Area Forecast Discussion
Issued by NWS Des Moines, IA
Issued by NWS Des Moines, IA
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496 FXUS63 KDMX 011950 AFDDMX Area Forecast Discussion National Weather Service Des Moines IA 250 PM CDT Mon Jul 1 2024 .KEY MESSAGES... - Widespread thunderstorms tonight and Tuesday. Heavy rainfall possible over parts of central and northern Iowa tonight into Tuesday morning. - A few severe storms and heavy rainfall possible over southern iowa Tuesday afternoon and evening. - Natures own fireworks likely on the 4th of July. A few strong storms may occur. && .DISCUSSION... Issued at 231 PM CDT Mon Jul 1 2024 Another active period coming up in the near term followed by more storms for the 4th of July holiday. Currently a slowly decaying MCS is moving into parts of central and west central Iowa with the main precipitation shield confined to north of Interstate 80 and west of Interstate 35. All lightning has ceased with this activity and the precipitation is having difficulty penetrating the dry air still moving into the area from the large Great Lakes high pressure system. A short wave over Colorado this afternoon, will move into the region tonight. A 50+ kt low level jet (LLJ will crank up quickly this evening and focus into the northwest half of the state before tilting more eastward later tonight and into Tuesday morning. Heavy rainfall is the main threat with the overnight precipitation into Tuesday morning as tremendous moisture advection fueled by the LLJ. Deep tropical like moisture is currently in place just to the west and south of the area and will move east into the area overnight. The main moisture plume with PWAT values of 2-2.25 inches will be over the northern half of the the forecast area then slowly settling south through the day Tuesday. There is the potential for a very heavy rain band to develop overnight along the nose of the LLJ feeding into the region with storms training over the same area for several hours. Storm motions near 40 kts are somewhat of a limiting factor by very efficient rainfall rates with the available moisture and with warm cloud depths over 14 kft may offset the shorter duration period of faster moving storms. A band of 3 to 5 inches with locally higher amounts could occur which will present a significant ponding and flash flood threat and may result in new or renewed flooding on area streams. Therefore, have issued a Flood Watch for tonight into Tuesday morning. The surface boundary will be into southern Iowa by Tuesday afternoon. Despite mostly saturated sounding profiles, descent instability will develop by the afternoon along and ahead of the boundary thanks to cooler temperatures aloft. Good deep layer speed shear will also be present that will aid in the potential for organized storms to develop. Damaging wind gusts will be possible with stronger storms. The hail potential will be limited due to the saturated profiles but a tornado could occur. Heavy rain may also develop if training of storms can materialize but if organized/supercell storms do develop, the right mover Bunker`s motions will move storms off the boundary which may limit the training potential. A quiet day will follow on Wednesday. Thursday will have thunderstorm potential return as an upper low descends into South Dakota. Climatologically, this upper level setup has been a tornado producer in July. Much more uncertainty exists at the low levels with the surface boundary possibly south into northern Missouri which would shift any surface based convection into that area and out of central Iowa. The main impact may be on the 4th of July outdoor holiday activities with showers and storms becoming likely by the afternoon and evening. Shower and isolated storm chances will persist into Friday under the cyclonic flow of the upper low to the north. This could turn into funnel cloud potential pending on how the low level thermal profiles play out. Another short wave arrives late Saturday and Saturday night which may more storms to Iowa. Iowa will be in the long wave trough cool region later Sunday and for early next week before another active period mid week. && .AVIATION /18Z TAFS THROUGH 18Z TUESDAY/... Issued at 1243 PM CDT Mon Jul 1 2024 Showers will move into the KDSM/KFOD areas early this afternoon and may reach KMCW mid to late afternoon though cigs will remain VFR and vsbys will remain AOA 6sm during this time. Thunderstorms will become more widespread overnight across central and northern Iowa and Cigs will lower to MVFR. The precipitation chances will persist through Tuesday morning with difficulty finding time windows to remove precipitation wording Tuesday morning. The wind will be breezy from the south/southeast for the period except near thunderstorm outflow. && .HYDROLOGY... Issued at 329 AM CDT Mon Jul 1 2024 A Flood Watch is in effect for parts of central and north central Iowa tonight into Tuesday morning which covers both the potential for flash flooding and new or renewed river flooding. The current river forecasts, which includes 48 hrs of forecast precipitation, do slow the fall along the West Fork Des Moines river. Rises are forecast across most other basins including the East Fork Des Moines, Shell Rock, Cedar and Iowa River basins including a return to Minor Flood stage at Shell Rock, Cedar Falls and Waterloo. Should a very heavy rain band materialize overnight into Tuesday morning, additional impacts may occur at areas including the Saylorville Lake storage area, which is currently forecast to reach 880 ft by the Corps of Engineers. Another area of heavy rainfall is possible over south central Iowa Tuesday afternoon and evening. Note, the QPF utilized by the RFCs for these forecasts may be on the low end and shifted to far northwest which may lead to some locations forecast being underdone. Rivers/streams and soil moisture in that part of the state are in better shape and can handle heavy rainfall compared to the recently impacted north. At this time, no streams are forecast to reach flood stage. For more details regarding the heavy rain potential, reference the main AFD section above. && .DMX WATCHES/WARNINGS/ADVISORIES... Flood Watch Tuesday morning for IAZ005>007-015>017-023>026- 033>037-044>048-057>060-070-071. && $$ DISCUSSION...Donavon AVIATION...Donavon HYDROLOGY...REV