460 FXUS63 KDMX 200804 AFDDMXArea Forecast Discussion National Weather Service Des Moines IA 304 AM CDT Sat Sep 20 2025
.KEY MESSAGES...
- Scattered showers south this morning, then north this afternoon. Thunder potential is highest north (25-30%) and only around 20% south. No severe weather expected at this time.
- Fog potential tonight, mainly in southwest Iowa.
- Lingering showers and thunderstorms will gradually clear out toward the east on Saturday, with a brief spell of quiet weather around Saturday night. Rain chances then return from around Sunday afternoon through much of next week. Severe weather chances remain low on Saturday, and for next week will be determined in the coming days.
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.UPDATE... Issued at 301 AM CDT Sat Sep 20 2025
Height rises will dampen vertical motion in the state today and poor lapse rates lower the convective potential. Rain overnight has come in delayed by around 2 hours, and has had little in the way of lightning activity, per NLDN, but still outpaces the MCS in the Central Plains. Regionally, it sits in a relative minima of MUCAPE (200 J/kg or less) so the chance for lightning is low. The warm front in the Central Plains will continue to inhibit the return of instability in the daytime, maximizing MUCAPE values at 500 J/kg or less. The H850 wind max will continue arch across the state today and give just enough moisture and kinematics to bring a return of showers and a few thunderstorms in northern Iowa, albeit also delayed (another change in the overnight hours. The chance for stronger storms continues to decrease as lapse rates become less favorable and the LFC-EL height shrinks in soundings. Any storm that does get going today, it would be most likely in northern Iowa.
Another short term concern is fog potential for tonight, mainly in the south, as a surface high centers over that part of the state. HREF probabilities already ping southwest Iowa with >50% of having fog with visibilities under 1/2 mile, this being driven by members that may be clearing skies too quickly, encouraging stronger radiational cooling. At the very least, there could be a stubborn low-level stratus deck in the wake of the rain from today, with more on the way from the northwest too.
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.DISCUSSION... Issued at 158 PM CDT Fri Sep 19 2025
A large upper low continues to spin over the region, primarily centered over eastern South Dakota down into northwest Iowa. A shortwave trough is rounding the low over central and eastern Nebraska, generating renewed cloud and precipitation development across that area this afternoon. Further east, a surface boundary (effective dryline) is sweeping through central into eastern Iowa at this time, generating a thin line of showers stretching from north to south roughly from around Mason City to Centerville. As we progress through this afternoon these showers will continue to push eastward producing spotty light rainfall, and probably isolated thunderstorms as instability gradually increases. While surface-based CAPE of 1000-1500 J/kg is forecast, deep-layer shear is very weak (10-15 KT over the 0-6 KM layer) and mid-level lapse rates are only around 6.0-6.5 C/KM, greatly reducing any severe weather threat. Weak low-level convergence along the boundary may provide a slight enhancement to justify isolated small hail/gusty winds however. Also, 0-3 KM CAPE of 100-125 J/kg and enhanced stretching potential near the boundary may support some brief weak funnel clouds, although widespread cloud cover and modest low-level lapse rates would seem to mitigate that potential. Overall any impacts from showers/thunderstorms today will be minimal, and POPs are capped around 40% in our northeastern and eastern counties.
Tonight, as the aforementioned shortwave aloft moves through the region, the showers and storms over Nebraska will translate east southeastward into parts of Missouri and Iowa. Most of the models have been having difficulty resolving both the timing and latitudinal position of the resulting swath of rainfall, with most of them initializing the precipitation too far west/slower than indicated by regional radar trends. In addition, a low- level jet is expected to develop this evening over Kansas and veer into Missouri overnight, which may generate an additional cluster of convection in that region that could cut off some of the push for storms further north in Iowa. However, there is near-unanimous agreement in a slug of Theta-E advection and region of diffluence extending over southern Iowa, and still believe rain will move through parts of our forecast area overnight accordingly. Have increased POPs to 60% in some of our southern counties, but resisted the temptation to go higher because confidence in the areal coverage and northward extent of the rain is lower. While severe weather is not anticipated with the overnight convection, locally heavy rainfall may be possible given the expected orientation of the band. Precipitable water values will only be around 1 inch however, and overall the rain should be progressive, mitigating the heavy rain potential.
One other item of note for tonight is that winds will become light and variable and near-surface moisture will remain high, with dewpoints in the upper 50s to lower 60s, so patchy fog may redevelop. Some high-resolution models are indicating this, but generally outside our forecast area. Any dense fog development would likely be contingent on areas of clearing that should be few and far between, and cannot be predicted with any confidence at this range, so will not mention fog in the official forecast but it will be something to keep an eye on as the night progresses.
On Saturday the upper low will finally begin to pull out to the northeast, taking the clouds and rain with it. Precipitation will move out slowly but steadily during the day, and there may be a bit of a resurgence in the afternoon/evening if diurnal heating is sufficient to destabilize enough for a few showers or storms to form. Once again severe weather will be unlikely, as deep-layer shear will be a bit stronger in the 20-30 KT range, but instability will be more limited. Saturday night will be relatively quiet, then on Sunday one last lobe of vorticity will dig down over Minnesota on the backside of the departing low, and may approach near enough to northern Iowa to generate some showers or thunderstorms late Sunday into Sunday night. Thereafter, the forecast for next week remains relatively (and unfortunately) active as another large-scale upper level trough develops right on the heels of the departing one, eventually forming a closed 500 MB low somewhere near Missouri by the middle of next week. This synoptic progression results in POPs being carried through next week, every day in at least portions of Iowa, with any details of severe weather/heavy rain hazards to be sorted out in the coming days. Meanwhile, temperatures will remain nearly steady with daily highs in the 70s to perhaps lower 80s at times.
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.AVIATION /06Z TAFS THROUGH 06Z SUNDAY/... Issued at 1056 PM CDT Fri Sep 19 2025
VFR and dry conditions prevail over the area late this evening, with showers beginning to creep into the southwest and drifting northeastward through the morning. Most likely location for showers and isolated thunder Saturday morning will be at KDSM and KOTM, while confidence continues to decrease in shower impacts at KFOD, KMCW and KALO. That said, have maintained PROB30 mention at KMCW and KALO for shower/storm potential mid- day through the afternoon. Lightning will be most likely with afternoon activity, but will be possible at any time of day. That said, thunderstorm activity will be fairly isolated, so have not included thunder in TAFs. MVFR ceilings and visibility may accompany showers/storms.
A low potential for patchy fog also remains, especially at KMCW and KALO, but expectation is for this to be fairly isolated with minimal impacts. Therefore, have kept fog mention out of TAFs at this time.
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.DMX WATCHES/WARNINGS/ADVISORIES... None.
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UPDATE...Jimenez DISCUSSION...Lee AVIATION...Dodson
NWS DMX Office Area Forecast Discussion