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Area Forecast Discussion
Issued by NWS Kansas City/Pleasant Hill, MO
Issued by NWS Kansas City/Pleasant Hill, MO
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996 FXUS63 KEAX 140900 AFDEAX Area Forecast Discussion National Weather Service Kansas City/Pleasant Hill MO 400 AM CDT Wed Aug 14 2024 .KEY MESSAGES... - Showers/Thunderstorms This Morning - Conditional Chances for Afternoon Redevelopment; More Overnight - Hot and Humid Wednesday Afternoon && .DISCUSSION... Issued at 359 AM CDT Wed Aug 14 2024 H5 ridge axis is shifting into the eastern Plains this morning with multiple short-wave perturbations and vort maxima upstream from the main axis. This has been promoting an extend period of dCVA from the Front Range into the High Plains over the last couple of days, with steady surface pressure falls extending eastward from the area of cyclogenesis in the Front Range. Low-level flow has been south- southwesterly the past few days and has provided several thermal boundaries to push through the area. Current surface analysis indicates a slow moving warm front oriented from near Salina, KS to Fayetteville, AR. This has setup a strong theta-e gradient from east- central Kansas into Central Missouri. Ahead of this warm front isentropic ascent along the 305K level has been promoting shower activity in the southwestern and southern portions of our forecast area. While there is plenty of heat and moisture around, strong WAA around 700mb has kept a stronger cap in place, which has largely limited updrafts from deepening. Meanwhile, a stronger convective cluster has been moving through southeast Nebraska during the overnight hours associated with stronger mid-level vort max and large scale lift under an area of strong upper-level divergence. As we head into the remainder of the morning hours Wednesday, anticipating more showers/storms to fill in along the area of isentropic ascent as well as increasing upper-level kinematic support for lift. The CAMs have not been handling this morning`s convective trends well. Early CAM runs developed very robust storms across our area for this morning, but then more recent runs have kept conditions very dry. It appears that although the cyclone was deepening over the Front Range, surface pressure falls stalled for a bit and thus pulled the thermal boundary further west. In addition, it looks like the CAMs were not depicting the timing of the mid- level short-waves riding through the ridge well. As a result, convection initiated further west than originally expected, and the CAMs also may have been struggling with the CINH present. With current radar trends, have kept rain showers and thunderstorms in the forecast for this morning, as the moisture and forcing is around. However, there is large uncertainty with how intense convection gets this morning, as well as how much QPF it will end up producing. Coarser synoptic scale ensembles still paint probabilities around 30 percent for seeing at least 0.50 inch of QPF with morning activity along the warm front and first passing mid- level vort maxima. The 00z HREF outputs a mean QPF for areas east of Interstate 35 of around 0.75 inches for the first round of rainfall, but keep in mind its inputs include the 00z HRRR run along with other CAMs that filled in much more robust convection compared what we have realized so far. As the low-level jet kicks in, expecting more to develop east of Interstate 35 during the mid and late morning hours, which could still drop around 0.75 inches of QPF, but confidence in this is low-medium. With respect to the severe threat, a higher CAPE environment should start shifting eastward, with current MUCAPE values between 500-750 J/kg. Effective bulk-shear values around 30-45 kts. This may support a few stronger updrafts this morning that could produce small hail and maybe wind gusts up to 60 MPH. However, with the large amount of CINH in place, updrafts may continue to struggle unless it can tap into the large scale ascent from the short-wave moving through. Overall, very uncertain how the remainder of the morning plays out. HREF probability matched means for morning convection QPF max out around 2 inches in some pockets. Not sure if storms will last long enough in any one location to achieve this, but if they do could result in some flooding issues. Thus have kept the Flood Watch in place for areas mainly east of Interstate 35, but will acknowledge that overall QPF trends have been downward the past several hours. As the first short-wave moves through, a stronger short-wave upstream from the ridge axis ejects across the Plains and will start to provide H5 height falls by the afternoon. A brief period of height rises may be possible in response to the initial ejection of the short-wave, and could perhaps clear conditions out from early morning isentropic upglide. With another round of dCVA, surface cyclone over the Front Range should continue to deepen with surface pressure falls continuing from the Central Plains into the lower Missouri River Valley through the afternoon, eventually sending the warm front thermal boundary further to the east-northeast. If we clear, we may see temperatures hit the lower 90s with dewpoints in the mid lower to mid 70s, especially for eastern Kansas and western Missouri. If we realize these higher temperatures, heat index values may reach near 105 during the afternoon hours. Thus have maintained the Heat Advisory for the southwestern third of the forecast area through the early evening hours. However, if we get more showers and cloud cover to backfill, temperatures may be cooler. Amongst NBM members, inner-quartile range for max temperatures this afternoon