Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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817 FXUS63 KLSX 231712 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 1212 PM CDT Sun Jun 23 2024 .UPDATE... Issued at 546 AM CDT Sun Jun 23 2024 The last of the pre-frontal showers and storms have exited the forecast area to the southeast. We`ve removed mention of these from the forecast. Kimble && .KEY MESSAGES... - A cold front this morning offers some modest heat relief today, but the heat returns in force, with humidity accompanying it, Monday into Tuesday. Heat Index values of 105 are expected at times, especially on Tuesday. - The next cold front Tuesday night provides the next opportunity for rain and thunderstorms as well as another brief break from the heat. But the heat and humidity look to return for the weekend. && .SHORT TERM... (Through Late Monday Afternoon) Issued at 247 AM CDT Sun Jun 23 2024 A cold front is moving through the region this morning, accompanied by some scattered showers and thunderstorms. These thunderstorms will continue to weaken and dissipate through the morning as the better forcing with the trough pushes off to the east and low level convergence along the front wanes. We will see a slightly less hot air mass moving in behind the front today as well as some drier air. This will give us a day`s break from the more extreme heat, with mostly upper 80s to low 90s expected. That`s right around normal for this time of year. The center of surface high pressure passes through the region tonight allowing the air conditioning units to get a bit of a break with lows falling into the 60s outside of the strongest urban influences. But as the high moves east on Monday it will open us back up to the return of the heat, this time accompanied by humidity. Ridging builds back in from the west as low level flow turns southerly. The core of the heat and humidity on Monday is focused to our west across the Plains and into western Missouri. But central Missouri will likely get in on this heat by the afternoon and heat index values will approach or exceed 105 degrees for a time. In coordination with neighboring offices to the west, we have issued a Heat Advisory for the areas most likely to see these heat index values on Monday. Further to the east, it will still be hot on Monday, but more normally so. East of the Mississippi River in Illinois it mainly tops out near 90 with much lower humidity. Kimble && .LONG TERM... (Monday Night through Saturday) Issued at 247 AM CDT Sun Jun 23 2024 After a balmy night Monday night, the heat and humidity spread across the entire forecast area on Tuesday. There`s still some uncertainty on the temperature forecast as NBM continues to run at the top end of the guidance spread and the trend the last several days has been for temperatures to run a little cooler than forecast. That said, it`s still going to be hot, whether we get to 101 or merely 98. And with dewpoints in the 70s, the humidity is going to be palpable. The entire area is likely to see heat index values above 100, with many areas reaching up to 105 or so. It is likely that another more widespread Heat Advisory will be needed for Tuesday. The next cold front will be driven through the area by a shortwave trough pushing east through the Great Lakes along the southern periphery of a larger scale trough in Hudson Bay. In the hot and humid air ahead of the front, thunderstorms are expected to form. This threat is greatest to our north on Tuesday. While there remains a slight chance (20%) of thunderstorms during the day on Tuesday for most of our area, models have trended drier which makes a lot more sense considering the emerging consensus that the front and better forcing don`t move in until Tuesday night. It`s this time frame, Tuesday night into early Wednesday, that we`ll see our best rain chances. While we can`t rule out severe thunderstorms with the very unstable air ahead of the front, the better focus for that threat is more to our north and east closer to the shortwave trough, better forcing, and stronger shear. The main story here is the chance for rain which has been harder to come by locally in recent weeks. Even that isn`t a slam dunk, though, as there are some indications that the shortwave comes through in two parts: the stronger wave to the north and a weaker one to the southwest. That may put us in between the two better-focused areas with locally less widespread thunderstorms Tuesday night. The front does provide at least modest heat relief as cool high pressure builds into the Upper Midwest Wednesday. The core of the push of modified Arctic air remains to the north, though, closer to and north of the surface high. Our temperatures are more likely to end up near normal, upper 80s to low 90s on Wednesday and Thursday. The cooler side of that range is more likely if we hang on to clouds and showers a bit longer on Wednesday. Dewpoints, though, will fall back significantly in the drier air. Even NBM is forecasting some 50s dewpoints which for this time of year is notable. Ridging builds back in from the west rather quickly later this week. As surface high pressure moves east we`ll see southerly return flow and heat back up again for the weekend. By Friday we`ll be solidly in the hot air again, with moisture return bringing dewpoints back to near 70. The ridge isn`t as strong this time, though, so the heat may be a little less intense than our last few rounds. Troughing over the northern Rockies and into the northern plains actually keeps the storm track fairly close to us, with waves on Friday and especially Saturday bringing thunderstorm chances. As the main trough axis passes by around Saturday we see another cold front moving in, knocking back the heat again. NBM interquartile range on max temperatures Friday to Sunday is rather large for this time of year, in the 5 to 10 degree range. This is a reflection of the uncertainty on timing and positioning of these waves and the resulting potential impacts on daytime temperatures. The deterministic NBM, and our going forecast, represents the upper end of the spectrum with highs getting back into the mid to upper 90s. Kimble && .AVIATION... (For the 18z TAFs through 18z Monday Afternoon) Issued at 1210 PM CDT Sun Jun 23 2024 Dry, VFR conditions with a clear sky are expected through Monday morning. Northwest winds will lighten up this afternoon before becoming light/variable this evening as a surface ridge approaches from the west. Winds are then forecast to swing around to the southeast Monday morning as the high departs into the Ohio Valley/Great Lakes. Gosselin && .CLIMATE... Issued at 247 AM CDT Sun Jun 23 2024 With hot temperatures forecast, here are some of the daily high temperature records: MON JUN 24 TUE JUN 25 ST LOUIS 102 in 1988 102 in 1954 COLUMBIA 103 in 1988 102 in 1988 QUINCY 99 in 1988 102 in 1931 && .LSX WATCHES/WARNINGS/ADVISORIES... MO...Heat Advisory from noon to 8 PM CDT Monday for Boone MO-Cole MO- Moniteau MO. IL...None. && $$ WFO LSX