Area Forecast Discussion
Issued by NWS Des Moines, IA

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000 FXUS63 KDMX 240445 AFDDMX Area Forecast Discussion National Weather Service Des Moines IA 1145 PM CDT Sat Mar 23 2024 ...Updated for the 06z Aviation Discussion... .KEY MESSAGES... - Light snowfall this afternoon and evening with accumulations generally under an inch. - Stronger system moving into the area tomorrow, initially bringing moderate to heavy snowfall rates in the northern half of the state then switching to rain area-wide by the evening. - Gusty conditions Sunday night into Monday with a few stronger thunderstorms possible during the day Monday. && .DISCUSSION... Issued at 316 PM CDT Sat Mar 23 2024 Increasing clouds across the area are the first signs of this weekend’s active pattern creeping into the area. Light snow has begun to fall over the western portion of the state and will continue to slowly make it’s way east through the evening. This precipitation has struggled initially to overcome the low level dry air, lagging slightly behind when it was expected yesterday. Better forcing and saturation is being realized further south which is allowing for precipitation in this area as well. Some accumulation is possible this afternoon and evening, but should be generally less than an inch. This snowfall is resulting from weak energy and Pacific moisture passing through the upper level flow, as an H500 trough begins to dig in over the west coast. This upper level trough and it’s associated surface low will be our main weather bringers starting early tomorrow morning through much of the day Monday. Large scale and mesoscale forcing will be plentiful as the surface low begins to churn to our southwest, and strong theta-e advection streams into the area. The primary challenge with this forecast lies with the progression and evolution of the precipitation, as snow falls initially before switching over to rain through the day Sunday. The million dollar question is how much snow will fall before temperatures increase and rain takes over as the dominant precipitation type. As of right now, this looks to be of no concern for much of central and southern Iowa as the transition will occur quickly and bring only a brief period of snowfall with minimal accumulations. However, the tricky locations will be further north where the forcing increases and frontogenesis is maximized. In northern Iowa, there looks like there may be multiple hours of moderate to heavy snowfall occurring before the switch to rain. Recent trends in CAMs and global models have shown a slightly southeastward trend in higher totals through the last 24 hrs, with probabilistic output from the HREF and LREF pinging a 50 to 70% chance of seeing snowfall greater than 3 inches near the Iowa/Minnesota border and increasing exponentially further north. This is reflected by forecast soundings near Mason City, which show a deep moist layer with strong lift through the DGZ and a near isothermal layer near 0C, suggesting intense snowfall rates and large aggregates. This would lead to heavy, wet snow, with temperatures near freezing in the lower levels and at the surface, making accumulations difficult to quantify. Right now, running with an expected snowfall of around 3 inches in our far northeastern counties. However, this will be a high “boom or bust” scenario, as a faster progression of the warm air would all but eliminate the snowfall amounts in NE IA and keep us well below the 3 inch forecast but a slower progression would shift the heavier snow totals further south and above the 3 inch forecast. Regardless of the bust potential, have decided to go with a small Winter Weather Advisory in this area, just to raise some awareness of the potential for some heavier snowfall on Sunday near the I-35 corridor and will allow future shifts to adjust this headline should the trend continue towards higher snow totals. The impacts from precipitation wane once the snowfall switches completely over to rain by Sunday evening, however, the system is far but done at this point. Winds will increase as the surface low draws nearer and pressure gradients tighten Sunday afternoon through Monday. The wind gust magnitudes have diminished some since this time yesterday, with Sunday looking to have widespread gusts of 30 to 35 kts and Monday having slightly higher values but only a few areas pushing over 40 kts. Therefore, have held off on any headlines due to the anticipated winter weather as well as the greater wind risk being on Monday. That being said, will want to continue to monitor wind trends and prepare for gusty conditions to start next week. Finally, models continue to hint at thunderstorm potential on Monday as the surface low passes through western Iowa and puts central Iowa right in the warm sector. Not much has changed in our thinking for severe threats since this time yesterday, as any stronger storms will have plenty of shear to work with, but will be limited by the amount of instability that can be realized. SPC issued a day 3 marginal risk for our area with the arrival of the warm sector and coincident cold front, which is reasonable given the conditional threat for severe weather. With it still being outside of the high- resolution guidance window, not going to speculate on too much at this time, but given the setup and parameter space, all severe hazards would be possible provided any strong storms develop. && .AVIATION /06Z TAFS THROUGH 06Z MONDAY/...
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Issued at 1145 PM CDT Sat Mar 23 2024 Light snow is ongoing across the northern terminals early this morning and is expected to continue over the next few hours or so, which will allow for continued MVFR/IFR conditions due to ceilings and visibility restrictions. There will be a brief dry period for a few hours across the state before sunrise, when the main wave associated with the large low pressure system circulating further west pushes into the region. This will bring returning widespread snow mainly over KMCW and KALO, while the remaining terminals see a wintery mix around 12z through at least mid to late morning. Following this, warmer air surging into the state with increasing gusty southerly winds will allow for a transition to rain around 18-21z from south to north. Overall, expecting continued periods of MVFR/IFR conditions with this precipitation. Rain will linger through the rest of the day, though may be on and off in nature.
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&& .DMX WATCHES/WARNINGS/ADVISORIES... Winter Weather Advisory from 9 AM to 7 PM CDT Sunday for IAZ006-007-016-017. && $$ DISCUSSION...Dodson AVIATION...Bury

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