textproduct: Des Moines

KEY MESSAGES

- Patchy morning fog dissipates, then ample sunshine and seasonally warm today. Staying dry through at least this evening.

- Storms chances increasing over northern northern Iowa through Tuesday and spreading southeastward gradually Tuesday night through Wednesday. Locally heavy rainfall remains possible.

SHORT TERM /THROUGH TUESDAY NIGHT/

Issued at 244 AM CDT Mon Aug 3 2026

GOES-East upper level water vapor imagery shows a trough lifting over the Appalachian Mountains early this morning while an upstream ridge axis oriented from just west of Lake Superior angles back to the Four Corners region. Farther upstream, broad scale troughing is noted over the western Canadian provinces and the Pacific Northwest into Montana with an upper level low pressure crossing from the Saskatchewan to the Manitoba province. For today, the ridge will stay in control as surface high pressure departs to the east. The clear sky and light winds this high has provided early this morning has allowed for patchy fog development over eastern and central Iowa, but this should dissipate shortly after sunrise. The surface flow over Iowa has responded to the high being east of the state with winds blowing from the south or southeast. which will continue through the day. It will turn breezy by this afternoon as well, particularly over the northwestern third to half of the state. This will aid in boundary layer mixing with forecast soundings continuing to show varying levels of dry air entrainment. While the initial National Blend of Models may still be showing a touch of its seasonal high dewpoint bias, the southerly flow and peak evapotranspiration season meant less adjustment to the dewpoints compared to yesterday. Temperatures will also respond upward, especially over western Iowa where highs will peak well into the 80s to near 90 degrees, given the ridge axis coming overtop the state with higher heights. Northerly flow aloft will continue to transport upper level wildfire smoke over the state creating a haziness most noted at sunrise and sunset.

Clouds will begin to increase tonight over the northwestern half of the state as the broad troughing to the northwest squashes the ridge southward and a weak, lead shortwave trough slips through its base. At the surface, a cold front will be approaching northwest Iowa and with the low level jet pointed into Siouxland along with increasing QG convergence and theta-e advection, there may be scattered showers and a few storms that reach northwest Iowa. A few of these may move over our far northwestern forecast area (e.g. Estherville, Algona, and vicinity) after midnight tonight. The chances are below 30% at this time and looking at forecast soundings, this may be generous given dry air that may still be present in the mid and low levels so any rain may be closer to sunrise Tuesday if not later after daybreak.

As the cold front slowly sinks further into the state on Tuesday, will see spotty showers and thunderstorms primarily over northern into portions of western Iowa with south central and southeastern Iowa staying dry through the day. Storm coverage looks to increase late in the afternoon over northern Iowa and then into Tuesday night over central and much of southern Iowa, in part due to phased kinematic and thermodynamic forcing along with perhaps a lobe of shortwave energy aiding in the additional development. Further, the low level level flow will intersect the front and bring seasonally high precipitable water values of 1.5 to 2 inches to pool along and ahead of this boundary. Deep warm cloud depths will be present for efficient rainfall and with 20 to 30 knots of 850-300mb flow aligned quasi-parallel to the surface front, rainfall may repeat over the same areas (aka training storms). The question that remains unanswered is how does the front progress through the state with the following possible scenarios - 1) the front keeps sinking southward resulting in less repeat rain round potential, 2) the front stalls with repeat rain rounds over the same area, or 3) the boundary sinks, but then lifts back northward resulting in rainfall over areas that had it earlier. Scenario 2 and 3 would be the more concerning scenarios for locally heavy rainfall and isolated flash flooding. However, offsetting these concerns are mature agricultural crops coupled with dry soil moisture profiles in parts of the state. Latest NASA SPoRT analysis shows the upper 16 inches below the 30th percentile for soil moisture over western Iowa while eastern Iowa has pockets above the 70th percentile due to late last week's rainfall. Thus, for rural areas this rainfall will be largely welcome, especially over western Iowa. Of course, any locally heavy rainfall falling over an urban area can quickly create at least low end flash flooding if rates start to exceed 2 inches per hour. Thus, the WPC excessive rainfall outlook (ERO) at marginal (level 1 out of a max of 4) seems reasonable for Tuesday into Tuesday night. This risk will increase some into Wednesday as discussed below due to the rain that falls on Tuesday lowering soil capacity for future rainfall.

LONG TERM /WEDNESDAY THROUGH SUNDAY/

Issued at 251 PM CDT Sun Aug 2 2026

A cold front will trail along a negatively tilted upper wave near the US-Canada border and has trended slower to exit over Iowa. Heavy rain is the main concern as PWATs rise above 1.5" and flow points parallel to the front's orientation. While CAPE totals over 1000 J/kg both Tuesday and Wednesday afternoon, the saturation of the profile keeps lapse rates closer to the moist adiabat, presenting an elongated and skinny CAPE profile. Uniform parcel acceleration will favor heavy rain over hail and the weak shear profile will make any wet microburst potential in storms relatively short-lived. The severe weather risk is low for these reasons on both days, but a Marginal and Slight Risk ERO have been issued. Rainfall will peak in the evening and overnight hours with deterministic runs all painting a band of 2"+ in northern and central Iowa. Given present convective processes highlighted above, would not be surprised if localized amounts over 5" are realized. Amounts will be better understood as the system enters the CAM window.

For hydrology impacts, soil moisture in northern and northwest Iowa is low (5-20%) and can take the most rain. National Water Model simulations using GFS QPF have most of the river responses in eastern Iowa where antecedent conditions are comparatively more wet. Only the high-end basin QPF outputs bring basins such as the Cedar into Flood Stage, so the chance for this is low (5%). For flash flood potential, Iowa is in peak crop maturity which further diminishes concerns for rural parts of the state. Concerns increase if the axis of heavy rain falls on an urban area.

AVIATION /18Z TAFS THROUGH 18Z TUESDAY/

Issued at 1242 PM CDT Mon Aug 3 2026

Cumulus coverage is expected to increase over the forecast area this afternoon, with periods of MVFR ceilings before coverage decreases this evening. Otherwise, slightly breezy winds are also expected to decrease this evening, followed by a gradual increase in cloud cover through the overnight as a front moves into north/northwest Iowa. While shower activity is possible into Tuesday morning over the northern terminals, confidence on occurrence remains rather low given remaining dry air overhead and progression of this feature, so have continued to leave out. Further monitoring will remain through the rest of today, with additions certainly possible in later issuances. Of higher confidence will be lowered ceilings into MVFR and near IFR across the north near the end of the current period, with further expansion beyond.

DMX WATCHES/WARNINGS/ADVISORIES

None.