Issue Date: 8/18/2018
Issue Time: 8:42 AM

A LOW FLOOD THREAT IS FORECAST FOR PORTIONS OF THE FRONT RANGE, SOUTHEAST MOUNTAINS, URBAN CORRIDOR, NORTHEAST PLAINS, SOUTHEAST PLAINS, PALMER RIDGE, AND RATON RIDGE.
A LOW FLOOD THREAT IS FORECAST FOR PORTIONS OF THE SOUTHWEST SLOPE AND SAN JUAN MOUNTAINS.

An upper-level disturbance, highlighted in the water vapor image below, will shift across Wyoming today, reaching western Nebraska by sunrise tomorrow. As it does so, it will provide the forcing for scattered showers/thunderstorms again today/tonight. In fact, an area of showers is already moving across western Colorado this morning associated with this upper-level disturbance. This shower activity will continue to move eastward, reaching the Front Range by 10-11 AM or so, and then reaching the Urban Corridor shortly thereafter. As this activity moves off the mountains, it will encounter better low-level moisture, especially east of I-25. This moisture, combined with the daytime heat and upper-level support, will result in a few strong-to-severe thunderstorms capable of heavy rainfall, hail, and strong winds.

Fortunately, storms should be moving at a good enough pace to mitigate much of the flood threat, so only a low threat is warranted. The areas with the most relative risk of flooding today will be those that received heavy rainfall yesterday, urban areas with poor drainage, and sensitive burn scar areas. Behind the initial line of storms, isolated-to-widely scattered showers and thunderstorms will dot the state during the afternoon and evening in response to a secondary disturbance associated with the upper-level low. All activity should come to an end, or exit the state to the east, by 1 AM or so.

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

Scattered showers/thunderstorms are expected again today/tonight, with the initial corridor of storms moving across the area from west to east, beginning around lunchtime and exiting the state around midnight. A secondary disturbance will kick off isolated-to-widely scattered showers/storms behind the initial round of storms. Most of the heavy rainfall will occur east of I-25 where low-level moisture is best, but there is enough moisture west of I-25 for brief periods of heavy rainfall. Maximum rain rates are as follows:

Urban Corridor: 1.0-1.5 inches/hour
Palmer Ridge and Raton Ridge: 1.2-1.6 inches/hour
Northeast Plains and Southeast Plains: 1.5-2.5 inches/hour

Timing: 11 AM – 1 AM

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

Scattered showers/thunderstorms are expected today/tonight, with an area of activity already ongoing this morning. West of the Continental Divide, moisture lacks a bit, so expect rain rates to be light-to-moderate. The only area of concern west of the Divide will be the sensitive burn scars in southwest Colorado; a low flood threat has been issued. East of the Divide, moisture will be sufficient for periods of moderate-to-heavy rainfall, but storm motions will mitigate much of the flood threat concern. Still, a low flood threat has been issued, with the primary concerns being sensitive burn scars. Maximum rain rates are as follows:

Northwest Slope, Northern Mountains, Grand Valley, and Central Mountains: 0.2-0.4 inches/hour
Southwest Slope, San Juan Mountains, and San Luis Valley: 0.4-0.7 inches/hour
Front Range and Southeast Mountains: 0.6-1.2 inches/hour

Timing: Ongoing – 10 PM, with a few lingering showers/weak storms over the Front Range and Southeast Mountains until Midnight

Issue Date: Friday, August 17th, 2018
Issue Time: 09:35 AM MDT

— A MODERATE flood threat has been issued for portions of the Front Range, Urban Corridor, Palmer Ridge, Southeast Plains and Southeast Mountains

— A LOW flood threat has been issued for the Front Range, Urban Corridor, Northeast Plains, Palmer Ridge, Southeast Plains, Raton Ridge and Southeast Mountains, 416/Burro burn scar

Another shortwave is rotating its way around the high pressure, which is helping spark some light showers and cloud cover over western Colorado. More dense clouds to the north should limit the amount of instability that can build over the area for this afternoon, so not expecting storms to be as convective. However, still expecting shower coverage to pick up through the early afternoon. Further south over the San Juans, breaks in the high clouds will allow a little more instability to build, which should result in a little higher rain rates and stronger thunderstorms this afternoon. Overall, fairly widespread, garden variety showers and thunderstorms are forecast over the over the higher terrains through this evening for western Colorado. With slow steering winds and increased moisture associated with the shortwave, the flood threat returns to the 416 and Burro burn areas. A Low flood threat has been issued with threats including mud flows, debris slides and local flash flooding of streams for storms that track directly over the burn scars.

