textproduct: Des Moines

KEY MESSAGES

- Patchy, low-lying frost for a few hours 3 to 7 am Thursday (tonight).

- Several chances for showers and thunderstorms from Thursday night through early next week. Highest chance up to 60 to 70% Saturday night over southern Iowa. No apparent signal for significant severe weather or heavy rainfall with any storm chances.

SHORT TERM /THROUGH THURSDAY NIGHT/

Issued at 128 PM CDT Wed May 6 2026

Winds from the northwest have continued to reinforce the cool and dry conditions across the region today as surface high pressure moves southeastward over the Rockies onto the Colorado Plains. While there was an area of clouds over northern Iowa to start this morning, the steep lapse rates have allowed for cumulus clouds to develop and become widespread as the profile saturates around 5- 7kft. Below this saturated layer, the boundary layer is quite dry with an inverted V sounding. This should result in efficient momentum transport and have increased winds above initial National Blend of Models along with lowering dewpoints and resultant RHs. These brisk breezes will be pared with RHs around or less than 30% over western into southern Iowa; however, increasing green fuels will mitigate any fast fire spread concerns.

The diurnal cumulus clouds this afternoon will dissipate after sunset. However, additional low and mid clouds will spread in after midnight as weak to perhaps modest theta-e advection and QG convergence begins to impinge on the state ahead of the next shortwave trough within the broad cyclonic flow. With the low dewpoints, temperatures may fall off quickly as the cumulus dissipate, but stabilize as the clouds move back in. The frost potential, therefore, is tricky with the incoming clouds. Current thinking is that frost that does form will not be widespread or prolonged and more transient. The favored locations for this short- lived, transient frost is in and around the Nishnabotna River Valley and low areas in the Iowa and Cedar River basins.

The aforementioned forcing on Thursday will pass through the state, but lacks much in the way of moisture in the atmosphere. Convective allowing models (CAMs) are painting model reflectivity returns, but not much in the way of QPF outside of the NAMNest, NAM, and GFS. Given the amount of dry air, especially sub-cloud, along with the generally weak forcing, have kept any shower mention out and may look at adding sprinkles in a future update. Given the dry boundary layer, this does raise the concern for virga gusty winds. Downdraft CAPE values never exceed 500 J/kg and CAMs are not showing any gusts higher than 30 knots in their model fields. In addition, the deep boundary layer mixing will once again lower dewpoints with resultant RHs dropping below 25% west and north of an Atlantic to Ames to Waterloo line. One to two months ago, this low of RH would have led to critical fire weather conditions; however, as discussed above increasing greenness along with the relatively strongest winds of 10 to 20 mph displaced from the lowest RHs, preclude any concern for rapid, uncontrolled fire spread.

LONG TERM /FRIDAY THROUGH TUESDAY/

Issued at 128 PM CDT Wed May 6 2026

Additional shortwaves will move through the region late this week into perhaps early next week. The rough timeframes and possible areas for scattered showers and storms are as follows: 1) late Thursday night into Friday morning, primarily southern Iowa, 2) late Saturday into Sunday morning with chances highest over southern Iowa, and 3) late Monday into Tuesday over the state. In all instances, there is no apparent signal for significant severe weather or heavy rainfall concerns from a pattern recognition and diagnostic standpoint. Examining the ML/AI guidance generally supports this idea with EC forced data pointing to up to 15% chance on Monday/Monday night while the GFS/GEFS is lower or none. Temperatures in this period will largely be around if not above normal.

AVIATION /18Z TAFS THROUGH 18Z THURSDAY/

Issued at 1223 PM CDT Wed May 6 2026

VFR conditions are expected through the end of the TAF period. Winds at the beginning of TAF period will be ranging between 10-15 kts but are expected to decrease to VRB at all sites by 03Z. Winds will slightly increase out of the SW during the morning hours. As we near the end of TAF period a few light sprinkles may develop near OTM but confidence is not strong enough to issue PROB30s at this time.

DMX WATCHES/WARNINGS/ADVISORIES

None.