Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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006 FXUS63 KLSX 181732 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 1232 PM CDT Wed Sep 18 2024 .KEY MESSAGES... - Dry and warm conditions continue for at least a couple more days. Highs near 90 are expected. - There will be multiple opportunities for rain through the weekend, although widespread drought relief is unlikely. && .SHORT TERM... (Through Late Thursday Afternoon) Issued at 324 AM CDT Wed Sep 18 2024 A broad trough over the western US and a blocking ridge in the east continues to be the story as it has been for quite some time now. The ridge centered over Texas expands its influence in our direction today and tomorrow, keeping conditions warm and dry. Temperatures nudge slightly upward today and again tomorrow with gradually more 90 degree readings being observed as the ridge asserts more influence. Dry air in the low levels will also promote efficient warming during the day and rapid cooling in the evening. Kimble && .LONG TERM... (Thursday Night through Tuesday) Issued at 324 AM CDT Wed Sep 18 2024 While the ridge persists over the Southern Plains, the trough in the west sees one potent wave lift out of the northern Rockies and into Canada while another digs in across the West Coast. The first wave is moving through Montana this morning, with a narrow corridor of moisture pulled northward through the western High Plains ahead of it. This has led to thunderstorms from the Texas Panhandle northward into the Dakotas early this morning. While the core of the trough continues northeast into Canada, the surface cold front pushes slowly eastward across the Plains with additional rounds of thunderstorms occurring within the band of moist advection ahead of the front. By Thursday this axis of instability will be just to our west from western Missouri north to Minnesota where the Day 2 SPC outlook has highlighted a risk for severe thunderstorms. While we are not expecting enough surface based instability and shear in our area for severe thunderstorms, we will see the low level moist advection move into central and northeast Missouri into western Illinois overnight Thursday night. Elevated instability within this low level jet will produce and maintain thunderstorms late Thursday night into Friday morning, although their intensity should be waning as they reach our area. Rain chances have increased as confidence on the positioning of the low level jet has increased, with now locally up to 60 percent chance of rain Thursday night in the northern portion of our forecast area. Thunderstorms should be relatively progressive, so overall rainfall amounts will not be excessive. The cold front enters our area from the northwest on Friday but will be gradually lose its definition as the trough moves further away and the ridge asserts dominance. The ridge will be strengthening on Friday in response to the digging trough over the Southwest US. Subsidence on the northeast side of the ridge is likely to suppress convection on Friday even with the decaying frontal boundary in our area and a moist, unstable air mass in place. The focus for convection Friday is likely to be to our west where another round of moist advection begins over the High Plains in response to the approach of the Southwest trough. Uncertainty grows as models differ in how to handle the interaction between a northern stream trough and the low over the Southwest US. There seems to be a growing consensus that the two will not phase, but the northern trough will influence the progression of the southern one as it moves out into the Plains this weekend. Another cold front stemming from the northern stream wave will move through the Northern Plains, with moist advection ahead of the southern wave interacting with this front to produce the most widespread shower and thunderstorm activity over the Central Plains late Saturday into Sunday. While the best rain chances will be to our west this weekend, the westerly flow around the top of the ridge will have a tendency to push some of this activity eastward toward our area. So we see rain chances becoming more widespread Saturday into Sunday, with the uncertainty deriving from where the cold front is positioned and thus where the dominant forcing mechanism to trigger convection will be. Eventually the cold front will make its way through our area and clear out the moisture and instability and end the rain chances. However models still vary widely on when that will be. On the early side, some solutions have it pushing through quickly on Sunday leaving Monday dry, while others are considerably slower with it not moving through until Monday. As a result, we still have some rain chances in the forecast on Monday if a slower progression occurs. Temperatures ahead of the front this weekend will be largely in the 80s to near 90, although do depend on clouds and timing of any thunderstorms. There is a solid cool down behind the front, though, with daytime temperatures only in the 70s even with good mixing. The greatest uncertainty is on Monday due to the timing of the cold front, so Monday`s temperature forecast currently represents something of a middle ground between the two extremes. Kimble && .AVIATION... (For the 18z TAFs through 18z Thursday Afternoon) Issued at 1208 PM CDT Wed Sep 18 2024 Confidence is high in dry and VFR flight conditions through the forecast period at all local terminals. Valley fog can`t be completely ruled out tonight, impacting KJEF, KSUS, and/or KCPS. However, the signal for fog right now is weak, so it has been left out of the TAFs until/if confidence increases. Elmore && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX