Area Forecast Discussion
Issued by NWS Des Moines, IA
Issued by NWS Des Moines, IA
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249 FXUS63 KDMX 271904 AFDDMX Area Forecast Discussion National Weather Service Des Moines IA 204 PM CDT Sat Apr 27 2024 ...Reissued to include Hydrology Discussion... .KEY MESSAGES... * Threat for strong to severe storms today, primarily south of I-80 this afternoon and evening. Primary threats for large/damaging hail and damaging straight line winds. A tornado or two cannot be ruled out. * Lingering hydrologic threat with potential for multiple rounds of storms over same areas of southern Iowa possibly yielding 3"+ of rainfall. Flood Watch has been issued. * Another round of strong to severe storms possible Sunday. Primarily over southern/southeast Iowa, with hail and winds leading threats. * Multiple opportunities for showers/storms next week as a handful of shortwaves progress through the large scale flow. && .DISCUSSION... Issued at 145 PM CDT Sat Apr 27 2024 A welcome quiet start to the day after the events of yesterday afternoon and evening. Multiple survey teams from the office are out in the field assessing damage and tornado paths. For today, another round of strong to severe storms remains expected over roughly the southern half of the area, namely in/around a surface warm front draped across the state. Compared to yesterday, trends have pushed the surface front, and by virtue the "juiciest" air, southward a bit. Aside from that, much of the advertised environment remains with moderate lapse rates and supportive deep layer shear for organization. SBCAPE values remain forecast into the 2000- 3000+ J/kg range with dew points into the mid 60s near/south of the front. Low level wind fields certainly are relaxed comparatively to yesterday, though hodographs still have suggestions of a tornado or two possible with good low level curvature and streamwiseness to go along with supportive low level CAPE, even as streamwise vorticity values are on the lower end/more marginal. That said, large hail and damaging straight line winds are primary risks this afternoon and evening as activity. Morning model and CAM runs have come into much better agreement as to the anticipated progression of convection this evening, favoring upscale growth out of NE Kansas and depicting a bowing complex of storms riding along the frontal boundary and instability gradients. This would thusly suggest that damaging winds may ultimately be the primary threat, with hail most likely early in the event with any initial discrete/semi- discrete cells. As additional lift overspreads the area, there remains the potential for additional storms, but that scenario appears more likely into areas of northern Missouri in/along outflow boundaries/southward shunted boundary. Regardless, wherever that scenario sets up will have an additional hydrologic/flash flooding threat. With that in mind, have hoisted a Flood Watch across the southern two tiers of the state. Into Sunday, some uncertainty remains with activity this afternoon/evening playing a role, but there does remain another potential window for strong to severe storms across southern Iowa as an upper wave and associated surface low track northeastward across the area. Cannot rule out any mode of severe weather, with tornado potential near the low as it tracks across NW Iowa, as well as hail and winds with any strong or organized convection within warmer/more unstable airmasses. Initially quiet start to the week next week will be disrupted by multiple shortwave troughs moving through the larger flow with periodic opportunities for showers and storms beginning as early as Tuesday afternoon and evening. && .AVIATION /18Z TAFS THROUGH 18Z SUNDAY/... Issued at 1235 PM CDT Sat Apr 27 2024 MVFR to VFR conditions will initially prevail across all sites this afternoon before anticipated convection and approach of next upper wave and surface low are expected to degrade conditions back towards IFR. Best opportunity for TSRA/SHRA will be in/around KDSM/KOTM around/after 21z, then expanding northward to at least encroach all TAF sites with SHRA/VCSH mentions. Ceilings should fall towards 1kft, but have held off on the most pessimistic of guidance for this issuance. && .HYDROLOGY... Issued at 145 PM CDT Sat Apr 27 2024 Last night`s rainfall, while welcome, was locally heavy in some areas. The highest amounts per MRMS QPE extended from our southeast to northeast CWA where widespread amounts of at least one inch occurred. Some areas especially southwest and northeast of Des Moines received 2-3 inches. River and stream response was as expected with us being a drought -- mostly minor responses with some more elevated responses in urban areas like Des Moines. Smaller streams showed more responses. Some larger streams such as the Iowa River are still rising. Last night`s rainfall also helped to increase our soil moisture especially in the heavier rainfall areas. Some locations within the heavier rainfall band especially southwest of Des Moines are showing upper-level soil saturation values approaching or slightly exceeding 50% per FLASH CREST data. The FLASH SAC-SMA data is slightly more muted which is expected as that model tends to take longer to saturate the soil. Regardless, when upper-level soil saturation approaches or exceeds 50% then the flash flood concern significantly increases. Thus we will have to monitor for runoff and flash flood issues overnight especially across the south and into the eastern CWA where the heavier rainfall occurred and upper-level soil moisture is higher. Thankfully the areas with 50% saturation in CREST are relatively small. Any issues we may see tonight would be primarily confined to smaller streams as well as urban areas. Looking further ahead, the combination of last night`s rainfall along with forecast rainfall tonight as well as later this coming week may lead to more substantial rises on area rivers. Probabilistic river forecast models are suggesting rises to or above action stage at multiple locations at the 50% exceedance probability level mainly in the Des Moines and Raccoon River basins. At the 30% level, several locations in the Des Moines, Raccoon, Cedar, Iowa and Skunk basins are showing rises to action stage or higher. While rises to or above flood stage are possible, the exceedance probabilities are relatively low at this time but will have to continue to be monitored. && .DMX WATCHES/WARNINGS/ADVISORIES... Flood Watch through Sunday morning for IAZ070-081>086-092>097. && $$ DISCUSSION...Curtis AVIATION...Curtis HYDROLOGY...Zogg