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A complex winter storm will bring the kitchen sink to my area later Tuesday and Tuesday night. Precipitation starts as snow and then transitions to sleet and freezing rain in the central and south. South of HWY 34 it even warms enough for a period of rain and drizzle where the freezing line inches further north.

At the time of this post these are the warnings and advisories in effect from the NWS. A large portion of my area (save for the far southeast) is under winter storm warnings.

Here's a larger perspective showing portions of 11 states under warnings or advisories for winter weather.

Most of the area north of I-80 has 90-95 percent odds of 2 or more inches of snow.

The winter storm severity index designed to give people an idea of the storms strength has moderate to even major impacts indicated for much of the general area near and north of I-80.

Here's the satellite showing the storm coming together late Monday night.

At the base of the longwave trough out west a closed low is situated and seen nicely on the GOES water vapor imagery. This short wave ejects across the region Tuesday allowing a surface reflection that tracks from Missouri into EC Iowa Tuesday night. Ahead of this feature strong convergence and warm air advection overspreads the region thanks to a 100 mph+ knot right entrance jet. That allows snow to quickly form Tuesday morning in the south and overspread the rest of the area by mid to late afternoon.

Strong lift will lead to high snow rates of up to an inch an hour and conditions will quickly deteriorate once snow begins. Moisture levels are quite high and liquid totals from the storm are likely to go over an inch in spots. If that were to stay all snow with ratios of 10:1 amounts would be in the 10-12" range. However, the strong warm air advection sticks a tongue of mild air just above the surface at 850mb into southern half of my area with time, especially near and south of HWY 30. That means snow in these areas will gradually turn to sleet, freezing rain, and perhaps even rain south of I-80 Tuesday night.

To make a long story short, this makes for a very tough snowfall forecast when transitions are involved. Snow ratios are impacted altering amounts and the timing of changeovers becomes extremely critical. Slower transitions mean more snow and quicker ones less. That could also increase the potential for higher ice and sleet accumulations. At this time, it seems as though the area northwest of a line from Des Moines to near Cedar Rapids and on to Dubuque should stay mainly snow and that is where the heaviest accumulations should be found on the order of 6-10". From that line to another one extending north of Ottumwa to Iowa City and on to Clinton. 3-6" amounts are possible with a transition to sleet and freezing rain at some point. South of that line where winter weather advisories are posted 1-3" snow totals are likely with sleet, freezing rain, and even rain expected. At some point where mixed precipitation develops, it will turn back to light snow before ending Wednesday morning but only minor accumulations are likely.

Again, I can't stress enough how important the freezing line at 850 will be to snow and ice accumulations. That will need to be monitored as Tuesday unfolds as it could certainly change the outcome of amounts in both of those categories. A shift north or south could make a significant difference in the placement of the heavy snow band and how much falls in any one location. Confidence in snow accumulations is definitely lower in the southeast half of my area due to this issue!

With that, here are the latest snowfall forecasts as of Thursday night. Remember, these are not official forecasts, just model guidance that official NWS forecasts are made from. Look for consistency in model trends to find the truth. I suspect the EURO will end up close when it's all said and done.



The Canadian GEM

The 3k NAM

The 12k NAM

The 3k HRRR

I wont get into this any deeper but the EURO shows this for sleet and freezing rain accumulations.


Freezing rain.

Once this gets out of our hair we'll have a short break before another storm arrives Friday. This too will have precipitation type issues that involve rain, snow, and perhaps ice transitions. Needless to say I've not gotten deep into it but wanted you to know it was on the table Friday and Friday night (new Years Day). It should be a quick hitter but does bring the potential for more accumulating snow in some part of the area depending on track and thermal profiles. Stay tuned and roll weather...TS


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