Issue Date: Sunday, July 3rd, 2016
Issue Time: 10:15AM MDT

— LOW flood threat for Palmer Ridge, Southeast Plains, Raton Ridge, Southeast Mountains and Grand Valley

A rather complicated weather map is found this morning across the western United States, as shown in the water vapor image below. The large-scale features include a low-pressure center across British Columbia and a distinct subtropical jet stream racing across the southern Rocky Mountain states. The latter feature will affect Colorado today as it drags a disturbance over the western part of the state eastward. Notable mid-level cooling was observed over the last 24-hours statewide and some more cooling today will help to create afternoon instability. Moisture continues to be abundant this morning with Precipitable Water (PW) values of about 1 inch, with pockets of higher values, across the state. However, this will be countered by significant drying being advected in from the north. For example, the Riverton (WY) sounding’s PW dropped from 0.74 to 0.43 inches over the past 24 hours. This air will affect northeast CO today, eliminating the heavy rainfall threat in that region.

Overall, expect a quick development of high elevation shower and thunderstorm activity to spread quickly into the lower elevations. Heavy rainfall will be possible across parts of the western slope and southeast parts of the state. Faster storm motion compared to past days implies that only a Low flood threat is warranted today for isolated flash flooding.

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.

Issue Date: 7/2/2016
Issue Time: 9:13 AM

A MODERATE FLOOD THREAT IS ISSUED FOR PORTIONS OF THE SOUTHEAST PLAINS AND RATON RIDGE.

A LOW FLOOD THREAT IS ISSUED FOR SURROUNDING AREAS, INCLUDING PORTIONS OF THE NORTHEAST PLAINS, PALMER RIDGE, URBAN CORRIDOR, FRONT RANGE, SOUTHEAST MOUNTAINS, NORTHERN MOUNTAINS, CENTRAL MOUNTAINS, SAN JUAN MOUNTAINS, SOUTHWEST SLOPE, GRAND VALLEY, AND NORTHWEST SLOPE.

Abundant moisture still sits across the state, evidenced by the IPW chart below. All 4 of the normal reporting stations (Boulder, Grand Junction, Pueblo, and Schriever AFB) are all above 1 inch this morning. This moisture will stick around throughout the afternoon and into the evening before drier air begins to move into Colorado from west to east. This means another day of scattered-to-widespread showers/thunderstorms over the High Country, with scattered activity over the lower elevations of eastern Colorado. The one exception will be Northeast Plains, east Highway 71 and north of Highway 36, where only isolated coverage of is expected.

Instability will be less than yesterday across most areas, and maximum rain rates will be down accordingly. Exceptions to this include portions of the High Country where sunlight is allowed to peek through the clouds and over the Southeast Plains/Raton Ridge where a small dry slot aloft has resulted in clear skies and allowing sunshine to warm the low-levels. The relative maximum in instability over the Southeast Plains/Raton Ridge means the strongest storms of the day will occur over these regions, producing the heaviest rainfall. A moderate flood threat has been issued for the Southeast Plains for this reason, plus the consideration of the last few days of heavy rainfall.

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, Southeast Mountains, Urban Corridor, and Palmer Ridge:

Scattered showers/thunderstorms expected today/tonight, ending for most locations by 10 PM. Instability will be less than yesterday, and rain rates will be down accordingly. There is still a risk for heavy rain underneath a stronger thunderstorm due to the shear amount of moisture available. Thus, the low flood threat. Maximum rain rates are as follows:

Front Range and Southeast Mountains: 1.0-1.4 inches/hour
Urban Corridor: 1.0-1.6 inches/hour
Palmer Ridge: 1.2-1.8 inches/hour

Timing: A few showers are ongoing, coverage/intensity will increase through the afternoon/early evening, ending by, or just after, 10 PM.

Northeast Plains:

Isolated-to-scattered showers/thunderstorms expected, mainly west of Highway 71 and south of Highway 36. A couple isolated showers/storms may develop in the far northeast corner, but probabilities are low. Maximum rain rates will be 0.8-1.4 inches/hour.

Timing: Current – 10 PM

Raton Ridge and Southeast Plains:

Scattered showers/thunderstorms expected, a few will be strong/severe. The main threats from stronger thunderstorms will be strong wind, hail, lightning, and heavy rain. Maximum rain rates will be 1.8-2.5 inches/hour for the Raton Ridge and 2.5-3.0 inches/hour for the Southeast Plains.

Timing: 11 AM – 2 AM over the Southeast Plains, 11 AM – 10 PM for the Raton Ridge

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

Scattered-to-widespread showers/thunderstorms expected again today and into this evening, ending from west to east as drier air works in. Pockets of stronger instability will develop where sunshine is allowed to peak through, so there is a threat of heavy rain once again today. The threat is less than previous days, and only a low threat will be issued today. Maximum rain rates are as follows:

Northern Mountains, Central Mountains, San Juan Mountains, and Grand Valley: 0.8-1.4 inches/hour
Northwest Slope: 0.6-1.0 inches/hour
Southwest Slope: 0.8-1.2 inches/hour

Timing: Ongoing – 10 PM, ending from west to east

Issue Date: 7/1/2016
Issue Time: 10:11 AM

2:30 PM UPDATE SUMMARY: After monitoring observational trends, and consulting with updated model guidance, the High Flood Threat has been extended southward. Additionally, the Moderate Flood Threat has been extended slightly to the south and east, now including more of the Arkansas River Valley.