had larger spread then typically seen for a forecast less than 12 hours out, with temperatures ranging from the 85F to 93F in some locations. This uncertainty is largely attributed to changes in some members that have kept robust convection and lingering cloud cover, and other members that were completely dry and clear. Attention then turns into the late afternoon and evening hours as the main mid- level vort max moves into the lower Missouri River Valley and the associated surface cyclone begins to propagate eastward across the Plains. This sends the warm front eastward out of the forecast area, with a cold front coming in from the Central Plains. Both large scale ascent from upper-level kinematics and surface convergence along the cold front should start to initiate storms late into the afternoon and through the evening. The timing of the mid-level forcing is about 4-6 hours later than was originally depicted in guidance from yesterday`s model runs. The CAMs has been focusing development in our area starting between 02-03z now. However, will note that with the struggles with this morning activity, not sure how reliable the CAM output really is for late afternoon and evening development. The warm sector ahead of the deepening cyclone appears to be capped with stronger WAA between 850mb-700mb, which would require longer daytime boundary layer heating and destabilization to realize surface based parcels. However, if we see more convection this morning that places an outflow boundary across the area, we may see better forcing near the surface that could work through this cap. With mid-level height falls and subtle cooling between 700- 500mb, could realize better SBCAPE that could get a few updrafts going. Increasing mid-level flow should help to yield 0-6km bulk shear values between 30-40 kts, which could lead to the organization of storms, and present a hail and wind threat. But, if the boundary layer needs more time to mix and destabilize, we may have to wait until later in the evening with the approaching cold front for enough forcing. Once we get past 00z though, mid-level lapse rates are progged to steepen with the mid-level short-wave trough approaching, and surface pressure falls and backing surface winds should increase overall shear values, as well as low-level storm- relative helicity values to close to the front. Not exactly sure how far into the warm sector this will extend at this point, but initial convection along the front could be discrete and take on a supercellular mode, capable of hail at least quarter size and wind gusts in excess of 60 MPH. Initial storms could present a brief tornado threat, but it appears storm motions may be more parallel to the boundary as low-level jet kicks in, and storms congeal into a line. From there, main threats then become hail and gusty winds. Important mesoscale features to watch this afternoon will be CINH and CAPE environment during the afternoon hours IF there is an outflow boundary hanging around. Then, as the cold front approaches, watching how the shear evolves and storm motions relative the boundary will determine convective mode. As for QPF, we are still dealing with an environment with PWATs of 2.00 to 2.25 inches. HREF mean QPF paints 1.25 to 1.75 inches for mainly our north-central area, with 0.25 to 0.75 for central and southward. HREF probability matched mean (PMM) values max out between 2.5 to 3.0 inches. If the thermal boundary stalls early Thursday morning, areas east of Interstate 35 could potentially see in excess of 2.0 inches of rainfall, that could result in flooding issues if that same area sees heavier rainfall from the Wednesday morning activity. Coarser scale ensemble seems to favor heavier rainfall into Thursday morning, likely with a more pronounced short-wave trough and surface cyclone moving through. Again though, the CAMs struggle this morning make the CAM output QPF hard to trust, along with what the 00z HREF is has produced. If the cold front remains progressive and limits training, this will help mitigate most of the hydro related concerns. Heading into the weekend, medium-range deterministic guidance depicts a deep closed system dropping south form Hudson Bay will an amplified mid-level ridge develops over the Rockies. Several vort maxima may work through the flow that will bring additional rain shower and thunderstorm chances. Differences in timing of the short- waves though have resulted in large spread amongst ensemble suites, which right depict low-end probabilities throughout much of the weekend. For right now will keep the low-end NBM POPs toward the end of the week, and will refine once this Wednesday-Thursday system clears the area. && .AVIATION /00Z TAFS THROUGH 00Z THURSDAY/... Issued at 1154 PM CDT Tue Aug 13 2024 A line of storms is likely to develop over eastern KS and western MO in the next few hours and shift east with time in the pre-dawn hours. This area of storms will likely affect all TAF sites from 8-9Z through 11-12Z. Outside of the storms, IFR to low MVFR conditions are expected through the overnight hours. As a warmer and more humid air mass builds into the area, storms shift east and ceilings begin to raise so that by the afternoon hours, VFR conditions become likely. Easterly winds initially will become southeasterly this afternoon and southerly this evening. && .EAX WATCHES/WARNINGS/ADVISORIES... MO...Heat Advisory from 1 PM this afternoon to 7 PM CDT this evening for MOZ028-037-043-044-053-054. Flood Watch until 3 PM CDT this afternoon for MOZ003>007- 013>017-022>025-030>033-038>040-045-046. KS...Heat Advisory from 1 PM this afternoon to 7 PM CDT this evening for KSZ025-057-060-103>105. && $$ DISCUSSION...Krull AVIATION...CDB