877 FXUS63 KDMX 221112 AFDDMX Area Forecast Discussion National Weather Service Des Moines IA 612 AM CDT Sun Jun 22 2025 ...Updated for the 12z Aviation Discussion... .KEY MESSAGES... - Heat headlines remain unchanged. Dangerous heat continues through Sunday night. - Marginal Risk of severe weather late Sunday night, followed by a Marginal/Slight for continued storms throughout Monday. - Heavy rain anticipated through at least the first half of the week with hydro impacts possible (see hydro discussion). && .UPDATE... Issued at 404 AM CDT Sun Jun 22 2025 Not much change in thinking regarding the forecast during the day tomorrow, as hot humid and windy conditions are still expected today. Most of the deliberations from the overnight forecast pertained to handling of the heat advisory over the eastern portions of the area. Heat indices of 105F or greater will end this afternoon, as does the current Heat Advisory and Extreme Heat Warning. However, while the warmest temperatures end today, overnight temperatures still remain quite warm with lows only falling into the mid to upper 70s tonight. Likewise, we see heat indices climb into the upper 90s to near 100 again on Monday afternoon, primarily in southeastern Iowa. This sparked conversation into extending the current Heat Advisory through tonight and potentially into Monday given the prolonged nature of the heat and little relief overnight. That said, there remains some uncertainty regarding the approaching front and coincident convection tonight through Monday, which will have an impact on temperatures as it moves into the area. A faster progression of the front and/or outflow boundaries could bring relief to the hot temperatures overnight, especially in the areas of the advisory farther north and west. Similarly, the expanse of convection mid-day Monday could squander how warm temperatures get into the afternoon as well. Therefore, while it stands to reason that warm, humid conditions will persist into Monday for some, they will likely be sub-advisory values and potentially even cooler should convection be more bullish. Given the low confidence in heat beyond today, have held off on any changes to the current headlines for now. If confidence increases in the progression of the front and coincident convection, could see a need to extend at least some of the advisory into tonight and even Monday, especially in the southeast. As I mentioned in the previous paragraph, precipitation and thunderstorm chances return to the forecast late tonight into Monday as the upper high pressure to our southwest begins to form a sort of "ring of fire" setup through the beginning of the work week. The first of these chances will arrive with a surface boundary being pulled behind a wave revolving around the ridging. This boundary first lights up storms primarily over Nebraska, South Dakota, and Minnesota late this afternoon and evening, staying primarily out of Iowa through midnight. As we get into the early morning hours on Monday, the surface front and any associated convection will slowly leak southeast into Iowa. Instability will be waning as this occurs, which should help to limit any severe convection festering over northwest Iowa overnight. But of some concern will be the 40 to 50 kt low level jet nosed up into the state on Monday morning which could help provide forcing for storms to take off overnight. That said, the saving grace to overnight storms will be the elevated warm layer still over the area Monday morning, which would need to be overcome for any parcels to take off. This is likely the main reason no convection is being modeled in the CAMs, despite the stout low level jet. Still, just it`s existence demands some monitoring, especially in such a moisture rich environment. After day break on Monday, the boundary will continue to sag southeastward into the area and reignite through the afternoon. This timeframe will bring a much higher likelihood for storms. This environment will boast 2000 to 3000 J/kg of instability which becomes surface based by the mid-afternoon hours. Fortunately, with primarily unidirectional flow through the layer, 0 to 6 km shear values will be under 30 kts ahead of the boundary, but then increasing behind where surface winds are more northwesterly. Low level shear will also be meager, only showing around 15 to 25 kts. Some near-severe hail may be possible, but the warm profile and lack of organization will make large hail hard to come by. However, with a well mixed boundary layer and inverted V soundings, wind will be a concern with any collapsing updrafts and/or propagating cold pools. As is also mentioned in the forecast and hydrology discussions from yesterday, efficient rainfall will be a concern, further enhancing the threat for downburst winds/cold pool production but also posing a hydrology threat for both flash flooding and river flooding. This will be especially true with a mean wind generally oriented along the boundary, promoting training storms and prolonged periods of rainfall. Both the storm prediction center and weather prediction center now have slight risks for severe weather and excessive rainfall, respectively, further solidifying the threat for strong storms and heavy rainfall/flooding concerns on Monday. && .DISCUSSION... Issued at 254 PM CDT Sat Jun 21 2025 Moisture was on the rise during the day and the environment reflected this at more than just the uncomfortable, muggy conditions at the surface. Mid-level Water Vapor imagery shows the plume of moisture propagating around Deep South ridge, angling into Texas and across the Central Plains. To quantify this moisture, DDC`s 12z sounding this morning was right at the 90th percentile for 850mb dew points (the value this morning was 14C). The seasonally strong 50kt LLJ has been transporting this moisture into the Upper Midwest, bearing responsibility for the multiple rounds of convection across Minnesota and Wisconsin. Pair this with a seasonably strong upper jet atop the ridge, contributing to organization from shear and intensity of winds at the surface. Enough convective precip loading is happening above the stout 16+C EML for showers and storms to form above it in Nebraska and into Southwest