Area Forecast Discussion
Issued by NWS Kansas City/Pleasant Hill, MO
Issued by NWS Kansas City/Pleasant Hill, MO
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204 FXUS63 KEAX 010501 AFDEAX Area Forecast Discussion National Weather Service Kansas City/Pleasant Hill MO 1201 AM CDT Wed May 1 2024 .KEY MESSAGES... - A Few Severe Storms Possible Tonight - More Rain Showers and Thunderstorms Wednesday/Thursday - Flooding Possible With Heavy Rainfall && .DISCUSSION... Issued at 326 PM CDT Tue Apr 30 2024 Compact H5 ridge axis has shifted eastward of the area and has pushed the surface anticyclone into the Ohio River Valley. A strong PV anomaly has resulted in a closed-low system over the Pacific Northwest that has been ejecting multiple vorticity maximums across the Central CONUS. The one of interest over the next 8-10 hours is an H5 short-wave trough axis that has acquired a negative tilt moving across the Northern Plains. Height falls extend southward with enhanced vorticity southward into the High Plains and Central Plains, and has promoted dCVA across the Front Range resulting in surface Lee Cyclogenesis. This has been promoting strong WAA across the area along with stronger theta-e advection into the lower Missouri River Valley. The warm front with this system has already pushed into Iowa this afternoon with cold front currently analyzed through west-central Kansas behind the main surface wave. There is also a dryline across western Kansas but the cold front should overtake this by the time it approaches our forecast area. With deepening surface cyclone, surface winds have been slowly backing over the past few hours across our area. H5 height falls have expanded eastward into eastern Nebraska and eastern Kansas late this afternoon and convergence has been steadily increasing in the area of pressure falls. A few showers have already developed in eastern Nebraska as of 19z. The boundary layer has been mixing quite a bit this afternoon, and the theta-e advection has helped MLCAPE values approach 1500 J/kg, with 12z HREF mean favoring this increasing to between 2000-2500 J/kg by the early evening. As the mid-level streak ejects ahead of the main trough axis with approaching vorticity maximum, deep layer shear will readily increase along the cold front and several kilometers ahead of it into the warm sector, with 0-6km bulk shear values generally around 40 kts. This bulk-shear vector is also oriented perpendicular to the cold front, which will support an initial discrete storm mode, as well as support a supercellular storm mode. Mid-level lapse rates have been steepening most of the afternoon which will support buoyant updraft development in the axis of convergence along the cold front and surface pressure falls with the approaching cyclone. RAP soundings suggest mid-level lapse rates near 8.0 C/km along several points with the approaching cold front. Thermodynamically these updrafts would suggest support for large hail, as there is also CAPE concentrated with the sub-freezing layer that could allow for hail embryo growth. The main question will be how fast storms move off the boundary, and if low-level storm relative flow is too strong, may eject hail stones out of the favorable growth. However with the magnitude of shear and shape of the hodograph the favorable pressure perturbations along the flanks of the updrafts may be enough to sustain large hail. With the larger degree of boundary layer mixing, some RAP soundings are showing increasing DCAPE values and drastically increasing theta-deficits from the surface to the top of the boundary layer that would suggest stronger cold pools and damaging wind potential. However, RAP/HRRR may be over producing the mixing, as stronger theta-e advection has maintained higher dewpoints and thus theta-e deficits are currently weaker than progged by deterministic CAM output. As for the tornado threat, we still need to remove some of the inhibition present from a weak inversion around 850mb. Thinking with the clear skies we have this afternoon we should be able to achieve this, giving us near- surface based parcels as convergence continues to increase. Low- level hodographs demonstrate notable curvature, with 0-500m SRH values near 100 m^2/s^/2, and 0-1km values between 150-200 m^2/s^/2 later this evening as the low-level jet ramps up. For both a storm motion with the mean wind and Bunker`s Right Mover estimation, storms will be able to realize large amounts of streamwise vorticity. Coupled with stronger storm-relative inflow providing strong updrafts, this could support a tornado threat. However, unsure how far east this will last. Storms will likely need to stay closer to the cold front to realize this kind of shear environment, and may quickly move off the boundary prior to entering our area. Therefore, will mainly be concerned with large hail and damaging wind if stronger supercells develop. Experimental WoFS models may be able to provide some guidance on how long supercells can last if they move off the boundary. As the low-level jet continues to