SPM 06-20-2018: Multiple Rounds of Supercells-Turned-Thunderstorm Complexes

Issue Date: Wednesday, June 20th, 2018
Issue Time: 9:30 AM MDT

Summary:

The previous FTB period was certainly an eventful one, as conditions came together to produce the most active severe weather day of the season thus far. Multiple rounds of supercells developed over/near the Front Range, tracking in a general eastward direction with time, impacting the Urban Corridor, Palmer Ridge, Northeast Plains, and Southeast Plains. The initially discrete supercells eventually morphed into multiple thunderstorm complexes over the eastern plains, resulting in plentiful rainfall and periods of heavy rain. Check out the following impressive heavy rainfall observations:

3.72 inches: Flagler (Kit Carson County)
1.5 inches: 3 miles NE of Idalia (Yuma County)

The big story yesterday, however, was the amount and size of hail produced by the severe thunderstorms. Up to 3 inch diameter hail was reported in El Paso County and Arapahoe County, 2.75 inch diameter hail in Yuma County and Park County, 2 inch hail in Weld, Denver, and Larimer Counties, plus a smattering of 1-1.75 inch hail across other portions of eastern Colorado.

Flash flooding was not reported yesterday, but street/field ponding occurred under the heaviest rainfall in conjunction with hail accumulation and poor drainage. For a look at precipitation estimates in your area, please see our State Precipitation Map below.

Click Here For Map Overview

The map below shows radar-estimated, rainfall gage-adjusted Quantitative Precipitation Estimates (QPE) across Colorado. The map is updated daily during the operational season (May 1 – Sep 30) by 11AM. The following six layers are currently available: 24-hour, 48-hour and 72-hour total precipitation, as well as maximum 1-hour, 2-hour and 6-hour precipitation over the past 24 hour period (to estimate where flash flooding may have occurred). The 24-hour, 48-hour and 72-hour total precipitation contain bias corrections that are not disaggregated into the hourly estimates, so there will likely be some differences. The accumulation ending time is 6AM of the date shown in the bottom right corner. Also shown optionally are fire burn areas (post 2012), which are updated throughout the season to include new burn areas. The home button in the top left corner resets the map to the original zoom.

SPM 06-19-2018: Incredible Severe Thunderstorms and Overnight Rainfall in Weld and Morgan Counties

Issue Date: Tuesday, June 19, 2018
Issue Time: 10:20AM MDT

Summary:

The atmosphere yesterday was primed for severe weather and heavy rainfall. The Denver cyclone helped pull in high moisture over the Northeast Plains and Urban Corridor with stations recording dew points between 55F-60F. Thunderstorms kicked off a bit later than usual (5PM) over the Front Range, so the atmosphere was able to maximize its parameters for severe weather. A line of convergence provided lift for several supercells along and just north of I-76. Thanks to upper air support and a high moisture environment, the storms continued to redevelop to the west. Storm motion was to the northeast along the convergence line and lasted through 5AM this morning. There are still some lingering showers and heavy fog in their wake this morning.

As expected, these storms produced very high rainfall, gusty winds and large hail. Hail reports yesterday ranged from quarter (1 inch) to tennis ball (2.5 inches) and caused quite a bit of damage in Louisville and Lafayette. For more storm reports you can visit the NWS Boulder page. Around 9:00 PM one of the storms became a small bow echo and produced a wind gust of 71 mph in Fort Morgan. As for rainfall, a CoCoRaHS gage recorded 4.84 inches in Keenesburg with 2.47 inches falling in 40 minutes. Radar estimates just north of this area were just under 5 inches and max 1-hr rain rates reached 2.75 inches/hour! The total rainfall over Weld County translates to about a 1 in 100 year event or has a 1% chance of occurrence each year.

Not surprisingly, there were two flood warnings and an areal flood advisory. The frontage road at I-76 and Weld County Road 53 was washed out and many other roads in the area were impassable. A CoCoRaHS member in Weld County reported the dry soils were able to soak up an incredible amount of the rainfall yesterday. Today, these areas will have to be monitored closely as saturated soils will produce increased runoff. There were no other flooding reports as of this morning. River levels should be able to recover starting this morning and remained well below Action stage yesterday.

To see how much precipitation fell in your area on Monday, scroll down to the State Precipitation Map below.

Click Here For Map Overview

The map below shows radar-estimated, rainfall gage-adjusted Quantitative Precipitation Estimates (QPE) across Colorado. The map is updated daily during the operational season (May 1 – Sep 30) by 11AM. The following six layers are currently available: 24-hour, 48-hour and 72-hour total precipitation, as well as maximum 1-hour, 2-hour and 6-hour precipitation over the past 24 hour period (to estimate where flash flooding may have occurred). The 24-hour, 48-hour and 72-hour total precipitation contain bias corrections that are not disaggregated into the hourly estimates, so there will likely be some differences. The accumulation ending time is 6AM of the date shown in the bottom right corner. Also shown optionally are fire burn areas (post 2012), which are updated throughout the season to include new burn areas. The home button in the top left corner resets the map to the original zoom.

Note: We have identified a possible underestimation in QPE over the southwest part of the state. We are working to on this issue, and will provide an update as soon as possible.

