Issue date: Sunday, May 31st, 2015
Issue time: 9:10AM MDT

For the second day in a row, Colorado remains under the influence of an upper-level ridge, as shown in the water vapor image below. True to its nature, the high pressure is inducing downward motion resulting in a fairly dry atmosphere. Though dewpoint temperatures this morning are in the 40s and even 50s, the moisture is very shallow. Just a few thousand feet off the ground, the relative humidity values are only 10-20%. There is, however, a weak disturbance noted on the water vapor image. As this disturbances treks southeastward, it may help organize scattered thunderstorms in eastern Colorado. The limited moisture will keep rainfall rates below 1 inch per hour, and no flood threat is anticipated today.

The Arkansas River continued to slowly recede yesterday, and only one official gauge, at La Junta, remains in Minor flood status. We have removed our Low flood threat for the Arkansas because the hot and dry weather is expected to further promote the river’s lowering.

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.

Zone-Specific Forecasts

Southwest Slope, Northwest Slope, Grand Valley, and San Luis Valley:

Mostly sunny and very warm with temperatures rising into the low to mid 90s in the lowest elevations. No flood threat is expected today.

Northern Mountains, Central Mountains and San Juan Mountains:

Mostly sunny to partly cloudy with isolated to scattered thunderstorms possible by early afternoon. Hourly rain rates will be 0.3 inches or less, so no flooding is expected.

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

Mostly sunny early, then becoming partly cloudy with isolated thunderstorms developing by early afternoon. Maximum hourly rain rates will be 0.4 inches close to the mountains, increasing to 0.8 inches  farther east. However, the strongest storms will be very isolated in nature, and no flooding is expected.

Issue date: May 30th, 2015
Issue time: 9:10AM MDT

— LOW flood threat for Arkansas River from Pueblo through La Junta

Today’s water vapor image, below, shows something that we have almost forgot exists: a ridge of high pressure scooting eastward towards Colorado. This ridge will promote downward motion across our state, resulting in lower moisture content and a reduced chance of rainfall. This kind of pattern has certainly been a commodity this spring.

This morning, we see most of the state covered in sunshine with the exception of some low-clouds in the Northeast and Southeast Plains. These clouds will quickly evaporate in a few hours once the atmosphere begins mixing in dry air from above. We expect sunshine to rule the land this morning. By early afternoon, scattered clouds will develop mainly over the higher terrain. Scattered weak thunderstorms will develop by mid-afternoon mainly across the higher terrain east of the Divide. Precipitable water values are in the 0.4 to 0.6 inch range, while dewpoints are expected to fall into the 20s to near 40F later this afternoon. Thus, one-hour rainfall from these storms will be 0.5 inches or less, and no flood threat is required.

A Low flood threat remains in place for the Arkansas from Pueblo trough La Junta, but we expect levels to recede slowly today. The river may final subside below flood status by tomorrow.

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.

Zone-Specific Forecasts

Southwest Slope, Northwest Slope, Grand Valley, and San Luis Valley:

Sunny early with scattered clouds developing by early afternoon. Very warm with temperatures topping out in the low 90s in the lowest elevations. An isolated thunderstorm cannot be ruled out, however, little if any rainfall will reach the ground. No flooding is expected today.

Northern Mountains, Central Mountains and San Juan Mountains:

Sunny early, then partly cloudy with isolated showers and storms developing by early afternoon. Maximum rain rates will be 0.3 inches per hour. No flooding is expected.

Front Range, Urban Corridor, Northeast Plains:

Sunny early with scattered clouds developing by early afternoon. An isolated storm cannot be ruled out but rainfall will be limited to 0.2 inches per hour.

Palmer Divide, Southeast Plains, Southeast Mountains, Raton Ridge:

Sunny early, then scattered showers and thunderstorms developing by early afternoon. Hourly rain rates up to 0.5 inches are possible, along with some small hail and gusty winds. No flash flooding is expected. A Low flood threat continues to be in effect for the Arkansas River from Pueblo through La Junta.

Issue Date: 5/29/2015
Issue Time: 10:32 AM

A LOW FLOOD THREAT HAS BEEN ISSUED FOR PORTIONS OF THE FRONT RANGE, URBAN CORRIDOR, PALMER RIDGE, SOUTHEAST PLAINS, SOUTHEAST MOUNTAINS, AND RATON RIDGE. A LOW FLOOD THREAT CONTINUES FOR A PORTION OF THE ARKANSAS RIVER.

As is the case on most days, the water vapor imagery paints a good picture for today’s forecast. It doesn’t tell the whole story, but it is a good place to start. A relatively weak upper-level trough will move to the east-southeast across the state today, providing support for another day of isolated-to-scattered showers and thunderstorms. West of the trough axis (black line), the large-scale environment will generally be unsupportive of thunderstorm development; however, a bit of moisture embedded in the northwesterly flow (red arrow) will combine with orographic support for isolated-to-scattered showers and thunderstorms west of the Continental Divide. The number of storms will be noticeably less than yesterday west of the Divide, especially for southern areas. The most activity will occur across the Northern and Central Mountains.

