Storm Chances Sunday Through Tuesday
by Angela Schilling, Meteorologist
March 17, 2012 8:55 PM
The chance is still there for thunderstorms to develop this evening with the potential for storms increasing as we head into Monday afternoon. Not looking at anything severe Sunday evening, but gusty winds are possible with any storms that do develop. Right now the low is over Montana and will continue to move eastward over the next 48 hours. Storms on Monday could be on the strong side, especially east of the Spencer, IA line, with a slight risk just outside of our viewing area. Rain totals will be anywhere between a half inch and 1 inch, with higher amounts to our south and east. We’ll get a break from the rain early Monday morning before 7am, with storms redeveloping by the afternoon.
Storm Prediction Center keeps a slight risk for severe storms well to our south, however we
could still see some gusty winds associated with any storms that develop Sunday evening.
With a cold front just off to our west our chances for strong storms is a little bit higher
by Monday afternoon. As of now the Storm Prediction Center keeps a slight risk just
to the east of Spencer.
Rain totals once this system is said with and done will be around a quarter of an inch
...with higher amounts in northwest Iowa.
**************Make sure to read the rest of the blog below*******************Seems odd to be talking about thunderstorms so soon, let alone temperatures into the lower 80’s, however rain is in the forecast and thunderstorms are likely as we head into Monday and Tuesday. We could begin seeing thunderstorms in the Northern Plains as early as Sunday afternoon, with the majority of the rain falling on Monday. We have been under a very dry airmass lately with dewpoints rather low, but as of Saturday we are beginning to see a big increase in moisture content, with dewpoints close to 60 degrees. Typically 60 degrees is the cutoff for whether or not it’s sticky outside, and it is very uncommon for us to see such high dewpoints so soon in the season. With moisture in place areas along and east of the I-29 corridor woke up to low-level clouds Saturday morning before finally clearing out in the afternoon. Sunday afternoon looks to start out the same way with a mix of sun and clouds with gradual clearing as the day goes on. Once we get into Sunday evening we will still have moisture in place, and will be just enough to fuel a few storms as the night progresses, with more activity expected on Monday and Tuesday.
The jet stream will be key over the next several days, with a ridge of high pressure currently over South Dakota and a strong low over the Pacific Northwest. As long as we stay underneath the ridge we will continue to see the warm conditions and keep severe weather at a minimum. The Jetstream is found in the upper levels of the atmosphere and drives our weather. A dip in the jet stream can be referred to as an upper level low, or in simple terms a cold pocket of air. It is the collision between cold dry air and hot and humid air, which causes thunderstorms to develop.
In order to get strong storms you need a lot of instability as well as shear. Shear is a change in wind direction or speed with height. For example if you have southerly winds at the surface and northwesterly winds aloft, you would have a lot of shear. A strong cold front can oftentimes give you a lot of shear to work with. You don’t have to have the same amount of each, but they have to compliment each other. If you’re lacking in instability then you need more shear and vice versa. On Sunday we will have enough instability to get storms going but will be lacking in shear. Granted we won’t have as much instability as say July, with cape values around 4000 j/kg or so, but with enough shear, 2000 j/kg of cape would be more than enough to get storms going.
However, on Sunday shear values will be relatively low, as the trough will still to be too far out west to start anything. With low dynamics or forcing, you can still get a little bit of severe weather, with large hail typically the main threat. The reason why? Hail forms in the updraft of a storm, the taller the updraft, the larger the hail. The warmer or more unstable the air mass the taller the updraft or “stomach” of the storm. If we have a lot of shear in place we could get the storms to potentially rotate as well. Heading into Sunday, we will still have a ridge of high pressure in place, and will be rather unstable, which means the risk for severe weather is still there, however small, with a few hailers and gusty winds possible.
As we head into Monday the trough of low pressure in the upper levels of the atmosphere will continue to work its way eastward, which will be the forcing mechanism to get storms going. Our chances for severe weather on Monday are highly dependent on the amount of sunshine we receive throughout the day. If we see a lot of sunshine, we will be very unstable at the same time the trough moves through, if we stay cloudy all day we won’t be nearly as unstable. The image below is for Monday afternoon and shows the jet stream. Notice the bright reds just west of the I-29 corridor; the reds represent high wind speeds, or a jet streak within the jet stream itself. A jet streak can trigger storms as well.
The Storm Prediction Center has issued a slight risk for northwestern Iowa on Monday, with the main threats being damaging winds and large hail. At this point in time Sioux Falls and areas west of the I-29 corridor are not under a slight risk, however, we still need to stay weather aware.
So how much rain are we talking about? Precipitation amounts can be tricky to pin point when you’re dealing with thunderstorms and can vary drastically from one city to another. In general rain amounts will be around a half to one inch, with higher amounts in northwestern Iowa. Also, keep in mind if you get underneath a strong storm, amounts will be much higher.
Make sure to stay tuned to KDLT and KDLT.com for the very latest. If you want to see some of the records we broke this past week, make sure to check out the previous post, found in our archive, I think you'll find it rather interesting.