Sunrise 5:57 AM   Sunset 7:56 PM Moonrise 2:35 AM   Moonset 1:52 PM

  Print   HELP   Reset   Save   Recall

FXUS63 KLSX 030815 AFDLSX

Area Forecast Discussion National Weather Service Saint Louis MO 315 AM CDT Fri May 3 2024

KEY MESSAGES

- An active weather pattern will continue this weekend and through much of next week, bringing multiple chances of showers and thunderstorms to portions of the area.

- This pattern will lead to a chance of isolated strong to severe thunderstorms on Saturday, with additional strong to severe thunderstorm chances next week. However, details pertaining to any severe thunderstorm threat next week are unclear at this time.



SHORT TERM
(Through Saturday Night)
Issued at 313 AM CDT Fri May 3 2024

Current water vapor imagery shows a trough over the western CONUS with a shortwave moving through the Upper-Midwest and Northern Plains. At the surface, a low is in a similar location, with its cold front seen in surface observations moving into northeastern and central Missouri. Forcing from the front and a weak shortwave aloft continue to drive scattered showers across the area. As the shortwave progresses eastward and the front exits the area later this morning, rain chances will wind down and end west to east.

Despite easterly-northeasterly winds behind the front today as high pressure settles into the Midwest, mid- to upper-level flow from the south-southwest will reduce the potency of this post-frontal airmass and keep temperatures at to just above climatological normals.
Despite the front clearing the CWA, it will not have scrubbed southern portions the area of boundary layer moisture completely. As a weak shortwave moves through the Mid-South during peak heating this afternoon, this low-level moisture will yield upwards of around 1,000 J/kg of SBCAPE that will lead to isolated thunderstorm development mainly over far southeastern Missouri. Thunderstorms are not expected to be strong, as weak 0-6km bulk shear of roughly 15-20 kts will produce updrafts that are weaker and short-lived.
Convection will dissipate during the early evening as the sun begins to set and instability wanes.

Tonight, the aforementioned surface high will move eastward and return southerly low-level flow to the area. In turn, low-level moisture will begin advecting into the CWA ahead of a cold front moving across the Great Plains toward the Middle Mississippi Valley.
Convection associated with the front will approach the CWA early tomorrow morning, but will be in the process of decaying. So, while I can't rule out very isolated damaging wind gusts across northeastern Missouri, strong to severe thunderstorms Saturday morning are unlikely.

The better chance for severe weather comes Saturday afternoon as the cold front moves into the area and a subtle shortwave travels overhead. Then, guidance consensus is that roughly 1,500 J/kg of SBCAPE will be present among 25-30 kts of 0-6 km bulk shear. Another round of showers and thunderstorms is expected along and ahead of the front in this environment, leading to an chance for isolated strong to severe thunderstorms capable of large hail and damaging wind gusts during the afternoon and evening. The amount of instability is conditional on how much clearing of convective debris from the early morning storms can occur prior to peak heating. If clouds stay thicker and more widespread, we could see less instability and weaker storms.

Elmore

LONG TERM
(Sunday through Thursday)
Issued at 313 AM CDT Fri May 3 2024

At the start of the extended period, guidance consensus is that southwesterly flow aloft will still be in place over the Middle Mississippi Valley. Both deterministic guidance and ensemble clusters show a shortwave within this flow moving out of the Southern Plains and into the Middle Mississippi Valley through the day on Sunday and into Monday, though, there remain differences in the phasing of this feature. A quicker solution would lead to a faster surface low and rain chances during Sunday afternoon mainly along and south of I-70, while a slower solution would lead to much of Sunday being dry across the area with rain moving in during the overnight hours. Regardless, guidance consensus is that the surface low will weaken as a warm front stalls somewhere across the Midwest.

As we get into early next week, confidence in the evolution of the upper-level pattern and thusly local surface conditions decreases.
In general, a trough will deepen over the western CONUS as a shortwave swings through the Great Plains and into the Upper Midwest. In turn, a surface low will follow a similar trajectory and occlude as it swings a cold front through the Great Plains and toward the Middle Mississippi Valley Monday into Tuesday. Guidance differs on the progression of this front through the region, with some stalling it west of the CWA while others have it clearing the CWA However, if the latter solution materializes, the front will quickly lift northward as a warm front for Wednesday. Narrow temperature spread among ensemble guidance for Wednesday showing temperatures rising compared to Tuesday provides high confidence that in some way, the CWA will be in the warm sector on Wednesday as the western trough reloads. Additional shortwaves will round the trough as it begins to edge eastward, sending another surface low through the Midwest Wednesday into Thursday. This low will swing a cold front through the CWA, with a majority of guidance having it clear the CWA Through this stretch (Monday-Wednesday), the CWA at times will be in the warm sector multiple times beneath southwesterly flow and in proximity to a warm and/or cold front.
This does set the stage for the potential for strong to severe thunderstorms, and severe probability guidance from Colorado State University shows portions of the area with heightened probability through these days. However, severe thunderstorm potential will hinge on the location and timing of the fronts - something that is uncertain at this lead time.

As a result of the cold front passing through the area sometime Wednesday into Thursday, ensemble consensus is that temperatures will cool by the end of the week into next weekend as the trough continues eastward and northwesterly flow aloft sets up over the Midwest. By how much temperatures will cool is uncertain, however, as temperatures range by about 10 degrees between the 25th and 75th percentile. Confidence is high in temperatures being at or below climatology given the 75th percentile hovering around seasonal normals.

Elmore

AVIATION
(For the 06z TAFs through 06z Friday Night)
Issued at 1221 AM CDT Fri May 3 2024

Showers along and ahead of a cold front are gradually winding down across the area, with impacts from rainfall departing the terminals through the early morning hours. The front will slowly sag across the local terminals, leading to a period of variable winds until the front clears the sites with winds becoming northerly to northeasterly in its wake. Additionally, a relatively narrow band of low stratus is expected along the front brining MVFR to IFR ceilings to the area. Given the slow moving nature of the front and it pivoting across the region as it moves through, low stratus may hang around KCOU, KJEF, KSUS, KSTL, and KCPS longer than indicated into the mid to late morning.

Elmore

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

Full Color Weather Map

Link to Loop


Other links: GEOS-West     Contential US     Full GOES-East

St. Louis, MO,