SHORT TERM
(Through Late Friday Night)
Issued at 247 PM CDT Thu Sep 19 2024

While a more active weather pattern is shaping up for the next several days, quiet conditions have persisted this afternoon as we await this change. As expected, high pressure continues to hold sway over the area this afternoon, with mostly sunny skies (aside from a few afternoon cumulus across the Ozarks) and temperatures already climbing well into the 80s to even near 90 degrees shortly after the noon hour.

However, hints of the change on the horizon can be found just to our west, where widely scattered thunderstorms have percolated near the KS/MO border for much of the day. While this activity is moving very slowly and not likely to reach our forecast area, another round of scattered thunderstorms is expected to develop in the same area later this evening along a weak cold front. This activity is more likely to hold together long enough to impact parts of our area overnight and into Friday morning, likely in its weakening phase.
While instability will be much reduced overnight, enough MUCAPE may persist through the night (roughly 1000 J/kg) to maintain mostly elevated convection and a few weak thunderstorms into the morning. Best chances for beneficial rain with this round will likely be along and north of the I-44 corridor, as showers are likely to weaken as they move farther south into the Ozarks and southwest Illinois.

While it's likely that these showers will largely diminish before noon, the parent cold front will likely sink into the area and stall somewhere in the vicinity of I-44. This boundary is expected to become much more diffuse and likely a poor source for convective initiation, this will keep an unstable airmass draped across parts of southeast Missouri and southwest Illinois through the afternoon.
As such, we can't rule out a few additional showers and thunderstorms tomorrow afternoon and evening in these areas, although chances for rain at any given location remain low (20% or less).

Otherwise, temperatures tomorrow are likely to be similar today, as this cold front is very weak and is not expected to put a significant dent in the very warm airmass currently in place. Some areas that see persistent could cover through the afternoon may get a bit of relief, but otherwise expect another day in the upper 80s to near 90 for most.

BRC

LONG TERM
(Saturday through Next Thursday)
Issued at 247 PM CDT Thu Sep 19 2024

By Saturday, an upper ridge is expected to slowly build back into the central U.S. and lower Missouri Valley, along with deep southwesterly flow. However, while this typically would bring to mind warmer and drier conditions, an unusual combination of ingredients will actually lead to increasing chances for showers and thunderstorms over the weekend and into early next week.

What separates this pattern from a typical southwest flow regime is an unusual abundance of moisture. This is likely a combination of "monsoonal" mid-level moisture originating from the subtropical eastern Pacific, and Gulf of Mexico moisture in the low levels wrapping around the western periphery of surface high pressure and eventually pooling along an advancing cold front. While this is atypical for this flow regime, there is a clear signal for anomalously high precipitable water content arriving in the lower Missouri Valley by early Saturday, and steadily increasing to near the 98th percentile or even higher by Sunday morning (per GEFS/NAEFS/ECM ensemble means). Meanwhile, subtle shortwaves will likely traverse the northern periphery of the ridge over the weekend, followed by a more significant shortwave and cold front sometime between late Sunday and early Monday.

While the timing of these latter forcing mechanisms is somewhat uncertain, their presence along with the anomalously high moisture content is leading to an increasing potential for significant rainfall over the weekend and into Monday. While the potential for scattered showers and thunderstorms will exist Saturday, the stronger forcing and increased moisture content between Saturday night and early Monday makes this the favored time frame for significant rainfall, with at least some potential for heavy rain as well. At the present time the potential for heavy rain is maximized across northwest Missouri where a stalled frontal boundary and the highest moisture content are both likely to reside for a longer period of time. This area is also highlighted with reasonably high values (.7 to .8) in the ECMWF Extreme Forecast Index for precipitation. While this bullseye is just outside of our forecast area, there is also a reasonable potential for locally heavy rain to extend into our area as well, primarily as far south as the I-70 corridor. Again, the best potential for heavy rain is likely to occur sometime between late Saturday night and early Monday, and may occur across multiple waves.

Meanwhile, the potential for strong or severe thunderstorms will also exist during this timeframe, although there remains a great deal of uncertainty regarding this threat. While instability is expected to steadily increase over the weekend along with the increasing moisture content, wind shear is expected to remain relatively weak until the more significant shortwave arrives sometime late in the weekend. The strength and timing of this feature remain somewhat uncertain, but it will likely bring enhanced southwesterly mid and upper level flow with it, contributing to increasing wind shear. This wave will also drive a cold front southeast through the area sometime between late Sunday and early Tuesday, which will also provide a forcing mechanism for convective initiation. Both of these factors will lead to an increased potential for stronger, potentially severe thunderstorms, but there remain many timing and strength details that are unresolved.
LREF cluster analysis reveals that there remains a wide range of reasonable solutions regarding the timing and strength of the arriving shortwave, and these differences have a dramatic impact on the distribution of precipitation.

Once the cold front exits the area to the southeast sometime between Monday and Tuesday, northwesterly flow is likely to develop, and a cooler airmass will settle into the area. Temperatures are likely to eventually settle back to near or even slightly below seasonal averages as a result. Precipitation chances also appear to diminish overall, but there remains a considerable amount of variability among ensemble members regarding precipitation throughout the work- week. Operational NBM precipitation chances have diminished to only around 20-30% throughout the work week, but there remains a relatively high percentage of LREF ensemble members that produce precipitation through the week...albeit with generally low amounts.
As such, it would not be a surprised to see unsettled conditions persisting into the work week, but with a low potential for significant rain amounts.

BRC

AVIATION
(For the 06z TAFs through 06z Friday Night)
Issued at 1045 PM CDT Thu Sep 19 2024

It is becoming more evident that any potential for showers and thunderstorms will originate out of Iowa late tonight into early Friday morning. However, how long thunderstorms survive is in question as support is limited the further southeast of ongoing development. Latest trends look as though any activity that makes it into KUIN will be delayed by an hour or two, which is reflected in this update. Guidance has continued to show a void in activity over central Missouri with the far southwest tail of the convection skimming north of KCOU, but weakening as convection moves southeast into a relatively more stable airmass. VCTS was maintained at KCOU but the window was limited to just a couple of hours. I felt comfortable enough to pull VCTS out of KJEF given the latest trends. Held onto VCTS over the metro terminals with a slight delay in arrival and also reduced the timeframe by an hour as convection looks weak and scattered through early Friday morning.

The most notable chance was the addition of VCTS at metro terminals Friday afternoon/early evening. This may end up being the best chance with better instability available through peak diurnal heating. As stated in earlier forecasts, potential for direct impacts are low, but could result in a brief period of MVFR vsby. Otherwise, VFR conditions are forecast the majority of the period at all sites.

Maples

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