SPM 06-18-2018: Bud Moves East and Provides a Good Soaking for Northeast Colorado and the Northern High Country

Issue Date: Monday, June 18, 2018
Issue Time: 10:50AM MDT

Summary:

Bud continued to impact the weather over Colorado on Sunday. However, the moisture axis shifted to the east of the Continental Divide, which gave the burn scars over the San Juan Mountains a break from the heavy rainfall as it moved into a dry slot. A cold front moved through eastern Colorado on Sunday morning, which increased convergence along it and cooled high temperatures for the day. During the afternoon, the upper level low combined with the surface front to create widespread showers and severe thunderstorms over the northeast portion of the state.

Most of this region got a good soaking throughout the day and overnight at least receiving 0.25 inches of rainfall. Over the Front Range, Alert gages recorded up to 1.26 inches near Evergreen. While no mountain rivers indicated flooded yesterday with moderate rainfall rates, most showed a rise in the gage height thanks to the multiple rounds of rainfall. In Avon, a small mud flow closed the west bound lane of I-70 due to the all of the rain. Radar estimates and SNOTEL sites in the area indicated up to 1.4 inches of rain fell yesterday.

Over the Urban Corridor, an Alert gage in eastern Douglas County recorded 1.54 inches, which triggered an areal flood advisory around 2:40PM. CoCoRaHS gages in the area recorded up to 1.75 inches for the 24-hour period. Further east over the Northeast Plains, thunderstorms were more severe with hail up to 1.25 inches and a few tornado warnings. A brief tornado was reported in Sedgwick County near Julesburg at 2:20 PM, but no damage was reported. Max 1-hour rain rates were estimated up to 1 inch with 24 hour totals in the 1-1.5 inch range.

To see how much precipitation fell in your area on Sunday, scroll down to the State Precipitation Map below.

Click Here For Map Overview

The map below shows radar-estimated, rainfall gage-adjusted Quantitative Precipitation Estimates (QPE) across Colorado. The map is updated daily during the operational season (May 1 – Sep 30) by 11AM. The following six layers are currently available: 24-hour, 48-hour and 72-hour total precipitation, as well as maximum 1-hour, 2-hour and 6-hour precipitation over the past 24 hour period (to estimate where flash flooding may have occurred). The 24-hour, 48-hour and 72-hour total precipitation contain bias corrections that are not disaggregated into the hourly estimates, so there will likely be some differences. The accumulation ending time is 6AM of the date shown in the bottom right corner. Also shown optionally are fire burn areas (post 2012), which are updated throughout the season to include new burn areas. The home button in the top left corner resets the map to the original zoom.

Note: We have identified a possible underestimation in QPE over the southwest part of the state. We are working to on this issue, and will provide an update as soon as possible.

SPM 06-17-2018: 416 Fire Burn Scar Reacted Quickly

Issue Date: Sunday, June 17th, 2018
Issue Time: 9:30 AM MDT

Summary:

The remnants of Bud overspread the state yesterday, increasing moisture and providing plenty of cloud cover, especially west of the Continental Divide. A more robust dry slot than expected in the mid-level moisture held the main flooding concerns off until the overnight hours, when a burst of shower/thunderstorm activity over the Southwest Slope impacted the 416 Fire burn area. This burst occurred between the hours of 3 AM and 6 AM, with lingering showers continuing to impact the area this morning. At 5:53 AM MDT, trained spotters reported flash flooding with rock and debris across Highway 550 just north of Durango. Rain rates between 0.25-0.50 inches/hour were reported by the National Weather Service, showing just how quickly the burn scar reacted to moderate precipitation. CoCoRaHS observers across La Plata County reported more than 0.70 inches in the general vicinity of the 416 Fire burn area, with localized values greater than 1 inch.

East of the Continental Divide, scattered thunderstorms rumbled during the afternoon and evening hours, across two main areas: 1) The Northeast Plains, northern Front Range, and northern Urban Corridor, where an increase in moisture from the east/northeast and convergence along a surface boundary aided thunderstorm development, and 2) along the CO/KS border, where a plume of moisture from Bud overspread the area in conjunction with daytime heating. Some street ponding occurred across portions of the northern Urban Corridor, where the strongest thunderstorm complex of the day produced small hail and a period of moderate-to-heavy rainfall.

For a look at precipitation estimates in your area, please see our State Precipitation Map below.

Click Here For Map Overview

The map below shows radar-estimated, rainfall gage-adjusted Quantitative Precipitation Estimates (QPE) across Colorado. The map is updated daily during the operational season (May 1 – Sep 30) by 11AM. The following six layers are currently available: 24-hour, 48-hour and 72-hour total precipitation, as well as maximum 1-hour, 2-hour and 6-hour precipitation over the past 24 hour period (to estimate where flash flooding may have occurred). The 24-hour, 48-hour and 72-hour total precipitation contain bias corrections that are not disaggregated into the hourly estimates, so there will likely be some differences. The accumulation ending time is 6AM of the date shown in the bottom right corner. Also shown optionally are fire burn areas (post 2012), which are updated throughout the season to include new burn areas. The home button in the top left corner resets the map to the original zoom.

Note: We have identified a possible underestimation in QPE over the southwest part of the state. We are working to on this issue, and will provide an update as soon as possible.