Area Forecast Discussion
Issued by NWS Sioux Falls, SD
Issued by NWS Sioux Falls, SD
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413 FXUS63 KFSD 281250 AFDFSD Area Forecast Discussion National Weather Service Sioux Falls SD 750 AM CDT Sun Apr 28 2024 .KEY MESSAGES... - Periods of light to moderate showers spread north through tonight. Moderate to high (60-80%) probability of additional rainfall exceeding 0.50" for most through Monday morning, with a low (15-30%) probability of exceeding 1.00". - Chilly, below normal temperatures continue through Monday with highs in the 40s, 50s, and a few sites hitting the 60s. - Increased severe weather risk for Tuesday afternoon, with a Level 2 of 5 risk in northwest Iowa as a cold front moves into a warm and modestly moist air mass. Still some uncertainty in timing/location of storm development. && .DISCUSSION... Issued at 400 AM CDT Sun Apr 28 2024 TODAY-MONDAY: Focus for the next 24-36 hours will be mid-upper level trough spinning into Nebraska early this morning, as it slowly lifts northeast across the northern Plains through Monday. Broad lift ahead of the trough should result in increasing coverage of showers across the forecast area today, initially focused to the west of I-29 early this morning, but expanding east mid-late morning in response to broad warm advection and the mid level front lifting north ahead of the trough. As the trough deepens/becomes negatively tilted through over the Dakotas through this evening, precip distribution begins to resemble a winter system with a dry slot punching into Iowa/southern Minnesota this evening, while precipitation wraps around the north and west sides of the system. Eventually, this wrap-around precipitation will lift northeast with the departing trough, but could see light showers linger across our north into Monday morning. Overall, looking at additional rainfall amounts exceeding 0.50" over much of the area (60-80% probability of exceedance in recent ensembles) with lower (15-30%) probability of exceeding 1.00". Highest of these low probabilities would be in the wrap-around area across eastern South Dakota, with lower probabilities in northwest Iowa. With this strengthening system and aforementioned dry slot, still have to give some consideration to a low severe weather risk across the southeast portions of the CWA. Increased mid to upper level winds ahead of the trough provide ample bulk shear during the afternoon/evening. However, a couple of questions remain. First, is whether clouds/rain will decrease in time to allow for even modest destabilization. Latest HREF maintains around a 50% probability of surface-based CAPE exceeding 300-500 J/kg in our extreme southeast counties (southeast of Sioux City to Storm Lake), but soundings reveal instability profiles that are very thin, with near moist adiabatic mid-level lapse rates less than 7g/kg. Second question is exactly where the surface low and attendant warm front will be located by late afternoon. Models still vary quite a bit, though greater consensus still tracks the low through northwest Iowa. If greater instability can develop, juxtaposition with the weak convergence around the boundary could support an isolated stronger storm. Overall, the risk of severe weather appears low for this afternoon/evening, but will need to monitor trends through the day. As for temperatures, the next couple of days will remain on the chilly side of normal. Widespread cloud cover/rain across most of the CWA today should limit temperatures from moving much from current early morning levels, and expect highs ranging from mid to upper 40s across our west and north, to perhaps around 60 in our far southeast. Wedge of colder air wraps around the back side of the departing low on Monday, which will again limit highs to the 50s most areas. TUESDAY: Brief reprieve from this wet pattern later Monday into Monday night, as surface and brief upper ridging move across the region. Increasing southerly flow will bring much warmer air northward Tuesday, ahead of our next trough which moves east across the northern Plains later Tuesday-Tuesday night. While some uncertainty in timing of the expected trough/cool front, thermal ridge pushes 850mb temperatures into the 10+C range by Tuesday afternoon, with a few solutions showing 925mb temps nearing 18-20C. This would support highs into the 70s across parts of the eastern CWA, with some 80s not out of the question, while moisture advection draws dew points in the 50s northward. This lends itself to increasing instability by Tuesday afternoon, 1000-1500J/kg as mid-level lapse rates steepen to 7.5-8C/km, though soundings do show a weak capping inversion. Modest deep layer shear ahead of the approaching trough as well, but as we often see with cold fronts in this area, greater instability seems to be displaced ahead of the stronger shear. That said, could see a few strong to severe storms with large hail and damaging winds the primary threats. WEDNESDAY-SATURDAY: Mild air remains in place mid-week, but another deepening trough could bring a shot of cooler air back into the region by the end of this week. Details on the timing and strength of the late week system are still low confidence, as are temperatures with growing spreads between the NBM 10th and 90th percentiles from midweek onward. Periodic rain chances continue through the latter half of the week ahead. Chances at this time seem focused around later Wednesday into Thursday, and again toward Saturday, but similar to temperatures, details are uncertain and further refinement is likely in the coming days. && .AVIATION /12Z TAFS THROUGH 12Z MONDAY/... Issued at 620 AM CDT Sun Apr 28 2024 MVFR-IFR ceilings persist as we start this TAF period, and expect most areas to settle down into IFR range as showers become more widespread across the area this morning. Areas of MVFR visibility will also accompany the showers, locally IFR in heavier showers or thunderstorms. Speaking of thunder, have left out of KHON/KFSD as potential is low, but non-zero so cannot rule out a stray embedded storm. Better chance for thunder will come to KSUX TAF site and farther east in northwest Iowa, so have included there for a few hours this afternoon/early evening. Rain tapers off south to north after sunset tonight, though IFR ceilings are likely to persist most areas beyond daybreak Monday. Northeast winds occasionally gusting 20-25kt today will gradually turn to the north and northwest tonight, with gusts easing to 15-20kt overnight. && .FSD WATCHES/WARNINGS/ADVISORIES... SD...None. MN...None. IA...None. NE...None. && $$ DISCUSSION...JH AVIATION...JH