Issue Date: 7/25/2018
Issue Time: 8:50 AM

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

High pressure aloft will be centered to the southwest of Colorado again over Arizona, sliding slightly west/southwest as the day wears on. This will keep a mid-level moisture stream overtop Colorado, though the depth of this moisture has backed off a bit as compared to previous days. Even so, the remaining moisture will provide the fuel necessary for another day of scattered showers/thunderstorms across the state. The good news is that much of the moisture is confined to the mid-levels of the atmosphere, so precipitation efficiency will be reduced, and storm motions will be quicker than the last few days – this all means the flood threat will be mitigated a bit, but not completely.

Most showers and storms will be fairly run-of-the-mill, producing periods of moderate rainfall and gusty winds. Heavy rainfall from the past few days over vulnerable terrain (i.e., burn scars) will make for sensitive responses to additional rainfall. Along/east of the Front Range/Southeast Mountains, sufficient instability and wind shear will be in place for the development of a few strong-to-severe thunderstorms, capable of producing heavy rain, strong winds (gusts to 65 mph), large hail (up to 1.0-1.25 inches in diameter), and an isolated tornado or two. For more information on timing and rain rates, 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

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

Scattered showers/thunderstorms are expected again today/tonight, developing first over the mountains between 11 AM and Noon, spreading eastward over the lower elevations with time. A few storms will become strong-to-severe, with heavy rainfall remaining a threat. Areas that have received heavy rainfall, especially those that have experienced flooding from heavy rainfall, over the last couple of days should be particularly aware of any additional rainfall. Flash flooding, street/field flooding, and mud flows/debris slides from burn scars are potential impacts. Maximum rain rates are as follows:

Front Range and Southeast Mountains: 0.8-1.2 inches/hour
Urban Corridor, Palmer Ridge, and Raton Ridge: 1.0-1.5 inches/hour
Northeast Plains and Southeast Plains: 2.0-2.5 inches/hour

Timing: Noon – 10 PM for the mountains, 1 PM – Midnight for the lower elevations. A few showers/storms will linger into the early morning hours, mainly east of I-25 and south of I-70.

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

Scattered showers/thunderstorms are expected again today, beginning around lunchtime and continuing into the evening hours. Storm motions and relatively dry air below cloud bases will help mitigate the flood threat somewhat, but southern and central Colorado locations that have received heavy rainfall over the last few days will be more sensitive than usual to any additional rainfall. Therefore, a moderate flood threat has been issued for a few burn scars, with a broad low flood threat area elsewhere. Maximum rain rates are as follows:

Northwest Slope: 0.1-0.25 inches/hour
Northern Mountains, Central Mountains, San Juan Mountains, and Southwest Slope: 0.6-1.2 inches/hour
Grand Valley and San Luis Valley: 0.25-0.5 inches/hour

Timing: Noon – 9 PM, with a couple lingering showers over southern mountains until midnight

Issue Date: Tuesday, July 24th, 2018
Issue Time: 09:40AM MDT

— A HIGH flood threat has been issued for the Spring, Junkins and Hayden Pass burn areas

— A MODERATE flood threat has been issued for the 416/Burro Scar, Southeast Mountains, and portions of the Front Range, Urban Corridor and Raton Ridge

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

Low-level moisture will hang on again today, with the heavy rainfall threat shifting a bit further south. The threat for flooding today will be greatest over the southern high terrains and the recent burn scars in this vicinity. Currently, there is some heavy fog over the Urban Corridor and some showers over the Grand Valley and Central Mountains as marked in the visible satellite imagery below. The fog will begin to breakup this morning as the sun begins to heat up the surface and dry out the lower atmosphere.

The closed, but broad, upper-level high will continue to spin over New Mexico and Arizona throughout the day with slight movement to the west. This will pull in more subtropical moisture over the southwest corner of the state, as shown by the current showers. Not expecting this moisture to make it too far north, but showers should increase over the Central Mountains this afternoon. Over eastern Colorado, the slight movement west will produce more of a northerly component to the upper-level flow. This will begin to dry out the atmosphere over the northern half of the state; thus, confining the main threat to the Palmer Divide, Southeast Mountains and Raton Ridge. Some showers may move into the Northeast Plains that develop over the Cheyenne Ridge, but unless the storms can reach south (Palmer Ridge), they will produce more gusty winds than heavy rainfall.

Upslope flow will initiate showers and thunderstorms a couple hours later than yesterday. Movement of storms will again be to the SE, but at a bit faster pace today (20-25 knots). However, if yesterday proved anything, it’s that this deep moisture in the atmosphere can easily produce heavy rainfall and flash flooding in a 30 minute span. This is especially true if a storm tracks over a burn scar. The storms won’t have as much upper-level support this afternoon, so heavy, overnight thunderstorms are not forecast over the eastern plains. Soils over the Southeast Mountains are likely saturated from yesterday, so expecting an increase in runoff this afternoon. Thus, a Moderate flood threat has been issued for this area. Threats include mud flows, debris slides and flash flooding over the steeper terrains. A High flood threat has been issued for the Hayden Pass, Junkins and Spring burn scars due to high confidence rain rates in the area will exceed 0.5 inches/hour, which over saturates soils, will cause flooding problems. To the west, a Low flood threat has been issued for the San Juan Mountains. Max 1-hour rain rates up to 1 inch are possible with gusty winds and abundant lightning. A Moderate flood threat has also been issued for the 416 and Burro burn scars since there is high confidence rain rates in the area will exceed 0.5 inches/hour. Please scroll down below for your zone specific forecasts.

