STP 08-26-2016: A Few Stronger Storms Brought Locally Heavy Rainfall and Small Hail

Issue Date: Friday, August 26th, 2016
Issue Time: 9:00 AM MDT


The upper-level trough continued to affect Colorado’s weather pattern yesterday, sending a small disturbance across the Colorado during the afternoon/evening hours. This disturbance provided favorable dynamics for the development of scattered showers/thunderstorms, while good moisture and sufficient instability promoted locally heavy rainfall within stronger storms. Drier air over the far eastern plains helped keep rain rates down near the CO/KS border. Storm reports from the local National Weather Service offices include:

Thunderstorm Wind Gust: 51 mph, 5 miles NW of Mack (Mesa)
Hail: 0.25 inch in diameter, 2 miles NW of Ridgeway (Ouray)
Heavy Rain: 1.63 inches, 11 miles E of Delhi (Las Animas)
Heavy Rain: 0.55 inches, 2 miles ENE of Cortez (Montezuma)
Heavy Rain: 0.55 inches, Telluride (San Miguel)

For a statewide look at 24-hour precipitation totals, please take a look at the Storm Total Precipitation map below.

Storm Total Precip Legend

FTO 08-25-2016: Two Big Pattern Swings Ahead

Issue Date: 8/25/2016
Issue Time: 2:35 PM

Threat_Timeline_20160825Event #1 in this forecast is a continuation of Event #1 from Monday’s FTO; the event is winding down as the large-scale trough continues its push to the east. Before it is able to clear the state, however, it will provide a couple more days of cooler weather (Friday/Saturday). For Friday, a fair amount of subtropical moisture will still be in place across the state, while the upper-level pattern will promote a good bit of upward motion. Over the mountains, garden variety showers/thunderstorms are expected, with locally heavy rainfall a concern with precipitable water values still elevated. Recent fire burns, like the Hayden Pass Fire burn, will need to be monitored closely. Further east over the lower elevations, a few strong-to-marginally severe thunderstorms will mix in with scattered showers/thunderstorms. The main focus areas for these storms will depend on which area receives the best sunshine and daytime heating, allowing for enhanced instability. Storm motions will help to mitigate the flash flood threat somewhat as they should move fairly quickly to the east.

Activity will begin to wind down on Saturday, so to speak, as upper-level dynamics become less favorable and moisture begins to fade. Residual low-level moisture will keep isolated-to-scattered showers/thunderstorms in the forecast, but reduce the overall flash flood threat. The best chance for heavy rainfall on Saturday will likely be across southern Colorado, where the best moisture will reside. By Sunday, drier air will invade the state as upper-level ridging builds in from the west, ushering in a drier/warmer period.


During this period, there will only be limited storm activity, mainly confined to the higher terrain as orographic effects work on residual moisture. Temperatures are expected to climb above seasonal averages, while slightly enhanced fire danger returns over the higher elevations due to low relative humidity. This stretch of warm/dry weather will continue until Thursday, when the next pattern change begins to take shape. A large-scale trough will once again develop/deepen along the west coast, bringing southwesterly flow back to Colorado. This setup would imply rainfall chances for the High Country and Western Slope, with a few storms working across the adjacent lower elevations. Uncertainty regarding smaller-scale features, as well as the amount of monsoonal moisture that will make a return, will keep this event designated as “No Apparent Flood Threat” for the time being. If deep southerly develops in concert with the trough, then the heavy rainfall threat will increase. Stay tuned to Monday’s FTO for an update on this event.

Event #1: Friday (08-26-2016) and Saturday (08-27-2016)

Elevated Flood Threat due to Potential for Locally Heavy Rainfall and Strong Storms

A fair amount of subtropical moisture will still be in place across Colorado on Friday, while the upper-levels provide support for scattered showers/thunderstorms. Pockets of enhanced instability will develop over the plains where the best daytime heating takes place. Over the mountains, scattered garden variety thunderstorms are expected, with locally heavy rainfall remaining a threat. Burn scars, especially new burn scars, will need to be monitored, as well as urban areas with high amounts of impervious surfaces. A couple strong-to-marginally severe thunderstorms will mix in over the plains. Storm motions will help mitigate the flood threat somewhat. For Saturday, moisture and favorable dynamics will begin to fade as the trough continues to shift eastward. Stay tuned to the daily FTB for up-to-date information.


