The monsoon rains that impacted the Front Range on Thursday were nothing short of impressive.

Not only did some areas pick up extensive rainfall amounts leading to flash flooding, stronger storms developed in Boulder and pushed toward Denver in dramatic fashion when an ominous shelf cloud barrelled toward the city.

On top of that, there was an extremely well-defined rain shaft with these storms and most areas were treated to a fabulous double rainbow after the evening rains.

Impressive rain totals

Heavy rains led to street flooding in Greeley Thursday afternoon after a nearly stationary storm dropped over 3 to 4-inches of rain in an hour and a half.

1WNW Greeley: 4.40 inches
7.2 SSE Watkins: 3.41 inches
2.5 NW Louisville: 2.39 inches
2.5 S Boulder: 1.70 inches
0.1 SW Littleton: 1.08 inches
1.8 SSW Niwot: 1.02 inches

Overall, there were 23 flash flood advisories issued by the NWS in Boulder and eight flash flood warnings issued on Thursday.

Epic shelf cloud

An epic shelf cloud wound up forming and bee-lining it straight to Denver proper Thursday evening. Shelf clouds form as heavy rains produce wind and that wind spreads out away from the main storm creating low-hanging clouds that look ominous and dangerous. While these self clouds look scary, the main threats they bring are a big initial gust of wind (which can exceed 40 mph) followed by the heavy rain underneath.

We normally see clouds like this with severe storms that impact us but Thursday’s seemed a bit more ominous and since there was so much moisture in the atmosphere, the depth and darkness of this particular storm cloud looked way more intense. You can pick out a lot of storm structure with these clouds normally like the dark, almost tail-like feature that looks to be underneath the cloud, that is the inflow winds. So winds are being sucked up into the storm feeding it moisture and energy. The shelf cloud, or low-hanging leading cloud, is moisture getting pushed up and over the gust front which shows how vertical motion leads to cloud development. There’s a lot more to dissect there too but overall, it was a cool feature to see.

Rain shaft 

Rain shafts are a fairly common weather phenomenon. They occur when very heavy rain falls out of a thunderstorm. The thunderstorms in our area yesterday had this type of definition to them denoting that there was so much moisture in the atmosphere. With any monsoon setup, we get moisture that originates from the tropics so rain shafts like the ones we saw yesterday are generally seen in tropical, or warmer and more moist areas. This is a very cool feature to see because the rain literally looks like an impenetrable wall of water.

Double rainbow

Rainbows are pretty stellar to see normally but double rainbows are even more impressive.

The optics involved in the formation of rainbows have to be exact. When sunlight hits raindrops at a 23.5º angle, the light actually refracts in that water droplet to produce the colors of the rainbow that you see.

Under the right conditions, the light that travels through a raindrop will refract light not once but twice leading to the formation of two rainbows. When you look closely, you’ll notice that the main rainbow, which is the brighter of the two, goes from red on top to violet on the bottom. When you look at the more faint rainbow, the secondary rainbow, you’ll notice that the colors are inverse. Instead of red on top, red is on the bottom and now the violet light is on top. This is a classic example of double-refracted light.

In any case, Thursday’s weather in the Mile High City was nothing short of amazing. We may see more thunderstorms like that again today as the forecast calls for more rain and thunderstorms to produce heavy, possibly flooding, rains. Thankfully, the upcoming long weekend is shaping up to be a bit better with drier and warmer air moving in.

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