609 FXUS63 KDMX 090335 AFDDMX Area Forecast Discussion National Weather Service Des Moines IA 1035 PM CDT Tue Jul 8 2025 ...Updated for the 06z Aviation Discussion... .KEY MESSAGES... - An isolated storm possible this afternoon. Patchy fog possible tonight over eastern Iowa. - Thunderstorm chances increasing Wednesday night into early Thursday. Low chance of locally heavy rainfall. - Strong to severe storms, locally heavy rainfall that may lead to flash flooding Thursday afternoon into early Friday. Another round of storms, heavy rainfall is probable on Friday afternoon into Friday evening. - Rises or renewed river rises and possible river flooding by late this week or this weekend on portions of basins in central Iowa. && .DISCUSSION... Issued at 247 PM CDT Tue Jul 8 2025 Clouds have been clearing out of the state after storms rolled through much of western and southern Iowa early this morning. This afternoon, with the clearing sky, temperatures have once again warmed well into the 70s to low 80s. Have seen cumulus development over southern Minnesota ahead of a weak front and this has dropped into northern Iowa. The temperatures are pushing towards the convective temperature, but there is an elevated warm layer. A glancing shortwave trough to the northeast may provide enough to overcome this along with the surface boundary, but not expecting more than an isolated storm or two over northern Iowa where those cumulus clouds are dropping down this afternoon. High pressure will pass over the state tonight with very light winds as the surface front settles just south of the state. With an inversion setting up and the recent rainfall, this may foster an environment conducive for fog formation. However, forecast soundings show that full saturation is not achieved, but surface and low level dewpoint depressions are just a few degrees. For now, have added some patchy fog mentions in eastern portions of the forecast area. As the high moves off to the east Wednesday, will be looking at a more active period based on pattern recognition along with AI/ML guidance from late Wednesday night/early Thursday morning through late Friday. However, there is lack of strong synoptic forcing and thus the details on hazard type and timing will rely on mesoscale features like leftover boundaries or depend on the degree of convective cloud debris that are largely unknown at this time. For Wednesday night/early Thursday, low level warm air advection may bring some sort of convective cluster towards the state. While severity likely will be waning into Iowa, efficient, heavy rainfall parameters will be favorable with a 30 knot low level jet (LLJ) funneling moisture into the state. There is the potential for a corridor of higher rainfall totals, which are reflected in the 12z GFS, FV3, and CMC along with the latest HREF max field over a small portion of western into central Iowa. This may not be the most probable outcome given the rest of the guidance, which shows amounts around or less than an inch. However, WPC will have a marginal over portions of western into a bit of central Iowa for this potential. As we move into the day Thursday, depending on the amount of clearing and any outflow boundaries lingering, could have several thousand J/kg of instability by late afternoon with the front south of the state now lifting back into Iowa. A lead shortwave trough topping the ridge will drop into the state and provide for some higher deep layer shear values in the 30 to 40 knot range. This parameter space would suggest strong to severe storms by later in the afternoon into the evening hours being possible. The LLJ will be more pointed into Iowa Thursday night so expecting the severe storm potential will lessen, but the heavy rainfall and flash flooding concerns will be increasing into the nighttime hours. The aforementioned front will be perpendicular to the LLJ axis and thus provide for repeated rounds, or training, of storms over the same area with deep, favorable warm cloud depths and precipitable water values of 1.5 to around 2 inches. This will add additional rainfall, perhaps several more inches, and lead to flash flooding, especially if the rain falls over more vulnerable basins from the previous night`s rainfall. The Thursday night/early Friday storms should diminish to some degree in the morning hours of Friday, but a shortwave trough translating eastward from the West Coast will bring some additional forcing into the region. While cannot rule out stronger storms in the vicinity of the stalled boundary, deep layer shear will not be as high at just around 30 knots on Friday afternoon/evening with plenty of instability. The bigger concern will be flash flooding as the rainfall adds up with little change in the overall favorable setup for heavy rainfall on Friday into Friday evening. While there are timing differences in how quickly the shortwave trough exits, conditions will trend drier on Saturday. While details such as the location of the heavy rainfall cannot be certain this far out, the soil capacity is the least in northern Iowa, but has also now been reduced in southern Iowa with last night`s rainfall. River channels across the state also have reduced capacity with USGS streamflows at any of the 7, 14, or 28 day averaging period showing at least normal if not much above normal, especially over northeastern into north central Iowa. While the ensemble QPF hydrographs are likely suffering from not have the full window of rainfall as it only goes through early Friday, the experimental five-day Hydrologic Ensemble Forecasting Service (HEFS) does show more response at the 10% chance exceedance level. Five day forecast of the high magnitude flow from the National Water Model (NWM) forced by the GFS, which has the higher QPF, shows low annual exceedance probabilities (AEP)/higher streamflows on some streams in central Iowa. This is all pointing toward rises or renewed rises on some streams in central Iowa late this week if not this weekend. Whether it is river or flash flooding or severe weather later this week, Iowans should continue to monitor the forecast and have a means to receive weather alerts whether NOAA Weather Radio and/or a notification service like Alert Iowa. && .AVIATION /06Z TAFS THROUGH 06Z THURSDAY/... Issued at 1035 PM CDT Tue Jul 8 2025 VFR conditions are likely for much of the forecast period. Patchy light fog chances remain for Wednesday morning, but with low confidence in occurrence and placement have kept mentions from TAFs. Thunderstorms chances return beyond the current TAF period overnight Wednesday to Thursday and will addressed in later updates. && .DMX WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION...Ansorge AVIATION...Hagenhoff