DISCUSSION: The development of scattered-to-widespread showers/thunderstorms is underway, with a few stronger storms being noted across the eastern mountains/I-25 corridor. Moisture continues to be abundant, with all four of the normal reporting stations (Grand Junction, Boulder, Pueblo, Schriever AFB) showing IPW values greater than 1 inches, with Grand Junction, Boulder, and Pueblo all above 1.2 inches.

A Denver cyclone has indeed developed, setting up over northern Douglas and Elbert counties. The High Flood Threat and Moderate Flood Threat areas have both been extended to the south and east as those areas have developed the greatest instability; sunshine has warmed temperatures with surface dewpoints in the low 60s. Aside from these changes, the previous discussion remains valid.

A HIGH FLOOD THREAT IS ISSUED FOR PORTIONS OF THE URBAN CORRIDOR, FRONT RANGE, AND PALMER RIDGE.

A MODERATE FLOOD THREAT IS ISSUED FOR SURROUNDING AREAS, INCLUDING PORTIONS OF THE NORTHEAST PLAINS AND SOUTHEAST PLAINS.

A MODERATE FLOOD THREAT IS ALSO ISSUED FOR PORTIONS OF THE SOUTHWEST SLOPE, SAN JUAN MOUNTAINS, AND GRAND VALLEY.

A LOW FLOOD THREAT IS ISSUED FOR SURROUNDING AREAS, INCLUDING PORTIONS OF THE NORTHERN MOUNTAINS, NORTHWEST SLOPE, CENTRAL MOUNTAINS, SOUTHEAST MOUNTAINS, AND RATON RIDGE.

Moisture is abundant across Colorado this morning, and this morning’s soundings (below, click to enlarge) show this well. Precipitable water is anomalously high, setting 12Z records for the date at both Denver (1.17 inches) and Grand Junction (1.21 inches). Dewpoints across most of the state currently reside in the 50s and 60s, with 40s along the Continental Divide. This high moisture content, coupled with a warm layer depth of about 3 km, will allow for efficient rainfall processes. Heavy rain is a good bet underneath thunderstorms.

 

 

 

 

 

 

 

The heavy rain threat will exist across nearly all of Colorado, but of particular concern are portions of the Front Range, Urban Corridor, and Palmer Ridge. There is potential for a Denver convergence zone to develop today due to the southeasterly low-level winds; if it develops, this will be a focus for the development of stronger storms. Urban areas are also a heightened flood risk due to the amount of impervious surfaces and their associated drainage issues.

In an effort to be thorough, it is important to point out that there is some uncertainty regarding the evolution of today’s risk. Instability will be a limiting factor today, especially with the expansive cloud cover across the state. Meanwhile, a mid-level disturbance will work to counteract the lack of daytime heating by transporting cooler temperatures aloft over Colorado. There are quite a few competing factors in this period’s forecast. After consulting all of the above factors, the flood threats that have been issued are warranted.

In addition to the heavy rain threat, a few strong/severe thunderstorms are likely across eastern Colorado, mainly east of I-25. Threats from these storms will be strong/damaging winds, hail (up to 1.5 inches in diameter), heavy rain, and lightning. An isolated tornado cannot be ruled out.

For more details regarding timing, rain rates, etc., please see the zone-specific discussions below. Our team of meteorologists will be monitoring today’s unfolding situation, and will post a forecast update at 3 PM. You can stay tuned by checking back in, and/or following our Twitter account (@COFloodUpdates).

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, Northeast Plains, Palmer Ridge, and Southeast Plains:

Widespread showers and thunderstorms are expected throughout today and into tonight, generally ending from west to east. Heavy rain is a good bet underneath thunderstorms, and the potential for the development of a Denver convergence zone is ringing the High Flood Threat bells for portions of the Front Range, Urban Corridor, and Palmer Ridge. Urban street flooding is the highest concern, especially in intersections and poor drainage areas. Burn scars will need to be monitored for bursts of very heavy rain, especially the Waldo Canyon burn scar. The moderate flood threat extends well east of the high threat, encompassing plains areas that have received heavy rainfall over the past several days. Maximum rain rates are as follows:

Front Range: 1.5-2.2 inches/hour
Urban Corridor and Palmer Ridge: 2.0-3.0 inches/hour
Northeast Plains and Southeast Plains: 2.5-3.0 inches/hour

Timing: 11 AM – 1 AM, with isolated showers/storms over eastern areas lingering into the morning hours.

