 90s. The warmest temperatures will take place along and west of I-29. While most of the day will be dry, WAA will further strengthen as the low level jet (LLJ) ramps up during the evening hours. This could lead to another evening and night of showers and storms, some of which could be severe. While shear and instability will be more than sufficient for severe weather, other factors decrease confidence in this potential. While the nose of the LLJ may be in the CWA, there is still some uncertainty where the nose of the jet sets up. At the same time, BUFKIT soundings show that little moisture will be in the layer of the jet, suggesting that moisture could be a bit hard to come by even if instability is still present when lifting from the nose of the jet. That said, the GFS and NAM both show sufficient 850 mb moisture convergence which could overcome this issue. The GFS, Euro, Canadian, and REFS all show different probabilities for exceeding a tenth of an inch Thursday night with the Euro having the highest probabilities up to about a 50% chance while the REFS shows almost a 0% chance for the same amount of rain. Given the lower confidence in the coverage and location of storms, do agree with SPC`s marginal risk across the area. Will be something to keep an eye going forward. Upper level ridging begins to build aloft on Friday as warm air advection remains strong. This will result in the beginning of the heat wave for our area. The ensembles remain in decent agreement in 850 mb temperatures warming above 25C. In fact, the ensembles show moderate to high probabilities up to a 50 to near 100% chance for 850 mb temperatures exceeding 25C. This will result in hot high temperatures into the 90s to even low 100s across the Missouri River. Dew points in the 60s to perhaps touching 70F will aid in apparent temperatures reaching into the low to mid 100s. No break in the heat overnight as lows look to bottom out in the 70s. The heat continues through the weekend as the thermal ridge sits over the Northern Plains. Saturday will be the warmest day with highs again in the 90s to low 100s and heat indices in the upper 90s to mid 100s. Sunday will be the "coolest" day of this heat wave as a cold front begins to enter the forecast area. Highs on this day will still be in the 90s to low 100s with similar heat indices. These temperatures will result in a high Heat Risk and Wet Bulb Globe Temperature being reached. Thus, heat headlines will likely be needed. Rain chances could return to the area on Sunday from the though this potential is a bit uncertain due to differences in the fronts timing from the GFS, Canadian, and Euro. While the front looks to push through the area on Monday, this could result in continued chances for rain as the upper level jet shifts to over the Northern Plains. In fact, medium range guidance is in decent agreement in the right entrance of the upper level jet remaining parked over the forecast area for the first half of next week. Depending on how low level mesoscale details pan out, this could result in a stormy start to next week. As of now, this potential is supported by machine learning guidance so will keep an eye on this potential next week. Aside from storm chances, high temperatures will cool back to near seasonable thresholds as highs look to warm to the 70s and 80s. && .AVIATION /12Z TAFS THROUGH 12Z THURSDAY/... Issued at 623 AM CDT Wed Jun 18 2025 Shallow fog is present across a few locations this morning. Brief reductions in visibility is possible within this shallow fog as a few observations show visibilities down to MVFR levels. At the same time, light rain showers along with a few thunderstorms are currently along and west of the James River. While the shallow fog burns off over the next few hours, these showers and storms will drift eastwards throughout the day today. While confidence is high in rain affecting KFSD and KSUX, confidence is not high enough to include mention of thunder in their TAFs. Trends will be monitored throughout the day. Any chance for rain will come to an end this evening, leaving quiet conditions for the remainder of the TAF period. Winds begin today light and variable but will pick up out of the north/northwest, with speeds up to 5 to 10 knots, and will slowly back to out of the west/northwest by this evening. && .FSD WATCHES/WARNINGS/ADVISORIES... SD...None. MN...None. IA...None. NE...None. && $$ DISCUSSION...Meyers AVIATION...Meyers