Area Forecast Discussion
Issued by NWS Sioux Falls, SD
Issued by NWS Sioux Falls, SD
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204 FXUS63 KFSD 191727 AFDFSD Area Forecast Discussion National Weather Service Sioux Falls SD 1227 PM CDT Thu Sep 19 2024 .KEY MESSAGES... - Shower and thunderstorm chances (30 to 40%) continue this morning, mainly south of I-90. Additional shower and storm chances (less than 30%) this afternoon and early evening east of IA/MN State Hwy 60. Severe risk remains low, but hail to quarter size and 60 mph wind gusts are possible in the strongest storms. - An active pattern aloft leads to renewed shower and thunderstorm chances (30 to 60%) this weekend into early next week, with the greatest chances Saturday and Sunday. - Confidence continues to increase in cooler stretch of temperatures starting as early as Sunday. && .DISCUSSION... Issued at 346 AM CDT Thu Sep 19 2024 TODAY THROUGH TONIGHT: Elevated convection early this morning has continued to develop along the LLJ and elevated boundary from south central MN stretching southwest into southeastern NE. Meanwhile, further west, surface cold front is draped from near Aberdeen southwest to east of Huron, into northern NE west of O`Niell. Convection has struggled along the surface front due to capping in place and lack of upper level support. We may begin to see this upper level support (mainly with the 500 mb jet streak) aid in convection through daybreak. If storms can develop and strengthen, can`t rule out hail to quarter size. Aforementioned surface cold front continues to move east during the day. It is possible to see some storms develop along the front this afternoon into early this evening across portions of northwestern IA into southwestern MN; however, coverage may be limited in our area depending on how quickly we can destabilize and how far east the front is by mid afternoon. If convection can initialize, strong to severe storms are possible. Dew points in the 60s lead to instability around 1500 J/kg, and shear increases to around 35 knots. Mid level lapse rates are a bit marginal, around 7 deg C/km. If a stronger storm could develop near the boundary, there is a non- zero tornado risk, but with 0-1 km shear around 10 knots and LCL heights around 4000 ft, tornadoes are unlikely. Main hazards with any stronger storm would be 60 mph wind gusts and hail to the size of quarters. Rainfall amounts through this evening generally a tenth of an inch or less, although locations which see thunderstorms could see rainfall totals around 0.25" or more. Highs today climb into the 80s once again. Lows tonight fall into the 50s, with clearing skies. Winds may become light enough overnight thanks to weak surface high pressure that patchy fog may develop in low lying areas, near river valleys, and in areas which receive precipitation today. Confidence is too low for mention in the weather grids at this time, mainly due to uncertainty in where/when precip occurs today. FRIDAY: Dry and warm to end the week, as mid/upper level ridge builds in. WAA and the return of southerly flow at the surface aid in warming temperatures again into the 80s. Compressed surface pressure gradient as well as efficient mixing leads to another breezy day, with gusts up to 30 mph possible west of the James River. This, along with the warmer temperatures and lower dew points, will lead to RH values in the 25-35% range areawide. Although fuels are still largely green, localized areas of elevated fire danger are possible in areas with more cured fuels. Greatest concern will be areas West River, where lowest humidity and strongest winds align. FRIDAY NIGHT INTO THE WEEKEND: Ridge begins to break down late week into the weekend as a couple of low pressure systems/troughs aloft strengthen - one in the southwestern CONUS and one across southern Canada. 19.00z model runs have come into a bit more agreement with a couple of weak shortwaves moving across the north central Plains, as we are stuck in the middle of the spilt flow. By Sunday afternoon, the northern low is moving northward across southern Canada, while the southern low as tracked into western NE/KS. This active flow keeps rain and storm chances (30-60%) in place through the weekend, highest for areas south of I-90 and east of I-29. As finer details are resolved, expect pops to be refined. Areas with highest pops have a 30-70% chance of seeing more than 0.25" of rainfall in a 24 hour period, and a 30-50% chance of more than 0.50". This will change with any shift in track of the low, but those with outdoor plans this weekend may want to keep an eye on the forecast. Not expecting any day to be a total washout, though. Near to above average temps continue for the first half of the weekend as WAA prevails. Some uncertainty with highs on Saturday due to the timing of the surface cold front, but 70s to lower 80s are expected. Cooler temperatures move in for Sunday, with highs in the 60s. Confidence is increasing in these cooler temperatures for the second half of the weekend, as ensembles show a less than 10% chance of temperatures exceeding 70 degrees Sunday. Breezy winds expected on Saturday. EARLY TO MID NEXT WEEK: By Monday, models are struggling with the evolution of the weekend low pressure as is meanders east, as well as the next mid/upper low/trough progged to move through the northern and central Plains sometime Tuesday. Could see renewed precipitation chances during this time. Northerly flow should keep temperatures cooler than average for at least the early part of the week, with warmer temperatures mid week as ridging returns. Highs in the mid 60s to lower 70s with lows in the mid 40s to near 50. && .AVIATION /18Z TAFS THROUGH 18Z FRIDAY/... Issued at 1214 PM CDT Thu Sep 19 2024 VFR conditions with occasional MVFR ceilings are expected this TAF period mostly due to lingering low-level stratus. Taking a look at satellite imagery, a cold front continues to gradually push eastwards this afternoon roughly extending along a Windom, MN to Sioux City, IA line with MVFR stratus with a few embedded showers ahead of it and mostly clear conditions behind it. As this surface feature spread eastwards over the next few hours expect mostly clear conditions to continue for the rest of the TAF period. Otherwise, breezy westerly surface winds will continue through sunset before becoming more light and variable overnight to end the TAF period. && .FSD WATCHES/WARNINGS/ADVISORIES... SD...None. MN...None. IA...None. NE...None. && $$ DISCUSSION...SG AVIATION...Gumbs