The SPC now has St. Louis in a Slight Risk for severe weather for Sunday. Convective development is expected to be primarily linear with a severe squall line developing during the afternoon hours across MO. The initial mode over western into central MO may be discrete supercells that consolidate in the afternoon.
Strong wind fields will contribute to a QLCS (Quasi-Linear Convective System) across northern and perhaps central and east central MO during the late afternoon or evening hours. This squall line will be capable of producing damaging straight line winds, especially if LEWP (Line Echo Wave Pattern) structures develop. Embedded supercells are also a distinct possibility where shear is sufficient which may produce weak, short-lived tornadoes.
**Caveat** - Current (Saturday afternoon) model forecasts point at St. Louis being currently situated north of the maximum instability axis and a little south of the best shear, so forecasting the intensity of severe weather in the St. Louis area is a bit tricky. This should become a bit clearer in later model runs. I am currently leaving the mention of tornadoes out of the forecast below for now (it's never entirely known until storms develop).
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The updated forecast for St. Louis and vicinity:
Tonight - Partly to mostly cloudy, low in the mid 50's.
Saturday - Partly cloudy during the morning. Showers and thunderstorms developing by late afternoon. Some storms may become severe with damaging winds and large hail. High in the upper 70's.
Saturday evening - Showers and thunderstorms likely before midnight. Some storms severe with damaging winds and large hail possible. Low in the 50's.
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