Current Conditions at Sanborn Field
Tuesday Night: Clear. Low: 64
Wednesday: Cloud cover increasing throughout the day. Showers possible in the afternoon. High: 86
Wednesday Night: Overcast. Rainy with possible thunderstorms. Low: 67
Thursday: Cloudy. Rain throughout the day. High: 79
Thursday Night: Cloudy. Scattered showers likely. Low: 65
Discussion:
Tonight will be clear with increasing clouds throughout the day on Wednesday, and by Wednesday afternoon scattered showers are likely. Potential of rain increases over night, with the possibility of thunderstorms early in the evening. While the chances of thunderstorms diminish by late Thursday morning, rain will persist throughout the day. Keep the umbrella handy as scattered showers will continue into the overnight hours. Despite rain dominating the forecast, temperatures will remain seasonable.
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Forecasters: Duff, Watts, Orr
Date Issued: 09/28/21 5:00 PM CST
Technical discussion:
The 12Z run of the GFS was used to create this forecast due to the NAM output having a cold bias and being over-saturated. GEFS and SREF plumes were used to forecast temperatures and GFS soundings were used to assess rain timing and the potential for severe weather.
At the 250-mb level, plots of heights, winds, and divergence display very meridional flow dominating over the United States. Missouri is under the influence of a large ridge Tuesday and into early Wednesday. By Wednesday afternoon around 18Z, the ridge moves out of the our region, and the Middle Mississippi Valley falls under the effects of a large trough. Higher amounts of divergence associated with the trough pass through the forecast area
between 21Z Wednesday and 12Z Thursday. Smaller amounts of divergence remain in the area through Thursday and into Friday.
At 500-mb, plots of heights and vorticity show large amounts of vorticity associated with the incoming trough. As the ridge leaves Missouri, high amounts of vorticity enter the forecast region around 00Z Thursday. Weaker circulations remain in the area until 00Z Friday. Future forecasters should watch for the potential for another wave of vorticity to move through Mid-Missouri on Friday, providing opportunity for storm development.
700-mb plots of heights and relative humidity support rainfall associated with the vorticity and divergence seen in the upper levels. High RH values begin entering the region around 21Z Wednesday and persist until 00Z Friday. The presence of moisture favors rainy weather for Wednesday afternoon through the rest of the forecast period, with the potential for thunderstorms most prominent Wednesday night and into Thursday morning.
At 850-mb, plots of heights, temperatures, and winds depicts the source of the abundant moisture. The LLJ forms late Tuesday around 21Z and funnels in moisture from the Gulf for Wednesday and Thursday's rainfall. These southerly winds are also advecting warmer air into Missouri, providing us with the slightly above seasonable temperatures. The surface level also depicts the steady, southerly winds, further favoring these decent temperatures.
GFS soundings provide support for Wednesday night's thunderstorms. Negative Omega values, negative LI values, favorable K and TT indexes, and MUCAPE of around 1100 J/kg first appear around 21Z Wednesday and persist to around 09Z Thursday. After this time, favorable thunderstorm conditions slowly taper off. Thunderstorms are most likely in Mid-Missouri during this time frame, and severe conditions are unlikely, but should not be ruled out.
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