FTB 06-25-2015: Storm Coverage To Increase, Especially East Of Divide

Issue Date: 6/25/2015
Issue Time: 10:45AM

— Moderate flood threat for Northeast Plains, and parts of Palmer Ridge and Southeast Plains
— Low flood threat for San Juans
— Low flood threat for Arkansas River from below Pueblo dam through La Junta

Since yesterday, the subtropical high that has been scooting eastward with every passing day has weakened. As shown in the water vapor image, below, this has allowed the jet stream to encroach on Colorado from the north. At least three “shortwave” ripples are noted this morning. The easternmost feature is leaving the state, and taking with it some weak thunderstorms noted over the Arkansas River valley near the Kansas border. The western two features will dive into Colorado and promote scattered to widespread thunderstorm activity. Coverage will be highest the farther east one goes. However, adequate moisture will allow for thunderstorms west of the Divide also.

watervapor_20150625

For today, we expect morning sunshine to lead to thunderstorms by early afternoon, starting with the highest terrain of the San Juans, Central Mountains and the Front Range. Most of these storms will be relatively light rain producers, but a few will be capable of rainfall up to 1 inch per hour west of the Divide and 1.3 inches per hour east of the Divide. A Low flood threat has been posted. Later in the afternoon, storms will move eastward over the Plains. The heavy rainfall threat will increase as storms gain access to higher moisture. Torrential short-term rainfall, with rain rates up to 0.8 inches in 30 minutes and 1.4 inches per hour will be possible. However, over the northeast portions of the state, 1-hr rainfall up to 2.5 inches and 3-hr rainfall up to 3.5 inches is possible. Additionally, the evolution of strong atmospheric wind shear will support a potential for very, very large hail, up to 3 inches in diameter towards the Nebraska and Kansas borders. A Moderate flood threat has been posted for parts of Eastern Colorado.

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, jump below the map.
FTB_snapshot_20150625

Zone-Specific Forecasts

Grand Valley, Northwest Slope, Northern Mountains, Central Mountains:

Partly cloudy early, then scattered thunderstorms forming by early afternoon, with higher coverage and intensity as one moves eastward. Maximum one-hour rainfall is expected to stay below 0.8 inches, and no flooding is expected. Stronger storms may produce gusty winds up to 65 mph.

San Juans, Southwest Slope, San Luis Valley:

Sunny early, then turning partly cloudy. Scattered thunderstorms are likely by early afternoon with maximum one hour rainfall up to 1.0 inch possible. This could lead to isolated flash flooding, mud slides and debris flows. Gusty winds up to 65 mph may accompany the stronger storms. A Low flood threat has been posted.

Primetime: 1pm through 8pm

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

Partly cloudy early, with scattered storms developing by early afternoon over the higher terrain. Storms will the move eastward with additional redevelopment over the Plains. Very heavy rainfall will be possible, with generally higher intensity the farther east one goes. Closest to the mountains, 1-hr rainfall up to 1.4 inches is possible. Farther east, 1-hr rainfall up to 2.5 inches is likely. Moderate and Low flood threats have been issued.

Additionally, there is the potential for very large hail, up to 3 inches in diameter, in the Moderate threat area.

Primetime: 2PM through 11PM, except through 1AM near the Kansas border

STP 06-25-2015: Active Day, But Especially So For Denver Metro

Issue Date: 6/25/2015
Issue Time: 9:05 AM

Storm coverage and intensity increased on Wednesday as upper-level southwesterly “return flow” provided better conditions for storm formation. Although most storms across the state were not overly impressive, with maximum rainfall anywhere between 0.5 and 1 inch, two particularly storms were noteworthy. First, a storm near Pueblo dumped torrential rain over a short period of time, leading to street flooding in the city. This storm also produced 1.75 inch hail west of town. Second, a very impressive storm blossomed over downtown Denver and produced very heavy rainfall, hail up to 1.5 inches and even prompted a tornado warning. This storm dumped around 2 inches per hour at a few Urban Drainage ALERT gages. Needless to say, this much water falling over a highly impervious area caused tremendous runoff resulting in many flash flood and flood reports just east of I-25 near downtown. Fortunately, the storm was very localized, with only about a 150 square mile area receiving over 1 inch of rain. Nonetheless, the impact was very strong. The highest 24-hour rain totals in the area were just northeast of downtown Denver. About a dozen 1+ inch reports were received across Boulder, Denver, Adams, Arapahoe and Jefferson counties.