Over eastern Colorado, there is some moderate to dense fog over the southwest corner of the state and far eastern plains. This indicates that low-level moisture is still quite high over the area. Southeasterly surface flow today should begin to return moisture further north. Denver’s sounding this morning shows PW values at 0.91 inches, which is the 80^th percentile for this time of year and quite an increase since last night (0.72 inches). Instability should be able to build throughout the morning and early afternoon and the shortwave is expected to arrive over eastern Colorado around peak heating coinciding with the diurnal flow. This should return the chances of showers and thunderstorms to the Southeast Mountains, Raton Ridge and Palmer Ridge. Thunderstorms are also expected fire over the Front Range today.

As storms move off of the higher terrains into the adjacent plains this afternoon, they will be moving into an area with dew points in the mid to upper 50Fs. With slow steering winds aloft and little shear, storms will be pulse-like in nature with outflow boundaries helping to trigger additional storm development. Threats today include gusty winds, small hail and heavy rainfall. As the storms move east of I-25, it is likely they will form a mesoscale convective system (MCS), which will be capable of producing widespread, heavy rainfall overnight. This system will slowly move eastward and exit the state by 1 or 2AM. Slow steering winds and high surface moisture also means there is a flood threat for recent burn scars. All recent burn scars over the eastern mountains should be monitored carefully this afternoon and evening. There has been a Moderate and Low flood threat issued today. Threats include mud flows, debris slides and arroyo flooding over the higher terrains with local stream flooding, field ponding and road flooding threats over the lower terrains.

Today’s Flood Threat Map

For more information on today’s flood threat, see the map below. For Zone-Specific forecasts, scroll below the map.

 

Zone-Specific Forecasts:

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

High low-level moisture over the eastern plains will pair with an upper disturbance to bring widespread shower and thunderstorm coverage over the eastern high terrains. As the storms move off of the mountains, they will encounter dew points in the mid to upper 50Fs. Slow steering winds and a complex of thunderstorms along and just east of I-25 will make widespread, heavy rainfall the main threat today and tonight. Max 1-hour rain rates over the higher terrains up to 1 inch (southern) and 0.3 inches (northern) are possible. Over the adjacent plains max 1-hr rain rates could reach 1.75 inches. Localized totals over 2-3 hours exceeding 2 inches over the adjacent/eastern plains are possible. Threats include road flooding, field ponding and local stream flooding. Burn scars should also be watched closely this afternoon with slow steering winds aloft. A Low and Moderate flood threat have been issued. This is an overnight threat for the eastern plains.

Primetime: 1PM – 2AM

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

Garden variety of storms over the western higher terrains are forecast today with increasing coverage after the noon hour. High clouds again over the area this morning associated with a disturbance that moved in from the west. This should limit the more convective rain to the San Juan Mountains. Threats today include gusty winds and possibly some small hail (south). Slower steering winds will again return the flood threat to the 416/Burro burn areas. Max 1-hour rain rates up to 0.75 inches (south) and 0.5 (north) are possible. A Low flood threat has been issued for the 416/Burro burn areas.

Primetime: 11:30AM – 8PM

Issue Date: Thursday, August 16th, 2018
Issue Time: 09:00AM MDT

— A LOW flood threat has been issued for the 416/Burro and Spring Creek burn areas

Rinse and repeat pattern for this Thursday. The low pressure continues to slowly move to the east and drier air has been entrained into northern Colorado in its wake. Currently, there is some ongoing fog over the Northeast Plains, but it will begin to break up quickly with the morning heating. Smoke and haze being reported over western Colorado along with some high and mid-level cloud cover associated with a disturbance rotating around the high. Today, the 500mb ridge begins to build to the north of Colorado. This will create light northwesterly flow aloft over the state.

