6:45 PM Monday, June 15, 2015
The early week cool down will be replaced with more typical June weather Tuesday and Wednesday along with a small chance for afternoon thunderstorms. On Thursday and Friday the temperatures are likely to be above average. Friday may be the first day of the season with widespread 90-degree readings across eastern Colorado and the urban corridor.
This week is the 50th anniversary of the Great Flood of 1965. The flood caused severe damage in the Denver area and in eastern Colorado, especially along the South Platte River, Cherry Creek, Plum Creek, and Bijou Creek. Areas in and along the foothills of northern Jefferson, Boulder, and Larimer Counties were largely spared from the worst impacts. The Colorado History Center in Denver will have a commemoration of the great flood on Tuesday, June 16th, at 7PM. I will be the second of four speakers and I’ll cover a historical perspective of Front Range floods.
12:55 PM, Thursday, June 11, 2015
A fairly weak but moist storm system will move east across southern Colorado tonight. As it approaches there will be showers and some locally heavy thunderstorms throughout much of Colorado and southern Wyoming this afternoon and evening. Rainfall will be variable, with some locations getting relatively small amounts and others getting intense, downpours. Localized flooding along creeks and in some urban areas this afternoon and evening is likely.
As the storm passes to the south, there may be a period of steady rain tonight and/or Friday morning. The potential for a widespread soaking rain overnight does not look as certain in today’s guidance & observations as it did yesterday. But it still looks like more than a small possibility for a soaking rain tonight/Friday morning and so we should be prepared for a wet morning and the possibility of some regional flooding.
The weekend looks like a typical June weekend with near seasonable temperatures (seasonable is around 80 in the urban corridor), and a scattering of afternoon thundershowers.
6:00 AM, Wednesday, June 10, 2015
We are returning to cooler, wet weather today (Wednesday) into to Friday, with the peak possibility of local heavy rain along the Front Range coming late Thursday into early Friday. It appears we may return to more seasonal weather on the weekend.
A slow moving storm system from the west is bringing some Pacific moisture (including some remnant moisture from last week’s Hurricane Blanca) and drawing up moisture from the Gulf of Mexico too. Showers and thundershowers are already occurring in western Colorado at daybreak and moving slowly east. Sunshine will be very limited today, and a few areas (especially in western Colorado) may experience locally heavy rain.
Thursday afternoon into Friday morning may bring the peak threat to the Front Range and eastern Colorado (and southeastern Wyoming). A few thunderstorms may be severe late Thursday, but locally heavy rain and flash flooding may end up being a bigger story. The showers and thunderstorms Thursday afternoon may evolve into a period of steady rain Thursday night and/or Friday morning.
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.
2:45 PM, Thursday, June 4, 2015
Today is one of those days when our numerical weather models might really prove their value. Several of the local scale models today have been predicting strong or severe late-night thunderstorms. If these models are correct, thunderstorm development after dark may last until after midnight. The greatest risk according to these models is for areas east of I-25. Although the risk appears much lower for Boulder, Broomfield, and the central & west sides of Denver, don’t be too surprised if you experience a late night thunderstorm.
12:45 PM, Wednesday, June 3, 2015
After a few warm and dry days, the chance for thunderstorms is greater this afternoon and evening. In fact, a few spots will probably experience strong or severe thunderstorms with large hail, heavy rain, and/or strong wind this afternoon or evening.
Afternoon thunderstorms should develop again Thursday afternoon, but probably with lower intensity and less areal coverage than today.
The weather pattern will start to allow tropical Pacific moisture from off the Mexican west coast to move up over the southwestern U.S. this weekend, at least in the middle and upper levels of the atmosphere. This part of the Pacific is especially moist due to the current El Nino conditions. In fact, Hurricane Blanca will likely move toward the Gulf of Baja and add some extra tropical moisture to the pattern this weekend or early next week.
It’s too early to tell if, or how much, this moisture will impact Colorado, but we should at least expect a period with numerous afternoon showers or thundershowers this weekend into next week. At this point it appears that the greatest impact from that moisture fetch may be in southern and/or western Colorado, but it will need to be monitored as Hurricane Blanca (and it’s remnants) move northward.