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572 FXUS63 KLSX 010854 AFDLSX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE ST LOUIS MO 354 AM CDT Mon Sep 1 2014 .SHORT TERM: (Today through Tonight) Issued at 344 AM CDT Mon Sep 1 2014 There are three interconnected forecast issues covering today through tomorrow: convective timing, the potential for severe weather, and the potential for heavy rainfall. The main factors involved with the forecast are the ongoing early morning convection with its outflow boundaries, an upstream synoptic cold front, and a couple of shortwaves moving through a broad longwave trough. If the early morning convection dissipates across our area, then the air mass is likely to destabilize ahead of the approaching synoptic cold front. Models are forecasting up to 2000-3000 J/kg of CAPE and between 30-60 kts of 0-6 km bulk shear across most of the CWA during the afternoon/evening hours ahead of the front. These parameters are more than sufficient to support organized severe thunderstorms. With at least a few models depicting a small but nonzero perpendicular component to the shear vectors with respect to the H85 boundary orientation, it would not be surprising to see a few supercells somewhere across the region before convection starts to congeal and become more linear with time. If the models are correct with the orientation of the shear vectors with respect to the cold front, then aftn/eve convection should become linear very quickly. As of 0830z, early morning convection appears to be diminishing over northern/northeastern MO, lending credence to the above scenario. However, if the early morning convection does not dissipate and if the outflow boundary is pushed farther to the south, then the overall coverage of severe thunderstorms would probably be much lower and would tend to be limited to the southern CWA. Moving into tonight, the cold front becomes oriented nearly parallel to the 0-6 km shear vectors, and the primary forecast issue then shifts from severe weather to heavy rainfall. Overall, this looks like a decent setup for locally heavy rainfall. PW values increase to over 2" tonight, which is not only at +2SD and the 99th percentile for August, but also exceeds both of those thresholds for September. Models also depict broad lift from favorable jet coupling (between the RER of a jet at H25 and the LER of a jet at H85) which develops by 06z in response to a shortwave moving through NEB/IA. In addition, MUCAPE remains quite high overnight and the mid-level flow also appears to be nearly parallel to the cold front. Because of these factors, I`m reasonably sure that there is going to be an elongated band of convection overnight, but I`m not sure whether it will be with the true synoptic cold front (resulting in a band of rainfall near the I-70 corridor) or with the outflow/effective boundary (resulting in a band of rainfall located much farther south, possibly only affecting the southern CWA). Flash flood guidance numbers are around 2-3" for 1 hour, 2.5-3.5" for 3 hours, and 3-5" for 6 hours. There hasn`t been much rainfall over the last few days, therefore the ground is not saturated and can probably absorb a fair amount of water before widespread flooding becomes a concern. There is also a question about where the axis of heaviest rainfall will occur. For these reasons, we will hold off on any flood/flash flood watches for now. Kanofsky .LONG TERM: (Tuesday through Sunday) Issued at 344 AM CDT Mon Sep 1 2014 The front and/or effective boundary should linger across the region on Tue/Tue night before lifting northeastward as a warm front on Wed/Wed night. The precipitation threat will also lift northeastward with the warm front during the middle of the week. Model solutions then depict a cold front moving into the area towards the end of the week and into next weekend, bringing a chance of rain and a cooler air mass into the region. Kanofsky && .AVIATION: (For the 06z TAFs through 06z Monday Night) Issued at 1103 PM CDT Sun Aug 31 2014 Convection extending from IA southwest through northwestern MO and KS just ahead of a cold front will drop southeastward into the UIN and COU areas late tonight, and then eventually into the St Louis metro area towards morning as it weakens. New convective development is expected late Monday afternoon and early evening mainly in COU and the St Louis metro area between the approaching cold front and an outflow boundary left from the late night/morning storm complex. S-sely surface winds late tonight will gradually veer around to a swly direction Monday morning. Specifics for KSTL: Weakening showers and storms are expected to drop southeastward into STL area by early Monday morning. There will be a break in the convection with redevelopment of storms expected late Monday afternoon and evening. S-sely surface winds late tonight wlll increase to around 14 kts by late Monday morning from a swly direction. GKS && .LSX WATCHES/WARNINGS/ADVISORIES: MO...NONE. IL...NONE. && $$ WFO LSX