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FXUS63 KLSX 280856 AFDLSX

Area Forecast Discussion National Weather Service Saint Louis MO 256 AM CST Tue Nov 28 2023

KEY MESSAGES

- A chilly but dry day is on tap with well below normal temperatures

- A big rebound in temperatures is expected on Wednesday with above normal temperatures

- Widespread beneficial rainfall is still on track to impact the area Thursday into early Friday. Despite some track differences, the potential for rainfall amounts greater than one inch have increase markedly across the southeast half of the area

SHORT TERM
(Through Late Wednesday Afternoon)
Issued at 144 AM CST Tue Nov 28 2023

The axis of a deep longwave trof centered over eastern NOAM will shift eastward today and tonight resulting in weakly rising heights aloft but continued mid-upper level northwest flow. In response the weak surface ridge currently located in eastern KS will slowly shift eastward across the region, resulting in surface winds gradually backing to more southerly later this afternoon. The pattern will bring weak WAA this afternoon but much of this will be concentrated above the boundary layer. As such we are looking at a rather chilly day owing to the cold start, delayed onset of southerly winds/WAA, and some high clouds this afternoon. Highs today will be as much as 10 to 15 degrees below normal, especially along/east of the MS River.

Continued retrogression of the surface high and passage of a weak surface trof tonight will bring the return of west-southwest low level flow on Wednesday and attendant WAA. Aloft heights will be rising as well as ridging nudges into the region from the west, well ahead of a southern stream upper trof digging into the West Coast. The combination of the good WAA regime and height rises will send the temperature elevator up, and we should see a nice warm-up and rebound from today's cool readings, with above normal highs.

Glass

LONG TERM
(Wednesday Night through Sunday)
Issued at 144 AM CST Tue Nov 28 2023

A widespread beneficial rain is still on track to impact the area Thursday into early Friday. Despite some continued differences in the guidance with the speed/timing and intensity of the southern stream upper trof ejecting across the region, and the north-south position of the accompanying surface low, these differences are rather minor at this time in terms of sensible weather.

Strengthening and persistent south-southwest low level flow on Wednesday night will begin to transport moisture poleward, and the main thrust through 12z Thursday will be across eastern OK into southeast KS and western MO. As the southern stream trof advances into the southern Rockies on Thursday, downstream mass adjustments will result in a strengthening and backing LLJ which should promote lift and moisture transport into the CWA with predominately light rain blossoming along and south of I-70. The upper trof will eject across the southern Plains into the mid MS Valley by early Friday morning, with continued backing low-mid level flow ahead of it and the establishment of a well-developed warm conveyor belt. The resultant isentropic ascent, large scale ascent/forcing with the upper trof, and divergence aloft/ageostrophic circulation resulting from coupled ULJs will result in widespread rain Thursday night into early Friday. The model rainfall forecasts and the NBM probabilistic/exceedance QPF data have increased markedly. Both the NBM v4.1 and v4.2 have 50-80% probability of 1+ inches of rain across southeast and east- central MO and southern IL. Meanwhile we still continue to monitor the potential for light snow across far northeast MO on the northern edge of the precipitation shield for late Thursday night and Friday morning. The odds still look quite low and currently around 10% or less of the NBM membership suggests any potential for light snow accumulations. Precipitation with this system should be long gone by midday Friday as the deamplifying short wave trof moves rapidly into the Ohio Valley by 18z.

There is low confidence with forecast details for the upcoming weekend and into early next week. Southwest flow aloft will be present in the wake of the late week storm system, and there is uncertainty in the presence/position/strength of any low amplitude short waves within the flow, and also with a deeper migratory trof in the late Sunday-Monday time frame. We presently have 20-40% occupying this portion of the forecast, and while there might be rain there could also be a good deal of dry time. The interquartile spread from the NBM temperature data suggests temperatures will be near to above normal.

Glass

AVIATION
(For the 06z TAFs through 06z Tuesday Night)
Issued at 1038 PM CST Mon Nov 27 2023

VFR flight conditions are expected to prevail through Tuesday.
High pressure over eastern Texas will drift east over the next 24-30 hours. The ridge axis will move east of the Mid Mississippi Valley by Tuesday afternoon which will turn surface winds back to the south.

Carney

LSX WATCHES/WARNINGS/ADVISORIES
MO...None.
IL...None.

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