Issue Date: 8/4/2016
Issue Time: 10:05 AM

A MODERATE FLOOD THREAT IS FORECAST TODAY FOR PORTIONS OF THE SAN JUAN MOUNTAINS, SOUTHWEST SLOPE, CENTRAL MOUNTAINS, AND SOUTHEAST MOUNTAINS.

A LOW FLOOD THREAT IS ISSUED FOR SURROUNDING AREAS, AS WELL AS PORTIONS OF THE RATON RIDGE, SOUTHEAST PLAINS, NORTHERN MOUNTAINS, NORTHWEST SLOPE, GRAND VALLEY, AND SAN LUIS VALLEY.

The monsoonal surge continues from the south-southwest, as moisture values remain anomalously high across western and southern Colorado. If you take a look at the water vapor imagery below, you’ll notice that both Pueblo (pink line) and Grand Junction (green line) are above 1 inch; Grand Junction is reaching towards 1.4 inches, heightening the concern for flash flooding/flooding issues.

Embedded in this flow of moisture is a disturbance at 700 mb, which will work its way east-northeast across Colorado throughout today and tonight. This wave will support scattered-to-widespread showers and thunderstorms across the High Country and Western Slope, with the best coverage generally to the south of I-70. Locally heavy rainfall is a concern, especially in the areas denoted by the moderate flood threat. Particular concern exists for burn scars (including recent burns like Hayden Pass) and steep terrain with saturated soils from previous days’ rainfall (high terrain of the Southwest Slope and San Juan Mountains).

Boulder’s IPW values (blue line) has taken a sharp decrease in the last hour or so, thanks to the passing of a cool front through eastern Colorado. Behind this front, drier, low-level air is filling in across the lower elevations. This will lead to a dry day through the afternoon and into the early evening across the Northeast Plains, with only an isolated shower/weak thunderstorm possible over the Urban Corridor and Palmer Ridge. This will change later this evening/tonight as the 700 mb disturbance shifts across the area, bringing showers/thunderstorms to the area through the overnight hours and into tomorrow morning.

Further to the south, the cool front is nearing the Raton Ridge at this time. Behind it exists the same drier air and cool temperatures in the low-levels, which will stifle showers/thunderstorms behind the front through the afternoon hours. Along the front, i.e., across the Raton Ridge and southern portions of the Southeast Plains, scattered showers/thunderstorms will develop this afternoon. A couple of these storms will become severe, producing hail, strong winds, and heavy rainfall. The low flood threat takes this scenario into account. Just as areas to the north, more numerous thunderstorms will move across the Southeast Plains and Raton Ridge tonight and into tomorrow morning in association with the passing disturbance.

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

Urban Corridor, Northeast Plains and Palmer Ridge:

Mainly dry and cool through the afternoon hours, with only an isolated shower/weak thunderstorm or two expected. The disturbance will move overhead during the late evening/overnight/morning hours, bringing numerous showers and a few thunderstorms to the area. Efficient rainfall processes will lead to locally heavy rainfall, but not enough to warrant the issuance of a flood threat. Instead, we will mention to be alert to the possibility of street/field ponding underneath thunderstorms in poor drainage areas. Maximum rain rates will be 0.4-0.8 inches/hour.

Southeast Plains and Raton Ridge:

Scattered showers/thunderstorms are expected along the cool front this afternoon and evening, mainly south of Highway 50. A few of these storms will become strong-to-severe, producing hail (up to 1.5 inches in diameter), strong winds (gusts up to 65 mph), and heavy rainfall. During the late evening hours, shower/storm coverage will increase in response to the disturbance moving through the area. This activity will continue into tomorrow morning. Maximum rain rates will be 2.0-3.0 inches/hour for the plains, and 1.0-2.0 inches/hour for the Raton Ridge.

