Neutral conditions are to blame for an increase in moisture events
By KRISS NELSON
To say the weather has been unpredictable is an understatement, but yet, quite true.
Justin Gilsan, state climatologist for the Iowa Department and Land Stewardship said because we are in neither an El Nina (increased sea surface temperature in the Pacific) or a La Nina (a decrease departure of what we would expect in the Pacific) it puts us in what is considered “neutral conditions.”
“When we are in neutral conditions, there’s normal sea surface temperatures in the Pacific, there’s not a good forecast ability because when we are in either phase of those, that can give us pretty good guidance for one, two, three months out because we know what pattern to expect with the jet stream,” said Gilsan.
The growing season so far
According to Gilsan, when we go back to April, we had the coldest April on record. We got into May, and experienced the third warmest on record. Preliminary results for June are showing it looks to be one of the top 10 warmest and 13 wettest on record.
“In May and June, the jet stream has been what we call zonal almost a straight line across the United States but it is further north than where it usually is and that does a few things,” he said.
This zonal jet stream, Gilsan said allows a warm, moist air from the southern part of the United States.
“So, when the jet stream is further north than it should be, it allows a lot of moisture to come into the state,” he said. “That jet stream has been at the Canadian border, northern Minnesota on average for the past two months.”
Gilsan said what will happen is little troughs, or fronts, develop. These are low pressure systems that transverse across the northern part of the state causing stationary fronts, or warm fronts that tend to stall.
“They are slow moving fronts,” he said. “What they do is separate warm air from relatively cool air. When these frontal features are over the northwest part of the state, for example, and you have the moisture streaming from the south, once it hits that front, it has no where to go but up and so when it goes up you get thunderstorms at form.”
But what makes these thunderstorm events so dramatic?
“We have been in this stagnant pattern where we either have connective activity on a daily basis driven by the sun or overnight convection,” he said. “With these stationary warm fronts, and as you have seen on radar, these storms pop up and either move slowly and drop tons of rain on the same spot of the state, or they will move fast and they’ll train.”
Training, Gilsan explained is when you have a thunderstorm pop up and then another thunderstorm pops up directly behind it moving through the same area.
“That’s what happened in Des Moines and Central Iowa over the weekend that gave us an insane amount of flooding,” he said.
The weather seems to be changing
Gilsan said there have been some changes over the last 30 years.
“Instead of seeing these gentle, two to three hour, nice rain falls that you like to sit and watch or listen to, we’re starting to transition, trend-wise, into more frequent high intensity events,” he said.
Producers, Gilsan said have definitely taken notice of this change.
“They have been on their land forever,” he said. “They know what the weather patterns are. I get phone calls from farmers all of the time asking me ‘why we are getting these more frequent, high intensity events?'”
Gilsan said these are most likely caused by a warmer atmosphere that is able to hold more water vapor.
“Once you have instability in the atmosphere, there’s always potential to get these really high intensity rainfall rates because you have so much moisture in the atmosphere especially in the late spring and early summer time,” he said. “We saw this in April with all of the snowfall accumulation in the northwest and especially along the Minnesota-Iowa border. There’s that moisture in the atmosphere.”
Outlook for the remainder of the growing season
Gilsan said, currently, the one month outlook for temperatures appear to show a 60 percent chance we are going to have above normal temperatures. Average temperatures for July, he said is about 72 degrees. That average comes from the maximum high plus the minimum high then divided by two.
“What we have seen, trend-wise, over the past 20 to 30 years is, high temperatures are pretty much staying where they should be, but the overnight lows are getting warmer so that has bumped up the average temperature,” he said.
As far as what to expect for precipitation, Gilsan said July, typically sees about 3.8 to four-inches of rain.
“The one month outlook is equal chance, above or below that,” he said. “We don’t have good guidance on what we should expect.”
Ttrend-wise, Gilsan said, when looking at the northwest corner of Iowa, there appears to be a possibility of above average precipitation.
“The Climate Prediction Center, right now, their guidance is an equal chance of above or below precipitation, so they are trending towards the normal four inches you would expect,” he said.
Gilsan said looking at the three month outlook, the temperature kind of flips showing an equal chance for most of the state to see average temperatures.
“This outlook was issued on the 21st of June and is looking at July, August and September. You can imagine, with statistics that you could have a hot July, cool August and a normal September and that’s where you can get that equal chance. It could be any mix of those things.”
As far as precipitation, Gilsan said there is also an equal chance.
“There’s not a lot of confidence in the model guidance right now,” he said. “But, the good thing about the Climate Prediction Center is that they do put out the eight to 14 day outlook and the six to 10 day outlook so that gives us short term guidance and then they also update the one month outlook as next month comes out.”
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