Significant Rains and Potential Severe Weather On Tap for the Next Several Days
SPEAKERS
Jim du Bois, Kenny Blumenfeld
Jim du Bois 00:00
Well, somebody's about to get pounded. This is Way Over Our Heads. It's a weather and climate podcast. I'm Jim du Bois, Kenny Blumenfeld's a climatologist. Kenny, where you kind of roused from your sleep in the early morning hours of Tuesday, August 24 by rumbling and the sound of something we haven't heard very often this spring and summer...raindrops and, in my case, a NOAA Weather Radio awakened me before the storm actually got here.
Kenny Blumenfeld 00:28
Yeah, I'm glad that you have that available. Everybody should have a NOAA Weather Radio, just like you have a smoke detector. Because, you know, when the storms come up in the middle of the night, how else are you going to know, and sometimes you need to get to safety. So yeah, Jim I, I woke up to the sound of thunder and rain. And then when I looked out the window, there was some wind too. It was quite a storm. You know, and it dropped some decent precipitation across parts of southern Minnesota, right around an inch in most of the Twin Cities area, but areas to the south got anywhere from an inch and a half to three inches of rain, not far from Northfield over towards Red Wing, and then out into parts of Sibley and Renville Counties. So, it was a nice rainfall event. And of course, there had been some, some other rains over the past week in parts in northwestern Minnesota. So, not a bad week--at least in the context of the dry conditions--has not so far been a bad week.
Jim du Bois 01:33
Well, here in the backyard rain gauge, we had 1.22 inches of rain on Tuesday morning. And probably the most dramatic indication of the power of that rainfall was the effect on Minnehaha Creek which happens to flow right behind our house. And I had taken a photo literally standing in the creek bed the day before, that'd be Monday, August 23, And it was virtually entirely dry. And I took another photo in the exact same location, although not standing in the creek bed because the creek bed now had water in it and water flowing at a pretty impressive rate. So, that to me was even beyond what I saw on my rain gauge that really drove home just how impactful that rain was.
Kenny Blumenfeld 02:20
Yeah, you know, Minnehaha Falls had been basically shut off. I mean, it was reduced to just a trickle like a, you know, like a faucet leak essentially over the weekend. And so, to have it flowing and raging, I think the falls would probably be on the very low end of of what's kind of normal the day after that rain. But it's good to know that the creek came back to life a little bit anyway.
Jim du Bois 02:48
Yeah, it was pretty impressive. Well, Kenny, was that kind of the debut performance of a series of rainfall events and possibly severe weather events we will see over the next several days?
Kenny Blumenfeld 03:00
Yeah, it's a, that's a good way of putting it. Jim, I would say that the, you know, so that was Tuesday, then we kind of took Wednesday off, didn't have much. And now on Thursday, when we're recording this, we're kind of looking ahead to a fairly intense rainfall event in parts of southern Minnesota. Don't know exactly where, there's all kinds of uncertainty, even just hours ahead of it. And then kind of a, you know, on and off fairly busy weekend covering a little bit more of the state. So yeah, I think that, you know, you could say that that was a Roman numeral one or I. And now, and then, and then the main event is right now, is going to happen Thursday afternoon and evening and overnight. And then there'll be other follow up events. I mean, these all have some potential for severe weather. But, you know, I think meteorologically, the most interesting aspect is the potential for the first system, the one on Thursday night, to produce some extraordinary rainfall amounts.
Jim du Bois 04:06
Are we talking the possibility of flash flooding in certain areas?
