761 FXUS63 KTOP 160502 AFDTOP Area Forecast Discussion National Weather Service Topeka KS 1202 AM CDT Thu Aug 16 2018 ...Update to aviation forecast discussion... .SHORT TERM...(This evening through Thursday) Issued at 354 PM CDT Wed Aug 15 2018 Visible satellite loops and surface analysis reveal a decoupled, warm-core-structure cyclone over the forecast area, with the center of the corresponding surface low located between Lawrence and Kansas City. Given the equivalent-barotropic, warm-core nature of this system, and corresponding lack of any low-level dry-air intrusions, ample boundary-layer moisture exists within all quadrants of the cyclone. However, the richest moisture characterized by lower 70s surface dewpoints resides near and east of a diffuse baroclinic trough trailing southwest of the surface low into southeast KS. Pockets of insolation have contributed to minor destabilization ahead of this boundary, and a gradual uptick in generally shallow convection has been noted from Jefferson County to Anderson County. Poor midlevel lapse rates contributing to muted upward convective- scale accelerations are mitigating charge separation and lightning potential. These showers should move east of the forecast area in tandem with the slow motion of the boundary and surface low, with the CWA expected to experience rain-free conditions by early evening. By early in the overnight hours, fog potential will increase across the forecast area. Previous cloud coverage lingered during the diurnal heating cycle today, muting vertical mixing. This has supported maintenance of ample boundary-layer moisture, which should continue into tonight, given (1) an only very modest influx of dry air in the surface layer behind the aforementioned boundary, (2) reinforcing moisture fluxes atop the wet ground owing to recent precipitation. Gradually clearing skies tonight, combined with light winds in proximity to the post-boundary surface ridge, should support optimal conditions for radiational cooling along with surface-layer saturation and fog potential. Patchy to areas of fog have been included in the forecast, and confidence is relatively elevated that fog formation will occur. However, cloud coverage peripheral to east/southeastward-spreading regenerative convection across parts of SD and NE could eventually spread across the local area later tonight, interrupting diabatic cooling and fog potential. While some potential for widespread and/or dense fog cannot be ruled out, the eventual influx of convective debris casts considerable doubt on such potential at this time. Otherwise, low temperatures tonight are expected to be in the middle 60s. By late tonight into Thursday morning, a compact midlevel vorticity maximum -- presently located over western SD and embedded in westerly/west-northwesterly flow aloft -- will be approaching the forecast area. Its attendant midlevel speed maximum and preceding zone of midlevel height falls/differential cyclonic vorticity advection will begin overspreading north-central and northeast KS late tonight and continue to spread across the forecast area through much of the day Thursday. The low-level mass response to this perturbation will be limited, in association with the short wavelength between this feature and the leading warm-core system described above (effectively minimizing the low-level pressure gradient magnitude). Nevertheless, seasonably strong midlevel flow -- e.g. 35-40 kt at 500 mb -- combined with a veering wind profile within the lower troposphere will establish a favorable shear profile for organized convective structures. Uncertainty regarding the specific evolution of overnight convection extends to subsequent rounds of convection through the diurnal heating cycle on Thursday. However, the interaction of the vorticity- maximum- preceding lobe of ascent and a broad area of moisture suggests that this pattern will be favorable for multiple areas of convective activity anytime from Thursday morning into late Thursday afternoon. The lack of an appreciable low-level mass response may tend to temper boundary- layer convergence, favoring somewhat separated updrafts or perhaps eventually small line segments resulting from cold pools congealing. The eastward advection of an elevated mixed layer (EML) associated with near-7-C/km 700-500-mb lapse rates -- facilitated by mass fluxes accompanying the midlevel speed maximum -- should support notably stronger convective-scale accelerations compared to those today. However, the short-duration return period of EML influx, coupled with an antecedent lack of particularly strong heating upstream across the Western Plateau, are factors that should limit the overall intensity of convection across the region. Nevertheless, hydrodynamic perturbation pressure gradient forces associated with supercell structures/midlevel-updraft rotation will have the potential to enhance updraft intensity, with the most intense storms capable of producing quarter-size to ping-pong-ball-size hail. In circumstances where convection congeals owing to cold-pool amalgamation, locally damaging winds may occur especially in areas where low-level lapse rates most substantially steepen. However, the lack of steeper midlevel lapse rates and resultant limited DCAPE/downdraft potential, and limited ground-relative winds in the low levels, should preclude the occurrence of a more widespread severe-wind risk from occurring. The weak low-level mass response and associated minimal SRH render very low predictability regarding any tornado potential. Regardless, sufficient low-level buoyancy driven by the moist boundary layer -- e.g., surface dewpoints in the upper 60s to around 70F -- may support sufficient stretching of low- level vorticity for incipient updrafts in proximity to outflow boundaries to yield a non-zero, mesoscale-mass-field-driven, small- predictability tornado risk. The overall tornado potential is quite low. .LONG TERM...(Thursday night through Wednesday) Issued at 354 PM CDT Wed Aug 15 2018 During the evening hours of Thursday, a weak cold front associated with the compact midlevel vorticity maximum will move south across the region, followed an influx of drier air from the north. For Friday into Saturday, a post-frontal surface ridge will move across the region, with near-normal temperatures. The return of richer moisture will commence during the latter half of the weekend, as a midlevel shortwave trough amplifies along its approach to the central Great Plains. Medium-range model guidance suggests cyclogenesis occurring over the High Plains as the midlevel wave emerges over the central states. Ample warm/moist advection should spread across the central states ahead of this system, with accompanying precipitation likely spreading across the area -- especially on Sunday. A comparatively stronger low-level mass response associated with this disturbance could support a somewhat more favorable kinematic environment for organized convection, though the potential for widespread precipitation/cloud coverage could limit instability. Heavy rainfall may also occur, though the relative fast-moving character of this system may limit flooding potential. As such, weather hazards for Sunday have not been included in NWS Topeka forecasts/messaging at this time. For early/middle parts of next week, drier conditions behind the surface cyclone should affect the region, with the potential for somewhat cooler temperatures. && .AVIATION...(For the 06Z TAFS through 06Z Thursday night) Issued at 1150 PM CDT Wed Aug 15 2018 For the 06Z TAFs, have reduced the time frame for expected fog formation to around the 12Z window. Higher level cloud cover should continue to work into the area, so perhaps shallow fog may still develop. But, do not expect widespread dense fog formation at this time. Have introduced VCTS for likely time frame for thunderstorm development in the afternoon provided clearing can take place. That may be complicated for the afternoon time frame by morning showers or storms that work into the area from the north, but this is low confidence at the current time. && .TOP WATCHES/WARNINGS/ADVISORIES... NONE. && $$ SHORT TERM...Cohen/Bunker LONG TERM...Cohen/Bunker AVIATION...Drake