Timing: 11 AM – 11 AM

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

Scattered-to-widespread showers and thunderstorms are expected throughout the period, with the best coverage occurring across the higher terrain of the Continental Divide, San Juan Mountains, Southwest Slope, and Southeast Mountains. The Front Range will get a late start to the better activity, relying heavily on the passing disturbance to kick off activity. The Northwest Slope will be the furthest from the upper-level support, so only isolated showers/weak thunderstorms are expected there. Locally heavy rainfall is a concern, especially with slow storm motions expected. Maximum rain rates are as follows:

Front Range and Northwest Slope: 0.3-0.6 inches/hour
Northern Mountains: 0.4-0.8 inches/hour
Central Mountains, Southwest Slope, San Juan Mountains, Southeast Mountains, Grand Valley, and San Luis Valley: 0.8-1.3 inches/hour

Timing: 11 AM – 11 AM

Issue Date: 8/3/2016
Issue Time: 9:58 AM

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

A LOW FLOOD THREAT IS FORECAST TODAY FOR SURROUNDING AREAS AND PORTIONS OF THE CENTRAL MOUNTAINS, FRONT RANGE, URBAN CORRIDOR, SOUTHEAST MOUNTAINS, AND RATON RIDGE.

Instead of using the usual IPW chart or water vapor image, I have decided that today is a good time to show the “Chance of Precipitation” plot from our internal toolbox. As you can see below, it highlights western Colorado well, especially the higher terrain areas. This is due to the incoming surge of monsoonal moisture into the area, with IPW values pushing 1.0-1.3 inches this morning across southwestern Colorado. This influx of moisture will continue through the period, providing the fuel for efficient rainfall from all showers/thunderstorms.

Additionally, a weak 700 mb disturbance is noted in this morning’s observations, and is expected to move into the four corners region. This will provide support for more numerous storms and enhanced precipitation across southwestern Colorado. Steep terrain and burn scars are areas of highest relative concern as storm motions are expected to be slow, which could result in not only flooding issues, but also rock/mud slides, debris flows, etc. For areas further north, some drying has occurred over the last 12 hours or so, and this will limit rainfall rates in those areas.

For locations east of the mountains, weak westerly flow aloft will allow a few storms to move off of the higher terrain and over adjacent lower elevations. However, slight warming aloft will keep the environment capped over most of the area, and limit the instability available to any storms that do form/move overhead. The best opportunity for heavy rainfall will be across far western portions of the Southeast Plains and over the Raton Ridge, where orographic effects, better moisture, and proximity to the weak 700 mb disturbance will aid thunderstorm development/maintenance. Towards the morning hours, a cool front will begin to pass through the area from north to south, which will set the stage for tomorrow’s forecast.

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

Urban Corridor, Northeast Plains, Palmer Ridge, Southeast Plains, and Raton Ridge:

Isolated-to-scattered showers/thunderstorms expected along and near the higher terrain and along the Raton Ridge. Most storms will be garden variety, but the amount of moisture combined with slow-moving storms means locally heavy rainfall is a concern for areas highlighted in the low threat area. Maximum rain rates are as follows:

Urban Corridor: 0.8-1.2 inches/hour
Palmer Ridge and Northeast Plains: 0.4-0.8 inches/hour
Southeast Plains: 1.2-2.0 inches/hour
Raton Ridge: 1.2-1.6 inches/hour

Timing: 11 AM – 10 PM for the Raton Ridge and Southeast Plains, 1 PM – Midnight for the Urban Corridor and Palmer Ridge, and 3 PM – 1 AM for the Northeast Plains.

Front Range and Southeast Mountains:

Isolated-to-scattered showers/thunderstorms expected. Locally heavy rainfall is a concern over the Southeast Mountains and the southern Front Range. The maximum rain rate for the Front Range is 0.8-1.2 inches/hour and 1.0-1.5 inches/hour for the Southeast Mountains.

Timing: 11 AM – Midnight for the Front Range and 10 AM – Midnight for the Southeast Mountains. A few lingering showers will continue into the early morning.

