Area Forecast Discussion
Issued by NWS Kansas City/Pleasant Hill, MO
Issued by NWS Kansas City/Pleasant Hill, MO
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302 FXUS63 KEAX 210509 AFDEAX Area Forecast Discussion National Weather Service Kansas City/Pleasant Hill MO 1209 AM CDT Sat Sep 21 2024 .KEY MESSAGES... - Warm and Humid - Storms Overnight into Saturday Morning - Stronger Storms Possible Sunday - Cooler Temperatures Next Week && .DISCUSSION... Issued at 250 PM CDT Fri Sep 20 2024 There are three primary H5 features that will contribute to active weather through the remainder of this weekend. A 592dam high is centered over Texas, that has been amplified toward the lower Mississippi River Valley due to a deeper trough starting to come ashore the southwestern CONUS. Meanwhile, another trough is moving across the Canadian Prairie Provinces. The surface cyclone with the Canadian system is starting to move toward Hudson Bay, with the cold front stretching across much of the CONUS, and is currently moving across Central Missouri as of the late morning and afternoon hours. A brief period of H5 height rises has provided weak subsidence on the backside of this cold front, which has been keeping conditions dry through much of this afternoon across the forecast area. A weak vort max is expected to eject ahead the deeper mid-level trough over the southwestern CONUS later this evening, providing modest H5 height falls across our area and may interact with the cold front that will be parked somewhere in the southeastern third of our forecast area. The surface cyclone associated with this wave likely stays over the High Plains, but surface southerly flow south of the cold front should increase instability overnight for the southeastern third of the forecast area, and we may see some elevated instability develop further northwest. Current CAMs depict thunderstorm development after 04z this evening and entering our western counties around the 06z-07z timeframe. MUCAPE values off the 12z HREF are progged around 1500-2000 J/kg, with 0-6km bulk shear values in the ball park of 25-30 kts. This may be enough to support a few isolated severe storms producing wind gusts around 60 MPH and hail around quarter size early Saturday morning. Steeper lapse-rates between 700mb-500mb may foster a few stronger updrafts. However, current HRRR and RAP soundings are very dry in middle portions of the troposphere, which may be detrimental to deep convection initiation as the forcing is moving through. This may even be a potential source of later initiation time then what current CAMs are depicting in simulated reflectivity fields. This first wave pushes east by late morning. For the remainder of Saturday afternoon, another short-wave trough and vort max eject out of the southwestern CONUS and continue to develop a broad surface cyclone across the Plains. This enhances low-level southerly flow as far east as the Mississippi River Valley, and will start to push the cold front back northward as a warm front. Along the warm front, expecting isentropic ascent to promote additional shower development. If there is any lingering elevated instability, a few thunderstorms may become possible, though current CAM guidance suggest this travels further north, impacting only far northwestern portions of the forecast area. Regardless of storm mode, heavier rain showers are expected most of the day as ensemble probabilities for measurable rainfall are above 80 percent for most of the forecast area, and for a threshold of .50 inches of QPF is nearly 50 to 60 percent. Temperatures Saturday afternoon are expected to be generally in the 80s, with cloud cover keeping conditions slightly cooler, but WAA will prevent a drastic cool down through most of the day. Sunday, H5 trough lifts out of the southwestern CONUS into the High Plains. Stronger dCVA occurs in the southern Plains as this moves eastward, but mid-level height falls expand further eastward and inverted surface troughing should extend into eastern Kansas through Central Missouri. Stronger AVA over the Northern Rockies develops a surface anticyclone that will push southward out of the Northern Plains, and should set the stage for a stronger cold front to develop across the Missouri River Valley heading into Sunday afternoon. Amongst deterministic guidance, there is still discrepancy in how far north the primary H5 kinematic forcing travels, and this also impacts how far northward favorable surface pressure falls occur with the surface cyclone, and how quickly the front moves through the area. Overall ensemble probabilities for rainfall remain high, with over 80 percent probabilities for any measurable rainfall, and continued 50-60 percent probabilities for at least 0.50 inches of rainfall. Between Saturday and Sunday activity, some areas may see as much as 2-3 inches of rainfall by the end of this active pattern, which will be welcomed due to current drought conditions across the region. The main uncertainty is with how strong storms will be, and what the severe threat could look like. Instability should not be an issue to come by, with high probabilities for exceeding 1000 J/kg of MUCAPE. However, if the cold front and forcing sink further south, stronger updrafts could initiate closer to the Interstate 44 corridor Sunday afternoon into evening. If the system takes track that is further north, areas up to Interstate 70 or even Hwy. 36 could experience stronger deep layer shear as well as better surface convergence that could generate a few storms. Hail and wind would be the primary threat. If the surface cyclone ends up in a spot where surface winds back considerably, could see a concentrated corridor of enhanced SRH that presents a brief tornado threat if storms were to be discrete. However, uncertainty remains too high to confidently pinpoint any of these mesoscale details at this time. In addition, instability recovery from Saturday activity and cloud cover could also play a large role in boundary strength and differential heating. Again, even if severe storms fail to go, there should be enough forcing and moisture around to still provide rain showers across much of the area. With respect to any flooding concerns, most areas will be able to handle 2-3 inches of rainfall, especially if it is spaced out over 36 hours. Will only need to monitor a few urban areas for fast responding creeks, but even these will be able to handle a decent amount of rainfall given the dry conditions over the past few weeks across the region. Heading into next week, our area will be under prominent troughing, with potential for a closed-low to remain directly overhead. This will keep temperatures substantially cooler. Most points in the forecast area next week have an inner-quartile temperature range between the upper 60s and mid 70s. While there will be some lift in the presence of the troughing overhead, the front from the weekend likely pushes most of the moisture out of the area. Ensemble probabilities for measurable rainfall through most of next week remains lower. If we see long periods of cloud cover, we may see multiple periods of light sprinkles move through. Overall probabilities for any measurable rainfall after Monday are between 10 and 15 percent. && .AVIATION /06Z TAFS THROUGH 06Z SUNDAY/... Issued at 1159 PM CDT Fri Sep 20 2024 VFR conditions will likely prevail through the rest of the night with some potential for scattered storms due to the nocturnal jet early this morning. Expect storms to dissipate by mid- morning with the loss of night time jet. More storms are possible late Saturday afternoon with storms and showers lasting through Saturday night. Otherwise, expect the wind to prevail from the south to southeast between 5 and 10 knots. && .EAX WATCHES/WARNINGS/ADVISORIES... MO...None. KS...None. && $$ DISCUSSION...Krull AVIATION...Cutter