Kenny Blumenfeld 04:12
I think that's accurate. You know, there's a little bit of a disconnect in some of the official products that we're seeing. And when we say products, of course, we're talking about basically, the way that the weather gets summarized by some official forecasting entity, usually part of the National Weather Service. And one of the products that we look at a lot that anybody who follows the weather looks at is this excessive rainfall risk. And it actually is very similar to the severe weather risk categories that the Storm Prediction Center uses. This is a different arm within NOAA. It's called the Weather Prediction Center. It used to be the Hydro Meteorological Prediction Center, but I think that, that wasn't very, that wasn't very friendly to lay populations so they changed it to the Weather Prediction Center. But they do most of the, the large area rainfall forecasting, and they have, they have much of Minnesota, southern half or so of Minnesota and then the northern half or so of Iowa and the western third or so of Wisconsin in a slight risk for excessive rainfall. When they say excessive rain, they mean rain that exceeds the current flash flood guidance. And what that means is that the rainfall rates, there's a slight chance that the rainfall rate, and that would be a 10 to 20% chance that rainfall rates would actually be high enough to exceed capacity and cause some flash flooding. The reason I said that there's a disconnect is I think that that slight risk area that they're using actually is also being used to express some uncertainty. We don't know exactly where the heaviest rains will be. But some of the, some of the models, Jim, have been showing very, very high rainfall totals up to, you know, eight, nine inches in narrow swaths. And of course, these swaths have been anywhere from entral Iowa up to central Minnesota. And so, that's the uncertainty is we don't know exactly where the heaviest rain will be. If I had to guess it would be somewhere along or south of that western arm of the Minnesota River into far southern Minnesota. And it might not be as high as, you know, some of those models might be getting a little bit carried away, but there's a lot of moisture in the atmosphere. And so, the conditions are at least supportive for some training thunderstorms. Those are the repeat thunderstorms that kind of form and move over the same areas, and also for very high rainfall rates during those thunderstorms. So, you know, we could be looking at a major flood event in at least a small area. And of course, this is inside one of the driest summers we've had in a long time. So, it's kind of surreal to be talking about it.
Jim du Bois 07:01
Now, when we're running a significant soil moisture deficit as we are this summer, what happens when it rains heavily? Now, we would tend to think that if the soil is dry, it might absorb the rain, like a sponge, but the reality may be something very different. Is it like the first inch of rain maybe will be absorbed like a sponge, and then any rainfall amount beyond that begins to run off? I mean, it seems almost counterintuitive because you would logically think if soil was saturated with moisture, then additional rains would run off. But it sounds like the contrary might be the case.
Kenny Blumenfeld 07:40
Yeah, I mean, I, you know, honestly, there's going to be soil physicists and hydrologists, who who probably understand these processes better than I do, but I think the harder it rains, well first of all, when it's really dry, we've seen this before, too, when it's really dry, and you have dried dusty topsoil, which is certainly the case in some parts of Minnesota, then actually, the first thing that happens is the heavy rain doesn't even soak into that topsoil, it just washes it away. And that's definitely not great. That's something that you want. So, you actually get some erosion from the heavy rain because there's nothing kind of binding the topsoil down to the rest of the profile. But beyond that, it kind of depends on the rainfall rate. You know, if we were to get a really slow, you know, if we were to get several inches of additional rain but say distributed over a month, which is kind of what the DNR, that's sort of the DNR's message that if we made up five to eight inches over the course of a month, that would significantly alleviate drought conditions, because over the course of a month, there's probably enough time for the soils to absorb a lot of that moisture. I think another way you could look at it as you know, rather than getting it all at once, you'd prefer to have it over a prolonged period of weeks, or even months. So, when you get it all at once, I don't know what the exact quantity is that goes in, it's going to be determined by the rate but you're going to get some of the moisture into the soil but it can only go in so fast. And so then what doesn't make it into the soil while sort of waiting to percolate down, that is then going to start forming puddles and start running off, and that's when you get extra, and especially in urban areas where you have more pavement than soil, you're going to see a lot of runoff if, if it's raining really hard. So, I don't know the exact answer, Jim, but I would say you know, if you are getting several inches of rain in a 24 hour period, don't expect the majority of it to go into, you know, replenish your groundwater and and your soil moisture but expect a lot of it to run off instead.
Jim du Bois 09:51
So in other words, if we're thinking this great rainfall that's coming is going to pull us out of the drought situation, not going to happen, correct?
