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FXUS63 KDMX 302333 AFDDMX

Area Forecast Discussion National Weather Service Des Moines IA 533 PM CST Thu Nov 30 2023

Updated for the 00z Aviation Discussion

DISCUSSION
/Tonight through Thursday/ Issued at 249 PM CST Thu Nov 30 2023

Key Messages:

- Round 1: A wintry mix overnight into Friday morning with light accumulations mainly along and south of I-80 - Round 2: Light rain and snow Friday afternoon and evening once again mainly along and south of I-80 with little accumulation - Round 3: Light rain and snow manly east of I-35 on Saturday night with little accumulation

Discussion:

We are entering an active period of wintry weather over the next couple days. While three separate rounds of precipitation are forecast, each will only result in light accumulation.

Round One: A low is lifting out of the southern plains and into the central US this afternoon. By this evening the northern side of the low will approach southern Iowa with precipitation expanding across southern Iowa overnight. Models are in good agreement with timing as precipitation arrives after midnight and departs by late morning. Precipitation type gets a little messier with soundings indicating variations in ice introduction, surface temperature, and depth of the warm layer. With much of the lower portion of the column near freezing, small fluctuations will be impactful for precipitation type. The NAM remains most aggressive on freezing rain with coldest surface temperatures. GFS and Euro suites include better ice introduction, but with surface temperatures near freezing, it will be a thin line on rain and snow. As this system fights dry air north, expect that the northern cut off could be rather sharp from dry to the wintry mix. Better chances to any ice accumulations would be far south with rain and snow along and south of the I-80 corridor, trending towards all rain towards the southeast.

Round Two: As round one exits east midday Friday, a weak embedded shortwave will follow quickly on its heels bringing another quick shot at wintry precipitation on Friday late afternoon and evening.
With less available moisture and a little less forcing, QPF with this round is lighter. With warmer daytime temperatures, this round is expected as mostly rain, with some snow mixing in especially into the evening. Primary uncertainty with this round is timing of the second round, as some of the CAMs maintain distinct waves while the HRRR has trended towards a more seamless transition from round one to two. There is also some question as to snow accumulations. With the column remains deeply saturated thorugh the DGZ, near surface temperatures hover right at freezing in soundings from Lamoni to Ottumwa. Given good saturation to the surface, expect little wetbulb influence to cool the column and push things one way or the other.

Round Three: After a lull, the parent trough axis passes across the area Saturday evening and overnight. Precipitation here reaches further north, but recent model runs have pulled the system further east with main QPF along and east of the I-35 corridor.
The primary precipitation type here will be rain, however some light snow may mix in overnight Saturday as the column cools and ice introduction increases. Any accumulations with this final round would be very light, a dusting or so.

Into next week a quick moving shortwave swings across the area on Tuesday, delivering another shot at at wintry precipitation. This will be followed by a warming trend to finish out the week with above average temperatures.

AVIATION
/For the 00Z TAFS through 00Z Friday evening/ Issued at 531 PM CST Thu Nov 30 2023

Conditions will deteriorate across southern Iowa overnight and into Tuesday and will impact KOTM with lowering cigs and increasing chances for rain and snow. Conditions are expected to reach IFR or lower at times. KDSM likely will have cigs lower to MVFR for a period on Friday and those conditions could briefly reach KALO. Otherwise VFR at KFOD/KMCW for the period. Northern wind will become northeasterly overnight. The wind may become breezy to gusty at times Friday, especially in the vicinity of KOTM.

DMX WATCHES/WARNINGS/ADVISORIES
Winter Weather Advisory from midnight tonight to noon CST Friday for IAZ062-073>075-082>086-092>097.

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