FXUS63 KLSX 080809 AFDLSX Area Forecast Discussion National Weather Service Saint Louis MO 309 AM CDT Sun Sep 8 2024 .KEY MESSAGES... - Another pleasant day with below average temperatures is in store today. A warming trend begins tomorrow, with near 90 degree highs returning by mid-week. - A dry forecast is in store through mid-week. There is a chance (20- 30%) of showers and thunderstorms late this week, but confidence in this occurring is low. && .SHORT TERM... (Through Monday Night) Issued at 307 AM CDT Sun Sep 8 2024 An uncharacteristically dry airmass continues to spin over the region, with surface high pressure centered near the Iowa/Illinois/Missouri triple point. Yesterday, this airmass gave us a perfect day for outdoor activities, with temperate highs and record breaking low dewpoints (31 degrees at KSTL - the earliest sub- freezing dewpoint on record). This morning, the very low dewpoints in the upper 30s to 40 degrees are setting an equally low floor for temperatures. With surface high pressure centered so closely to the area, clear skies, and winds very light to calm, temperatures have been allowed to free fall for most locations. Some observation sites reported temperatures in the 40s as early as 03z. More urban areas like our climate sites, however, have been pretty persistently in the 50s as of 08z. METARs suggest that light winds are keeping the these sites from experiencing the fullest potential of radiational cooling. A great example of this is KJEF, who dropped 8 degrees in the hour that they went fully calm. So although we have sites like Mexico and Cook Station that will definitely see lows in the 30s this morning, our three climate sites may avoid setting record lows due to 5 kt winds and urban heat islands where applicable. See the climate section below for this morning's record lows. We'll lose the influence of cold air advection today, and our temperature forecast will marginally reflect that. Abundant insolation will allow us to warm a degree or two with most locations reaching the mid-70s. Our warming trend will begin on Monday as insolation pairs with southerly low-level flow to bring us back into widespread low-80s for highs. With the dry airmass still in place, humidity will not be a factor today or tomorrow. This dry air will also continue to create a low floor for nighttime temperatures through tonight - the last night of efficient radiational cooling conditions before high pressure shifts east. Lows won't be as cool as this morning, but widespread 40's are still expected. Jaja && .LONG TERM... (Tuesday through Saturday) Issued at 307 AM CDT Sun Sep 8 2024 Southerly low-level flow will be locked into the region by Tuesday as low-level ridging centers over the Mid-Atlantic. Despite the southerly flow, we won't see much in the way of warm air or moisture advection. Our warming trend will continue through mid-week with highs around 90 degrees forecast Wednesday afternoon. This won't be due to an obvious warming factor, only insolation and a lack of any cooling factor to offset it. The temperature forecast becomes less certain by late week. This is when we may have our only shot at measurable rainfall through next weekend. The latest NHC Tropical Weather Outlook highlights an 80% chance of a tropical disturbance in the western Gulf of Mexico becoming a tropical depression in the next seven days. Deterministic guidance pulls this feature north, making landfall somewhere from east TX to New Orleans. From there, it tracks northeast into the Ohio Valley by next weekend. Where exactly it tracks and how well it holds together are the two variables that will decide how warm we are into next weekend and if we receive rainfall or only cloud cover from the system. The latest NBM init is pulling the system close to our area and as such has entered 20 to 30% PoPs into the forecast. Although this is a jump from the non-mentionable PoPs we've been maintaining, this disturbance has only been in the Tropical Weather Outlook for under 48 hours, and deterministic guidance has accounted for it for less time than that. Ensemble guidance is also catching on, with 30 to 40% of members now showing light precipitation late this week at KSAR. This is a 20%+ increase from the last run, and matches decently the NBM init PoPs. With that, I think it's fair to keep the NBM init's chance PoPs until confidence is higher that we won't be side-swiped by this system. We'll receive cloud cover from the disturbance regardless, and that's, to some extent, accounted for in the temperature forecast from Thursday onward. Late week high temperatures are cooler than mid-week's as cloud cover filters in to the area, and low temperatures are warmer for the same reason. The degree to which we cool is uncertain based on the previous paragraph's discussion. This is highlighted well in a very wide NBM interquartile range for Friday (15 degree spread). The difference between the 99th and 1st percentiles is even more broad, ranging from 95 degrees to 66 degrees (29 degree spread). Will we see either extreme occur on Friday? Likely not. However, this does highlight how much uncertainty we're dealing with at this issuance. Jaja && .AVIATION... (For the 06z TAFs through 06z Sunday Night) Issued at 1041 PM CDT Sat Sep 7 2024 Calm winds tonight will not increase much on Sunday, likely staying light and variable through the day under a clear sky. There is at least a little bit better chance of river valley fog Sunday night into Monday morning. Kimble && .CLIMATE... Issued at 309 AM CDT Fri Sep 6 2024 RECORD LOWS ON 9/8 SITE | RECORD KSTL | 42 (1986) KUIN | 40 (1986) KCOU | 43 (1986) && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX