Being a budding meteorologist with limited forecast experience, I think it's important to be aware of my mistakes.
Yesterday, I relied way too much on having a persistent pattern in place, basically allowing for a repeat of Tuesday's setup. This wasn't the case as even while forecasting yesterday, I missed a surge of air that moved in from the north and changed a lot of the mesoscale dynamics. I've tracked the "surge" to an outflow boundary that traveled south overnight from severe storms that formed in northeast Wyoming. This northerly flow temporarily trumped the easterly upslope that I had expected. Eventually, however, the easterly winds began to overpower the northerlies, which led to a convergence zone (northwest vs. northeast winds) along a line from Thornton to Lakewood. The air from the north was just as moist as our easterly upslope, but it was a little cooler. As a result, the air over the metro area stayed a little more stable, a little longer. If it weren't for this push, I'm sure that the airport would have seen 90ºF once again. Unfortunately (fortunately), the streak will now stay at 24 days.
As expected, the mid-level moisture from the west continued to advect into the area throughout the day. Even though the surface got out to a slow start in the heating aspect, it soon began to cook nicely. By mid afternoon, cumulus began to form but was capped by a stable layer (which could be seen by a field of altocumulus). As the day wore on, the cumulus became more intense, and though not extremely strong, began to push through the stable layer. As the convergence zone strengthened over the urban corridor, the propensity for lift was very evident.
All eyes, however, were on an outflow boundary moving due-west from convection in eastern Colorado. On RADAR, it appeared as a flying-V formation (think migrating geese) and was pointing directly at Denver. Meanwhile, the cap over the city broke in spectacular fashion and stationary moisture-laden storms blew up. These storms were putting down 0.50" of rain in a half hour, right during evening rush hour.
The storms didn't really move, maybe drifting (or building) slowly to the west. At this time, however, the outflow boundary from the east was now in the metro area. In its wake, stationary storms were blowing up and dumping rain over Adams and Arapahoe Counties. When the outflow boundary finally met the convergence zone, they interacted in spectacular fashion. It was like watching a game of "red rover". Outflow from the original convergence line was now pushing east in opposition of the incoming boundary. Each boundary exploited the weaknesses in the other, causing an amazing scene.
As you can see in the radar capture, some areas of the line pushed through the other. On the edges of these opposing charges, rotation became evident. Soon, tornado warnings were issued as people began to see funnel clouds and even weak tornadoes. Again, the NWS has not yet issued any sort of statement or report after the fact and the SPC has no record of a tornado. The spin-ups were likely brief and weak and may have been gustnadoes or landspouts. It was very interesting to go out on the observation deck and hear the tornado sirens ringing in downtown Denver. That's only the second time in my life I've heard tornado sirens (the first being in Fort Lupton last year).
The convergence of the boundaries led to heavy, heavy rains in many areas. Storms continued to fire and remain stationary for the next few hours before lack of heating sounded the death knell. Another interesting sight was the broad circulation that seemed to hover about the area in the late hours. Moderate rain continued to fall in the metro area after dark while the plains saw heavier precip.
Here are some rainfall totals from yesterday:
Brighton (North): 2.48"
Aurora (Havana Park): 1.38"
Denver International Airport: 1.31"
Aurora (Toll Gate Creek at 6th): 1.18"
Aurora (Aurora Regional Pond): 1.14"
Denver (Diamond Hill): 1.10" (My location)
Arvada (West Woods): 1.06"
Thornton (Niver Detention): 1.02"
Denver International Airport (Third Creek): 0.98"
All in all, it was a fun day. I learned a lot and had a great time at the flood center, even though I was there until 10PM!
September 2018 blog update: upcoming videos
3 days ago