LEAST ACTIVE SUMMER FOR SEVERE WEATHER SINCE 1988...............BUT IT IS NOT A DROUGHT SUMMER:
We have had just three reports of severe weather in the June-July 10 period & will end up with none to at least July 14. The last time we had so few in June-July 14 period was 1988. Then, there was just one single report: tree damage just east of Brookston on July 7.
Lack of severe weather in June-July of 1988 makes sense. Some places were experiencing record dryness or drought to a degree not experienced since the 1930s. There was considerable capping & ridging aloft, an environment hostile to widespread storm development. Also, this was not a regional drought, but one massive extreme drought that extended over a good chunk of the country, so much of the area between Montana & the East Coast had well-below normal severe weather for the summer.
The lack of severe weather this summer (& really this spring, too) is a bit more elusive, however. Spring was cold here & rarely unstable with lots of snow, followed by a flash spring/summer with intense heat in May, especially late in the month. Forests went from bare to fully-foliated, seemingly in a week. We went from winter cold stability to stability of a different kind: upper ridging & capping aloft. This resulting in a lack of rainfall in May, which led to the corn & soybean crop getting out promptly, but also was a feedback for hot temperatures & low humidity. Thus, typical often violent severe weather development was hampered.
This summer has seen its share of rounds of rainfall & storms with intense heat & oppressive humidity, but there has been a lack of severe weather. By chance, a lot of it has been southwest (June derecho to our southwest & nasty bow to our south with widespread wind damage weeks prior to that), south & north & northwest. However, there is an overall trend towards lack of severe weather nation-wide. I was reading today that we reached a total of 63 tornado watches issued for 2018 on July 9. That seems like a lot, but in averaging out numbers, since 2000, the normal date for reaching 63 tornado watches is around April 9! In 2008, we hit that mark March 3! Even in 2012, a major drought year, we hit it March 12! We hit the mark on April 3 last year.
WHAT ARE SOME SET-UPS THAT QUELL SEVERE WEATHER HERE?
So, what's up? We know that a multitude of factors contribute to less severe weather in our region:
1. Upper ridging with heat & capping & usually drought,
2. Cold, dry northwest flow with consistent below normal temperatures & more stratocumulus/fair weather cumulus or clear skies than big cumulus towers to promote storms.
3. Rain & storms, but still hot with the best shear, upper winds aloft well north of the area.
4. May even have a cloudier-than-normal summer with lots of heavy rainfall from tropical systems or persistent frontal boundary that just frankly limits good destabilization.
5. No distinct "Ring of Fire" pattern that gets established to promote squall lines & derechos on its periphery.
6. A strong baroclinic zone in our area can promote more severe weather. If it is cold & wet just north of here & hot & dry just south of here, that gradient tends to increase the wind at all levels, which enhances shear. Also, organized storms like to followed those boundaries of heat, but also wetness nearby. The bigger the difference, the bigger the consequences. Lack of a baroclinic zone may decrease severe weather.
LARGER SCALE PATTERNS THAT QUELL SEVERE WEATHER:
You could go further into the overall pattern over a large area:
1. La Ninas tend to promote more severe weather from our area to the Plains. The Plains are most affected. They also tend to promote for severe weather in our area right into summer.
El Nino & La Nina influence (pinks & reds represent increases in tornado & hail frequency, largely during the warm season)...............wind events increase in La Ninas, too, especially here in our area (working on that map):
Data courtesy of Willis Research Network & Columbia University:
Obviously, the best ENSO state for tornadoes here is a moderate to strong La Nina (image courtesy of ustornadoes.com).
Moderate to strong La Ninas since 1900 occurred in 1917, 1925, 1933, 1956, 1971, 1974, 1976, 1989, 1999, 2000, 2008, 2011.
2. Pacific Decadal Oscillation (PDO) negative phase promotes more severe weather, specifically strong to violent tornadoes, in the U.S. Notice when the PDO goes into a longer positive phase, the less strong to violent tornadoes that tend to occur. Negative PDO, which was consistent in the 50s & 60s promotes stronger, violent tornadoes.
Image courtesy of NOAA Storm Prediction Center:
Lack of moderate to strong La Ninas in recent years, tied with more El Ninos & a trend toward El Nino Modoki this winter appear to be working in tandem with a positive PDO in quelling severe weather here. This trend can also be observed nation-wide.
There are other small-scale factor quelling severe, as well. Some of it is merely by chance, but this change from cold, stable in spring, which shut down a lot of severe to hot, ridging & capping in May led to very little opportunity to establish a severe weather pattern.
June saw a few big events just miss us by chance (derechos to our southwest & south), but the overall migration of the strongest winds & shear to our north (pattern has brought consistent above normal temperatures since May) has made all of the rain & storms we have seen lack good shear to promote widespread severe weather.