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FXUS63 KLSX 280536 AFDLSX

Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 1236 AM CDT Sun Apr 28 2024

KEY MESSAGES

- Thunderstorms late this evening and overnight have the potential to produce heavy rain and localized flooding across northeast MO and west-central Illinois, with a limited potential for damaging winds and an isolated tornado as well. A Flood Watch has been issued for parts of northeast Missouri and west- central Missouri from 7 PM tonight until 10 am tomorrow morning.

- Thunderstorms are likely late tomorrow afternoon through late evening in many areas, with damaging winds, large hail, and an isolated tornado or two possible.

- A few thunderstorms will be possible mainly along and south of I-44 Monday afternoon and evening.



SHORT TERM
(Through Late Sunday Night)
Issued at 318 PM CDT Sat Apr 27 2024

While conditions are quiet across the region as of this writing, this will not last long as multiple rounds of thunderstorms remain on the horizon over the next 48 to 60 hours. At 2 PM CDT, a surface low was located across southwest Kansas and the OK/TX panhandles, with a nearly stationary front extending northeastward into Kansas, northern MO, and southern IA. South of this boundary lies a broad warm sector characterized by surface dewpoints in the low to mid 60s, and precipitable water values between 1 and 1.25 inches, the latter of which is near the 90th percentile for this time of year. Meanwhile, a slow-moving shortwave and associated jet streak was located across the central high plains, providing ample upper forcing to overcome weak capping along both the dryline and previously mentioned warm front extending into northwest Missouri.

As the afternoon progresses, this convection is expected to expand and gradually drift eastward and eventually into parts of northeast/central Missouri and west central Illinois. While scattered weak thunderstorms ahead of this activity may produce a few heavy downpours and small hail this afternoon, the more impactful thunderstorms are likely later in the evening and overnight as more organized storms arrive from the west. Across northeast Missouri and west-central Illinois, training thunderstorms will be possible due to storm motions nearly parallel to the stalled front. Meanwhile, anomalously high moisture content will promote moderately efficient rainfall rates, which may fall in areas that have already observed significant rainfall in the past couple of days. Meanwhile, CAMs have indicated a potential for locally heavy rainfall across a narrow swath in these areas, and confidence is increasing in the potential for 1 to 3 inches of rain, and potentially higher amounts possible as well. As such, we've issued a Flood Watch for this evening through tomorrow morning across a few counties in northeast Missouri and west-central Illinois, in addition to many other counties added by neighboring offices to our north and west.

Meanwhile, the strengthening low level jet will also cause low level shear to increase through the evening and overnight ahead of these storms, and while instability will wane considerably overnight (SBCAPE falling to 500 J/kg or less), we can't rule out a few damaging wind gusts or even a tornado or two late this evening and overnight. This threat will steadily diminish after roughly 3 am as storms wane in intensity. Remnant showers and thunderstorms will continue to gradually spread eastward through a majority of the area through the morning hours, and eventually diminish as they move east of the Mississippi River.

Exactly when this occurs will play a big role in the potential for local strong/severe thunderstorms tomorrow. Considering the potential for widespread cloud cover and remnant showers, instability will be at a premium tomorrow afternoon locally and there remains some question as to how efficiently the warm sector will be able to destabilize. What is most likely is that a corridor of moderate instability will develop across western Missouri, and forcing from the departing shortwave and mid level jet streak will drive thunderstorm development here during the afternoon. This activity will then drift eastward during the late afternoon and evening as it matures, eventually reaching our portions of central and northeast Missouri, but encountering a steadily less favorable thermodynamic environment. However, should we maintain sufficient instability, 40 to 50 kt of 0-6km shear will be present, with gradually increasing low level shear late in the afternoon and into the evening as the low level jet increases. In other words, while confidence is high that much of the region will see more showers and thunderstorms late in the day/evening tomorrow, likely in the form of a broken linear complex, the threat of severe storms remains conditional.

Thunderstorms will move east of the area late in the evening and overnight tomorrow, and a cold front will lag this activity and stall across the area sometime during the day Monday. The implications of this will be discussed in the long term section below.

BRC

LONG TERM
(Monday through Next Saturday)
Issued at 318 PM CDT Sat Apr 27 2024

By Monday morning, the bulk of the remnant shower and thunderstorm activity from the previous night will likely have moved east of the area. However, as previously mentioned a cold front will remain draped across the region, with a modestly unstable warm sector draped across southeast Missouri and west-central Illinois by mid afternoon. Meanwhile, a belt of modest 500mb flow will remain in place aloft (20-35kt), but the strongest winds and forcing aloft will depart as the previously discussed shortwave and jet streak move into the upper Midwest. While this will not be much to work work, this may support at least some scattered convection along and south of the cold front by mid to late afternoon Monday, with a limited potential for some small hail or gusty downburst winds.

Beyond Monday, confidence in the day-to-day details diminishes as we emerge into a regime of largely zonal flow aloft. Tuesday remains the most likely day to remain dry during the work week, but the passage of several shortwaves along the periphery of a broad upper ridge across the central CONUS will maintain multiple chances for rain over the latter half of the week. Meanwhile, temperatures are likely to remain near or slightly above average overall, but the potential for widespread cloud cover and waves of rainfall will likely limit this potential. The NBM in particular is very warm with regards to temperatures late in the week, and in spite of the relatively narrow ensemble spreads, it remains to be seen whether or not these very warm temperatures (upper 80s to low 90s) can be realized.

BC

AVIATION
(For the 06z TAFs through 06z Sunday Night)
Issued at 1223 AM CDT Sun Apr 28 2024

A line of strong thunderstorms extends from west-central Illinois through central Missouri as of 0530Z this morning. Observations have shown gusts of 30-35 kts to be fairly common with localized gusts up to 45 kts around KCOU. The line is preceded by outflow that has briefly turned winds out of the northwest, which consequently will lead to continued weakening with eastward movement. Over the last 24-36 hours, guidance has been consistent in suggesting the line will weaken considerably as it approaches the metro terminals. Signals are that this process has already begun with the line becoming outflow dominant. While KCOU/KJEF and KUIN will continue to be impacted by SHRA/VCTS and MVFR ceiling, metro terminals may be spared of similar conditions with weakening showers and an intermittent clap of thunder early this morning.

Focus shifts to Sunday afternoon as a slowing frontal boundary could ignite additional showers and thunderstorms around and after 18z. The coverage and intensity will be affected by behavior of prior convection and how well the atmosphere can recover instability. Nonetheless, another round of thunderstorms could impact all terminals as they progress from central Missouri from mid-afternoon through eastern Missouri through early evening. A few storms could be strong to severe with heavy rain, gusty winds and frequent lightning. For now, this was handled with SHRA/VCTS mention until details can be fine tuned in subsequent updates.

Maples

LSX WATCHES/WARNINGS/ADVISORIES
MO...Flood Watch until 10 AM CDT this morning for Knox MO-Lewis MO- Marion MO-Monroe MO-Shelby MO.

IL...Flood Watch until 10 AM CDT this morning for Adams IL.

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