Issue Date: 5/27/2015
Issue Time: 10:33 AM

A LOW FLOOD THREAT CONTINUES FOR PORTIONS OF THE CACHE LA POUDRE/SOUTH PLATTE RIVER, AND PORTIONS OF THE ARKANSAS RIVER.

The broad-scale picture that the water vapor imagery (below) shows will help to define the forecast for today. The upper-level low (marked by the red “L”) will continue to dig to the south-southeast, placing Colorado under west-southwest flow for today. The water vapor imagery shows that the best mid-level moisture will remain to the south and north of Colorado, leading to a decrease in precipitable water values. Additionally, the westerly flow will mix down to the surface, eroding surface dewpoints and limiting the coverage/intensity of thunderstorms over most of the state. Expect the number of showers and thunderstorms to be slightly less than yesterday over and near the mountains.

The exception to all of this, of course, will be near the CO/KS border, where dewpoints just east of a developing dryline will remain in the upper 40s/low 50s. Furthermore, the dryline will provide support for strong-to-severe thunderstorms to develop, and this is where the heaviest rain of the day will be found, along with the possibility for large hail (> 1 inch), damaging wind gusts (> 60 mph), and an isolated tornado or two.

Finally, the last feature in the water vapor imagery to be discussed is a remnant upper-level low (marked by the purple line). This remnant low will transition into a shortwave trough and slide into western Colorado overnight tonight. This will likely keep a few showers/weak thunderstorms going until after midnight over the mountains.

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 and Urban Corridor:

Isolated-to-scattered showers and thunderstorms expected this afternoon and evening, producing mainly gusty winds and light-to-moderate rain. Rain rates will generally be in the 0.2-0.4 inches/hour range, but a stronger thunderstorm will hold the potential for 0.5-1.0 inches/hour. However, drying of the lower levels will discourage those maximum rates from being realized at the surface.

Timing: Noon – 8 PM

Palmer Ridge, Southeast Plains and Northeast Plains:

These are the regions of interest today, mainly east of the 104th meridian, particularly for strong-to-severe thunderstorms developing this afternoon and evening. West of the 104th meridian, showers and thunderstorms will be weak and isolated; east of the 104th meridian and near the CO/KS border, the environment will be favorable for the development of a few strong-to-severe thunderstorms. Maximum rain rates under these storms will be 1.5-2.5 inches/hour, but storm motions will limit any flash flooding potential.

Timing: 2 PM – 11 PM

Southeast Mountains and Raton Ridge:

Isolated showers and thunderstorms are on tap, resulting in rain rates in the 0.2-0.4 inch/hour range. High temperatures will continue their trend upward towards “normal” for this time of year.

Timing: 1 PM – 7 PM

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

Scattered showers and thunderstorms, slightly fewer than yesterday, are expected across these regions. Rain rates will generally be in the 0.2-0.4 inches/hour range, but a stronger thunderstorm over the San Juan Mountains will hold the potential to produce 0.3-0.6 inches/hour.

Timing: 11 AM – 9 PM, with a few showers and thunderstorms continuing until after midnight.

Issue Date: 5/26/2015
Issue Time: 10:02 AM

A LOW FLOOD THREAT HAS BEEN ISSUED FOR PORTIONS OF THE FRONT RANGE, URBAN CORRIDOR, PALMER RIDGE, SOUTHEAST PLAINS, AND SOUTHEAST MOUNTAINS. A LOW FLOOD THREAT CONTINUES FOR PORTIONS OF THE CACHE LA POUDRE/SOUTH PLATTE RIVER, AND PORTIONS OF THE ARKANSAS RIVER.

As the upper-level trough slides to the east of Colorado today, weak mid-/upper-level ridging will move in to take its place. This weak ridging will add a bit of subsidence to the mix, bringing a downtick in showers and thunderstorms for this period. However, this is not to say that showers and thunderstorms won’t occur (they will), there will just be slightly fewer than previous days. Providing the fuel for today’s activity will be residual moisture (slightly greater than yesterday, see IPW chart below), marginal instability, and orographic influences, as well as a weak disturbance that will move west-to-east across Colorado. This weak disturbance is currently located over central Utah and, as it moves eastward, will be the focal point for kicking off today’s activity.

The greatest coverage of showers and thunderstorms will be over the higher terrain where orographic influences provide the best forcing on a day marked by a relative lack of upper-level support. Further east, the lower elevations will rely on storms moving off of the higher terrain and marginal instability to develop/maintain showers and thunderstorms. A stronger thunderstorm or two will be possible along the Front Range, Urban Corridor, Palmer Ridge, and western portions of the Northeast/Southeast Plains, with hail, lightning, and brief heavy rain being the main threats. A low flood threat will accompany this due to saturated soils lowering the maximum rain rate needed to produce any flash flooding/mudslide/rock slide issues.

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

Warmer and drier overall as compared to yesterday, but isolated-to-scattered showers and thunderstorms will make an appearance again this afternoon and evening. A stronger thunderstorm or two is likely, and will produce hail, gusty winds, lightning, and brief heavy rain. Storm motions aren’t particularly quick; couple this with areas that have saturated soils from previous days of rain, and we have reason to include a low flood threat area. Maximum rain rates from the stronger thunderstorms will be in the 1.0-1.5 inches/hour range.

Timing: 11 AM – 10 PM, with a few showers/thunderstorms lingering until 2-3 AM near the CO/KS border.

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

Scattered showers and thunderstorms expected again today, mainly over the higher terrain. Even so, storm motions will allow for storms to drift over lower elevations with time. Rain rates will mainly be in the 0.2-0.4 inch/hour range, but a few thunderstorms will produce maximum rain rates in the 0.4-0.7 inch/hour range.