Event #2: Friday (09-02-2016) through Thursday (09-08-2016)

No Flood Threat Apparent as Moisture Return Isn’t Clear

The stretch of warm/dry weather will come to an end as southwesterly flow aloft returns to the state ahead of the next upper-level trough. Early in the period, the focus for showers/storms will be across the High Country and Western Slope, mainly along and west of the Continental Divide. Uncertainty remains regarding smaller-scale features and moisture return, so expect the precipitation map to change a bit over time as details become clearer.


FTB 08-25-2016: More Sunshine = More Storms; Heavy Rainfall Possible

Issue Date: Thursday, August 25th, 2016
Issue Time: 10:20AM MDT

LOW flood threat for Palmer Ridge, parts of Southeast Plains, Raton Ridge, Urban Corridor and Front Range

The trough that has been affecting Colorado’s weather pattern for the better part of 3 days continues to stay put while small disturbances rotate through it on a daily basis. The water vapor image, below, shows another such disturbance located across western Colorado. As this feature moves east, it will provide favorable dynamics in the form of cool temperatures aloft and mid-level upward motion. With clear skies nearly statewide this morning, temperatures will be able to warm into the 70s and 80s over lower elevation locations. This will be enough to cause scattered to widespread thunderstorm activity mainly over the Palmer Ridge and surrounding areas. A Low flood threat has been posted here. One negative factor today is the relatively dry air in eastern Colorado that will try to advect westward as upslope flow develops by early afternoon. This will likely limit heavy rainfall to the foothills and immediate areas. Due to the cold air aloft, hail up to 0.75 inches will be possible with the strongest storms.


Today’s Flood Threat Map

For more information on today’s flood threat, see the map below (hover over threat areas for more details). For Zone-Specific forecasts, scroll below the map.

Flood Threat Legend

Zone-Specific Forecasts:

Front Range, Urban Corridor, Palmer Ridge, Northeast Plains, Raton Ridge, Southeast Plains, Southeast Mountains:

Sunny early and much warmer with scattered thunderstorms developing by early afternoon. Coverage will be highest along the Palmer Ridge and surrounding areas. Max 30-minute rainfall up to 0.9 inches is possible, with max 1-hr rates up to 1.4 inches. Isolated flash flooding, debris slides and mud flows are possible, especially due to the wet soils from yesterday’s steady rain. Hail up to 0.75 inches is possible under the strongest cells. A Low flood threat has been issued.

Primetime: 1PM to 9PM

San Juan Mountains, San Luis Valley, Northwest Slope, Northern Mountains, Central Mountains, Grand Valley, Southwest Slope:

Mostly sunny this morning then scattered to showers and thunderstorms developing by early afternoon. Max 1-hr rain rates up to 0.5 inches possible with max 24-hour rainfall up to 0.9 inches. Hail up to 0.5 inches could accompany the strongest cells. Flooding is not expected today.

Primetime: 12PM to 7PM

STP 08-25-2016: Widespread Showers, But Fewer Storms

Issue Date: Thursday, August 25th, 2016
Issue Time: 9:00AM MDT


Widespread rainfall made it across the Continental Divide to affect almost all of eastern Colorado, courtesy of a compact but strong shortwave moving across the state. Many Coloradoans along the entire I-25 corridor woke up to find light to moderate rain showers. Rain rates up to 0.25 inches per hour were common, providing beneficial rainfall without causing a runoff threat.

Farther south where more sunshine and warmer temperatures were present, some thunderstorms were found though coverage was reduced compared to the past few days. Highest rainfall rates occurred in three areas. The first was in the southern part of the San Juan Mountains where close to 1 inch of rainfall fell. The second was along the central and southern parts of the Sangre de Cristos where 1 inch fell at a SNOTEL site on the CO/NM border. The third was in the far southeast part of the state where a warm front provided a source of convergence and warm moist air, resulting in instability. Just over 2 inches of rainfall was observed at a CoCoRaHS station in Baca County with even higher amounts possible as indicated by radar. A flood advisory was posted for the region, but fortunately no flooding reports were received as of this morning. Finally, we should note that the Hayden Pass burn area only received 0.1 to 0.3 inches of rainfall yesterday, which was fortunately not enough to cause any runoff.

For rainfall estimates specific to your area, check out our Storm Total Precipitation map below.

Storm Total Precip Legend