002 FXUS63 KLSX 210235 AFDLSX Area Forecast Discussion...Updated National Weather Service Saint Louis MO 935 PM CDT Thu Sep 20 2018 .UPDATE... Issued at 935 PM CDT Thu Sep 20 2018 PoPs were boosted into likely category for most areas later tonight and into Friday morning. CAMs and now the 00z NAM have bought on to a re-invigoration of convection between 07-09z for areas N and W of STL metro thanks to a low level jet, with this rain then pushing through during Friday morning with cold front, all the while the original area of convection now entering far northwest MO loses its punch. This should give many areas a period of showers with spotty thunderstorms overnight into Friday morning. The forecast is otherwise on track with cold front to attempt to strengthen its thunderstorm coverage heading into Friday afternoon with increased heating/de-stabilization ahead of it, but the front should be southeast of STL metro by this time and will soon be exiting the forecast area. Summer is over! The cold front Friday morning will bring to a close this latest round of summer heat and--I can dare say it now--bringing to a close summer itself, with the autumnal equinox set for 8:54 pm CDT Friday evening. Temperatures look to be seasonable to below average heading into next week. TES && .SHORT TERM... (Through Late Friday Afternoon) Issued at 320 PM CDT Thu Sep 20 2018 Isolated to scattered SHRA/TSRA have developed this afternoon, although the coverage thus far has been slightly lower than was previously expected. SHRA/TSRA are expected to continue percolating through the early evening hours and then dissipate after sunset because diurnal heating is the primary driver for these storms. A lull of several hours is then expected before an approaching cold front brings another area of SHRA/TSRA into the area late tonight, probably not before midnight in the northwestern CWA. Precipitation will then spread southeastward overnight and tomorrow as the front moves through the region. The front will have moved through most of the CWA by late Fri afternoon before it stalls across southeast MO. Models are forecasting approximately 25-35 kts of 0-6km shear along with sufficient instability to support thunderstorms. A few strong to severe storms will be possible across parts of southwestern IL on Fri afternoon. Lows tonight should be similar to or a few degrees cooler than last night, especially in the northwestern CWA where the aforementioned cold front will have made some progress overnight. Highs on Friday will be 10-20 degrees cooler than today due to a combination of clouds, precipitation, and a cooler post-frontal air mass. The coolest high temperatures on Fri will be in the northwestern CWA which will already be within the cooler air mass before day break. Kanofsky .LONG TERM... (Friday Night through Next Thursday) Issued at 320 PM CDT Thu Sep 20 2018 Precipitation chances will linger across the southern CWA on Fri night and Sat due to the stalled frontal boundary in southeast MO. Elsewhere, no precipitation is expected. Quasizonal to weakly ridged flow aloft will prevent the cold front from making much northward progress until early next week when a new surface low develops over the plains in response to an approaching upper PV anomaly. This low pressure system will help lift the stalled boundary northward as a warm front early next week, and the system's trailing cold front is forecast to move through the LSX CWA on Tue. Periods of SHRA/TSRA are expected from Monday through Tuesday night until the cold front has moved through the area. A brief warm-up is expected early next week before another cold front moves through the area and produces cooler highs/lows for Wed and Thu. The GFS, ECMWF, and GEM offer different solutions regarding the evolution of the upper trough and the placement of important surface features. This leads to higher uncertainty regarding temperature and precipitation trends during the middle and late part of next week. Kanofsky && .AVIATION... (For the 00z TAFs through 00z Friday Evening) Issued at 700 PM CDT Thu Sep 20 2018 Strong cold front moving through our region late tonight and Friday morning will be the main driver of rain chances with all TAF sites expected to get a brief period of an hour or two of rain as the front moves through with the threat of additional rain lingering for several hours after. The better chances of this rain being thunderstorms will be where it affects earlier, such as KUIN and KCOU, with thunder chances expected to diminish by the time it reaches STL metro sites to go with just showers. By the time the front attempts to re-invigorate with thunderstorms Friday afternoon, it should be through STL metro sites. Models depict a 35-40kt low level jet moving out ahead of the front later tonight, but given the relatively strong surface gradient winds, the LLWS setup looks marginal and have declined to include in the TAFs. While VFR conditions are expected to prevail at all TAF sites thru the valid period, a brief instance of MVFR is possible either with rain or immediately behind cold frontal passage as lower cloud bases. For now, kept it all VFR until confidence can increase on these items. Otherwise, look for S-SW surface winds to veer NW with cold frontal passage late tonight or Friday morning, continuing to veer more N by Friday night. SPECIFICS FOR KSTL: Strong cold front moving through on Friday morning will be the main driver of rain chances with a brief period of an hour or two of rain expected late tonight ahead of the front with the threat of additional rain lingering for several hours after until just after cold frontal passage. Thunder chances look low by the time rain does move in, and have kept it as showers. By the time the front attempts to re-invigorate with thunderstorms Friday afternoon, it should be sufficiently southeast of the terminal. Models depict a 35-40kt low level jet moving out ahead of the front later tonight, but given the relatively strong surface gradient winds, the LLWS setup looks marginal and have declined to include in the TAF. While VFR conditions are expected to prevail thru the valid period, a brief instance of MVFR is possible either with rain or immediately behind cold frontal passage as lower cloud bases. For now, kept it all VFR until confidence can increase on these items. Otherwise, look for S-SW surface winds to veer NW with cold frontal passage Friday morning, continuing to veer more N by Friday night. TES && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX