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

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

A LOW FLOOD THREAT IS FORECAST TODAY FOR PORTIONS OF THE SOUTHWEST SLOPE AND SAN JUAN MOUNTAINS.

Flow aloft will turn more westerly today, thanks to the upper-level high settling over the southern Rockies. This will bring in drier air from the west, which will lessen the threat for flash flooding, especially west of the Continental Divide. From a statewide perspective, IPW values have fallen below 1 inch this morning at three of the normal reporting stations (Schriever AFB is not reporting currently). There will still be the opportunity for heavy rain east of the Continental Divide, as residual moisture and surface dewpoints in the 50s and 60s provide the fuel for efficient rainfall production. Storm motions are expected to be a bit quicker today, as well, which will also mitigate the flooding threat. The main concern areas for any flooding issues will be areas that have received heavy rainfall over the past few days, as well as burn scars like the Hayden Pass Fire, which will react quickly to any heavy rain.

A couple severe storms are possible east of the Front Range/Southeast Mountains, where ample instability will be in place this afternoon/evening. Sufficient wind shear, mainly in the mid-levels will allow for a couple organized storm clusters, with the main threats being strong winds and periods of heavy rainfall. Small hail may mix in, as well. The best opportunity for these storms will be across the Urban Corridor, Palmer Ridge, and Northeast Plains.

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:

Widely scattered showers and thunderstorms are expected this afternoon and evening, moving west to east with the steering flow. Most areas will see any storm activity come to an end by 8-9 PM, with only a couple lingering storms over the far eastern Plains until 11 PM-Midnight. Maximum rain rates are as follows:

Urban Corridor and Raton Ridge: 0.6-1.0 inches/hour
Palmer Ridge: 1.5-2.0 inches/hour
Northeast Plains and Southeast Plains: 2.0-2.5 inches/hour

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

A few scattered showers/thunderstorms are ongoing this morning due to their proximity to the upper-level jet. New development of showers/thunderstorms will begin around 11 AM, with coverage increasing through the 5 PM, before decreasing again as daytime heating wanes and drier air helps choke off convection. Most of the moisture exists above 3 km, so the moisture profile is a bit “top-heavy” today. Still, dewpoints in the 40s and 50s will keep the threat of moderate-to-heavy rain in the forecast, mainly for southern zones. For this reason, the low flood threat has been issued for portions of the San Juan Mountains, Southwest Slope, Southeast Mountains, and Front Range. Maximum rain rates are as follows:

Front Range: 0.8-1.2 inches/hour
Southeast Mountains and San Juan Mountains: 0.8-1.4 inches/hour
Southwest Slope and San Luis Valley: 0.6-1.0 inches/hour
Grand Valley, Northwest Slope, Northern Mountains, and Central Mountains: 0.4-0.8 inches/hour

Issue Date: Sunday, August 7th, 2016
Issue Time: 10:10AM MDT

— MODERATE flood threat for Front Range, Urban Corridor, Palmer Ridge and Northeast Plains
— LOW flood threat for Southeast Plains, San Juans, Grand Valley, Central Mountains and Southwest Slope

Although precipitable water (PW) values have dropped off to around or just under 1 inch for many areas, there are several factors today that will continue to support a heavy rainfall threat. First, PW measures the water vapor in total column. For consideration of heavy rainfall, it is the lower-level moisture content that is typically more important. This remains high for all areas east of the Divide as well as many parts of the western slope. For example, morning dewpoints were as high as the low 60s F. Second, aside from a very thin low-cloud deck in the northeast part of the state, most areas are under clear skies: a big difference from the past few days. This will allow maximum heating to be realized, promote a moderately strong upslope flow from the climatological heat low circulation and result in overall more instability than past few days. The one other important difference today is the approach of a large-scale trough from the west (see water vapor image below). This will provide for rainfall chances even in northwest CO, although fast steering winds will reduce the chance for heavy rainfall. Overall, we see a similar pattern as yesterday but with the possibility of heavier short-term rainfall rates due to higher instability. A Moderate flood threat is warranted for parts of eastern Colorado with a Low flood threat for parts of the southwest.

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.

Zone-Specific Forecasts:

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

Sunny early, then scattered to widespread shower and thunderstorm activity developing by noon. Storms will first occur over the higher terrain and then propagate and expand in coverage farther east. Max 1-hour rain rates up to 2.5 inches will be possible, which could cause isolated flash flooding, mud flows and debris slides (in higher terrain). A moderate flood threat has been posted for parts of the region with a low flood threat surrounding that. Hail up to 0.5 inches and winds up to 55mph will be possible with the strongest cells.

Primetime: 12PM to 9PM except 11PM for eastern areas.

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

Sunny early with scattered showers and thunderstorms developing by noon. Max 15-minute rainfall up to 0.6 inches possible, max 1-hour up to 1.0 inch. Isolated flash flooding and debris slides are possible. Antecedent rainfall is also a factor that will cause higher runoff. A low flood threat has been posted for the San Juans and surrounding areas.

Primetime: 12PM to 8PM

Northwest Slope, Northern Mountains:

Sunny early then scattered showers and thunderstorm developing by noon. Max 1-hour rain rates up to 0.7 inches possible through the day and into the evening. An approaching disturbance will also support a chance for light to moderate rainfall (<0.4 in per hour) into the overnight hours. Flooding is not expected today.

Primetime: 12PM to 8PM

Issue Date: Saturday, August 6th, 2016
Issue Time: 10:45 AM MDT

— MODERATE flood threat for parts of Urban Corridor, Palmer Ridge and Southeast Plains
— LOW flood threat for Front Range, Southeast Mountains, San Juan Mountains, Central Mountains

It has taken longer than expected, but low level moisture is now firmly entrenched east of the Divide. Although the precipitable water (PW) chart, below, appears to not have changed much over the past 24 hours, there has been an important redistribution of moisture so that what started as upper-level monsoonal moisture has now been transported more into the low-levels. This promotes higher instability and potential for heavy rainfall. Meanwhile, higher moisture continues to be in place west of the Divide, although the moisture spigot will slowly come to an end as substantial drying is seen over Utah and Arizona.

For today, we expect a very active day with showers and thunderstorms developing by noon over the higher terrain. West of the Divide, coverage outside of the San Juans will be more limited than past days. East of the Divide, we expect a significant increase in coverage especially over the Palmer Ridge and the Urban Corridor. A moderate flood threat is in place for parts of the region for very heavy 1-3 hour rainfall. Antecedent rainfall is also an important factor over the Southeast Plains warranting a moderate flood threat there as well. Morning low/mid-level cloud cover will likely cause weaker instability over the Northern Mountains and Northeast Plains so flooding is not expected in these regions.

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.

Zone-Specific Forecasts:

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

Partly to mostly cloudy early, then scattered to widespread shower and thunderstorm activity developing by early afternoon. Storms will first occur over the higher terrain and then propagate and expand in coverage farther east. Max 1-hour rain rates up to 2.5 inches will be possible, with max 3-hour rates up to 3.8 inches. A moderate flood threat has been posted for parts of the region with a low flood threat surrounding that. Hail up to 0.5 inches and winds up to 55mph will be possible with the strongest cells.

Primetime: 1PM to 9PM except 11PM for eastern areas.

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

Partly cloudy early with scattered showers and thunderstorms developing by noon. Max 15-minute rainfall up to 0.6 inches possible, max 1-hour up to 1.0 inch and max 24-hour up to 2 inches. Isolated flash flooding and debris slides are possible. Antecedent rainfall is also a factor that will cause higher runoff than with dry soils. A low flood threat has been posted for the San Juans and surrounding areas.

