Fall is finally here! A brief boost in the high temperature for Monday will lead us into the upper 70s. A low pressure system to the south will bring in rain and cloudy conditions. As clouds start to move in Monday night, they will keep daytime high temperatures cooler for Tuesday and Wednesday. Increased cloud cover Tuesday night will give way to showers Wednesday into Wednesday night. Total rain accumulations will range from 1/4" to 1/2".
- Cade, Baker
Forecasters: Cade, Baker, Travis
Date Issued: 10/4/21 10:00 AM CST
Fall-like weather is in store for mid-Missouri this week. The main focus of this forecast period is a cut-off low pressure system over the Lower Mississippi River Valley that will bring sufficient moisture northward to support rain near the end of the period on Wednesday. Both the GFS and NAM had a good handling of the large-scale, upper-level pattern, but the GFS was too warm with initialized temperatures in the Central Plains (by ~10-15 degrees F). This warm bias appeared to continue into the forecast period when compared with the NAM and GEFS members. As such, went primarily with NAM guidance, while looking at the SREF and GEFS plumes to analyze model performance.
The NAM 250-hPa height field depicts another period of unsettled, meridional flow similar to that of last week. The longwave trough sitting over the eastern CONUS that brought a weekend of heavy rain is not quite done with the Midwest yet. As an amplified ridge moves in from the southwest, it pushes the jet stream further into Canada and closes off that pesky LW trough cutting it off from the storm track. That newly cut-off low will park itself over the Lower Mississippi Valley allowing active weather later in the week.
NAM 500-hPa plot of absolute vorticity shows a weak shortwave propagating along the back side of the LW trough during the day on Monday. This weak disturbance will move in from the north before quickly getting absorbed into the upper low as the system becomes cut off, after which the circulation will sit to the south, bringing in vorticity maxima from the SE.
NAM 700-hPa plots of RH favor moisture, albeit very little, lagging behind the weak disturbance on Monday night into Tuesday morning. This will bring in clouds Monday night that will stick around for the remainder of the forecast period. A brief clearing of the saturated air at this level will be cut short by, our friend, the cut-off low. Widespread saturated air will surge NW into Missouri as the low sits to the south. The NAM omega field favors that the bulk of the forcing for ascent will be confined to the Tennessee and Ohio River Valleys, the reason for which becomes evident at 850 hPa.
A noticeably lackluster low-level setup is evident in the height/MSLP fields 850-hPa and the surface, respectively. At 850 hPa, a weak LLJ is present over the river valleys mentioned above and terminates and does not continue into Missouri. Moisture convergence associated with the LLJ sheds some light on the abundant saturation and ascent seen at 700-hPa. As such, current thinking is that, while mid-Missouri will see rainy conditions all day, Wednesday, precipitation totals should not approach the >3" that Sanborn Field recorded over the weekend. Much is the same story at the surface where no obvious surface features are evident.
NAM soundings show an increase in low-level saturation Monday night into Tuesday morning associated with the first, weaker disturbance seen at 500 and 700 hPa. Low-level saturation in the lowest 3km and weak UVM would suggest that isolated light showers cannot be ruled out for Tuesday. The low levels dry out Tuesday night before the entire column rapidly moistens Wednesday morning. Deep saturation exists for the entire day Wednesday not drying out until overnight Wednesday. PW ~1.5 inches and K indices ~30 would suggest that rain may be heavier at times. However, little if any instability does not currently favor convection. Accumulation totals could range from 0.25" to 0.5", but if heavier showers set up, amounts could be locally higher.
Future forecasters should watch the evolution of this system carefully to narrow down precipitation timing and accumulation as we saw over the weekend that the models drastically underperformed on accumulations.