textproduct: Des Moines

KEY MESSAGES

- Areas of fog this morning, some of which will be dense.

- Mostly cloudy to overcast conditions today. Highs near 70 degrees.

- Patchy fog possible east tonight.

- Confidence remains high in a prolonged, widespread rainfall event late Tuesday through late this week, which raises concerns for potential flash flooding and river rises.

SHORT TERM /THROUGH TUESDAY/

Issued at 248 AM CDT Mon Sep 28 2026

Deep low-level saturation created a blanket of thick stratus overnight. The saturated layer had essentially no lift or flow, so the surface saw fog as opposed to drizzle. Fog will be locally dense near the Iowa-Minnesota state line. This is because of clearing stratus, allowing for the surface to cool more. The deep saturation mentioned above will make the fog difficult to burn off and it may well last past 9am. Southeast winds will try and advect in more dry air today, which may allow for peaks of sunshine in southeast Iowa by the afternoon. This is also where temperatures have the best chance to reach 70 degrees. For tonight, winds will increase in the west. This is in response to the increased influence of Western CONUS troughing. Troughing at H85 arrives in the Plains which will increase the LLJ and moisture transport in western Iowa. Midlevel clouds will form in response. In eastern Iowa, flow will remain weaker due to the influence of high pressure. Fog will be possible in this region, but will be more patchy in nature due to less low-level moisture.

LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/

Issued at 130 PM CDT Sun Sep 27 2026

GOES-East upper level water vapor imagery shows the key features that will be important for the longer term forecast. A broad cyclonic spin is along the central West Coast associated with a longwave trough. Ahead of this is remnant moisture from Odalys, which is being drawn northeastward over the Four Corners region already. And finally, Hurricane Polo is readily apparent off the southern portion of Baja California as it begins its north and later northeastward track and weakens. By Tuesday morning, this trough will be over the Southwest with the weakening Polo cresting the Sierra Madre Occidental. There will already be pieces of energy spilling northeastward into our region along with surges of low level thermal lift and modest QG convergence to support showers and storms spreading into at least some portions of Iowa later Tuesday if not Tuesday night.

There is uncertainty on where the highest rainfall of this long duration rain will set up over the region. AI guidance has shown promise for highlighting the location of the band of heaviest precipitation and over the last day, the AIFS has stabilized in the 48 hour rainfall total axis through midday Thursday. Meanwhile, the AIGFS continues to trend northward in its placement. Looking at deterministic and ensembles, these have generally trended northward in the last several runs as well. These shifts compared to 24 hours ago do not lend themselves to forecast confidence, but investigating some of the causes for these shifts does provide elements of the weather pattern to monitor this week. First, what degree of interaction will the approaching trough and tropical moisture have with a northern stream wave over the Canadian prairies. Yesterday's 12z suites had minimal interaction, which resulted in the northward surging moisture being met with dry air in the mid-levels. Overnight and so far today, models are showing more play with the northern stream and thus a change of rainfall being farther northward and oriented from southwest to northeast vs west to east. Second, a zone of high pressure over the upper Mississippi River Valley at the start of this week will slide eastward and sprawl over the Eastern Seaboard. This may leave some degree of mid-level dry air over the region, though uncertainty is where and how long that may last against the northward push of moisture. Third and finally, last night's and today's GFS and CMC have a northern stream trough advancing slower than at least last night's ECMWF, which allows the GFS and CMC to pick up the approaching trough and take it through the state more quickly. However, the ECMWF is faster with the northern trough, which causes the approaching trough to break off into a closed low. Thus, the rainfall does not progress through as fast, but in the end the closed low does pull it away from the state by late week. Depending on where this rain falls will have implications on river flooding and possible flash flooding, which are also highlighted in this morning's National Water Center's hydrologic discussion (NMDNWC).

While the details are more murky beyond Tuesday night in location of highest rainfall, confidence is high that it will be raining over some portions of Iowa from late Tuesday through Wednesday night. In addition, the rain that does fall will be in an environment that will support efficient rainfall given the atypical deep warm cloud depths and above 90th percentile precipitable water values. This will lead to areas of rainfall of 2 inches or more somewhere in Iowa by week's end. 7z/13z NBM probabilities of 2"+ rainfall in a 48 hour period ending Thursday morning are already above 50% over much of western and central Iowa with 4"/48 hour probabilities between 20 and 30% over parts of western Iowa. And ending on a bit of good news, the weak lapse rates will minimize any severe weather risk this week.

AVIATION /12Z TAFS THROUGH 12Z TUESDAY/

Issued at 639 AM CDT Mon Sep 28 2026

Fog is common in two areas of the state this morning: locally dense fog is common in northern Iowa and in southeast Iowa. KMCW and KOTM are the primary impacted terminals at this time. LIFR stratus is common across most of the area, MVFR cigs moving in from the east. KDSM poses the most uncertainty in the short term as it is at the edge of the fog and stratus. Have kept a tempo for cigs and vsbys as nearby obs have worsened. Expecting vsby impacts to be transient here.

Further into the period, have delayed the departure of IFR and MVFR cigs in the north. Eventually stratus will break apart and midlevel clouds will replace them overnight. While not mentioned in the planning period, patchy fog will be possible at KALO and KOTM, but is not expected to be as impactful as the fog this morning.

DMX WATCHES/WARNINGS/ADVISORIES

None.