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

Scattered-to-widespread showers and thunderstorms are expected, with the best coverage occurring across the Southwest Slope and San Juan Mountains. Deep moisture, slow-moving storms, and locally heavy rainfall are a concern for flash flooding issues, as well as rock/mud slides, debris flows, etc. Maximum rain rates are:

Southwest Slope and San Juan Mountains: 1.0-2.0 inches/hour
Grand Valley: 1.0-1.5 inches/hour
Central Mountains and San Luis Valley: 0.8-1.2 inches/hour

Timing: 10 AM – Midnight, with isolated-to-scattered showers/storms continuing into the morning hours, especially across the higher terrain of southwest Colorado.

Northwest Slope and Northern Mountains:

Isolated-to-scattered showers and storms. A bit of drying has taken place overnight and this morning, and this will help limit rain rates over these regions. Maximum rain rates will be 0.4-0.8 inches/hour, which is below flood threat thresholds.

Timing: 11 AM – 11 PM

Issue Date: 8/2/2016
Issue Time: 9:40 AM

A LOW FLOOD THREAT IS FORECAST TODAY FOR PORTIONS OF THE FRONT RANGE, SOUTHEAST MOUNTAINS, URBAN CORRIDOR, PALMER RIDGE, SOUTHEAST PLAINS, AND RATON RIDGE.

A LOW FLOOD THREAT IS FORECAST FOR PORTIONS OF THE SOUTHWEST SLOPE, SAN JUAN MOUNTAINS, GRAND VALLEY, AND CENTRAL MOUNTAINS.

Moisture content (IPW) as measured by the four normal reporting stations is fairly close to average for this time of year. In fact, this morning’s soundings from Grand Junction and Boulder show precipitable water values within 0.02 inches of average for the date. It is expected that moisture will bump upward somewhat during the day, mainly across southern Colorado where monsoonal moisture continues to nudge northward. East of the mountains, IPW values will increase slightly due to a surface moisture surge from the east this afternoon/evening. All told, these “bumps” in this morning’s moisture will push IPW values back to near yesterday afternoon’s levels.

A weak disturbance will move across the state this afternoon and evening, coinciding with peak heating and moisture values, increasing coverage of showers/thunderstorms. Storms associated with this disturbance will first form over the High Country and Western Slope, generally move eastward, with a few pushing over the lower elevations of eastern Colorado during the late afternoon/early evening hours. Slow-moving storms due to light winds aloft are the main concern behind the low flood threat, as efficient rainfall over one area for extended period of time can cause issues, especially for steeper terrain and urban areas.

During the later evening hours and just after dark (7 PM – 11 PM), there is a slight chance (20%) of new thunderstorm development over the Front Range/Urban Corridor owing to continued upslope flow. The areas that will most likely be affected by any of this development will be eastern Larimer, western Weld, Boulder, eastern Gilpin, Broomfield, Denver, western Adams, western Arapahoe, and northern/eastern Jefferson counties. This is accounted for in the low flood threat area outlined in the map 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, Southeast Mountains, Urban Corridor, Northeast Plains, Palmer Ridge, Raton Ridge, and Southeast Plains:

Isolated-to-scattered showers/thunderstorms expected, with the best coverage expected over the mountains and Raton Ridge. Much of the plains will remain weakly capped by warm air in the mid-levels, but a few storms are still expected in associated with the disturbance aloft. As mentioned above, redevelopment along the Front Range/Urban Corridor between 7 PM and 11 PM is possible (20% chance). Maximum rain rates are as follows:

Front Range, Urban Corridor, Palmer Ridge, and Southeast Mountains: 0.8-1.2 inches/hour
Northeast Plains: 1.0-1.5 inches/hour
Southeast Plains: 1.5-2.2 inches/hour
Raton Ridge: 0.9-1.3 inches/hour

Timing: 11 AM – 11 PM over the mountains, 1 PM – Midnight over the plains and ridges

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

Isolated to the north, scattered to the south, showers/thunderstorms are expected today. Most will be garden variety, producing mainly gusty winds and light rainfall. A few will be stronger, however, and will likely produce locally heavy rainfall. The higher terrain of the San Juan Mountains, Southwest Slope, Grand Valley, and Central Mountains have the highest relative chance of experiencing heavy rain; these areas are outlined in the low flood threat area. Maximum rain rates will be:

