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FXUS63 KLSX 112104 AFDLSX

Area Forecast Discussion National Weather Service Saint Louis MO 304 PM CST Mon Dec 11 2023

KEY MESSAGES

- Temperatures through the week are forecasted to be near or above normal.

- Dry conditions are expected through the weekend.



SHORT TERM
(Through Late Tuesday Night)
Issued at 303 PM CST Mon Dec 11 2023

A surface ridge to the east of the CWA is favoring above normal temperatures amid southerly flow and sunny skies. Dry air and south/southwesterly flow is allowing for relative humidity values to drop into the upper-20s and low-30s across portions of southeast Missouri. Despite these lower relative humidity values, winds are too light to create conditions that are favorable for fire weather concerns. Tonight, under clear overnight skies, temperatures are forecast to drop into the upper-20s and low-30s.

A shortwave upstream of a surface low pressure system over the Hudson Bay will push a weak cold front across the Mid-Mississippi River Valley Tuesday morning. There is high confidence in this being a dry cold frontal passage due to a lack of low-level moisture and given that 100% of ensemble guidance members indicate no precipitation. The primary impact felt with this cold front will be slightly cooler temperatures on Tuesday across northeastern Missouri with highs in the 40s. These cooler temperatures will also be due to increased mid-level cloud cover across northern Missouri. High temperatures across the southern half of the CWA are forecasted to be in the 50s, where there will be more sunshine and less time behind the cold front.

MMG/BSH

LONG TERM
(Wednesday through Next Monday)
Issued at 303 PM CST Mon Dec 11 2023

There is consensus among ensemble guidance that an upper-level low will develop over the southwest CONUS with an upper-level trough over southeastern Canada on Wednesday. Deterministic guidance is signaling a vorticity maximum rounding the southeast Canadian upper trough across the northern most portions of the CWA early Wednesday morning. As this does so, a slight decrease in mid-level temperatures will allow for dewpoints to increase and will favor increased mid-level cloud cover on Wednesday. Despite an increase in mid-level moisture, model soundings continue to lack low-level moisture so a dry forecast continues to be maintained.
Temperatures on Wednesday are forecasted to be near normal with highs in the 40s.

As the upper low over the southwest CONUS treks eastward through the end of the work week, ridging building in over the Mississippi River Valley and surface high pressure will favor near to above normal temperatures Wednesday onward. Confidence in these near to above climatological normal is high (90%+) given that the lowest quartile of the NBM members are upwards to 5 degrees above normal for mid- December.

As we get into the weekend, the track of the upper-level low and associated trough varies among difference guidances. Deterministic guidance is showing that a northern stream upper-level low will develop as the upper low over the southern CONUS treks eastward.
There is uncertainty in the exact tracks of the upper-level lows but there is confidence in no weekend rainfall given that only 20% of ensemble guidance members indicate at least a trace of rainfall.

MMG/BSH

AVIATION
(For the 18z TAFs through 18z Tuesday Afternoon)
Issued at 1122 AM CST Mon Dec 11 2023

Dry and VFR conditions will prevail through the TAF period.
Southerly winds today will continue to increase through the afternoon and diminish around sunset. A dry cold front will move through the region Tuesday morning and veer southerly winds to northerly.

MMG/Pfahler

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

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