By Justin Poublon
September 2, 2016 - 10:00 AM CST | 107 0
Wisconsin had a 65-78% chance of above average tornado, severe hail and wind reports in 2016. Thus far we have defied the odds...
- 2016 severe weather reports (351). Great distribution from south to north. Tornado reports across central Wisconsin.
As of September 1 Wisconsin has 351 total severe reports in 2016. This is 37% below the seven year rolling average (564) making it the 3rd weakest season in 20 years behind only the dud seasons of 2009(-59%) and 2003(-52%). Since 2012 Wisconsin has been at or below average for total reports each year. The years of 2012 (4) and 2016 (11) were suppressed in total tornado reports via the Storm Prediction Center database. The National Weather Service in Wisconsin has shown that 15 tornadoes actually occurred this year so far; the difference we believe being the four weak <75 mph max gust EF0 "tornadoes" that were reported days later. The volume of severe weather appears to be decreasing yearly which is incredible given the increased potential for more reports each year due to improved technology and increased sensitivity.
To help understand the trends we are seeing we've conducted our own research, including a high-resolution reanalysis of spring severe thunderstorm events in 2006. Even though it is not entirely over we have three high resolution conclusions on this season:
1. More Warmth but not More Severe Storms
Warm, moist air at the surface contributes instability to the atmosphere and is critical to severe supercell thunderstorms development and maintenance. We often discuss the four ingredients of severe thunderstorms and some have higher priority over the others; Instability, Lift, Shear, and Moisture. You can have severe storms with instability, lift and no wind shear but not vise versa. This summer was expected to be warmer, more moist, and more unstable consistent with departing strong El nino pattern. Our hypothesis was that thunderstorms would be more severe. It was definitely warmer and more moist but t-storm severity did not increase, it actually did the opposite.
- EAU CLAIRE - Year, hourly counts, % of total hours between the months of June and August orderd by dewpoint >60 hours decending. Right most column is temperature.
Dew point temperature represent the moisture content of the air and an approximation of the energy available to thunderstorms. Warmer air can hold more moisture but not all warmer years are more moist. We analyzed weather observations from Eau Claire back to 1980 and found that the summer of 1995 recorded 1406 hours where minimum dew point was greater than or equal to 60F. This was the moistest of any year within the period analyzed. Further analysis showed that air temperatures were greater than or equal to 60F for 1248 hrs(64% of total hours), tied for the warmest of any year with 1983 (El Nino). In other words 1995 was one of the warmest, moist years since 1980 which is what we expect. It just barely beat out the El nino years of 1983 and 2010. These three years have distinct severe weather characteristics as being weak in the hail column but solid wind report producers. 1995 had less tornadoes than 2016.
Interestingly there were years were the percent of hours with temperature >= 60F matched that of 1995, 1983, and 2010 but were much less humid(moist). Years such as 1988 and 2006. Just because it is warmer does not mean it will be more moist. I find this to be a very interesting relationship.
2. Heavy Rain the "Big Story"
We know that as we move from late spring through late summer, naturally the mid-upper level temperatures get warmer as cold air retreats north. This will contribute to weakening wind fields (less difference in air pressure). Weakening wind fields will contribute to more disorganized thunderstorm updrafts. By the time we reach late July and August t-storms are slow moving and loaded with moisture, this is a great recipe for flooding rains. This is the natural trend. If you were to increase the warmth of the atmosphere relative to average, not just at the surface but top to bottom you would drive(enhance) this trend further.
- 30 Day Precipitation Estimates on Aug 31, 2016
Statistically more warmth and moisture at the surface will produce more rain and thunderstorms. In 2016 there were 123 with measurable precipitation at Eau Claire (147 @ Black River Falls) . 2002 was very close to 2016 with respect to temperature and dew points. There were 163 hours of with measurable precipitation that summer, second only to 2010 at 166 hours. Our data was limited to 1998 - 2016. The one near-constant when it comes to precipitation might be the opportunities. Past research revealed that there is usually 29-34 cold front passages (and associated low pressure systems) May-Sep. It's really a question of efficiency.
- Most hours with precipitation per year(left to right). 2016 = 129
As of August 31 all of western, most of northern, and much of southern Wisconsin were facing a significant surplus of rainfall during over the last 30 days. Some locations were estimated at nearly eight inches above average during that period! Heavy flooding rain events started with northwest Wisconsin in early July, then three more times coincidentally occurred further south along the Mississippi River region each time through August.
June 1995 was on the dry side state wide relative to the 30 year average. During the month of August northern Wisconsin was deluged more than once with heavy rain. The location of heaviest rain was centered across northern Wisconsin. In 2016 the axis of heaviest rains fell along the Mississippi River.
- Summer cumulative precipitation 1995 in Neillsville, WI located just southeast of Eau Claire.
