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May is the month when tornado activity typically peaks in the United States, but that may not be the case this year.

The first part of the month has been headlined by an omega block – also known as an atmospheric clog – which has featured well above-average temperatures in the Plains and Rockies between cooler and wetter periods on both the East and West coasts.

This omega block is generally not conducive for severe weather in the Plains due to the locked-in high-pressure system in the central part of the country.

(MORE: Atmospheric Clog Cools East and Warms Plains)

Underneath these areas of high pressure, air sinks toward the ground, which limits thunderstorm growth over a large area.

This pattern looks to remain well-established through the middle of May with intermittent disturbances. At least one area of low pressure is expected to temporarily bust the omega block and move across the Plains in the middle of the upcoming week with a chance of severe weather, including tornadoes, but the ridge should soon return.

There have been 28 reports of tornadoes in the U.S. so far this month through May 6, according to NOAA's Storm Prediction Center. That total could grow during the midweek severe weather threat in the south-central U.S., but thereafter, the threat of severe weather should be low into next weekend as we near the midpoint of the month.

(MORE: GOES-16 Satellite Detects Thousands of Lightning Strikes During the Midwest/Ozarks Flood Event)

It's too far out in time to determine how conducive atmospheric conditions will be for severe storms and tornadoes in the second half of May, but a slow start to the first half of the month could tip the scales toward a below-average overall number of May tornadoes.

Other Times When May Was Less Stormy

The number of tornadoes is typically high in May, but there have been four Mays in recent history when twister activity was well below-average. May 2005, 2006, 2012 and 2014 had the lowest tornadoes counted for the month of May since 1996.

In 2012, only 121 tornadoes were confirmed, according to the Tornado History Project, compared to an average of 276 tornadoes from 1996 to 2015.

A growing drought and a migrating ridge that oscillated between the Rockies and the East dominated the weather that month. Temperatures and pressures were both well above normal for much of the country east of the Rockies.

In 2005, 122 tornadoes were counted, but conditions were similar to 2012. An omega block even made an appearance for a short time in early May before transitioning more to generally higher-than-average pressure in the South fo the rest of the month. Drier-than-average conditions also dominated the central and southern Plains much of May 2005.

All four of these months of May occurred in neutral months between El Niño and La Niña. Several other years – 1996, 2001, 2002 and 2009 – that had lower-than-average tornado activity also occurred in neutral months. Two years, 1997 and 2016, had weak El Niño conditions present, and 2000 had weak La Niña conditions present during May, when fewer tornadoes than average occurred.

(MORE: Above-Average Tornado Activity Doesn't Necessarily Mean Much for Climatological Peak of Tornado Season)

Neutral conditions are expected through at least mid-2017, according to the CPC.

May is typically the peak in tornado activity nationally due to the best setup of warm air, jet-stream disturbances swinging out of the Rockies and plentiful moisture east of the Rockies.

MORE ON WEATHER.COM: Tornado Risk by Month