East of the Continental Divide, another factor will be in play – a passing cool front. This cool front will do a few things:

1.  Provide upslope flow after passage, reinforcing moisture against the mountains, albeit a small net increase over yesterday
2.  Provide a forcing mechanism for thunderstorms to develop along the front
3.  The wind shear due to easterly flow behind the front with northwesterly flow aloft will support a few strong-to-severe thunderstorms, mainly across the Palmer Ridge, southern reaches of the Urban Corridor, Southeast Plains, Raton Ridge, and Southeast Mountains. The probability of occurrence will increase the further southeast you go.

Due to these factors at play, scattered showers and thunderstorms are expected east of the Continental Divide, with heavy rain, hail, and gusty winds being the main threats. For more details on timing and maximum rain rates, be sure to jump down to the Zone-Specific forecasts.

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.

Zone-Specific Forecasts

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

Scattered showers and thunderstorms are expected today, and a few will be strong-to-severe. Rain rates will generally be in the 0.5-0.8 inches/hour range, but the strongest storms will produce rainfall at rates of 1.2-2.0 inches/hour. The likeliest location for heavy rainfall at that rate will be across the Southeast Plains and Raton Ridge, but strong storms across the Urban Corridor and Southeast Mountains will push rain rates near 1.0-1.5 inches/hour. Burn scars like Waldo Canyon and Black Forest will need to be watched, as well as areas that are still saturated.

Timing: 11 AM – 9 PM, a few thunderstorms continuing over the Southeast Plains until midnight. Storms will develop from north-to-south with the passage of the cool front, and will diminish from north-to-south this evening.

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

Considerably fewer showers and thunderstorms are expected today, with the most activity occurring across the Northern and Central Mountains. Instability will be fairly low, so any showers/thunderstorms will rely on orographic influences to develop/maintain themselves. This means that the higher terrain will see the majority of all precipitation, with lower valleys scraping by on the remainders as the storms move off of the mountains. Above 10,000 feet, it is likely that snow will fall, producing light accumulations mainly across the Northern and Central Mountains. Don’t expect heavy rain to cause any problems, with storms only being able to muster maximum rain rates of 0.2-0.4 inches/hour.

Timing: 11 Am – 8 PM, diminishing quickly thereafter.

Issue Date: May 28th, 2015
Issue Time: 10:10AM MDT

— LOW flood threat for parts of Northeast Plains, Palmer Ridge and Southeast Plains
— LOW flood threat for South Platte River from Greeley through Fort Morgan
— LOW flood threat for Arkansas River from Pueblo through La Junta

A disturbance is moving into Colorado from the northwest, shown by the water vapor image below. The disturbance’s passage will coincide with the maximum heating of the day, which will increase thunderstorm activity compared to yesterday. However, despite favorable upper-level dynamics, one important missing piece to heavy rainfall today is low-level moisture. Although morning dewpoints are in the 40s and 50s, precipitable water values are only around 0.6 inches. More importantly, since the disturbance is arriving from the northwest, this will allow dry air to mix down the eastern side of the Divide. The end result is that only eastern Colorado will have enough low-level moisture to warrant a low flood threat. There, in addition to heavy rainfall, large hail up to 1.75 inches and wind gusts up to 60 mph are all possible with stronger storms. Away from the threat area, expect scattered thunderstorms with one hour rain rates in the 0.2 to 0.5 inch range, along with small hail up to 0.5 inches.

In addition to the flash flood threat today, the Arkansas and South Platte Rivers continue to run in minor flood stage. Today’s activity is very unlikely to significantly impact either river. Nonetheless, we expect both rivers to stay elevated due to the abundant upstream runoff.

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.

 

Zone Specific Forecasts

Southwest Slope, Northwest Slope, Grand Valley, and San Luis Valley:

Partly to mostly cloudy with scattered showers and thunderstorms. Hourly rainfall up to 0.4 inches will be possible, along with small hail up to 0.5 inches and gusty winds up to 60 mph. No flooding is expected today.

Northern Mountains, Central Mountains and San Juan Mountains:

Scattered showers and thunderstorms developing by early afternoon. Hourly rainfall up to 0.6 inches will be possible, along with small hail and gusty winds. No flooding is expected today.

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

Becoming partly cloudy with thunderstorms developing by early afternoon. Closer to the mountains, hourly rain rates up to 0.7 inches will be possible. Farther east, stronger storms will be capable of up to 1.8 inches per hour of rainfall. A Low flood threat has been posted for the eastern part of the Palmer Ridge and the entire Kansas border. The strongest storms will be capable of 1.75 inch hail, and winds up to 60 mph.

Prime Time: 2pm through 9pm

Southeast Mountains, Raton Ridge:

Mostly sunny early, then becoming partly cloudy. An isolated high based thunderstorm cannot be ruled out. However, the passage of a dryline will lower dewpoints into the 20s and 30s preventing heavy rainfall. Gusty winds up to 35 mph will be likely in the afternoon, but the fire danger is not of concern due to wet soils. No flooding is expected today.