Noon, Friday, June 5, 2015
From a climatological perspective, early June is peak time for severe weather along the Front Range, and this year seems to be an overachiever.
Ample moisture from both the Gulf of Mexico (lower part of the atmosphere) and from the tropical Pacific (mid level of the atmosphere) are combined with an atmosphere characterized by a wind structure that favors severe “rotating” thunderstorms. Today and Saturday will likely be a little cooler than Thursday which could lower the atmospheric instability a little. But today is also more moist than Thursday. So all things considered, expect intervals of sun, and periods of thunderstorms. Some local areas may receive severe weather (large hail or brief tornado) and/or intense downpours that trigger flash floods. Saturday and Sunday are likely to see more thunderstorm activity, possibly some locally severe. Some sunny periods are likely too, mainly in the mornings to midday hours. The West Slope of Colorado was warm and dry yesterday, but today and this weekend are likely to bring numerous thunderstorms and a flash flood risk there too.
The remnants of the Pacific’s Hurricane Blanca are likely to spread unseasonable moisture into the Desert Southwest early next week. It appears that Colorado may be north of that moisture and see some warmer and drier weather move in from the west. But that’s too far off for high confidence and we should watch for possible impacts from that moisture later next week.
What happened yesterday?
The larger scale weather models leading up to yesterday did not indicate an extraordinary event was unfolding. But as I mentioned in yesterday’s blog, the local scale models (such as the HRRR and the WRF) yesterday morning were consistently forecasting a severe thunderstorm outbreak late in the day and into the night. What those models were weak on was the amount of rain and severe weather west of I-25. Many of the initial thunderstorms triggered rain-cooled “outflow” winds surging southward and westward toward the foothills. This caused the newer thunderstorms to develop toward the west and southwest as the individual storm cells were trying to move north and northeast. That meant that thunderstorms grew explosively in some locations along the foothills and caused repeated bursts of rain and hail in the same local areas. The local scale models did predict this “backbuilding” but not to the extent of what actually took place.