Primetime: 12PM to 9PM

Northwest Slope, Northern Mountains, San Luis Valley:

Mostly cloudy and with isolated to scattered showers and weak thunderstorm possible. Max 1-hour rain rates up to 0.6 inches possible. Flooding is not expected today.

Primetime: 12PM to 7PM

Issue Date: Friday, August 5th, 2016
Issue Time: 10:35AM MDT

— HIGH IMPACT flood threat for Palmer Ridge and parts of Front Range (threat persists after sunset)
— HIGH flood threat for San Juans
— MODERATE flood threat for parts of Urban Corridor, Northeast Plains, Southeast Plains, Raton Ridge and Southeast Mountains
— LOW flood threat for Grand Valley, Southwest Slope, Central Mountains and San Luis Valley
— Afternoon update possible today. Check back by 3PM.

A very active weather day is expected for most of Colorado as monsoonal moisture that has largely been held west of the Divide makes it eastward. The water vapor image, below shows the factors at play. A weak ridge over the southeast US will continue to direct moisture into Colorado. Nearly the entire state is now measuring over 1 inch of precipitable water with many locations seeing values up to 1.4 inches. A cyclonic circulation is expected to strengthen over the higher terrain, which will cause an increase in low-level winds providing more moisture transport from both the southwest and southeast direction.

Ongoing showers and storms over the western slope will quickly grow in intensity in coverage. Heavy rainfall will be possible today across the region, with especially high flood threat concerns over the south faces of the San Juans where over 3 inches of rainfall has fallen over the past 3 days. It will take most of the afternoon before sufficient low-level moisture develops east of the Divide. However, at least one round of slow moving storms is likely in the late evening before activity gradually subsides. A High Impact flood threat has been posted for the Palmer Ridge due to the proximity of expected heavy rainfall to the I-25 corridor.

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.

Zone-Specific Forecasts:

Front Range, Urban Corridor, Palmer Ridge:

Partly sunny early, with variable cloudiness expected through the afternoon until scattered to widespread shower and thunderstorm activity rapidly develops by mid to late afternoon. Max 1-hour rain rates up to 2.5 inches will be possible, with max 3-hour rates up to 4.0 inches. A High Impact flood threat has been posted for Colorado Springs and areas to the north (including fire scars) for flash flooding and debris/rock slides. The brunt of the activity is expected towards sunset and into the overnight hours as it takes some time to moisten the initially dry lower levels of the atmosphere. Hail up to 0.5 inches will be possible with the strongest cells.

Primetime: 3PM to 3AM

San Juan Mountains:

Mostly cloudy with scattered showers and thunderstorms this morning gaining in coverage and intensity. Max 15-minute rainfall up to 0.6 inches possible, max 1-hour up to 1.0 inch and max 24-hour up to 2.9 inches. Substantial antecedent rainfall warrants a high flood threat for the fire scars in the region with a moderate threat for surrounding areas.

Primetime: 11AM to 9PM, though threat of mud flows and debris slides will last the entire 24-hour forecast period.

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

Partly sunny early then isolated to scattered showers and thunderstorms developing by early afternoon. Max 1-hour rain rates up to 2.5 inches possible, leading to isolated flash flooding. Antecedent rainfall in the southeast parts of the region are especially vulnerable to high runoff. Hail up to 0.5 inches is possible with the strongest cells. A moderate flood threat has been posted for parts of the region, with a low threat elsewhere.

Primetime: 2PM to 11PM

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

Mostly cloudy with scattered showers increasing in coverage and intensity by early afternoon. Max 1-hr rain rates up to 1.1 inches possible. Isolated flash flooding, debris slides and mud flows possible, especially for regions with high antecedent rainfall. A low flood threat has been posted for the region.

Primetime: 11AM to 9PM

Northwest Slope, Northern Mountains:

Mostly cloudy and cool with scattered showers and an isolated weak thunderstorm possible. Max 1-hour rain rates up to 0.7 inches possible. However, flooding is not expected today.

 

Primetime: 11AM to 8PM