Area Forecast Discussion
Issued by NWS Sioux Falls, SD
Issued by NWS Sioux Falls, SD
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788 FXUS63 KFSD 171731 AFDFSD Area Forecast Discussion National Weather Service Sioux Falls SD 1231 PM CDT Tue Sep 17 2024 .KEY MESSAGES... - Increasing southerly winds expected today and tonight, strongest west of the James River. Gusts 30 to 45 mph late this afternoon through tonight. - Low (generally < 30%) rain chances through this afternoon. Confidence in timing and placement is low, but expect amounts to remain light, less than a quarter inch through this period at any location. - Greater chances for rain (40-70+%) tonight through Wednesday, mainly focused late tonight west of I-29. Moderate (~ 50%) probability of rainfall exceeding 0.25", but low (< 30%) probability of exceeding 0.50". - Moderate chances (40-60%) for showers/storms late week into the weekend, with highest chances currently focused Friday night through early Sunday. Exact timing and location remain uncertain. && .DISCUSSION... Issued at 353 AM CDT Tue Sep 17 2024 TODAY AND TONIGHT: Another morning of isolated to scattered showers developing along the convergence of the LLJ and a weak short wave moving across the area. Severe weather is not expected, but these storms have been producing frequent lightning. A stronger wind gust to 50 mph and small hail may occur in the strongest storms. Isolated to scattered showers/storms will continue to develop, mainly east of the James River, through daybreak. Morning showers and storms move east and slowly dissipate through the late morning and early afternoon as the subtle short wave moves east and we begin to mix out, losing the forcing from the LLJ. Meanwhile, the main low pressure/trough begins to move east into the Rockies. Lee side cyclogenesis occurring through the day and tonight leads to the compressing of the surface pressure gradient (SPG), and with mixing into stronger winds aloft, a breezy day and night are expected. Strongest winds are anticipated west of the James River where the gradient is the tightest and LLJ is the strongest late this evening. Gusts in this area may reach up to 40 to 45 mph; however, given marginal conditions, held off on a Wind Advisory for now. Ensemble probability of exceeding 45 mph gusts is less than 10%. Elsewhere, wind gusts through tonight of 30-40 mph are expected. Sustained southerly winds of 15-25 mph prevail. Warm conditions continue, with highs today in the 80s to near 90 in south central SD and mild lows overnight in the 60s. Expect a lull in precipitation this afternoon into the evening hours, as the aforementioned trough begins to lift northeast into ND, dragging an elevated boundary and short wave into the central and eastern Dakotas overnight through Wednesday morning. Greatest rain chances will be overnight west of I-29, where the best dynamics line up (boundary, LLJ, upper level forcing). Not expecting any severe weather with activity overnight and early Wednesday; however, stronger storms may produce wind gusts to 50 mph and small hail in addition to frequent lightning and locally heavy rain. Some locations tonight could see a quarter inch or more, with probability of over 0.25" between 30% and 60% west of I-29. Lower end chances (< 30%) of more than a half inch. WEDNESDAY AND WEDNESDAY NIGHT: On Wednesday, low pressure deepens as it slides north from eastern MT and western ND. Initial boundary from the overnight and early morning slides east, with another short wave and lower/mid level front poised to slide through the area during the evening and overnight hours. Surface low and attendant cold front won`t be too far behind the mid/upper level support Wednesday evening and night. Models show some variance in exact timing of the front, and there is uncertainty in where any convective outflows are. Depending on the timing of the boundary and how much the atmosphere can recover from any early day convection, a stronger storm or two may be possible in the late afternoon through the evening hours. CAPE around 1200-1500 J/kg and shear around 30 knots may support wind gusts to 60 mph. With lapse rates less than 7 deg C/km, quarter sized hail appears to be a lesser threat. Rainfall amounts of less than a quarter of an inch expected Wednesday into Wednesday night. Slightly cooler with highs in the lower/mid 80s and lows in the 60s. Southerly winds remain on the breezier side, but not as strong as today. THURSDAY ONWARD: Showers and storms linger into the day Thursday east of I-29 as the front and surface low track east. Confidence is low in the location and timing, but severe threat on Thursday is very low. Weak/flat mid and upper ridge builds for the end of the week as low pressure deepens across the southwestern US, leading to continued above normal temperatures and mostly dry conditions. Models diverge toward Saturday with the evolution of this low and another across southern Canada, but all generally lean toward these features moving east through the weekend. This brings a return to the more active pattern with short waves and more defined lows/troughs moving across the north central Plains into the early part of next week. Ensembles indicate that a return to near or even below average temperatures is on the way, with ensemble probabilities of exceeding 80 degrees less than 30% on Saturday, and less than 20% into early next week. With confidence low on the details and periodic rain chances, did not make any changes to the NBM in the extended. && .AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/... Issued at 1231 PM CDT Tue Sep 17 2024 Outside of a few lingering showers across NW Iowa/SW Minnesota, have seen largely quiet conditions return to the region. Expect this to continue through the afternoon and evening, along with breezy S/SE winds. Currently have gusts between 20 to 40 mph, with the strongest along and west of the James River. As alluded to in the previous discussion, could see some LLWS during the overnight period, especially if winds taper off a bit. However, confidence concerning this remains low, so have opted to exclude mention for the time being. Otherwise, look for additional showers and storms to develop west of our area overnight, pushing east toward the I-29 corridor through daybreak. Outside of convection, VFR conditions are anticipated. && .FSD WATCHES/WARNINGS/ADVISORIES... SD...None. MN...None. IA...None. NE...None. && $$ DISCUSSION...SG AVIATION...SST