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FXUS63 KDMX 091129 AFDDMX

Area Forecast Discussion National Weather Service Des Moines IA 529 AM CST Sat Dec 9 2023

Updated for the 12z Aviation Discussion

DISCUSSION
/Today through Friday/ Issued at 200 AM CST Sat Dec 9 2023

Key Messages:

- Strengthening northwest winds today, with gusts to around 40 mph northwest by this afternoon.
- Light snow showers at times today, mainly north and northeast but possibly into central Iowa. Only a light dusting of accumulation expected.
- The forecast is dry and quiet from tonight through at least Thursday, then some rain chances return at the end of the week.

A large storm system will track eastward across Minnesota today and over Wisconsin by tonight. Currently this system is producing a swath of accumulating snow and blowing snow over the eastern Dakotas, while ahead of the low light rain is falling from Northern Missouri across southeastern Iowa and into Illinois and Wisconsin. The latter is producing light, spotty rain in our southeastern counties but with no real accumulation or impact, and this will move out over the next few hours. As the parent system progresses eastward today, a tighter pressure gradient and advancing cold air advection will support substantially stronger winds spreading across Iowa, particularly from around midday into the afternoon. Forecast soundings indicate potential for gusts to around 40 mph in our northwestern counties at times, which is just below advisory criteria, but if this this potential increases it is possible a relatively small, targeted Wind Advisory may become necessary today. Aside from the windy conditions, the other forecast issue with the passage of this storm system today is the development of light snow showers over about the northern half of Iowa. The low pressure center to our north will be slowly pulling away and weakening during the afternoon, but will still be capable of producing a shield of light snow that will likely affect at least our northern and northeastern counties, with flurries or light snow showers perhaps reaching as far south as I-80. Any accumulation or impact from the snow showers should be minimal however, as air and surface temperatures will hover above freezing and QPF is very light. Some areas may see a dusting, but other than briefly reduced visibility as snow showers move through, blowing snow should not be an issue with bare ground and so little accumulation.

As the system moves off to the east tonight, we will see gradually diminishing winds overnight and any lingering snow showers will exit the service area by this evening. From Sunday through Tuesday Iowa will reside beneath more benign northwesterly steering flow aloft, resulting in a rapid series of surface ridges and troughs that will delay appreciable airmass modification and keep temperatures relatively cool, with daily highs generally in the 30s to lower 40s and no additional precipitation forecast.

By Wednesday a large cutoff 500 mb low will develop over the southwestern U.S., and the weakly cyclonic northwesterly flow over Iowa earlier in the week will shift northeastward over the Great Lakes. Between these two systems, weak ridging will pinch off over the Midwest and we will see a return of south/southwest low-level flow and warm air advection, pushing temperatures up into the 50s in most areas by Thursday afternoon. Meanwhile, the southwest U.S. gyre will slowly drift eastward, moving over or near Kansas and Missouri and approaching Iowa at the end of this week. This will bring a return of precipitation chances on Friday, likely in the form of rain given the forecast temperatures. However, long range models are not handling the details of the system evolution very well, and precipitation is hardly a sure bet and would likely be relatively light even if it does pan out. Sorry to burst the bubble of those hoping to put any appreciable moisture back into the ground this week.

AVIATION
/For the 12Z TAFS through 12Z Sunday morning/ Issued at 529 AM CST Sat Dec 9 2023

Low ceilings will spread across the area this morning and persist into tonight. This will result in prolonged periods of MVFR conditions, and brief IFR conditions may be possible at MCW but probability of reaching that level is too low to advertise in TAFs at this time. Light SHSN is also expected at MCW and ALO later today, however any impacts to visibility would be brief so have only included VCSH at those terminals. Finally, strong WNW winds are forecast from late morning into this evening, including gusts approaching 30 KT, which could result in strong crosswinds on some runways.

DMX WATCHES/WARNINGS/ADVISORIES
NONE.

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