Mothers Day AND Graduation weekend for some will be a slightly cool and damp period especially on Sunday morning. Clouds will be on the increase Friday night as a low pressure system develops to the southwest over the Southern Plains. Scattered showers Saturday could dampen graduation celebration plans as a warm front pushes northward across the state Saturday afternoon. The rumble of thunder overnight Saturday into Sunday is possible as the ensuing cold front associated with the low pressure system pushes through central Missouri by mid-morning on Mothers Day. Rain will clear out quickly behind the front and leave behind falling temperatures back into the upper 40's by Sunday night. Monday will be a calmer but unseasonably cooler day after the previous days frontal passage. Rainfall accumulations look to be around 0.5 inches for the weekend.
Forecasters: Travis, Bongard
Date Issued: 05/07/21 5:30 PM CST
The main focus of this forecast is an upper-level shortwave expected to bring showers and thunderstorms into the region this weekend. A blend of the GFS and NAM was used for this forecast as they both had good placements of the surface lows in Wyoming, Colorado, and Michigan. The NAM also looks to be shying away from its typical bullish tendencies regarding daytime high temperatures.
The strong upper-level low at 250 hPa over Ontario and Quebec retreats north in the coming days as the LW trough parked over the eastern CONUS begins to flatten out. This will result in a break down of the upper-level ridge over the western and central CONUS with flow eventually transitioning to a less amplified trough ridge pattern by the end of the forecast period. This breakdown of the ridge will allow an upper-level shortwave move into the region Saturday night into Sunday. GFS/NAM plots of 500-hPa vorticity show an increase in widespread circulation well out ahead of the approaching shortwave. The bulk of the upper-level energy associated with this disturbance will move through the region Saturday night and into Sunday morning. This is expected to be the main time frame of concern.
Model plots of 850-hPa winds have a prominent LLJ terminating in southwest Missouri for Friday night through Saturday afternoon. As the surface cyclone moves off of the Rockies, it will deepen and allow this LLJ at 850 hPa to strengthen with it. A synoptically driven LLJ coupled with the enhancing from the nocturnal nature of the LLJ will allow for strong (>50 kt) winds to infiltrate well into Missouri. This will help transport plenty of warmer, moister air for the bulk of the upper-level energy to make good use of. Plots of 850 and surface dewpoint temperatures back this up.
GFS and NAM soundings suggest sufficient saturation of the column to support brief passing showers Saturday afternoon associated with the above mentioned initial circulations. There are some discrepancies between the two models that are worth noting. The GFS favors a fully saturated boundary layer up to 700 hPa with a 200 hPa deep layer of dry air. The NAM is almost the opposite favoring a saturated upper atmosphere with a near-surface dry layer. As such, the lack of deep moisture with either model suggests brief passing showers throughout the day. The presence of roughly 300 to 400 J/kg of upper-level CAPE hints that rumbles of thunder, though unlikely, are not out of the question.
Attention then turns to overnight Saturday into Sunday where the earlier mentioned bulk circulation associated with the upper-level shortwave will begin to move in. As mentioned above, the presence of a strong south-southwesterly LLJ will provide plenty of moisture for this system to feed off of. Both the GFS and NAM favor large elevated instability (1500-2000 J/kg)above the boundary layer overnight Saturday into the early morning hours on Sunday. Looking likely is an early morning MCS that will blow through the region ahead of the surface cold front around 12z Sunday. NAM soundings suggest a slightly slower frontal passage than those of the GFS with moderate, post-frontal, instability sticking around until ~15z where the GFS has none. DCAPE values >1000 J/kg and Corfidi storm motions of ~75 knots suggest the potential for strong winds associated with the initial line of thunderstorms. With all this in mind, the SPC's Day 2 Convective outlook places mid-Missouri in a marginal risk. As such, strong storms are possible, but severe weather, though possible, is not very likely. Heavy rain looks to be the main threat with the MCS with PWATs >1.5 inches and K indices around 30 to 35. The GEFS mean QPF for Columbia is 0.70 inches. Seeing as the MCS and cold front is quick to blow through, current thinking is rainfall totals slightly less than that at around 0.5 inches.
Upper-level flow turns zonal behind the exiting shortwave and associated surface front. Temperatures will fall well below average for May.