MISSISSIPPI FLOOD THREAT RISES
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ONE MORE FOR THE ROAD...
The system that brought widespread snow to the region Thursday has passed leaving the region with a quiet but cold start to the weekend. Clouds will thicken again Saturday and more snow is expected to develop Saturday night. Accumulations of 1-2" are possible. More on the potential later in this post.
First, the snow totals from Thursday's event, the 20th winter storm to impact some part of Iowa this winter.
Here's a regional perspective. Much of SE Wisconsin picked up 8-12" of new snow.
With the recent addition of snow across Minnesota and Wisconsin, along with a forecast containing more snow and below normal temperatures through March, flooding concerns on the Mississippi have increased in the latest spring flood outlook from the NWS.
This new report from the NWS in Minneapolis states, "the outlook for spring flooding in the upper Mississippi has been upgraded to well above normal, particularly on the Mississippi from St. Paul downstream. The addition of more rain and snow over the past two weeks has raised the amount of water in the snowpack to very high levels for this time of year".
The report goes on to say, "Factors that could help alleviate at least some of the snowmelt threat are becoming less relevant the longer the snowpack holds on. And as always, the threat of seeing major flooding will still depend on what kind of rainfall/temperature patterns we get later in March and April. Due to the nature of this year's flood potential, we will issue an additional spring flood outlook on March 23rd, 2023".
With approximately a month left in the snow season over the upper Midwest, snow totals continue to mount over the headwaters of the Mississippi. The NWS in Minneapolis reports that the seasonal snow total there has reached 91.3 inches with 4-5 more inches possible Saturday night. Below you can see the yearly snowfall totals through March 10th. Note the extremely heavy band of 65 to 90 inches from SC South Dakota through central Minnesota into NW Wisconsin.
Here's a closer view of seasonal snow totals centered on Minneapolis.
Most of the Mississippi tributary basins have 3 to 7 inches of Snow Water Equivalent (SWE) on the ground.
Comparing this year's SWE to normal: this winter ranks in the 90th percentile across the upper Mississippi River basin. That means over the past 70 years, we've only seen 5 to 10 years where snow water content was comparable to this year through early March.
By adding to the snowpack in February and early March, the flood probabilities have risen in response. Alleviating factors such as soil moisture and low water levels are going to have less relevance as we move through March, while shallow frost depth will help a little. The weather outlook through March over the upper Midwest is for continued cooler than normal conditions, which could allow for the snowpack to remain in place as we move toward April and and increase the probabilities of seeing a warm and/or wet weather system after that. Thus, the spring flood threat is well above normal, especially for the Mississippi River from St. Paul downstream. It's now not so much a question of if we will see flooding, but how severe and widespread will it be? The severity of flooding will depend on if we receive heavy rainfall and/or very warm temperatures during the melt.
As it stands now, the conditional simulation of the Mississppi River at lock and dam 15 in Rock Island, shows a 50 percent chance of a 20 foot crest. The threshold for major flooding is 18 feet. Odds are at 25 percent for a 22 foot crest. (the record crest is 22.7' in 2019) Again, this is all based on current conditions. These numbers will fluctuate up or down based on additional snow, the speed of the thaw, and spring rains.
MARCH MADNESS AT MY AIRBNB IN GALENA CLICK THE BANNER FOR MORE...
ANOTHER SNOW SYSTEM...
Snow (generally light) is expected to return to the region Saturday night thanks to warm air advection ahead of an elongated trough. One surface low tracks through Minnesota and a second crosses Oklahoma. Between the two, pressures lower across the Plains enhancing lift and drawing moisture northward. That sets the stage for a round of snow.
Thermal profiles look sufficient for snow in most areas, although the south could start with a bit of rain or a mix before quickly changing to snow. Models had been consistent in generating .20" to .30" of liquid precip. However, the latest runs have come in more in the range of .10 to .20". There is also a trend for a weakening snow band east of the Mississippi. With relatively mild temperatures, snow ratios will again be low, no higher than 10:1. That should produce snows of 1-2 inches over many areas Saturday night. By no means is this a high impact event but it is one that could create some low end travel impacts.
Here's what models are suggesting for snowfall totals.
The 12k NAM
The 3k NAM
Any lingering snow moves out quickly Sunday morning leaving us with cool quiet weather through Tuesday. A warm-up is anticipated Wednesday that could lead to some showers by Thursday. Hopefully models will come to better agreement on how energy is resolved later Saturday. Until then, have a fine weekend and roll weather...TS