593 FXUS63 KDMX 121739 AFDDMX Area Forecast Discussion National Weather Service Des Moines IA 1239 PM CDT Thu Jun 12 2025 ...Updated for the 18z Aviation Discussion... .KEY MESSAGES... - Isolated showers and storms this afternoon and early evening with limited severe chances - Not a washout, but chances for storms at times into early next week with organized storms possible next week && .UPDATE... Issued at 209 AM CDT Thu Jun 12 2025 The effective surface frontal boundary, reinforced by outflow from Wednesday night storms, has sunk into central Iowa overnight and lies near the Highway 30 and Interstate 80 corridors. However, the 925-850 MB frontal zone lies well to the north, near or just north of the Iowa/Minnesota border. A few clusters of thunderstorms are lingering in our northeastern counties around Waterloo and Waverly early this morning, but these are slowly moving out and otherwise the bulk of convective activity this morning should reside in the northern zone near the border. Meanwhile, a broad upper low is drifting northward across Arkansas and into Missouri where it will stall through Friday. As of this writing the rain associated with that feature remains well south of Iowa. Later this morning, 500 MB steering flow over the Upper Midwest will back just slightly to west southwest, allowing the frontal zone near the IA/MN border to move north into Minnesota. Modest but organized southerly surface flow will also push the shallow surface boundary back up into northern Iowa. As a result, there should be a break in the rain across most of our area in the late morning and have lowered POPs during that time accordingly. By this afternoon however, POPs increase again as diurnal heating will allow for destabilization to occur, with forecast soundings indicating sufficient CAPE and lack of capping to support convective initiation. However, there is no real forcing mechanism and flow is very light through the column, with the NAM sounding for Des Moines showing that at noon the maximum wind speed below 400 MB is only 13 KT. That means the metaphorical "popcorn" storms, which should remain benign and fairly limited in coverage. The possible exception to this will be in our far northern/northeastern counties, where a subtle shortwave moving overhead will provide a bit more forcing, and the retreating surface boundary may focus convection a bit more. Have carried higher POPs (40-60%) in those areas accordingly, with lower POPs (20-30%) farther south. SPC has also outlined a Marginal Risk of severe hail/wind across our far north/northeast which is appropriate. From late Thursday night through Friday it appears we will see a period of weak ridging which should allow for quieter and drier weather overall, and have lowered POPs to 10-20% during this time. && .DISCUSSION... Issued at 218 PM CDT Wed Jun 11 2025 A more active period of thunderstorm chances is in store over the next week largely driven by shortwave troughs being carried through the low amplitude flow. This flow can be viewed on the upper level water vapor imagery from GOES-East with one wave over the Dakotas and Nebraska and another nearing the northern Rockies. This first shortwave is reaching the state this afternoon interacting with a stationary front that is draped over far northern Iowa from just north of Sheldon to just north of Algona to north of Waterloo. This front has provided some level of convergence despite the weak surface flow with a storm along the Iowa/Minnesota border. Expectation is that additional storms will be able to form along northern Iowa this afternoon and push eastward into early this evening. Now, the weak flow does result in weaker effective shear despite the traditional deep shear values looking favorable for storm organization. Surface based CAPE values over 1500 J/kg are common south of the front already this afternoon and with the dry mid-level air, downdraft CAPE values are over 1000 J/kg. The steep low level lapse rates and still more than sufficient SBCAPE values should allow for good parcel acceleration on storm development phase that pushes past the 12kft freezing level and into the hail growth zone. This will result in a large hail threat. Further, the large downdraft CAPE values, inverted V soundings, and surface dewpoint depressions over 20 degrees F also point to the hazard of damaging wind gusts. This is shown in various convective allowing models (CAMs) such as the 12z HRRR, 0/12z ARW cores, and 12z FV3, which have small, but varying swaths of severe wind gusts (>50 knots/58 mph) over northern Iowa this afternoon into early this evening. Already have seen severe gusts up in WFO Sioux Falls service area with the one storm this afternoon. This first wave of storms will exit into northeast Iowa mid to late evening, but regeneration of additional storms is expected over Siouxland as a weak low level jet points into that region, especially late tonight into the overnight. While severe weather is not expected, locally heavy rainfall will be possible as these storms track roughly eastward following the weak 850-300mb flow along the stalled front over northern Iowa. Efficient rainfall is certainly possible given the deep warm cloud depths and precipitable water values over 1.5 inches pooling along the boundary. Rain totals of 1 to 2 inches are possible somewhere along this boundary whether in northern Iowa or southern Minnesota. A few CAMs (e.g. ARW, HRRR, FV3) also show amounts over 4 inches in varying locations, with these CAMs impacting the localized probability matched mean of the 12z HREF. Flash flood guidance (FFG) values over northern Iowa range from 1.75 to 2 inches for 1 hour, 2 to 2.5 inches for 3 hours, and 2.5 to 3 inches for 6 hour. With relative soil moisture (RSM) percentiles in the normal range (30th to 70th percentile) at the 0-10cm and 10-40cm layers, the FFG seems a bit low from an empirical sense for mid-June. So, urban areas and/or smaller basins, if the higher rainfall moves over those areas, could have isolated flash flooding. As for rivers, monthly average USGS streamflows are average and there is capacity and expectation at this point is mainly within stream bank rises. The exception would be if one of those higher QPF CAM solutions verifies and this could result in more than within stream bank rise. As we move into Thursday, there should become a minimum in coverage of any showers or storms in the morning hours that lasts into at least part of the afternoon. The second shortwave nearing the northern Rockies today looks to pass largely north of the state Thursday afternoon and night. This would keep the focus for much of the storm activity north of the state, though a few storms may move across northern Iowa mid to late afternoon into the early evening hours. These could pose an isolated hail or gusty winds threat and is currently captured in the day 2 SPC outlook. Further south outside the marginal risk area, a few storms may develop during peak heating over central Iowa. Instability may reach 1000 J/kg, but shear is weak so outside of perhaps some sub-severe hail and wind gusts as storms collapse, the activity over central Iowa looks to be more of the sub-severe variety. Depending on where the rain falls tonight (Wednesday night/early Thursday) will determine what kind of flash flooding or river flooding risk, if any, will exist later on Thursday. Low shower and storm chances will persist in parts of the state, mainly northern and eastern Iowa on Friday as shortwave energy passes north of the state and a 500mb closed low drifts northeastward over Missouri. A front may clear the state by Saturday with this weekend favoring drier, though not fully dry conditions. The machine learning/artificial intelligence (ML/AI) severe probabilities are also low or non-existent in this period. Chances for more organized storms may develop as a warm front lifts over the region and Iowa returns to the warm sector early next week. Instability and shear parameter space would point to some strong to severe storm risk with this supported in the ML/AI severe probabilities as well. && .AVIATION /18Z TAFS THROUGH 18Z FRIDAY/... Issued at 1239 PM CDT Thu Jun 12 2025 The primary challenge in this cycle are the chances for isolated showers and storms to develop mid and late afternoon over the terminals. With the exception of FOD where probabilitieshave decreased, have maintained and slightly expanded in some cases the PROB30 group for lowered ceilings and/or visibilities. Otherwise, VFR conditions are forecast to prevail through tonight. Some lowered ceilings are forecast to move over southeast Iowa and OTM Friday morning for a short period. && .DMX WATCHES/WARNINGS/ADVISORIES... None. && $$ UPDATE...Lee DISCUSSION...Ansorge AVIATION...Ansorge