Southeast Mountains and Raton Ridge:

Scattered showers and thunderstorms are expected throughout today and tonight, with a couple isolated showers continuing into tomorrow morning. These areas are the most distant (relatively speaking) from the best mid-level support, so rain rates will be a bit lower than other locations. Maximum rain rates are:

Southeast Mountains: 0.8-1.2 inches/hour
Raton Ridge: 0.9-1.4 inches/hour

Timing: 11 AM – 11 PM, with a couple isolated showers lingering into tomorrow morning.

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

Isolated-to-scattered showers/storms are currently ongoing across the area, mainly along and south of the I-70 corridor. Coverage will increase through the afternoon as daytime temperatures warm and a mid-level disturbance pivots in from the southwest. The areas that will receive the best support from this disturbance are included in the moderate flood threat area. In areas where rain has fallen recently, runoff will be enhanced, and that is an added concern today.

Generally speaking, with the amount of moisture available, an expansive low flood threat is warranted. Scattered showers and storms will continue into tomorrow morning, mainly over the higher terrain. Maximum rain rates are as follows:

Northwest Slope, Northern Mountains, Central Mountains, and San Luis Valley: 0.8-1.2 inches/hour
Grand Valley, Southwest Slope, and San Juan Mountains: 0.9-1.4 inches/hour

Timing: 11 AM – 1 AM, with lingering showers/storms into the morning hours tomorrow

Issue Date: 6/30/2016
Issue Time: 10:15 AM

A MODERATE FLOOD THREAT IS ISSUED FOR PORTIONS OF THE FRONT RANGE, URBAN CORRIDOR, NORTHEAST PLAINS, PALMER RIDGE, AND SOUTHEAST PLAINS.

A LOW FLOOD THREAT IS ISSUED FOR SURROUNDING AREAS, INCLUDING PORTIONS OF THE NORTHERN MOUNTAINS, NORTHWEST SLOPE, CENTRAL MOUNTAINS, GRAND VALLEY, SOUTHWEST SLOPE, SAN JUAN MOUNTAINS, AND SOUTHEAST MOUNTAINS.

Instead of discussing the water vapor imagery, today is good opportunity to show one image from our model guidance that gets to the nitty-gritty of this forecast. The image shows the chance of precipitation across Colorado, and it’s fairly easy to see just how widespread the activity will be today. Monsoonal moisture, plus the presence of a cold frontal boundary/reinforcing outflow boundaries, will combine to leave a moist air mass in place across the state. Precipitable water (PWAT) values will be at (or above) 1 inch nearly everywhere along/east of the Front Range and Southeast Mountains, as well as far western Colorado. This signals just how moist the air mass is, and heightens the concern for flash flooding across the state.

Scattered showers are ongoing over the high country and over the eastern plains. Activity will increase around 11 AM, first over the mountains, and then spreading eastward over the Urban Corridor and then across the plains. A couple storms across the plains will be strong/severe, with strong winds, hail (up to 1.5 inches), and heavy rain being the main threats. Scattered showers/thunderstorms will continue overnight and into tomorrow morning. For more details, including timing and maximum rain rates, please see the zone-specific discussions below.

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, Northeast Plains, Palmer Ridge, and Southeast Plains:

Widespread showers/thunderstorms are expected today and tonight, with isolated-to-scattered showers/thunderstorms continuing into the morning hours tomorrow. With precipitable water values at, or above, 1 inch, heavy rain is a heightened concern. Storm motions will help mitigate the flood threat somewhat, but that “help” is neutralized a bit with the past few days of rain saturating soils. Maximum rain rates are as follows:

Front Range: 1.0-1.5 inches/30-45 minutes
Urban Corridor: 1.2-2.0 inches/30-45 minutes
Palmer Ridge, Northeast Plains, and Southeast Plains: 1.8-2.5 inches/30-45 minutes

Timing: 11 AM – 4 AM

Southeast Mountains and Raton Ridge:

Scattered showers/thunderstorms expected to begin around lunchtime and continue through the evening, with a few isolated-to-scattered showers/storms lasting into the early morning hours. Although surface moisture will be a bit lower here, and it will be a bit removed from the best mid-level forcing, the environment will still support periods of locally heavy rainfall. Maximum rain rates will be 0.8-1.4 inches/hour, but most rain will fall at a rate of 0.4-0.8 inches/hour.

Timing: 11 AM – 3 AM

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

Widespread showers/thunderstorms are expected today and tonight, with scattered showers/thunderstorms continuing into tomorrow morning. Gusty winds, lightning, and locally heavy rainfall are the main threats. Maximum rain rates are as follows:

Northwest Slope and Grand Valley: 0.8-1.4 inches/hour
Northern Mountains and Central Mountains: 0.6-1.2 inches/hour
San Luis Valley: 0.6-1.4 inches/hour
Southwest Slope and San Juan Mountains: 1.2-1.6 inches/hour

Timing: 11 AM – 11 AM (tomorrow)

Note: Notice that some portions of central Colorado are not included in the low flood threat. This is due to the best instability/moisture existing east and west, so heavy rainfall causing flash flooding is not enough of a concern to warrant inclusion in the low flood threat.