Not much change in the low-level moisture, so there will be enough residual moisture under the ridge to fire another round of showers and thunderstorms over the southern higher terrains. Expecting coverage to ramp up over the Central and San Juan Mountains into the early afternoon as more upper-level energy rotates around the 500mb high in tandem with the diurnal flow and a little bit more low-level moisture. Later this afternoon, this disturbance will move east and help increase coverage over the Southeast Mountains and Raton Ridge. With slower steering winds today, rainfall totals over an area are expected to increase. The majority of the activity will be over the San Juan Mountains, along and near the Continental Divide and over the Southeast Mountains/Raton Ridge. Some more isolated storms are expected over the southern Front Range and Palmer Ridge. Moisture won’t be quite as high further north, so expecting totals to decrease and gusty outflow winds to increase. There is also a slight chance for some thunderstorms over the Southeast Plains thanks to a surface low in the area. Better moisture will be to the east of this surface trough, so 1-hour rain rates should stay under flood threat criteria once again.

With slower steering winds aloft and increased coverage over the San Juan Mountains this afternoon, there is higher confidence storms in the area will have rain rates exceeding 0.5 inches/hour. It is also feasible for some localized 24-hour totals up to 1 inch, which could cause issues if this occurs over the recent burn scars. There is less confidence 1-hour rain rates will reach this threshold over the Southeast Mountains, but again, slow steering winds make this scenario more plausible. A Low flood threat has been issued for the 416/Burro fire and the Spring Creek fire. Threats include mud flows, debris slides and flash flooding of local streams for storms that track directly over the burn scars. The threat should end a couple of hours after sundown, but a few showers could linger over the higher terrains and Southeast Plains tonight.

Today’s Flood Threat Map

For more information on today’s flood threat, see the map below. For Zone-Specific forecasts, scroll below the map.

 

Zone-Specific Forecasts:

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

High clouds over the area associated with a disturbance that moved in from the west. A lot of haze and smoke being reported this morning, which should stick around today. Storms are forecast to increase through the early afternoon with the San Juans and along/near the Continental Divide receiving the majority of the rainfall with a slight increase in low-level moisture. Threats today include gusty winds and small hail. Slower steering winds should increase totals. Max 1-hour rain rates up to 0.75 inches are possible. Localized 24-hour totals near 1 inch are also plausible over the San Juan Mountains. A Low flood threat has been issued for the 416/Burro burn areas. Some light showers may linger over the high terrains into tomorrow morning, but the flood threat should end a few hours after sundown.

Primetime: 11:30AM – 11PM

Southeast Plains, Raton Ridge, Southeast Mountains, Front Range, Palmer Ridge:

With a similar pattern to yesterday, showers and thunderstorms will return to the southern high terrains this afternoon. Decreased moisture to the north will limit rain rates with higher rain rates over the Southeast Plains due to increased low-level moisture. An isolated severe storm is possible over the Southeast Plains with small hail and gusty winds being the main threats. Some showers may linger over the Southeast Plains through 2AM. Max 1-hour rain rates up to 0.7 inches (SE Mountains/adjacent plains), 0.5 inches (Front Range) and 1 inches (SE Plains) are possible. A Low flood threat has been issued for the Spring Creek burn area.

Primetime: 1PM – 9PM

Northeast Plains, Northern Mountains, Northwest Slope, Urban Corridor:

Arid air associated with the passing trough will again nearly eliminate the chances for measureable rainfall this afternoon. High clouds will increase over the higher terrains this afternoon when compared to yesterday. It is possible for a few light showers over the Northern Mountains/Northwest Slope near the Roan Plateau, but totals are expected to be under 0.1 inches. Smoke will continue to inundate the area and reduce visibility throughout the day. Tomorrow morning the near surface smoke looks to decrease a bit when compared to this morning. Expecting high temperatures to be similar to yesterday.

