851 FXUS63 KTOP 042334 AFDTOP Area Forecast Discussion National Weather Service Topeka KS 634 PM CDT Fri Sep 4 2020 ...Update to aviation forecast discussion... .UPDATE... Issued at 630 PM CDT Fri Sep 4 2020 Added a slight chance of thunderstorms to the forecast for late tonight after 09Z through midday Saturday in conjunction with a wedge of weak theta-e advection working northward on the return southerly flow. RAP/HRRR/NAM forecast profiles all show the base of the EML saturating by 09-12Z along and southeast of the Turnpike with a resulting 1500-2000 J/kg of MUCAPE residing above this layer. The overall kinematic support for convective initation is weak to modest, with an H850 jet of <25kts, but a subtle H500 shortwave noted in water vapor imagery upstream over the Black Hills that is progged to amplify slightly as it drops southeastward and reach the forecast area around 12Z. The low to mid-level flow is weak and any cells that do develop will be poorly organized and likely present a low threat outside of lightning. The threat for convection should shift eastward after midday as the upper tropospheric ridge and H850-700 thermal ridge build from west to east over the forecast area. Also increased cloud cover for late tonight and through the day on Saturday east of Manhattan given the moistening BL and likelihood that convective temps will be reached late Saturday morning. This overall forecast is supported by several recent CAM/CPM runs and leveraged the SREF for PoPs and HREF for sky cover trends for the next 18 to 24 hours. && .SHORT TERM...(This evening through Saturday) Issued at 359 PM CDT Fri Sep 4 2020 We continue to see northerly mid to upper-level flow over the forecast area as the trough in the Great Lakes region slowly moves east and ridging remains in the western third of the CONUS, putting us between the two features. Sfc high pressure has already started to slide eastward and this will be the trend through the evening into Saturday. This will allow for southerly flow in the low levels to advect moisture back into the area, bringing a return of heat and humidity for the holiday weekend. Have increased overnight lows as a result of the moisture advection with temps only falling to the low to mid 60s. Highs tomorrow are expected to reach the low to mid 90s as dew points rise through the 60s and into the low 70s for some parts of the area. This puts heat indices in the upper 90s to around 100. .LONG TERM...(Saturday night through Friday) Issued at 359 PM CDT Fri Sep 4 2020 Saturday night into Sunday, lee troughing deepens and strengthens the pressure gradient across our area. This in addition to downslope flow and strengthening WAA will allow for temperatures to warm further, with Sunday expected to be the hottest day in the forecast period. Highs range from the low 90s in the east to upper 90s in north central KS , but with more mixing farther west, heat indices should still be in the upper 90s across most of the area. The pattern then changes drastically through the rest of the week, as an upper trough swings down from western Canada and ultimately becomes a cutoff low by mid-week. This brings the much-advertised cold front Monday into Tuesday. This morning's GFS/NAM had shown hints of the sfc boundary starting to pass through during the day on Monday, while the 00Z EC lagged behind with fropa occurring Monday night into early Tuesday. This is likely due to the EC keeping the upper trough a bit further north into Tuesday and ultimately tracking the upper low into the Four Corners region, which was much farther west than the GFS/NAM which tracked the low right over KS. Now, the 12Z EC run is more in line with the GFS/NAM both with the track of the upper low and in the timing of the front. This slightly earlier frontal passage results in highs on Monday in the 80s as dew points start to fall back to the 50s. CAA strengthens Monday night and especially into Tuesday, yielding highs in the low to mid 50s in north central KS to the 60s in east central KS. The much cooler air sticks around through the week as the upper low pivots around the state (although the track diverges after Thursday between the GFS/EC). This keeps Wednesday's highs only in the 50s before managing to get back to the 60s Thursday and Friday. Regarding precip associated with the front, the greatest likelihood of showers and thunderstorms is on Tuesday behind the front, possibly lingering into Wednesday. Small shower chances continue into Thursday and Friday with wrap-around moisture from the upper low, although we'll need to continue monitoring its track for run-to- run consistency before we can increase confidence on those. && .AVIATION...(For the 00Z TAFS through 00Z Saturday evening) Issued at 630 PM CDT Fri Sep 4 2020 Predominantly VFR conditions are forecast for the TAF period with low VFR ceilings and an isolated thunderstorm possible along and southeast of an EMP to TOP line late tonight through Saturday morning. Light and variable winds tonight shift to the south- southwest at 5 to 10 kts for the day on Saturday. && .TOP WATCHES/WARNINGS/ADVISORIES... NONE. && $$ UPDATE...Skow SHORT TERM...Picha LONG TERM...Picha AVIATION...Skow