Meanwhile, there was also action across the Divide in the San Juan mountains where many popcorn type thunderstorms dumped short-term, but heavy rainfall before dissipating. Although highest official 24-hr rainfall was only about 0.4 inches in the region, our radar estimated product suggests near 1 inch of rainfall fell over remote parts of Dolores and San Miguel counties.

For estimated rainfall in your area, be sure to check out the rainfall map below.

STP_snapshot_20150625

FTB 06-24-2015: Storm Chances Continue, Now Across the Whole State

Issue Date: 6/24/2015
Issue Time: 10:10AM

— Low flood threat for Northeast Plains, Palmer Ridge and Southeast Plains
— Low flood threat for San Juans
— Low flood threat for Arkansas River from below Pueblo dam through La Junta

The upper-level high pressure system continues to retreat eastward, allowing for a return flow to develop across the Four Corners region. Today’s water vapor image, below, shows that this flow has some mid and upper-level moisture with it. As such, thunderstorm coverage is expected to increase across the state. However, one main limitation will be the surface moisture. Though surface dewpoints continue to run very high with readings over 60F in many locations, this moisture is incredibly shallow. A nice example comes from Boulder this morning: the Foothills weather station showed a dewpoint temperature of 61F, while the Mesa weather station, only 800 feet higher in elevation, had a dewpoint of 36F!

watervapor_20150624

For today, we expect sunny to partly cloudy skies this morning to lead to scattered thunderstorm activity by afternoon. Coverage will be highest in the San Juans stretching northeast through the Central Mountains and eastward across the plains. Over the San Juans and parts of the Plains, a Low flood threat has been issued. Storms will also be capable of producing very gusty winds, up to 65 mph. Activity will quickly subside after sunset, except through 11pm near the Kansas border.

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, jump below the map.
FTB_snapshot_20150624

Zone-Specific Forecasts

Grand Valley, Northwest Slope, Northern Mountains, Central Mountains, San Luis Valley, Front Range, Urban Corridor:

Partly cloudy early, then scattered thunderstorms forming by early afternoon. Maximum one-hour rainfall is expected to stay below 0.5 inches so no flooding is expected. Stronger storms may produce gusty winds up to 65 mph.

San Juans, Southwest Slope

Sunny early, then turning partly cloudy. Scattered thunderstorms are likely by early afternoon with maximum one hour rainfall up to 0.8 inches possible. This could lead to isolated flash flooding, mud slides and debris flows. Gusty winds up to 65 mph may accompany the stronger storms. A Low flood threat has been posted.

Primetime: 1pm through 8pm

Northeast Plains, Palmer Ridge, Southeast Plains, Raton Ridge and Southeast Mountains:

Sunny early, with scattered storms forming by early afternoon. Maximum 1-hour rainfall up to 1.2 inches will be possible in the stronger storms, along with hail to 0.75 inches and gusty winds up to 65 mph.

Primetime: 2PM through 9PM, except through 11PM near the Kansas border

STP 06-24-2015: Storms Dot The Landscape Once Again

Issue Date: 6/24/2015
Issue Time: 8:55AM

Though conditions were not optimal for their development, thunderstorms took full advantage of the high moisture content in the low levels of the atmosphere on Tuesday. Storms formed mainly east of the Divide by mid-afternoon, and progressed farther eastward towards the late evening hours. As expected, most thunderstorm rainfall was quite light, generally 0.25 inches or less. However, a few storms were able to reach maturity and produce over 1 inch of rainfall. This was observed for several separate thunderstorms, covering Washington, Lincoln and Las Animas counties.

In addition to the rainfall, many storms produced hail. Most of the hail was pea-size to penny-size but one particularly strong cell in eastern El Paso County produced 1 inch hail.

Meanwhile, river levels continued to slowly subside as the snowpack is virtually all gone, except in deeply forested areas.

No flash flooding was reported yesterday. For estimated rainfall in your area, be sure to check out the rainfall map below.

STP_snapshot_20150624