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FXUS63 KDMX 052333 AFDDMX

Area Forecast Discussion National Weather Service Des Moines IA 533 PM CST Tue Dec 5 2023

Updated for the 00z Aviation Discussion

DISCUSSION
/Tonight through Tuesday/ Issued at 315 PM CST Tue Dec 5 2023

Key Messages: - Warmer days Wednesday and more so Thursday with record highs challenged in a few places - Rain and light snow chances trending upward over southeast Iowa this weekend - Return to seasonal conditions this weekend into next week

Details: Early afternoon visible satellite imagery shows stratus clouds, which are slowly eroding and giving way to sunshine over western Iowa. The clouds are associated with a departing surface low, which has created gusty winds this afternoon. Winds will decrease into tonight with high pressure pressing into the region.
With the winds become light and the clear sky, could see the potential for patchy fog over eastern portions of the state per HRRR mean visibility probabilities. As the longer wave trough pushes east tonight, strong low level warm air advection (WAA)
will ensue with rising mid- level heights and a ridge moving over the central US Wednesday into Thursday. BUFKIT soundings show this strong WAA with 850mb temperatures going from around 0C at 12z Wednesday to around 10-11C by 0z Thursday 12 hours later. Surface winds will also turn breezy during the day as the high slides off to the east and the surface pressure gradient increases. QG convergence arrives in concert with the low level WAA as it pushes eastward across the state and while forecast soundings and cross sections show moistening in the mid-levels, not expecting more than some mid or high level clouds on Wednesday as the overall column is not saturated. Highs should out perform, especially across western Iowa where more full sunshine is expected compared to farther east in the state. Thus, increased temperatures to 75th percentile of National Blend of Models (NBM) resulting in 40s over eastern Iowa to 50s over western Iowa. Thursday will feature breezy winds again during the daytime and more sunshine with a dry atmospheric column per forecast soundings and cross sections.
This will result in even higher temperatures as they surge well into the 50s to around 60 degrees. Record highs still look to be challenged in some places, with Mason City and Waterloo the more favored locations based on NBM probability of exceeding the record high.

A northern stream lead shortwave will flatten the ridge Thursday night into Friday and send a cold front through the state. While Iowa will stay dry, temperatures will begin a downward trend Friday with more typical early December temperatures this weekend in the middle 30s to around 40 degrees. Another shortwave piece of energy will follow closely behind the first, but will dig into the southern Plains before lifting northeastward toward the Ohio Valley and Great Lakes. Deterministic data shows the trough becoming neutrally tilted over or just east of our longitude, but there are differences in the deterministic and ensemble suite of data as how far equatorward it digs and whether it will be an open or closed wave. That said, there is growing consensus in deterministic models (GFS, CMC, ECMWF, and now NAM) and slowly in the ensemble land of precipitation over at least some portion of the state (namely southeastern Iowa) starting later Friday night through Saturday. This is shown also in the upward trend in the initial NBM precipitation chances, which are now up to 60% over southeastern Iowa (from 40% yesterday at this time)
and 20 to 40% over a good portion of the state trailing off with northwestward extent.

The 0z/12z CMC provides more of a glancing blow with QPF over the southern half or so of the state, but the GFS and now the 12z ECMWF bring two waves of precipitation over at least the southeastern half of the state - the first with the low level thermal lift and the second by the deformation zone. NBM unconditional precipitation type probabilities do favor rain over the southeastern half or so of the state into Saturday and this seems supported by GFS and ECMWF ensemble soundings at Des Moines and Ottumwa. Colder air will wrap in and result in light snow on the northwestern side of the precipitation shield and there are a handful of ensemble members that show a stripe of higher snowfall draped somewhere across the region. However, these are the minority at this time noting that the 12z ECMWF deterministic snowfall at Des Moines lies around the 75th percentile when looking across its members and above the ensemble mean. This is not to say the higher snowfall could not occur, particularly if the column dynamically cools causing a faster transition to snow, but this solution is far from certain given uncertainties in placement of lift, thermal structure, and northward extent based on low pressure track. While data seems to be trending upwards precipitation chances on Iowa, 0z cluster analysis shows that about 40% of ensemble members have a weaker, less deep trough and would result in less than the ensemble mean QPF. Winds will increase later Saturday into Sunday as the surface low passes southeast of Iowa and up into the Great Lakes. Following behind this system will be northwesterly flow favoring drier conditions and temperatures remaining near seasonal levels.

AVIATION
/For the 00Z TAFS through 00Z Wednesday evening/ Issued at 532 PM CST Tue Dec 5 2023

Lower ceilings in eastern Iowa will depart with widespread VFR conditions for much of the area this evening. However, some fog may develop across eastern portions of the forecast area tonight, possibly affecting Mason City, Waterloo and Ottumwa airports and will monitor closely. Otherwise, winds remain light overnight but increase quickly on Wednesday morning from the south to southwest.
Widespread VFR conditions are also expected into Wednesday.

DMX WATCHES/WARNINGS/ADVISORIES
NONE.

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