- Interesting to note close proximity of above average, below average precip in upper midwest
(RANT ALERT) All thunderstorms produce a by-product called heavy rain. While chasing storms I have never encountered a thunderstorm that did not contain heavy rain. "Heavy rain" is a term that is widely miscommunicated and forecasts now will contain the term 100% of the time if possible. From my perspective heavy rain is a matter of duration and location, not the frequency of the rain drops. It's like constantly worrying about the mouse in the wall while the lion waits in the shadows; the lion perhaps being a catastrophic EF-5 tornado. When I say heavy rain (rarely) it should mean something more.
Rant over, theoretically assuming a trigger in the form of the low pressure system moving through; the warmer and more moist the airmass, the easier it will break inhibition, and the more storms there will be. The storms that form will have a relatively higher potential to produce "flooding rain" over other severe weather impacts.
3. Hail Needs Cold Air Aloft
Wisconsin had only 44 severe hail reports(0.7" diameter) this year as of Sep 1, the least since the 1980s! This is an incredible trend.
In our July post we noted the lack of northwest flow upper level flow regimes. We mentioned the three-way relationship to hail production in severe thunderstorms. I believe one of those components has emerged as most important for Wisconsin, cold air aloft. We've learned that El nino summers and summers of transition between El nino to La nina are very poor for total hail report volume beating the seven yr average only 29% of the time.
In 1995 between three severe events (one in each month from June to August) Wisconsin saw five hail reports of 3" or larger. This suggests that just very warm, moist surface conditions does not correlate with large hail trends mid-summer. We mentioned that hail production in warm year may need fresh t-storm development within state before water-loading processes can take over. What the data shows are that warm summers have notable reduction in smaller hail (0.7-2.0" diameter). Because the majority of hail reports are < 2", this means a large decrease in total volume of hail reports.
The spring of 2006 was insane for hail reports in Wisconsin (514!), including one of the most damaging hail storms in state history. We conducted high-resolution reanalysis of five events such as 4/13 and 6/6. We decided to study 2006 because tornado and wind report counts were the same as 2016. For some reason the hail reports were out of this world. We found what was happening at mid-upper levels; notably temperature, had a strong influence on lower atmosphere. This makes sense but was never something we thought about in great detail. We should really compare weather observations in 2006 to 2016 on a daily basis. 2006 was a La Nina year.
So much emphasis is placed on ENSO phase (El nino vs. La nina) and for good reason. Our findings only support it as a possible tool for forecasting severe weather seasons. Future research will look at the connection between the mid-upper level atmosphere and the ENSO global weather pattern. I'd go as far to say that the mid-upper level's drive the surface temperature/dew point trends and this is where ENSO phase connects, but it is not the only process involved.
We had 60-78% likelihood of meeting or exceeding the seven year rolling average with regards to severe weather reports. That's a pretty good chance. Instead of going the path of 1998 and 2010, we went 1995. As you can see in the table below 2016 fell through the cracks statistically. This goes to show there is plenty more research to be done beyond ENSO phase.
- Note right most column showing % variance of tornadoes to 7 yr rolling average per year
The tornadoes reported this year were definitely characteristic of environments with strong surface instability but poor overall storm organization. All tornadoes were weak (EF1 or less). We saw many bookend vortex, mesoscale systems producing weak short lived tornadoes on the front end. Wind reports were near average. With damaging wind I look for difference in temperature and humidity within the atmosphere as well as strong mid-upper level winds to get pushed down to the surface by downdrafts. Wet microbursts seemed to be the primary source of wind damage on our chases as opposed to widespread damaging straight-line winds.
From my storm chaser perspective, I would describe the Wisconsin severe weather season as stagnant. It was a lost year as I accomplished nothing in-state. Wisconsin didn't have the spring outbreak this year (we didn't expect to) and haven't for several years now. I found May and June to be active and exciting but most chases were out-of-state. July and August were frustrating. Western Wisconsin was very active in July and August in addition to the heavy flooding rains that were observed. This likely contributed to my frustration with multiple missed opportunities living in the eastern half of Wisconsin. Storm setups were okay in the west but not great enough to justify the four hour drive. By the end of the season you are often looking for something special but my season like most others gradually faded into mediocrity.
I volunteer my time to forecasting weather and chasing storms. I've spent the last 4385 hours (183 days) on edge. I'm constantly watching and aware. I suppose it's understandable that by the 184th I might be a little agitated. Why?
I do this only because I'm passionate. I care, and it would be selfish to withhold the knowledge I have. I do it because the next 182 days will not provide the storm chasing opportunities we have in summer. I cannot go to Walmart and buy a storm to enjoy in November when nothing is happening. Secondly, the minute I turn my back there will be a severe weather outbreak. Experience has taught me not to count on others to get that forecast right. It is my responsibility to you to be there and to tell the truth.
Today we sit near the end of the season. It's day 184 and I know it's not over. As the upper level jet starts to strengthen next week and into fall we should certainly have one or more severe weather events to deal with. The biggest tornado outbreaks in state history came in August when cold mid-upper level air encountered mid-late summer warmth. August is gone but I still won't sleep until October. All along this was the time of year I was most concerned about.
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