Area Forecast Discussion
Issued by NWS Des Moines, IA
Issued by NWS Des Moines, IA
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604 FXUS63 KDMX 210501 AFDDMX Area Forecast Discussion National Weather Service Des Moines IA 1201 AM CDT Fri Jun 21 2024 ...Updated for the 06z Aviation Discussion... .KEY MESSAGES... - Isolated to scattered showers and storms possible through the night. Severe weather risk is low. - Showers and thunderstorms possible over northern Iowa tomorrow afternoon. Heavy rainfall, wind and hail are the primary concerns, although a tornado can`t be ruled out. - Additional severe weather is possible on Saturday afternoon, with all severe hazards possible. && .UPDATE... Issued at 922 PM CDT Thu Jun 20 2024 There was some concern this evening regarding the latitudinal placement of the heavy rain axis overnight and into tomorrow, and whether the Flood Watch needed to be started earlier. The convection allowing models (CAMs) were not handling the O`Neill, NE supercell well, which is currently rooted fairly low and in the heart of the 925-850mb moisture transport and developing low level jet. It was previously tracking slowly eastward per the Bunkers supercell motion, and could potential continue that movement along the Highway 20 corridor per the Corfidi upwind propagation vector, but after weakening is now starting to lift slowly northeastward following more of the mean wind. The overall SW-NE moisture plume in the water vapor imagery suggests a gradual northward focus into the night, with 0-2km moisture convergence also targeting those areas right now. So, long story short, confidence is current insufficient to move the headline and higher PoPs farther south at the moment, although MCS trends will continue to be monitored into the night as the airmass remains ripe for heavy, efficient rainfall, especially northwest. && .DISCUSSION... Issued at 307 PM CDT Thu Jun 20 2024 The front that brought a healthy amount of rainfall to portions of southern Iowa overnight hasnt budged much through the day, keeping plenty of moisture pooled over the area. This boundary has begun to develop convection again early this afternoon, but storms have been relatively benign with little to no shear present through the layer. Isolated to occasionally scattered storms may fester along and south of this front through the afternoon and evening hours, but not expecting any impacts beyond small hail and locally heavy rainfall. As we head into tonight, we turn our attention to the northern CWA as surface low pressure building to our west continues to pull large amounts of moisture into the region. The evolution of this low will bring a prolonged period of rainfall to the north starting tonight and lasting intermittently into Saturday morning. Most of the heavier rainfall tonight will stay north and west of the area, mainly impacting SD and MN, however, still expecting some showers and occasional thunderstorms in our northern counties overnight and into early tomorrow morning. Although we arent expecting the heaviest rainfall with this wave, PWATs will be increasing through the night and into tomorrow, leading to moist profiles and efficient rainfall for those that are impacted by precipitation. Through Friday morning, the surface warm front will have caught up to the better forcing and moisture aloft, increasing the magnitude of showers and storms in northern Iowa by Friday afternoon. This will result in both an increased chance for severe storms, as well as an increased chance for heavy rainfall as the instability increases along the boundary. First looking at the severe chances on Friday, the parameter space isnt off the charts, but a few stronger storms still look possible with warm, moist low levels and 2000 3000 J/kg of SBCAPE during the peak heating hours. The limiting factor will be that most of the shear in the warm sector will be fairly weak and unidirectional, resulting in messy hodographs. Therefore, wind and hail would be the primary threats, while a tornado would require a storm that is tied into the vorticity along the boundary. This is reflected well in the SPC Day 2 outlook, which places a slight risk for severe weather along the expected boundary in northern Iowa. In addition to the severe threat on Friday, heavy rainfall will also be a concern. PWATs will have continued to increase through the day, resulting in anomalously high values exceeding 2 over southern Minnesota and into northern Iowa. This, combined with the better instability present in the warm sector, will lead to very efficient rainfall over throughout this region. To make matters worse, storm motions will be out of the west, resulting in training convection along the boundary. These heavy rain features have been well captured by guidance, with ensembles and deterministic guidance all in pretty good agreement for a swath of rainfall amounts of 3 to 5 inches along the SD, MN and IA borders and locally higher values in the 6-8 range. Where the uncertainty resides is in the placement of the surface front and the influence of mesoscale features on the location of this heavy rainfall. As has been stressed in previous discussions, setups of this nature during this time of year tend to trend towards the location of the more favorable environments, as cold pools/outflow boundaries spread out and produce new convection south of the original boundary and in the more unstable air. This is important because higher QPF amounts that had been over southern MN the last few days are now trending south as CAMs begin to pick up on the smaller scale features and leading to higher rainfall amounts in northern Iowa. Unfortunately, these smaller scale features are often not well forecasted by models, making the location of the heavy rainfall uncertain. WPC has shifted the moderate risk for excessive rainfall further south into Iowa to account for this, and we have locally issued a flood watch for Friday afternoon into Saturday morning to account for this heavy rain potential. Of course, its an active summer in Iowa, so this is far from the end of our convective fun for the week. As the surface low pressure tracks over the region, the cold front will drag behind it and be the focal point for convection on Saturday as well. Instability, moisture and forcing will be easy to come by as this passes through, with PWATs along the boundary well exceeding two inches and modeled SBCAPE values again exceeding 2000 J/kg. The deep layer shear will also be a bit higher (30-40 kts) on Saturday afternoon, which will increase the chances for all modes of severe weather. As of right now, SPC has a day 3 marginal risk over much of the area, and a slight over northeast Iowa nearer to the low and where the better shear will reside. This seems reasonable at this timeframe, but will want to keep an eye on severe chances as this event starts to be captured by higher resolution models. && .AVIATION /06Z TAFS THROUGH 06Z SATURDAY/... Issued at 1201 PM CDT Fri Jun 21 2024 IFR stratus was developing across northern IA, and that should persist through at least 12z with some contributions from fog development as well. VFR conditions should return area wide later Friday morning however, with mainly dry conditions through the afternoon. Additional convection is expected to develop across northern IA Friday evening however, with sufficient confidence to mention thunder even at this lead time. && .HYDROLOGY... Issued at 326 PM CDT Thu Jun 20 2024 With heavy rain still expected to fall from MN into northern IA over the next few days, more rises on area rivers are expected with renewed or increasing flooding possible. The river basin of most concern is the Des Moines River basin above Saylorville Lake and secondarily the upper Cedar/Winnebago River basins because those two basins have their headwaters in southern Minnesota. Minor to moderate flooding is expected per QPF ensemble hydrograph and HEFS guidance. Our official river forecasts today included 48 hrs of QPF whereas they typically include 24 hrs this time of year. The QPF time horizon was extended because the longer term nature of this rainfall event. && .DMX WATCHES/WARNINGS/ADVISORIES... Flood Watch from Friday afternoon through Saturday afternoon for IAZ004>007-015>017. && $$ UPDATE...Small DISCUSSION...Dodson AVIATION...Small HYDROLOGY...Zogg