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FXUS63 KLSX 031950 AFDLSX

Area Forecast Discussion National Weather Service Saint Louis MO 250 PM CDT Fri May 3 2024

KEY MESSAGES

- Isolated strong to severe thunderstorms are possible from 2pm to 7pm Saturday. Hazards include up to 60 mph winds and up to quarter sized hail.

- Confidence is increasing in a multi-day severe event taking place in the central U.S early to mid-next week. However, there is still uncertainty regarding details like timing, exact location, and how one day's thunderstorms will impact the next day's severe potential.



SHORT TERM
(Through Late Saturday Night)
Issued at 249 PM CDT Fri May 3 2024

Thursday night/Friday's cold front will continue to slowly buckle northward through the bi-state tonight as a quasi-stationary boundary. Meanwhile, a mid-level shortwave and surface cold front will push east through the Plains, arriving in Mid-Missouri early Saturday afternoon. This system will push the boundary to the northeast and leave us within its warm sector. Because of this, temperatures Saturday will be much warmer than they were today, reaching the mid-70s to mid-80s. Moisture will also pool into the area, mostly along and just ahead of the incoming cold front.
Instability will be maximized here with SBCAPE values of 1500-2000 J/kg. Deep layer shear will be lacking, though, ranging from 20-30 kts. Modest CAPE and bulk shear values are expected to result in a severe weather threat much like yesterday - disorganized clusters of outflow dominant cells. Although we're confident in thunderstorm development, strong to severe thunderstorms are expected to be isolated in nature due to them being outflow dominant and likely quick to intensify and even quicker to die. The greatest potential for severe weather will be between 2pm and 7pm as the front is passing through the CWA In any thunderstorms that become severe, damaging winds of up to 60 mph and up to quarter sized hail are possible. We're confident in the end time of the severe potential, as the front will be close to exiting to the southeast, and that coincides with the downfall of daytime instability. With the lack of appreciable shear, diminishment in instability will be detrimental to the life of any thunderstorm.

Saturday night will be relatively chilly for portions of the areas surface high pressure passes to our north. Areas in northeast Missouri and west-central Illinois are forecast to fall into the upper 40s. To the south, further from the influence of the high and under more dense cloud cover, low temperatures may not fall under 60 degrees.

Jaja

LONG TERM
(Sunday through Next Friday)
Issued at 249 PM CDT Fri May 3 2024

Sunday will be a relatively quiet opening to the week, almost literally the calm before the storm as we face a potential multi-day severe event between Monday and Wednesday. Despite the relative quiet, wet weather is expected beginning Sunday afternoon and continuing into overnight. This is due to a shortwave passing through the Mid-Mississippi Valley from the southwest and driving a weak surface low through our CWA This surface low will cause Saturday's remnant cold/stationary front to buckle back north as a warm front with showers with embedded thunderstorms along it and within the system's warm sector. Severe weather is not expected, even earlier in the afternoon, due to weak lift, weak moisture convergence along the front, mid-level subsidence, and minimal instability (<250 J/kg MUCAPE). High temperatures Sunday will be near normal, landing in the 70s areawide.

Monday kicks off the potential for multiple days of severe weather extending from the central and southern Plains into the southwestern Great Lakes and western Ohio Valley. A deep mid-level closed low will be responsible for this as it ejects northeast into the Plains on Monday. At the surface, two low pressure systems will develop lee of the Rockies, advecting warm air and moisture into the region ahead of a dryline and developing cold front. The dryline will act as a trigger for convective initiation in the Plains on Monday. On Tuesday, the system as a whole will be further east, and a shortwave and vorticity lobe will swing through our region. The cold front will be further east as well, shifting the severe threat into our area. Guidance across the board is in consensus that instability, deep layer shear, moisture, and synoptic level dynamics will all be favorable for severe weather on Tuesday and again on Wednesday as the cold front continues its eastward advancement. With the environment that may be in place, no hazard can be discounted at this point.

Of course, this multi-day event begins four days out from now, so there's uncertainty to discuss. Firstly, although WPC cluster analysis shows consistency amidst guidance in the mid-level trough/closed low early next week (positioning, strength, size), there are differences in the phasing that impact the mid-level flow and thus impact the timing of the overall system. For a multi-day system like this, a faster or slower passage of the system could increase or decrease our potential for severe and/or lengthen or shorten our window for it. A faster frontal passage may shift Wednesday's severe threat further east and out of our CWA, whereas a slower frontal passage may extend our severe threat into Thursday.
There's also the question of how/if severe weather to our west impacts our severe weather threat for the following day (Monday into Tuesday and Tuesday into Wednesday). Overnight low-level jets may cause ongoing convection to continue into the next morning, potentially dampening the potential for severe that day. The opposite scenario is also equally as likely - thunderstorms dying in the evening could lay down outflow boundaries and open up the potential for rapid destabilization the next day, increasing our chances for severe. All of this to say despite the very favorable synoptic set-up for severe, there's still plenty of variability that could sway our severe potential and its timing.

Jaja

AVIATION
(For the 18z TAFs through 18z Saturday Afternoon)
Issued at 1204 PM CDT Fri May 3 2024

IFR/MVFR ceilings have been slow to scatter this morning.
Observations have shown very gradual improvement, but restricted flight conditions will likely last into early afternoon. Otherwise, winds will veer to southeasterly overnight and to southwesterly by 18z Saturday. By this time, a cold front will be approaching the forecast area, introducing the chance of showers and thunderstorms and a wind shift to northwesterly to all terminals Saturday afternoon into evening.

Jaja

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

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