Issue Date: Wednesday, August 15th, 2018
Issue Time: 09:10AM MDT

— Flooding is NOT expected today

Partly cloudy skies over the western higher terrains with some weak showers over the southwest corner of the state associated with the disturbance over southeast Utah. Mostly clear everywhere else with some lighter cloud cover over the eastern high terrains as the trough passes to the north. As the upper-level trough continues to move to the ESE throughout the day, drier air will be pulled into northern Colorado. This should nix the chances for precipitation over the northern third of the state this afternoon and evening. The northerly surface winds from this boundary will meet more southerly surface winds over the Palmer Ridge and Southeast Plains, which will help enhance thunderstorm activity over the adjacent plains as storms move off the higher terrains.

As the slightly positively tilted trough passes to the northeast of Colorado, a jet streak will move into the Northeast Plains. This will help increase thunderstorm activity over the Southeast Plains and this afternoon and evening by enhancing lift. The faster steering winds associated with this feature will also keep heavy rainfall to minimum. However, severe hail (up to 1.5 inches) and damaging winds are possible with these stronger thunderstorms as shear and instability look to support a couple severe thunderstorms over the Southeast Plains. As far as initiation today over eastern Colorado, storms will favor the Southern Front Range/Palmer Ridge intersect and Southeast Mountains. A passing shortwave, currently over southern Utah, is expected to increase storm coverage. Again, quicker storm motion and drier low-levels (when compared to yesterday) should keep the flood threat marginal at best. Confidence rain rates will reach 0.5 inches/hour over the recent Southeast Mountain burn scars is low, so there is no flood threat. Over the eastern plains, rain rates should remain under flood threat criteria and dry soils and crops will gladly soak up the moisture and decrease runoff. However, some light field ponding in low-lying areas may be possible under the stronger storm cores.

Over western Colorado, enhanced lift from the shortwave over Utah has already sparked a few weak showers over the southwest corner of the state. Expecting coverage to increase by early afternoon over the Southwest Slope, Grand Valley, Central Mountains and San Juan Mountains. Swift westerly flow aloft and dew points in the 40Fs should limit rainfall totals as the subtropical moisture remains to the south and west. Storms today will be capable of producing strong winds once again with the inverted-V soundings. Flooding is not expected.

Today’s Flood Threat Map

For more information on today’s flood threat, see the map below. For Zone-Specific forecasts, scroll below the map.

Zone-Specific Forecasts:

Southeast Plains, Raton Ridge, Southeast Mountains, Front Range, Palmer Ridge:

Showers and thunderstorms return to the southern high terrains this afternoon. Decreased moisture and swift steering winds should keep the flood threat to a minimum. A couple severe storms are possible over the Southeast Plains with large hail and damaging winds being the main threats. Max 1-hour rain rates up to 0.5 inches (mountains), 0.75 (adjacent plains) and 1.25 inches (east plains) are possible. Flooding is not expected today.

Primetime: 2PM – 11:30PM

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

Light showers currently over the southwest corner of the state. The shower and thunderstorm activity should increase through the early afternoon with the San Juans receiving the bulk of the activity this afternoon and evening. Threats today include gusty winds and small hail.  Max 1-hour rain rates up to 0.15 inches (north) and 0.5 inches (south) are possible. 24-hour totals up to 0.75 inches are possible over the San Juan Mountains. Flooding is not expected though storms that track over the 416/Burro burn scars should be monitored closely.

Primetime: 11AM – 10PM

Northeast Plains, Northern Mountains, Northwest Slope, Urban Corridor:

Dry air marked in the water vapor imagery above should nix the chances for rainfall this afternoon as northerly surface winds pull the arid air into the state. There is an outside chance for some isolated storms to clip the far northeast corner of the state, but the more likely scenario is those storms will remain in Nebraska. Highs today around 90F over the lower elevations with 60-70Fs in the mountains. Smoke will likely decrease visibility a bit, similar to the last couple of days.