Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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944 FXUS63 KLSX 151057 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 557 AM CDT Sun Sep 15 2024 .KEY MESSAGES... - Today will be another warm day with scattered afternoon showers and thunderstorms. - Warm and dry conditions are expected through the upcoming week. && .SHORT TERM... (Through Late Monday Afternoon) Issued at 322 AM CDT Sun Sep 15 2024 A large ridge of high pressure is centered over the Great Lakes today while an elongated trough related to the remnants of Francine lingers across the southern US extending into southern Missouri. While the trough continues to weaken and provide only localized support for ongoing showers across the Midsouth, its broader axis extending up into Missouri does represent an area of cooler temperatures aloft over the top of a continued warm and moist surface air mass. So while we don`t have a persistent area of thick cloud cover or showers to contend with this morning, as daytime heating begins and we warm well into the 80s, we`ll see cumulus bubble up and eventually sprout into showers and thunderstorms this afternoon in the absence of any appreciable capping. This is similar to what occurred yesterday when temperatures warmed a bit more than expected and air mass showers and a few thunderstorms developed for a brief few hours during the mid to late afternoon. These pop up showers and storms may occur anywhere across the forecast area this afternoon, but we believe the favored areas will be west of the Mississippi River, especially toward central and southwest Missouri where instability is likely to be greater. Some low level dry advection on the easterly flow will limit instability further east at least somewhat. Models vary substantially on just how much instability develops this afternoon, but the consensus is for surface based CAPE to exceed 1000, perhaps even 2000. Winds aloft are weak and shear is almost nonexistent, so thunderstorms that form today will be of the pulse variety, going up and down fairly quickly, possibly organizing around a common cold pool. If we do manage to realize some of the higher end instability, it wouldn`t be out of the question that a strong to severe storm could occur with downburst winds and hail the threats. But due to the lack of shear to organize the thunderstorms we do not anticipate messaging a severe weather threat today. Storm motion will be from east to west, the opposite of what we typically see, a factor of the easterly flow through the low and mid levels. As for Monday, afternoon shower and thunderstorm chances are even lower and shifted further west as the low level dry advection on easterly flow continues. Dewpoints drop into the 50s along and east of the Mississippi River, but there remains just enough low level moisture to keep a low (20 percent) chance for afternoon showers and thunderstorms mainly in central Missouri. Temperatures continue to run 5 to 10 degrees above normal for mid September, in the mid to upper 80s. Kimble && .LONG TERM... (Monday Night through Saturday) Issued at 322 AM CDT Sun Sep 15 2024 Our weather this week will be dominated by the broader scale ridge across the eastern US leading to dry and warm conditions through the week. Highs stay within a relatively small range from 85 to 90 each day. The trough over the South eventually reorganizes around a low developing off the Southeast coast which then gets pulled back to the northwest through the Mid Atlantic into the Appalachians where it stalls and weakens. While there is some variability on how far west the remnant low wobbles through midweek, it stays far enough east of us to avoid any rain from it. Looking to the west, persistent troughing over the western US maintains southwest flow over the central and northern Plains with multiple embedded troughs ejecting northeastward through the week and into the weekend. Over time there is a gradual drift eastward of this trough with the southwesterly mid level flow getting close to our area by week`s end. It`s this southwest flow with embedded troughs and moisture pumping northward that represents our only real chance of rain in the next week or so. Uncertainty increases in the overall flow pattern as we get toward this weekend and into next week. That`s understandable considering the retrograding low over the Appalachians, its effect on the ridge to the north and west of it, and the cumulative effect of many troughs emanating out of the western US through the week. As such, low rain chances (20 to 30 percent) do begin to sneak into the western part of the forecast area Friday into Saturday. Although there is more uncertainty in the broader flow regime, there is general consensus that ridging in some form continues across our area which would lead us to favor the persistent dry and warm forecast. Even among the guidance sources that do depict some rain scraping our area, it doesn`t come in a particularly widespread or heavy form. Overall ensemble probability of 0.25 inch of rain in any 24 hour period never gets much above 20 percent anywhere in our forecast area right through the weekend. Because of this we do not expect any improvement in the developing drought conditions across the region. Kimble && .AVIATION... (For the 12z TAFs through 12z Monday Morning) Issued at 552 AM CDT Sun Sep 15 2024 VFR conditions prevail this morning, outside of some patchy river valley fog which will dissipate shortly after sunrise. Extensive high clouds exist this morning, but we expect cumulus development this afternoon with showers and thunderstorms popping up from mid to late afternoon before dissipating near sunset. These thunderstorms are most likely in central Missouri although they could affect any TAF location. Valley fog is possible again tonight depending on how extensive cloud cover is overnight. Kimble && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX