Day 5 of St. Louis, MO (KSTL)

Very involved and challenging forecast for day 5. Day 5 will be somewhat similar to day 3, but also somewhat different. Although it is relatively cool and rainy now, a broken line of storms this evening will push north effectively as a warm front, with the warm sector “untainted” by convection arriving by 06z or so, leading to a warmup then due to low-level warm advection, stiff southerly winds, and clouds, despite it being nighttime. GFS and USL temperatures look reasonable.

 

Source: Aviation Weather Center

 

Source: PivotalWeather

 

Source: Meteogram Generator

Late tonight into tomorrow morning, the cold front will bring a period of likely heavy rain and/or thunderstorms, with potentially slightly stronger winds (perhaps up to 25 kt) with any stronger thunderstorms able to mix down stronger winds from aloft enhanced by cool downdrafts, though the lack of low-level instability at night and generally moist adiabatic profiles will prevent any gust fronts from being too strong. The Storm Prediction Center gives a marginal risk of severe thunderstorms mainly for wind threat, though isolated tornadoes are not out of the question. A short period of heavy rain with the squall line will transition to a several-hour period of lighter, more stratiform rain afterward, taking its time to traverse as the storm motion is almost parallel to the squall line, with the front moving quite slowly. It might still be raining by early afternoon, with clouds persisting for the rest of the day, preventing a daytime high temperature any higher than temperatures tonight. Rainfall amount is quite uncertain with the squall line possibly not being a solid line and/or weakening tonight (leading to potentially lower amounts if KSTL ends up in holes) but also being rather slowly-moving, allowing for large rainfall amounts if KSTL gets nailed in a stronger portion of the line for a while. Since there’s little systematic way to know which way it will go, going in the middle is usually the safest bet. 

 

Source: Storm Prediction Center

 

Source: PivotalWeather

 

Dry air will sweep in the mid to upper levels tomorrow evening, but there may still be enough low-level moisture for at least scattered to broken low cloud cover tomorrow night, which could complicate the low which will occur at the end (Wednesday 06z). Winds will lighten just enough for better (but not quite ideal) radiational cooling then if it can clear out more, though KSTL isn’t known for particularly rapid radiational cooling. There will be some synoptic-scale cold advection regardless of radiational cooling, but the uncertain cloud cover adds some more uncertainty.

 

Source: PivotalWeather