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:

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

Drier air will begin working its way into the atmosphere this morning and afternoon as shown at the 600mb level in Denver’s sounding this morning. This should limit widespread shower and thunderstorm activity over the Front Range and Urban Corridor, minus the southern portions of these areas. Some thunderstorms may move into the state that form over the Cheyenne Ridge, but with dew points in the 50Fs and fast storm motion, gusty winds and large hail are more of a threat than local, heavy rainfall. Max 1-hour rain rates up to 0.7 inches (north) and 1.0 inches (south) are possible. Over the northwest corner of the state, very dry air will work its way into the area, which will only allow for some cloud cover over the higher terrains this afternoon. Flooding is not expected today.

Primetime: 3PM to 10PM

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

A very moist atmosphere will support heavy rainfall again this afternoon. With saturated soils from yesterday’s multiple rounds of thunderstorms, expecting increased runoff this afternoon. Max 1-hour rain rates up to 1.1 inches/hour are possible, which over saturated soils, could trigger mud flows, debris slides and local stream flooding this afternoon. A Moderate Flood threat has been issued. There is high confidence storms in the area will produce rain rates greater than 0.5 inches/hour, so all recent burn scars are under a High flood threat again today. Some storms may wander into the adjacent plains this afternoon, but only storms located near the Palmer Ridge and Raton Ridge have the potential for flooding. Over this area, max 1-hour rain rates up 1.3 inches/hour are possible. A few stray thunderstorms may make their way into the Southeast Plains later this evening, but they are not expected to cause flooding.

Primetime: 2PM to 10PM

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

Low-level moisture won’t be quite as high over western Colorado, but the shift in the upper-level high will pull some subtropical moisture back into the area. With westerly flow aloft, the majority of the storms this afternoon will be confined to the higher terrains. Max 1-hour rain rates up to 1 inch/hour are possible over the San Juan Mountains. A Low flood threat has been issued with a Moderate flood threat over the 416 and Burro burn scars due to storms in the area having rain rates that will exceed 0.5 inches/hour. Threats include mud flows, debris slides and local stream flooding. Over the Central Mountains, max 1-hour rain rates decrease to 0.25 inches/hour. Thunderstorms here and over the Grand Valley will produce gusty winds and dangerous lightning.

Primetime: 1PM to 8PM

 

Issue Date: Monday, July 23rd, 2018
Issue Time: 09:10AM MDT

— A HIGH flood threat has been issued for the Spring Creek, Junkins and Hayden Pass burn areas

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

— A MODERATE/LOW flood threat has been issued for the Front Range, Urban Corridor, Northeast Plains, Palmer Ridge, Raton Ridge and Southeast Plains

The closed, upper-level high pressure continues to spin over New Mexico again today, which will bring westerly flow aloft into the state. This flow will continue to hold high moisture in-place to the east and rotate upper-level disturbances from Arizona into the state. The disturbances can been seen in the water vapor imagery below (green) and will help enhance lift later this afternoon. Upslope flow is expected to initiate numerous showers and thunderstorms over the higher terrains early this afternoon as low-level winds become perpendicular to the higher terrains. Best initial coverage of storms today will be along the Front Range and Southeast Mountains. Low-level moisture over the San Juan and Central Mountains decreases throughout the day, so storms will produce more gusty winds and dry lightning this afternoon and evening.

Ample moisture remains east of the Continental Divide, which brings the threat of heavy rainfall again this afternoon. Dew points are expected to remain in the 50sF (west) to 60sF range (east), so rainfall efficiencies will intensify when compared to yesterday. Precipitable Water at Denver this morning was 1.21 inches, which is in the 90^th percentile for this date and 0.1 inches below the daily max. Numerous outflow boundaries from yesterday’s convection will also help trigger thunderstorms over the eastern plains in the high moisture environment. As storms move into the adjacent plains off the mountains, their movement will be to the southeast at about 20 knots. This should help limit the threat to an hour, although the potential mesoscale convective system (MCS) will still produce rain rates capable of heavy rainfall and flooding of low-lying areas, roads and local streams. There will likely be a second round of showers this evening for the adjacent plains as outflow boundaries will continue to kick off showers and thunderstorms in the moist environment. The thunderstorms will eventually turn into showers as instability decreases, but will likely linger over the Southeast Plains and Palmer Ridge into tomorrow morning. The flood threat will also decrease overnight.

Recent burn scars over the Southeast Mountains will need to be monitored closely today. Rain rates up to 1 inch/hour are possible, which would cause flash flooding, mud flows and debris slides. A High Flood threat has been issued for these scars and portions of the Urban Corridor, Palmer Ridge and Southeast Plains. Threats include flash flooding of roads, local streams and low-lying areas. Over the higher terrains threats include local stream flooding, mud flows and debris slides.