Minnesota. This being driven by a surface low deepening over South Dakota. The synoptic pattern won`t deviate much for the next several days, at least as long as the ridge in the Deep South is stalled. The short-term impact from this is the muggy heat. There could be some spots in the west today that reach the century mark for highs, this being less common due to the moisture content in the air that is difficult to warm. Regardless, temperatures statewide will be in the mid to upper 90s the next two days, highest in the north and west. The moist air also holds heat well, which creates a scenario where there is little to no reprieve overnight. Lows will be in the upper 70s and Apparent Temperatures around 80 degrees. Not much deviation in the temperature forecast for this weekend and consequently headlines. The bottom line is that dangerous, prolonged heat is here to stay today and tomorrow. The other aspect to the short term is the winds, which may provide marginal relief to the heat. Sustained winds up to 30mph will be common this afternoon. The jet keeps going through the day so winds not far aloft in the PBL are valued at and above 40kts, some of this will be able to mix down the rest of this afternoon and on Sunday as well, especially in the north. This will be sporadic enough to not issue a Wind Advisory, at this time as the WAA regime and already warm temperatures will inhibit momentum transfer. Now for the more loaded part of the forecast, that being the precipitation, beginning with today. Elevated thunderstorms will be possible across northwest Iowa this afternoon, some of which could have small hail with them that will have mostly melted in the warm air mass. Chances of storms is below 10%. Moving on to Sunday night, a deepening trough from the Intermountain West will take the surface low to the northeast, setting up its cold front over northern and northwest Iowa. Wind shear will be parallel to the front and will be weaker due to upper-level ridging amplifying in response to the trough. It will also be poorly oriented for storms that make their way southeast towards central Iowa. Over central Iowa, NAM soundings indicate the potential of elevated hailers ahead of the front after 3am. The sustainability of damaging winds and hail with these is going to be short-lived given the shear profile mentioned before. The storm mode on the front will likely be a line given the shear orientation, which will drop an outflow boundary southeast towards central Iowa. Inverted-V soundings and CAPE values around 2000 J/kg offers a window of severe weather along the outflow, but with the better shear remaining along and behind synoptic front; storms that fire along this boundary will have an early window for more intense severe weather, but quickly become disorganized as they grow upscale. This covers the marginal risk in place for Monday, both with damaging winds and hail in play. Storm modes will be messy clusters. Theta-e differences exceeding -20K will generate stronger, but disorganized cold pools which will only add to the mesoscale forcing mechanisms for CI. Heavy rain, perhaps with localized flash flooding, will be possible due to slow storm motions and moist profiles. That brings our main focus: the heavy rain setup for the week. PWAT values are over 2 inches (175% above climo), warm cloud depths are pushing 12kft, and the mean wind parallels that of the LLJ. On the synoptic front left behind by its parent low: it will be draped SW to NE beneath weak synoptic flow. Since its orientation mimics that of the ridge, the flow that does exist runs parallel to it. This is a synoptic/frontal setup primed for multiple days of heavy rain. The combination of mesohighs and nocturnal jets will wobble this front north and south. A wave that arrives late Wednesday will finally make the boundary more progressive in nature. However, zonal flow sets in following this front`s departure, leaving the window open for more rainfall into next weekend. So far, there are slight risks for excessive rain through the first half of the week. It is likely (80-100%) that most of northern and northwest Iowa will see at least 2 inches of rain total this week. High end amounts right along the front itself could be 5+ inches, the location of which will be better understood as mesoscale features are captured in the CAM window. The flash flooding confidence is increasing for this time frame and a flood watch will likely be needed in the coming days. && .AVIATION /12Z TAFS THROUGH 12Z MONDAY/... Issued at 608 AM CDT Sun Jun 22 2025 VFR conditions prevail through the period, with gusty south southwest winds being the primary impact to TAFs this period. Sustained winds of 20 to 25 kts expected with gusts up to 35 kts possible. && .HYDROLOGY... Issued at 254 PM CDT Sat Jun 21 2025 Current precipitation trends have the heaviest rain starting late Monday into Monday night on a line extending from Creston up to Waterloo. As mentioned in the discussion above, high confidence in 2 inches of rain for the week across much of the state with some of the highest amounts found somewhere in this corridor. Rainfall amounts exceeding 1 inch per hour possible Monday night. HEFS guidance reflects the higher sensitivity of the Cedar and Raccoon River basin which has received the most rain recently. The 25th-75th percentile curve comes into minor flood stage for sites such as PROI4, MCWI4, and ALOI4. More runoff is possible in the Cedar compared to the Raccoon as flash flood guidance in northeast Iowa is some of the lowest in the state right now (the 6 hour guidance is 2 inches). Less certainty of flood potential on the Des Moines and Skunk basins as these have been less impacted in the past week, making their flood potential more dependent on where the heavy rain axis ends up. The high-end scenarios bring both of these into flood. && .DMX WATCHES/WARNINGS/ADVISORIES... Extreme Heat Warning until 9 PM CDT this evening for IAZ004- 005-015-023-024-033>035-044>046-057-058-070-071-081-092. Heat Advisory until 9 PM CDT this evening for IAZ006-007-016- 017-025>028-036>039-047>050-059>062-072>075-082>086-093>097. && $$ UPDATE...Dodson DISCUSSION...Jimenez AVIATION...Dodson HYDROLOGY...Jimenez