Timing: 11 AM – 9 PM, with a few showers lingering after midnight and into the early morning hours on Wednesday.

Issue Date: 5/25/2015
Issue Time: 10:33 AM

LOW FLOOD THREAT IS FORECAST FOR PORTIONS OF THE SOUTH PLATTE AND ARKANSAS RIVERS. A LOW FLOOD THREAT IS ALSO FORECAST FOR POTIONS OF THE FRONT RANGE, URBAN CORRIDOR, PALMER RIDGE, SOUTHEAST PLAINS, RATON RIDGE, AND SOUTHEAST MOUNTAINS.

The mid-/upper-level low that moved across the state yesterday is centered over NE Colorado/W Nebraska this morning. As this continues to push to the east-northeast, Colorado will come under the influence of weak subsidence behind the low. Despite this, there will be enough residual moisture, instability, and orographic influences to produce another afternoon and evening of isolated-to-scattered showers and thunderstorms across Colorado.

The most numerous storms will occur over and near the higher terrain as orographics play a key role in development. Above 8,000-8,500 feet, snow/graupel will fall, with rain below those elevations. Further east over the Plains, storms will rely on relatively weak instability and residual moisture to develop/maintain isolated showers and thunderstorms. The low flood threat area is as much a result of saturated soils as it is rain rates; overall, rain rates will not be particularly impressive (most between 0.25-0.5 inches/hour), but it will not take much more than that over certain areas to create flash flooding issues. Rain rates, as well as timing, will be broken down in the zone-specific forecast discussions.

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:

Isolated-to-scattered showers and thunderstorms will develop once again around noon, continuing through the evening before diminishing after sunset. Southern portions of the regions will have the best chance for a storm or two to become more organized, producing rain rates of 0.7-1.1 inches/hour. A storm with rain rates in this range will be capable of producing flash flooding, especially considering the already saturated soils, thus the low flood threat.

Timing: Noon – 10 PM

Northeast Plains, Southeast Plains, and Raton Ridge:

Isolated showers and thunderstorms are on tap again this afternoon and evening, with the best relative chances near the interface with the higher terrain. Western portions of the Southeast Plains and the Raton Ridge will have the best chance for a strong thunderstorm to develop, producing rain rates in the 0.7-1.2 inches/hour range, which is the reason for the low flood threat in those areas. Otherwise, rain rates will mainly be in the 0.25-0.5 inches/hour range.

Timing: 1 PM – 9 PM

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

Scattered showers and weak thunderstorms are expected again today and tonight, producing snow/graupel above 8-8,500 feet and rain below those elevations. Rain rates will typically be in the 0.25-0.5 inches/hour range, but a stronger thunderstorm over the Southeast Mountains will hold the potential to produce rain at 0.7-1.1 inches/hour.

Timing: 11 AM – 9 PM, a few showers lingering until 11 PM.

ISSUE DATE: May 24, 2015
ISSUE TIME: 10:10AM

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

After three days of drifting, the upper-level disturbance has finally reached Colorado, as shown by the water vapor image below. However, this system has already seen its best days and will continue to slowly weaken. Nonetheless, there are many smaller disturbances embedded within the main system (black dashed lines on water vapor image) that will continue to generate precipitation across the state.

Overall, we anticipate a decrease in rain intensity today. There are several reasons for this. First of all, the upper-level system has weakened. One way to measure this is the temperatures in the upper-levels, say 15,000-20,000 feet above the ground. Yesterday, those temperatures were a chilly -20 Celsius, but today, they have warmed to -15 to -17 C. This implies less overall instability. Second, precipitable water values, measuring total water vapor in the overhead atmosphere, have declined into the 0.4 to 0.6 inch range. They are expected to stay in this range, except for the far eastern portions of the state. Finally, there will be more downsloping flow today, especially in the in the Urban Corridor and down through the Southeast Mountains. This will tend to disrupt thunderstorm formation.

For today, we anticipate early thunderstorm formation across almost all areas of the state. Thunderstorm rainfall will be quite light, generally in the 0.25 to 0.5 inch range. Once again, the only exception to this will be in the eastern portions of Colorado, where instability will be higher. There, one-hour rainfall up to 1.6 inches will be possible, warranting a Low flood threat.

Finally, the South Platte and Arkansas continue to run high. We have downgraded the South Platte into a Low flood threat, putting it in the same category as the Arkansas. We do not anticipate today’s rainfall being a serious impact to either river.

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:

Variable cloudiness with showers and thunderstorms developing by early afternoon. Hourly rainfall up to 0.3 inches will be possible, along with small hail and gusty winds. No flooding is expected today.

Northern Mountains, Central Mountains and San Juan Mountains:

Mostly cloudy with showers and thunderstorms developing by early afternoon. Hourly rainfall up to 0.5 inches will be possible, along with small hail and gusty winds. Snow levels will range from 10,000 to 12,000 feet.

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

Mostly sunny early, but increasing clouds with showers and thunderstorms developing by early afternoon. Hourly rainfall up to 0.8 inches will be possible, except higher amounts in eastern areas towards the Kansas border. There, one hour rainfall up to 1.6 inches and 3-hour rainfall up to 2.5 inches will be possible. Hail up to 1.25 inches and gusty winds may also accompany the strongest storms. A Low flood threat has been issued. Additionally, the South Platte remains under a Low flood threat from Greeley through the NE state line. The Arkansas is in a Low flood threat from Pueblo through La Junta.

Prime Time: 2pm through 8pm

Southeast Mountains, Raton Ridge:

Mostly sunny early, then variable cloudiness with showers and thunderstorms developing by early afternoon. Hourly rain rates up to 0.7 inches are possible. No flooding is expected.