Sunrise 6:05 AM   Sunset 7:52 PM Moonrise 12:00 AM   Moonset 9:05 AM

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FXUS63 KLSX 291921 AFDLSX

Area Forecast Discussion National Weather Service Saint Louis MO 221 PM CDT Mon Apr 29 2024

KEY MESSAGES

- Patchy fog is expected across portions of southeast Missouri and southwest Illinois, as well as in the main steam river bottoms through tomorrow morning.

- Many river forecast points have exceeded minor flood stage, with a few forecast to reach moderate flood stage this week.

- Temperatures are expected to be above normal through the period.
There are numerous chances for rain through next weekend with the best chance (60-80%) Thursday into Friday.

SHORT TERM
(Through Late Tuesday Night)
Issued at 220 PM CDT Mon Apr 29 2024

Many of our forecast points along the main stem rivers and associated tributaries have risen above flood stage thanks to the widespread rainfall received over the weekend. Depending on the basin and river, points may remain above flood stage for several days. Additionally, some rivers like the Illinois, may be slow to respond and may not go above flood stages for a few days.

A cold front is moving through the forecast area behind a stratus deck. As of 2 PM the cold front is stretched from roughly Pittsfield through the St. Louis metro to Farmington. Behind the cold front a surface high is pushing into the region. Under the influence of the high, a clear sky and near calm winds overnight will promote patchy fog development. This is particularly the case across portions of southeast Missouri and southwest Illinois where the sky will clear much later in the day and subsequent near surface moisture will be higher. Additional patchy fog development in the valleys of the main stem rivers and in the hollows of southeast Missouri are also possible.

The high will move eastward overnight and by tomorrow winds will be from the south, increasing in speed as the surface pressure gradient tightens ahead of another, weaker cold front. Warm air advection and a clear sky will push high temperatures on Tuesday into the upper 70s to low 80s across the area. Cloud cover will begin to increase Tuesday evening as a shortwave disturbance and weak surface low slide across the upper Mississippi River Valley overnight. The previously mentioned weak cold front will move through northern Missouri bringing a chance (20-40%) of showers and thunderstorms to the area. Coverage and intensity of the showers and thunderstorms is expected to be light and weak given the weakness of the front as it moves into our forecast area.

MRM

LONG TERM
(Wednesday through Next Monday)
Issued at 220 PM CDT Mon Apr 29 2024

A mid-level ridge will strengthen over the Ohio River Valley Wednesday, and to the northwest of the forecast area, a mid-level trough will slide along the US-Canada border in the upper Plains. At the surface a low will form and strengthen over the southern Plains, with a warm front developing to the north of the forecast area and a cold front sagging into Texas. The forecast area will stay within the warm sector ahead of this system on Wednesday and Thursday, increasing temperatures and low-level moisture.

By Thursday the mid-level trough axis will swing into the mid- Mississippi Valley and the surface reflection will progress through the region during the evening and overnight. Showers and thunderstorms are expected ahead of and along the attendant cold front as it moves east to west through the forecast area overnight Thursday into Friday. Ahead of the front, the warm sector will be characterized by high instability (1000+ J/kg of MUCAPE via deterministic members), minimal cap, and high precipitable water (90th percentile via NAEFS situational awareness tables), and weak effective shear. Conditions are favorable for widespread showers and thunderstorms, particularly once the near surface dry layer can be eroded. However, the weak shear limits the organization of strong to severe thunderstorms. The cold front is expected to be progressive enough to also limit slow moving thunderstorms and the associated heavy rain potential. The cold front will clear the forecast area Friday, bringing a relative and brief cool down to the region before temperatures warm over the weekend.

The mid-level trough will continue to push east along the US-Canada border into and through the Great Lakes region during the end of the work week. With the system so far to the north, the region remains open to mid-level disturbances through the weekend. Ensemble and deterministic guidance varies on handling the mid-level pattern beyond Friday, and we certainly cannot rule out disturbances moving across the forecast area and bringing additional chances of rain.

MRM

AVIATION
(For the 18z TAFs through 18z Tuesday Afternoon)
Issued at 1204 PM CDT Mon Apr 29 2024

The IFR/MVFR stratus is continuing to push eastward ahead of a cold front. The western edge of the stratus has started to scatter, speeding up clearing even further. The St. Louis metro sites are the only terminals remaining at MVFR and will be VFR by the early afternoon. Ceilings will remain VFR through the remainder of the TAF period.

However, a clear sky tonight combine with high low level moisture and weak surface winds increases the potential for fog overnight.
Widespread fog is most likely southeast of the St. Louis metro terminals, but sites near rivers (KCOU, KJEF, KSUS, and KCPS) will be at risk for IFR fog overnight and into the early morning hours.

Winds will become west to northwesterly today behind the approaching cold front, becoming increasingly light and variable this evening and overnight as a surface high pushes through the area.

MRM

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

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