Current Conditions at Sanborn Field
Tuesday Night: Mostly cloudy. Low: 61
Wednesday: Overcast. Rain beginning late morning. Thunderstorms likely throughout the afternoon and evening. High: 71
Wednesday Night: Cloudy. Rain ending after midnight. Low: 61
Thursday: Mostly cloudy. Foggy in the morning. High: 73
Thursday Night: Partly cloudy in the early evening. Sky clearing after midnight. Low: 60
Discussion:
Throughout the forecast period, cloudiness will help keep high temperatures around the lower 70s. This is cooler than the past few weeks, though still above average for this time of year. Tuesday night will be calm, but mostly cloudy. Clouds increase Wednesday, due to the approaching low pressure system. Rain will start late Wednesday morning, so be sure to bring an umbrella, because thunderstorms will start by afternoon and will continue into the evening. These storms will end around midnight, leading to fog early Thursday morning. Cloud cover persists throughout the day Thursday, but sky conditions will begin to clear after midnight.
- VanUnnik, Watts
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Forecasters: VanUnnik, Watts, Orr
Date Issued: 10/5/21 06:00 PM CST
Technical discussion:
The 12Z run of the NAM was used to create this forecast due to its more accurate initial conditions and more accurate placement of the low pressure system over Louisiana. SREF and GEFS plumes were used to aid in forecasting temperatures, and NAM soundings were consulted for rainfall timing and thunderstorm potential.
At 250-mb, plots of heights, winds, and divergence depict a large low pressure system over the Lower Mississippi Valley. Flow over Missouri is dominated by the system for almost the entirety of the forecast period. The low moves over Mid-Missouri around 15Z Thursday, and flow soon falls under the influence of an upper-level jet. There is little divergence associated with the passage of the low, and the most abundant amounts are seen Wednesday around 20Z.
500-mb plots of heights and vorticity depict the incoming low pressure system's dominance over Missouri's weather over the next few days. The low advects high amounts of vorticity into the region as early as 08Z Wednesday. The abundant vorticity will remain in Mid-Missouri until the low exits the region around 00Z Friday. A ridge begins entering Missouri at this time, and future forecasters should watch for its impact on the forecast area.
Plots of heights, winds, and relative humidity at 700-mb show the low pressure system funneling abundant moisture into Missouri. Winds associated with the low will bring in moisture from the Gulf, placing Missouri under high RH values from around 11Z Wednesday to 06Z Thursday. PW values favor Wednesday's rainfall amounts ranging from around 1" to 1.5". Moisture moves out of the region along with the low around 00Z Friday, leading to clearer sky conditions Thursday night.
At 850-mb, plots of heights, winds, and temperatures favor slightly above average temperatures for this time of year. Winds begin in a northeasterly direction, but are shifted to westerly due to the passage of the low around 00Z Friday. The shifting winds not only lead to Thursday's higher high, but they also cut off moisture flow from the low pressure system, leading to Thursday's clearing conditions.
At the surface level, plots of winds, MSLP, and thickness show the deepening and development of the low pressure system as it passes through Missouri. At this level, the low isn't as defined as it is at upper levels until around 00Z Wednesday. The low continues to mature as it makes its way over Missouri, becoming much more defined by the time it exits the forecast region.
NAM soundings were consulted to assess potential for thunderstorms since abundant vorticity and high RH values coincide Wednesday and into late Wednesday night. Thunderstorms are favored to begin around 18Z Wednesday, negative Omega values, LI values of up to -6, and MUCAPE over 1800 J/kg lasting until 03Z Thursday. SRH values in the SFC-1km region are also favorable for thunderstorm development.
-Orr
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