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:

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

Very moist low-level will support heavy rainfall this afternoon. Showers and thunderstorms should kick off just after noon over the higher terrains. Over the higher terrains max 1-hour rain rates up to 1.25 inches are possible, which could trigger mud flows, debris slides and local stream flooding. Rain rates increase over the adjacent plains with 1-hour rain rates around 1.75 inches/hour. Localized 1-hour rain rates over 2 inches/hour are not out of the question in areas of the highest low-level moisture. A High flood threat has been issued for portions of these regions with the largest threat over the Urban Corridor, Palmer Ridge and Southeast Plains. A High flood threat has also been issued for all recent burn scars over the Southeast Mountains as storms could trigger debris slides and local stream flooding that track over these areas. Thunderstorms and showers will continue to rumble into the night, but the flood threat should decrease after midnight.

Primetime: 12PM to 7AM

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

Low-level moisture won’t be quite as high over western Colorado. Max 1-hour rain rates up to 0.4 inches/hour are possible over the higher terrain this afternoon. Upslope flow will be the main driver of thunderstorm development today, and with westerly winds, the rainfall should remain over the mountains with the greatest cover near the Continental Divide.

Primetime: 1PM to 8PM

Issue Date: 7/22/2018
Issue Time: 8:02 AM

A LOW FLOOD THREAT IS FORECAST FOR PORTIONS OF THE NORTHERN MOUNTAINS, FRONT RANGE, CENTRAL MOUNTAINS, SOUTHEAST MOUNTAINS, URBAN CORRIDOR, PALMER RIDGE, RATON RIDGE, NORTHEAST PLAINS, AND SOUTHEAST PLAINS.
A LOW FLOOD THREAT IS FORECAST FOR THE 416 FIRE AND BURRO FIRE BURN SCARS IN SOUTHWEST COLORADO.

The upper-level high pressure is centered over central New Mexico this morning, placing most of Colorado under moist west-southwesterly flow aloft. The high pressure is not expected to shift much throughout the day, so expect this mid-level moisture stream to continue into tomorrow. This moisture has already fueled a couple isolated showers over the northern high country this morning, and will set the stage for another day/night of scattered showers/thunderstorms across Colorado.

Due to hot temperatures expected again today, low relative humidity near the surface will once again work against the moisture aloft. This will reduce the precipitation efficiency of storm activity, especially during the afternoon and early evening hours when temperatures are at their warmest. Just as yesterday, it will take storms saturating the lower levels themselves to get heavy rainfall to the surface. This is most likely for storms east of the mountains, where outflow from adjacent storms will aid nearby storms’ efforts to moisten the near-surface air.

Dewpoints in the 50s and 60s across eastern Colorado suggest the threat for periods of moderate-to-heavy rain, as well as a few strong-to-severe thunderstorms. Relatively quick storm motions to the east at 15-20 mph will limit the flood threat somewhat, so a low flood threat is all that is warranted for those areas. For areas west of the Continental Divide, it looks like another day of more wind than rain. However, due to the higher terrain of the San Juan Mountains and Southwest Slope receiving about 0.05-0.1 inches of rainfall this morning, a low flood threat has been issued for the 416 Fire and Burro Fire scars.

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

Another day of scattered showers/thunderstorms expected. During the afternoon, activity will produce more wind than rain. As the day wears on, outflow from adjacent storms will help to moisten the low-levels, increasing the risk of heavy rain, especially for areas east of the mountains. Maximum rain rates are as follows:

Front Range and Southeast Mountains: 0.8-1.1 inches/hour
Urban Corridor, Palmer Ridge, and Raton Ridge: 1.0-1.4 inches/hour
Northeast Plains and Southeast Plains: 1.5-2.5 inches/hour

Timing: Noon – 10 PM for the mountains, 1 PM – 11 PM for the Urban Corridor, and 2 PM – Midnight for the Palmer Ridge, Northeast Plains, and Southeast Plains. A couple showers/thunderstorms will linger into the early morning hours near the CO/KS border.

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

Scattered showers/thunderstorms are expected again today. A few showers are ongoing north of I-70 already this morning. Due to warm temperatures, relative humidity remains low in the low-levels, and the monsoonal moisture remains confined to the mid-levels. This will lead to storms producing more wind than rain, with the exceptions being eastern portions of the Northern Mountains and Central Mountains, due to their proximity of better low-level moisture from the east, thus their inclusion in the low flood threat area. The 416 Fire and Burro Fire have been issued a low flood threat, due to the light rainfall that fell this morning adding a bit of moisture to the sensitive soils. Generally speaking, rain rates will be 0.1-0.3 inches/hour, with stronger storms in the low flood threat area having maximum rain rates around 0.8-1.0 inches/hour.

Timing: 11 AM – 8 PM for the Southwest Slope, San Juan Mountains, San Luis Valley. 11 AM – 10 PM for all other regions. A few showers will linger into the early morning hours over the higher terrain (mainly north of I-70 where moisture will be best).