826 FXUS63 KDMX 140809 AFDDMX Area Forecast Discussion National Weather Service Des Moines IA 309 AM CDT Sat Jun 14 2025 .KEY MESSAGES... - Storm chances increasing later this afternoon into the overnight and Sunday. A few strong to severe storms may be possible. - High confidence in stormy pattern starting late Sunday into next week. While lacking specifics such as timing, location, and hazards, one and likely more than one round of strong to severe storms will be possible over the state in this period. - Increasing confidence in heavy rainfall next week that could lead to flash flooding and/or river flooding later in the week. Similar to above, the location is not clear, but chances will increase if rain can repeat over the same basins and as time goes on in the week. - Staying warm and humid most days through late next week. && .UPDATE... Issued at 309 AM CDT Sat Jun 14 2025 Patchy dense fog has developed in far southern Iowa into Missouri early this morning with plenty of available low level moisture in place. Areas that have seen fog development so far have been where winds have gone calm in a bit of a saddle region between the low to the southeast and the shortwave off to the northwest. By 3 AM fog was not widespread in the area, but this will be closely monitors through the morning for expansion in case an advisory is warranted. Convection this afternoon is expected to develop along the warm front that has been draped across northern Iowa for several days. Available instability is higher than it has been the past couple days, but shear continues to remain fairly weak. A notable difference in the most recent model runs (00z to 06z so far this morning) is that while earlier runs developed convection along the boundary that would remain anchored there and percolate into the late Avignon before fading out. That has been the case the past several nights. The latest data, however, has escalated the situation as it were. The boundary is further south into Iowa than previous forecasts and there is a weak perturbation in the flow that, while subtle, helps to weakly fuel convection across the area resulting in less isolated and more scattered to perhaps widespread convection in northern to central Iowa this evening. Severe potential remains low, but not non-existent. HRRR soundings across northern Iowa show 2000+ J/kg of ML and SB CAPE, however entrainment CAPE reduces these values by 600-800. With dry air being entrained and weak shear in place the sever threat remains low, however some hail of gusty winds would certainly be possible with these storms. Convection will percolate along the boundary in northern Iowa thorugh the overnight and on Sunday morning. By afternoon the western shortwave drops across the area, reinvigorating convection across the area. This sweeps across much of the state through the afternoon and evening. The better instability axis at this time remains off to the west and shear is, again, pretty weak so the severe potenitial remains fairly low with this activity. As we look ahead to the upcoming week the forecast is active with multiple chances for severe storms and heavy rain. The setup remains similar to that outlined below in the Friday afternoon discussion. && .DISCUSSION... Issued at 257 PM CDT Fri Jun 13 2025 Upper level water vapor imagery shows an active northern stream with multiple shortwave impulses traversing north of the state this afternoon. To the south of Iowa, the upper closed low continues to slowly spin east-northeastward. This latter feature has helped to provide a focus for agitated cumulus and light showers over our extreme southeastern forecast area. In the low levels, the effective front is over southern Minnesota into far northeastern Iowa and this may be the focus for a few storms yet this afternoon to early this evening. SPC mesoanalysis shows decent surface based instability of 1000 to 2000 J/kg, but as has been the case the last few days effective shear is rather marginal at or under 30 knots. Thus, not expecting anything severe with the storms that do form this afternoon. In addition, storms will be efficient rainers and slow movers at 10 to 15 knots at most and thus could produce a very isolated 1 inch or so of rainfall. After this activity diminishes early this evening, winds will be light and with the more humid airmass may see patchy fog develop afd midnight. For now, guidance points towards southern Iowa and have focused the fog mention in those locations. Into the day Saturday, the boundary to the north should begin to sag into Iowa and as such, so should the storm chances return late in the afternoon into Saturday night. There should be more surface based instability Saturday afternoon than Friday and low level lapse rates are favorable. However, the wind profile through the column has around or less than 20 knots and thus there is not enough shear for organized storms and the severe risk remains quite low. With the weak flow, pinpoint rainfall amounts around an inch once again will be possible. The upper level pattern from Sunday into next week is looking quite supportive for several rounds of thunderstorms. The first shortwave trough will ride overtop the ridge on Sunday and while there are higher amounts of instability, shear looks to remain more marginal and highest out over western Iowa. This latter location is closer to the day 3 SPC outlook. Additional shortwaves will ride over an increasing favorable airmass characterized by several thousand J/kg of instability and deep layer shear supportive of storm organization. Thus, strong to severe storms will be possible over some portion of the state and some of the time next week. Details on hazard types, location, and more specific timing will be better defined in the coming days. Further, the airmass will also be supportive of efficient rainfall with precipitable water values at least 1.5 to at times around or a bit above 2 inches, deep warm cloud depths, and 850-300mb flow that will be slow early in the week and see increasing speed later in the week. ECMWF extreme forecast index is honing in on Wednesday into Wednesday night with some level of ensemble support and NAEFS/ECMWF low level specific humidity is above the 97.5th percentile. Thus, heavy rainfall could lead to flash flooding with WPC excessive rainfall outlooks marginal most of the week over some portion of central Iowa to slight risk over northern Iowa by Tuesday. Obviously, these locations can change as the rainfall forecasts change, but if rainfall can repeat, or train, over the same area this will increase the chances for flash flooding. Further with the rainfall, the 12z GFS forced National Water Model (NWM) annual exceedance probabilities (AEP) at 5 days and 10 days show low values in parts of the Cedar and upper Des Moines River basins. Similar to the WPC outlooks, the NWM AEP does highlight the potential for increased flows and possible river flooding. While there will likely be a day in next week that may be less stormy, which currently looks like Thursday, it will certainly be an active weather week across the region with most days being warm and humid. && .AVIATION /06Z TAFS THROUGH 06Z SUNDAY/... Issued at 1121 PM CDT Fri Jun 13 2025 VFR conditions currently prevail over the area, although stratus and patchy fog is expected to develop through the morning leading to MVFR to IFR conditions at most sites. Confidence in these impacts is highest at KMCW, KALO, and KOTM, but KDSM will also be right on the fringe of MVFR ceilings/visibility. Conditions improve through mid-day. Showers and thunderstorms then develop in the afternoon and evening, mainly at KMCW and KFOD, but may drift further south later in the evening. && .DMX WATCHES/WARNINGS/ADVISORIES... None. && $$ UPDATE...Hagenhoff DISCUSSION...Ansorge AVIATION...Dodson