Kenny Blumenfeld 09:59
Yeah. I mean, you know, it's certainly going to have the potential to undo, or at least, maybe start reducing some of those major deficits in whichever areas get hit the hardest. I think that the long term deficits numerically might get reduced or even in small areas, erased. But that wouldn't necessarily end the drought for those people. Because, again, those are just precipitation deficits. It doesn't account for the this, you know, we've had months and months and months to deplete soil moisture and to deplete the groundwater, the aquifers, those kind of underwater lakes, or underground lakes. And we've had months and months and months to drop the river stream and lake levels. And that's going to all take time to get back. And those are, of course, the major indicators of drought. So yeah, I wouldn't, you know, pin your hopes on getting out of drought with one event, even if the numbers in small areas do support, you know, the deficit being gone. Remember, if it rains really hard, somewhere, within a few days, all that water that fell, or a lot of that water that fell is going to be off in some stream or river 500 miles away. So. it's not entirely local.
Jim du Bois 11:19
Well, Kenny, we are recording this on the morning of Thursday, August 26. And it looks like for each of the next three days, we in parts of Minnesota are going to have the risk of severe weather. And it looks like from the forecast discussions from the National Weather Service that all modes of severe weather potentially might come into the mix. So, what do you think, is this going to be pretty isolated, popping up in very isolated areas? Or could it be something on a grander scale?
Kenny Blumenfeld 11:52
Yeah, I think that, you know, for Thursday, the Storm Prediction Center definitely is looking at the Minnesota-Iowa border as having the highest risk. And that's the area that's going to be the warmest, have the most instability, and that's the area where I think the strong winds in particular, from the squall line would, would tend to be most likely. And that also seems to be the case as we get into Friday because there will be an additional round of storms. But there's, on both of those days, there's a marginal risk for severe weather that does extend up into the Twin Cities area roughly. And then we get into the weekend. And it appears that we have another risk for severe weather on Saturday afternoon, and that covers more of Minnesota. That, that covers the St. Cloud and Twin Cities area. And this, of course will change also, as we get more information, and as some of the forecast confidence increases on where the storms will be. But yeah, I think that in addition to heavy rains over the next three days, you're going to see the potential for some storms to produce hail, high winds, and and even isolated tornadoes. The winds aloft, especially in central and northern Minnesota, are actually pretty strong. And that would be conducive to organizing some of the thunderstorms so that they could either form really large, powerful storm complexes, or even in some cases become supercell thunderstorms. The only real limitation there is that where those strongest winds aloft occur, we might not see the most instability, might be a little cool, not quite humid enough. So, there's not a perfect match there. But it looks like a busy, kind of active weekend. I would say the biggest rainfall event looks like it would be Thursday into Friday morning where there is a potential for some flash flooding and very heavy rains, at least in narrow axes of southern Minnesota somewhere with maybe a more general one to two inch soaking rain type event surrounding that generally in the southern third, maybe southern half of Minnesota, but then additional rainfall chances as we get into Friday afternoon, and then again on Saturday afternoon. And we, you know, might take a couple days off and get back in business again on, as we get into next week. So it's a busy, active time.
Jim du Bois 14:17
Well, definitely keep an eye on the sky and an ear to your favorite source or an eye to your favorite source of weather information and be aware of the potential for severe weather. Let's hope we get some much needed rain but obviously not to the extreme point where we get flash flooding. So, hopefully it hits, it hits that sweet spot.
Kenny Blumenfeld 14:37
Yeah, I wish I could promise that. One of the things that the storm has is a huge amount of what meteorologists called precipitable water. That's basically if you take all of the moisture in the atmosphere, if you were able to just turn that into rain, some of the precipitable water quantities coming off of these climate models will be approaching, you know, all time highs for the date. And so, there's a lot of moisture, it's kind of highly anomalous or unusual quantities of moisture. And because of the mechanisms with these storms, they will have the ability to stay in place. So, I do think that at least localized flash flooding is very likely somewhere in southern Minnesota. But, you know, let's hope that surrounding that there's just a kind of general widespread soaking rain that could be quite beneficial to at least start, you know, or help dent further the ongoing drought conditions.
Jim du Bois 15:38
Well, Kenny, always great chatting with you and look forward to talking with you again early next week.
Kenny Blumenfeld 15:45
Very good. Thanks, Jim. You have a good rest of your day.
Jim du Bois 15:48
You as well, Kenny. This is Way Over Our Heads. It's a weather and climate podcast. I'm Jim du Bois. Kenny Blumenfeld is a climatologist. We'll see you next time.