Northwest Slope and Northern Mountains: 0.15-0.3 inches/hour
San Luis Valley: 0.4-0.8 inches/hour
Central Mountains, Grand Valley, San Juan Mountains, and Southwest Slope: 0.8-1.2 inches/hour

Timing: 11 AM – 9 PM, with a few isolated showers/weak thunderstorms lingering into the early morning hours over higher terrain in southern zones

Issue Date: 8/1/2016
Issue Time: 10:17 AM

A LOW FLOOD THREAT IS FORECAST FOR PORTIONS OF THE GRAND VALLEY, CENTRAL MOUNTAINS, SOUTHWEST SLOPE, SAN JUAN MOUNTAINS, SOUTHEAST MOUNTAINS, RATON RIDGE, AND SOUTHEAST PLAINS.

Drier air is poised to move into the northern third of Colorado behind the shortwave trough that passed through Colorado last night. The drying is showing up in the morning’s IPW trace (chart below) with Boulder falling below 0.8 inches. A bit of mid-level warming will accompany the drying as the ridging builds slightly over the area, weakly capping the environment. A few isolated showers/weak thunderstorms will still develop, mainly over the mountains, owing to the influence of orographic effects. Most of the Northeast Plains will remain capped and dry, especially areas north and east of a line from Alvin to Fort Morgan to Hereford, which is a nice change from recent days of scattered rainfall.

Further to the south, there will be plenty of moisture available to showers/thunderstorms, resulting in efficient rainfall from any activity. Over the mountains, isolated-to-scattered showers/thunderstorms are expected, with the best coverage expected over the higher terrain of the Southwest Slope, San Juan Mountains, and Central Mountains. Instability/shear values will be sufficient enough to produce a couple of stronger storms capable of locally heavy rainfall; thus, the issuance of the low flood threat.

To the east, the Southeast Mountains and Raton Ridge will be dotted with scattered thunderstorms capable of heavy rainfall. The IPW value (~1.2 inches) measured at Pueblo is the highest of the four normal reporting stations (see chart above). A weak disturbance will work across the area and then over the Southeast Plains this afternoon/evening, which will trigger isolated-to-scattered showers/storms over the plains. A couple storms will be strong/marginally severe, with the main threats being hail, strong winds, and periods of heavy rainfall. For more information regarding rain rates and timing, please see the zone-specific forecast 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

Urban Corridor and Northeast Plains:

The environment will be weakly capped today, resulting in a dry day for a majority of the area. Locations near the mountains and Palmer Ridge will see the most coverage, relatively speaking, with isolated showers/thunderstorms. Maximum rain rates will be 0.2-0.4 inches/hour.

Timing: Noon – 11 PM

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

Scattered showers/thunderstorms expected over and near the mountains, with more isolated-to-widely scattered coverage elsewhere. Locally heavy rainfall will attend the strongest storms, thus the issuance of the low flood threat. Maximum rain rates are as follows:

Southeast Mountains: 1.0-1.4 inches/hour
Raton Ridge: 1.2-1.6 inches/hour
Southeast Plains: 1.5-2.2 inches/hour
Palmer Ridge: 0.6-0.9 inches/hour

Timing: 11 AM – 1 AM

Northern Mountains and Northwest Slope:

Much of the area will remain dry today, with only isolated showers/weak storms over the higher terrain. Drier air will keep rain rates down, resulting in gusty winds and lightning as the main threats. Maximum rain rates will be less than 0.25 inches/hour.

Timing: Noon – 9 PM

Front Range, Central Mountains, San Juan Mountains, Southwest Slope, Grand Valley, and San Luis Valley:

Isolated-to-scattered showers/thunderstorms expected, with the best coverage expected over the Central Mountains, San Juan Mountains, and Southwest Slope. Locally heavy rainfall is a concern with IPW values near, or just above, 1 inch. Maximum rain rates will be 1.0-1.4 inches/hour, with areas near the CO/NM border having the highest relative probability to achieve these rates.

Timing: 11 AM – 11 PM, with a few showers/storms continuing into the morning hours tomorrow, mainly over the higher terrain