Temperatures at Sanborn peaked at 74.5 F at 2:19pm CDT. Winds have since shifted from a WSW direction to a NNW direction and pressure has begun rising. All indications that cold frontal passage already occurred. A look at the METAR plots around the state of Missouri, overlain with RAP surface analysis loops indicate FROPA occurred at KCOU between 15z and 16z. A weaker front, void of moisture, this cold front mainly brought with it an increase in winds as KCOU METAR observed a wind increase from 5-10 kts prefrontal passage to ~20kts post frontal. Behind the front, high pressure from the surface to 850 hPa has already begun setting up and will keep our skies clear going forward.
The main concern for this forecast period is the potential first frost of the season as temperatures will continue to plummet to end off the week. Attention then turns to the next system that will drop out of the north into the Midwest that will bring with it chances for rain into Saturday. A blend of the GFS and NAM was used as there was consensus between the two models in terms of timing of the precip Friday night.
Much of south-central Canada and the Upper Midwest remain on the poleward side of LW trough for the entire forecast period so suggests GFS and NAM plots of 250-hPa heights and wind. This will allow cold air to surge south. Both GFS and NAM 700-hPa RH plots are in good agreement that moisture associated with the parent LW trough will stay confined to the Upper Great Lakes Region. Winds at 850 hPa will stay out of the NNW, allowing the cold air to penetrate further south into the Mid Mississippi Valley. Thursday night is where the GFS and NAM consensus diverges slightly. The GFS wants to keep the 850 hPa 0-C isotherm to the north of Columbia while the NAM is slightly more aggressive with the southern surge, bringing the isotherm further south than Columbia. Temperatures could very well dip into the upper 30s and possibly lower if the NAM has its way. Would not be surprised to see the potential for frost early Friday morning as skies will remain clear and winds relatively calm.
Attention turns to a SW seen in 500-hPa heights and vorticity that both models want propagating along the parent LW mentioned earlier. As the aforementioned high pressure at 850 hPa and the surface slides to the east over the Middle Mississippi Valley, winds will see a shift from northwest to southwest coinciding with the wave's arrival. The LLJ originating from the Gulf sets up over the Southern Plains that will advect plenty of moisture into the Middle Mississippi Valley during the day on Friday. The moisture, combined with the dynamically primed SW will result in high chances for precip Friday evening into Saturday. GFS soundings show a moistening of the lower levels with a closing "zipper" at the surface occurring around 21z Friday afternoon. Saturation from 700 hPa to the surface continues through Friday night and into Saturday morning. Will defer to later forecasting shifts to narrow down the timing of both the precip's arrival and departure.