Marine Weather and Tide Forecast for Des Moines, IA

DISCUSSION
Issued at 228 PM CDT Sat May 17 2025

GOES-East upper level water vapor imagery shows that the upper low that brought the strong, gusty winds and spotty light showers yesterday has moved over the Great Lakes region. With this pulling away, this has allowed the pressure gradient to relax so while it is still a rather breezy May day, certainly not like yesterday. To the west, a low amplitude ridge is over the High Plains while a shortwave trough is slipping out ahead of a deepening trough over the West Coast. In the lower levels, weak cold air advection continues this afternoon with the stratus clouds having eroded largely with the exception of northeastern Iowa. With the sky becoming clear and winds further decreasing tonight as the Great Lakes upper low moves farther away, temperatures will drop into the upper 30s over far northern Iowa to the low and middle 40s over central and southern Iowa. The initial forecast looks on track and is around the 25th percentile of available guidance. Winds look to remain above 5 mph and temperatures are marginal so not anticipating widespread frost. However, sheltered, low lying areas like river valley over northern Iowa may see patchy frost in those favored locations and have added patchy frost to the forecast over this area as well. To the south of Iowa, the aforementioned lead shortwave will generate showers and storm south of the state tonight into Sunday morning.

As we head further into Sunday, the deepening West Coast trough will eject one shortwave out of its base while another one descends the backside Sunday night into Monday with these two waves dancing around one another on Monday into Tuesday before essentially phasing into one upper low. These features will bring a more active pattern to the state with thunderstorm chances starting Sunday night as waves of low level theta-e advection move over the region through Tuesday night. We'll first dive into the severe thunderstorm potential followed by a discussion of hydrology concerns. Looking at Sunday night, a mesoscale convective complex (MCS) looks to blast across Missouri scraping perhaps southern Iowa as it moves along the warm front. This is shown in the 17/0z C-SHIELD and MPAS runs as well as the latest ARW and FV3 core convective allowing models (CAMs). Additional convection north of this warm front is possible, particularly over southern into perhaps central Iowa, though to what extent remains to be seen. Deep layer shear is in ample supply while there should be sufficient instability and assuming the MCS stays south, any storms over Iowa would be elevated. This would then mean that the main hazards would be large hail and perhaps gusty winds if the MCS does scrape southern Iowa Sunday night/early Monday.

As we move into the daytime hours of Monday, the upper low will be near the Black Hills region with a sub-1000mb surface low moving across Kansas towards Iowa through the day. Things we'll be monitoring closely on Monday will be how much cloud erosion occurs and the location of the warm front. Much of the guidance (6z/12z)
would point to the warm front staying south of the state with a few solutions bringing it close or into a few of our southern tier of Iowa counties. If the warm front does stay south, then temperatures may need to be adjusted lower and there could even be a bit of drizzle at times given the depth of low level saturation in the soundings. Further, breezy winds are also on tap for the day. With the warm front expected to stay south of the state given current guidance, thinking is that storms will be elevated and largely a large hail risk and perhaps gusty storm winds with these hazards focused over southern into perhaps central Iowa. Timing wise, it looks like severe weather potential would be from mid to late afternoon into the Monday evening hours with shower and more so non- severe storm chances continuing overnight.

As these storms shift away Tuesday morning, the upper low will slowly move over the region Tuesday into Wednesday. This will keep cloud cover and shower and storm chances around over the state with storm chances more probable on Tuesday compared to Wednesday. There will still be some instability up into southeastern Iowa on Tuesday afternoon, though the shear magnitude will be lower and thus any severe risk looks to more marginal and in line with the SPC day 4 outlook with the higher probabilities southeast of the state.

For hydrology concerns, let's first set the stage. There has been around or less than a half an inch of rainfall over the last 14 days. This has led to 14 day National Water Model (NWM) streamflow anomalies showing below normal to much below normal over southern Iowa with a mixture of below normal and normal streamflows over central and northern Iowa. This is also viewed in the USGS average streamflow compared to historical data. Further, NWM soil moisture analysis shows 50 to 80% capacity remaining with the latest NASA SPoRT 0-10cm layer relative soil moisture (RSM; this layer to assess rapid onset flooding) and 10-40cm layer RSM (this layer to assess river response) showing comparable values to absorb rainfall. To sum this up, it's been dry and there is capacity in the soils for a decent rainfall.

Warm cloud depths and precipitable water values look seasonal through much of the event, though above normal later Monday into Monday night. This is when the latest NAEFS and ECMWF 850mb specific humidity are pinging at the 90th to 97.5th percentile with precipitable water values above 1.5 inches. ECMWF extreme forecast index (EFI) shows 70 to 80% of its ensemble members pointing to an anomalous, but not extreme event relative to its model climate and that the event will outperform the ensemble mean. While the 6z run of the NAM showed a 1 to 2 inch event Sunday night into Monday morning, much of the 12z guidance is pointing to amounts under an inch if not a half an inch that is focused south of I-80 Sunday night. This is also shown in the ensemble data with the 17/12z HRRR showing the higher rainfall amounts closer to I-70 in Missouri Sunday night. This should keep river responses more muted and flash flooding concerns near zero given the dry conditions discussed above. Monday night into Tuesday morning looks to contain the higher rainfall amounts with ensemble means showing 1 to 2 inches of additional rainfall over southern into central Iowa with amounts likely under an inch over northern Iowa. Of course, convective elements will have locally higher rain totals. All told, southern Iowa has anywhere from a 50 to 80% chance of at least 2 inches of rainfall for the event total (Sunday night through Wednesday) from the various and grand ensemble means.

Given the rainfall amounts, WPC has issued slight risks of excessive rainfall outlooks (EROs) over some portion of Iowa both Monday and Tuesday. With 1 hour flash flood guidance (FFG) values ranging from 2.7 inches over southern Iowa to 1.75 inches over northern Iowa and 6 hour FFG ranging from 4.5 inches over southern Iowa to 1.75 inches over northern Iowa, thinking is that dry soils and these high FFG values will limit flash flooding and river flooding concerns. The exception would be urban centers, such as the Des Moines metro, and if these areas were able to accumulate 2 or more inches in a short period of time this could lead to flash flooding and urban stream concerns. Of course, the rainfall would have to fall over these basins and placement issues with the highest rainfall amounts remain yet to be resolved. For rivers, latest QPF ensemble hydrographs and experimental Hydrologic Ensemble Forecasting Service (HEFS) show a half a dozen to a dozen forecast points above action stage with most of those not reaching flood stage. These assume the 95th percentile rainfall (QPF ensemble hydrograph) and 10% chance of exceedance (HEFS). With this guidance in mind and the capacity in the larger waterways and dry conditions able to absorb some of this rainfall rather than route directly to the streams, mainly within bank rises are expected on gaged rivers and streams.

As rainfall ends Wednesday, drier air will arrive as high pressure pushes into the region for Thursday into at least a part of Friday.
North and later northwesterly flow aloft will keep conditions on the cooler side through Friday with a low chance of a few showers or storms by Friday as a shortwave trough moves through the flow.

AVIATION /00Z TAFS THROUGH 00Z SUNDAY/
Issued at 629 PM CDT Sat May 17 2025

MVFR stratus continues to skim KMCW but is expected to lift north in the next couple hours. Then VFR conditions will prevail across all sites through the rest of the period. Wind will continue to diminish this evening shift through the overnight and Sunday morning, becoming light and out of the east by Sunday afternoon.

DMX WATCHES/WARNINGS/ADVISORIES
None.