strengthen through the evening, discrete storms along the front should congeal and eventually become an MCS or QLCS storm mode. This should eventually generate stronger cold pools capable of wind gusts, and perhaps a localized hail threat. As for rain, most deterministic CAM solutions are generally around 0.25 to 0.50 inches, with mean HREF values around 0.30 inches. The probability matched mean values from the 12z HREF do show a few high pockets of rainfall that exceeds one inch. From coarser resolution ensembles, mean QPF values in our far southwestern CWA is generally around 0.75 inches but a few members to produce more than one inch, and probability for exceeding one inch are around 30 to 40 percent. Therefore, have issued a Flood Watch for Flash Flooding potential in Bates County , MO and Linn County, KS this evening. Other areas could experience flash flooding with localized heavy rainfall, but will be difficult for it to become widespread. Rivers already experiencing high flow may rise with heavier rainfall this evening. Wednesday morning, subtle H5 height rises are progged through the morning which should help to end overnight activity along the front with just enough subsidence. Another short-wave coming out of the Intermountain West will provide another round dCVA and surface cyclogenesis across the Plains, maintaining southerly low-level flow. RAP/HRRR depict this stalling out the surface cold front roughly along the Hwy. 36 corridor, and starts another period of WAA by Wednesday morning across the area. Expecting isentropic upglide that could produce shower activity Wednesday morning into the afternoon. There will still be some instability under steep-midlevel lapse rates that perhaps could produce more convective structure, but there may not be enough large scale ascent support for this. Further, any subsidence with the H5 height rises my suppress this development. Have kept some POPs through Wednesday as current ensemble probabilities for measurable precipitation remain between 20- 30 percent, though probabilities rapidly drop off for a threshold QPF of 0.10 inches. By Wednesday evening, H5 height falls will pick up in pace, and the surface cyclone deepens as stronger dCVA occurs over the Central Plains and the system begins to move eastward. Convergence will increase across the lower Missouri River Valley and another cold front will begin to move across the area. There is some potential for convection to develop, but this will largely depend on boundary layer destabilization through the afternoon. If insentropic ascent keeps skies overcast, there may not be enough instability for convection structure. However, clear skies may allow for steepening boundary layer lapse rates, and improving kinematics with the approaching short-wave trough could produce stronger thunderstorms with some severe potential. Given how conditional Wednesday Night`s setup is on with tonight`s activity, will not dive to deeply into mesoscale details at this time. Right now, it appears the more favorable spot forcing and instability to line up will be west of the CWA. However, continued rainfall though may start to increase Flash Flood Risk, especially into Thursday morning. By Thursday Morning, ensemble probabilities for exceeding 0.50 inches of QPF is around 50 percent for a large portion of the forecast area, with around 20 percent for exceeding 1.0 inches. If a stronger convective storm mode is realized Wednesday Night into Thursday Morning, the higher end of the QPF spectrum could result in further flash flooding issues, as well as other hydrologic issues on area rivers and streams. Rain shower activity continues into Thursday as boundary slows down as well as propagation of H5 short-wave trough axis, with most of the activity exiting the area by late Thursday Night. The main vorticity max driving the Wednesday/Thursday activity should be cleared of the forecast area by early Friday morning. Progressive mid-level flow being rerouted around Canadian closed-low system continues through the weekend and into the next work week bringing additional chances for rain showers and thunderstorms. At this time, there is still a large amount of spread that makes it difficult to pinpoint details at this time, as well as comment on any severe weather potential. However hydrological concerns will remain with heavier rainfall events. && .AVIATION /00Z TAFS THROUGH 00Z THURSDAY/... Issued at 1201 AM CDT WED MAY 01 2024 VFR conds are expected thru 19Z with just sct-bkn high clouds expected thru then. Aft 19Z...bkn cigs around 4kft are expected with the chc for thunderstorms in the VC thru much of the rest of the TAF pd. Right now the best chc for storms at the terminals is btn 01Z-04Z however conf is too low for inclusion at this time. Winds to begin the pd will be lgt and vrb at the TAF sites except MCI which will be out of the north btn 5-10kts. Winds by 11Z-13Z will shift to the ENE around 10kts before becmg easterly aft 19Z. && .EAX WATCHES/WARNINGS/ADVISORIES... MO...None. KS...None. && $$ DISCUSSION...Krull AVIATION...73