Area Forecast Discussion
Issued by NWS Sioux Falls, SD
Issued by NWS Sioux Falls, SD
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756 FXUS63 KFSD 111727 AFDFSD Area Forecast Discussion National Weather Service Sioux Falls SD 1227 PM CDT Thu Jul 11 2024 .KEY MESSAGES... - Wildfire smoke aloft continues to advect into the region. Smoke should track further east as the upper ridge builds in over the next few days. - Isolated showers and storms are possible late this week, with additional chances early next week. Confidence is very low in the details. - Temperatures and dew points increase late week and into the weekend. Moderate to high (> 70%) chances of seeing heat indices above 100 degrees by Sunday afternoon. && .DISCUSSION... Issued at 334 AM CDT Thu Jul 11 2024 Widespread areas of patchy, dense fog this morning, especially in low laying areas and along river and creek beds. Any fog is expected to burn off quickly after sunrise. Water vapor imagery shows the upper ridge pushing east this morning. Winds at the surface have begun to turn to the south as surface high pressure builds in. Expect light and variable winds to increase to 10-15 mph. Aloft, winds continue to be north-northwesterly bringing smoke south from the Canadian wildfires. Skies will be hazy with some late morning/early afternoon cumulus forming. Highs will be limited a bit by the smoke, in the low-mid 80s. Smoke should track further east as the ridge builds in during the next few days. Looking into the overnight, there is some indication a weak mid- level wave will trigger showers early in the morning Friday. These look to form roughly over central South Dakota and track sharply southeastward, taking them out of the region within a few hours. There is risk for some of these storms to become strong to severe, with lapse rates approaching 7 deg C, roughly 500-1200 J/kg MUCAPE, and 25-30 kts bulk shear. Model agreement is low on timing and placement, so will leave PoPs at Isolated chances (15-24%). Location most likely to see showers is a line from Chamberlain south into Wagner. Showers should clear out later Friday morning. 500 mb heights continue to rise into Friday afternoon, coupled with strong WAA this will boost surface temperatures into the mid 80s to low 90s. Strong moisture influx from the south will begin to tick our dew points up into the upper 60s to low 70s. A low pressure system moves into the region Friday night into Saturday morning, but guidance is in low agreement on track. The GFS keeps rain east of our area of responsibility, while the Canadian has isolated showers sweeping across from northwest to southeast. With this uncertainty in mind, left what the NBM loaded for PoPs. The trend of increasing dew points and high temperatures continues into Saturday and especially for Sunday. WAA brings 850 mb temperatures into the mid to upper 20 deg C range. Good mixing to the surface will translate into widespread highs in the low to mid 90s Saturday. Dew points increase to the low to mid 70s, especially over northwestern Iowa. The combination of heat and humidity will result in the heat index approaching 100+ degrees along the James and Missouri Rivers. Sunday is a very similar story with slightly warmer high temperatures: low 90s to low 100s. Heat index looks to be 100-105 along the southern Missouri counties. There remains some uncertainty in the heat forecast, as the ensemble guidance has backed off somewhat on the probability of widespread greater than 90 degree highs. In addition, evapotranspiration may boost dew points higher, but to what extent is uncertain. Considering this, have opted to forego headlines at this time. Even so, those with outside plans this weekend should continue to be weather and heat aware. As noted in the previous discussion, a strong cap remains in place for the weekend. A few weak waves are expected to move through, and if they can break through it there is plentiful instability for storms to tap into. But given the strength of the cap, confidence is low. Sunday night/Monday morning a cold front moves through turning winds at the surface northwesterly. Increasing CAA will offer us a respite from the heat, dropping highs Monday into the upper 80s to low 90s, and then into the low to mid 80s for Tuesday. By midweek a more organized trough looks to bring our next chances at widespread rain. && .AVIATION /18Z TAFS THROUGH 18Z FRIDAY/... Issued at 1217 PM CDT Thu Jul 11 2024 Through the period, winds generally will be southerly to southeasterly with gusts 15-20 knots. Gusts drop off after sunset, but should pick up fairly quickly after sunrise Friday. Winds may be light enough along/east of US Hwy 75 into early Friday morning for some patchy fog development in low lying areas, but confidence is low. Diurnal cumulus clouds are expected each afternoon/evening, with wildfire smoke aloft creating some hazy skies. Isolated showers and storms are possible west of the James River overnight, with additional chances after daybreak Friday in southwestern MN. Confidence in timing and coverage is low (especially for later chances). Outside of any fog development and convection, expect VFR conditions to prevail. && .FSD WATCHES/WARNINGS/ADVISORIES... SD...None. MN...None. IA...None. NE...None. && $$ DISCUSSION...AJP AVIATION...SG