Issue Date: 7/31/2016
Issue Time: 9:15 AM

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

Deeper moisture has overspread much of western Colorado, as evidenced by the sharp rise in Grand Junction IPW (see chart below, green line). Values above 1 inch, especially across the Western Slope, signify a developing flood threat, and such is the case today. East of the mountains, moisture is less, mainly due to a drier low-level profile. This morning’s (6 AM MDT) sounding from Denver shows that much of the moisture resides between 3.5 km and 5.5 km aloft, which will temper rain rates along the Front Range/Southeast Mountains/adjacent lower elevations a bit. Storm coverage will be scattered-to-widespread across the mountains, with areas along/near the mountains seeing scattered showers/storms.

Further east, low-level moisture is better across the plains. Precipitable water values above 1 inch will be widespread across the eastern third of Colorado, with the far eastern plains pushing 1.25-1.4 inches. Thunderstorm coverage will be fairly isolated across the eastern third of Colorado due to weak capping aloft and generally meager instability, so no flood threat attends much of the area.

The exception is the far Northeast Plains, and perhaps far enough south to graze the Palmer Ridge and Southeast Plains, where a weak disturbance is forecast to move overhead this afternoon and evening, providing the best opportunity for strong storms across eastern Colorado. The moisture described above will fuel fairly efficient rainfall, and slow-moving storms are a concern as the steering flow will only be about 10-15 mph. This scenario is the driving factor behind the low flood threat for these areas. For more information regarding 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, Urban Corridor, Raton Ridge, and Southeast Mountains:

Scattered showers/thunderstorms are expected, producing garden variety results. Brief periods of moderate rainfall, gusty winds, and lightning are the main threats. Slow-moving storms may result in street ponding in areas with poor drainage, but flooding that would warrant the issuance of a flood threat is not expected. Maximum rain rates will be 0.4-0.6 inches/hour.

Timing: 11 AM – 1 AM for the mountains, 1 PM – 9 PM for the Raton Ridge, and Noon – 10 PM for the Urban Corridor

Northeast Plains, Palmer Ridge, and Southeast Plains:

Isolated-to-widely scattered storms are expected, with the best coverage occurring across the Northeast Plains. A disturbance aloft will help kick off stronger thunderstorms, producing bouts with heavy rain, strong winds, and hail. Slow-moving storms are the main concern behind the low flood threat. Maximum rain rates are 1.4-1.8 inches/hour.

Timing: 1 PM to 1 AM for the Northeast Plains, 3 PM – 10 PM for the Palmer Ridge, and 1 PM – Midnight for the Southeast Plains

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

Scattered showers/thunderstorms are expected throughout today and into tonight as deeper moisture fuels activity. Locally heavy rainfall is a concern, especially for steeper terrain areas that received rainfall yesterday and/or this morning. Rain rates will not be particularly impressive due to generally weak instability, but slow-moving storms are a concern. Maximum rain rates will be 0.8-1.2 inches/hour.

Timing: 11 AM – 10 PM, with a few showers/storms lingering over the higher terrain in southern regions into the early morning hours

Issue Date: 7/30/2016
Issue Time: 9:20 AM

A LOW FLOOD THREAT IS FORECAST FOR PORTIONS OF THE NORTHEAST PLAINS, PALMER RIDGE, AND SOUTHEAST PLAINS.

Current water vapor analysis (below) shows the upper-level high centered just off the southwest coast of California, an upper-level trough (red line) digging along the west coast of Canada, and the mean flow pattern of the jet stream (black line and arrows). The upper-high will shift to the east today as the upper trough digs, nudging better moisture into western Colorado. This will lead to an uptick in thunderstorms over the High Country and Western Slope, especially across the higher terrain of the Southwest Slope, San Juan Mountains, and Southeast Mountains. Further to the north and east, the atmosphere will be a bit drier, and only isolated thunderstorms are expected.

With the jet stream positioned well to the north and east of Colorado, small disturbances in the flow will not have the same magnitude of effect on eastern Colorado storms. Instead, this period’s thunderstorms will mostly originate over the mountains and move generally eastward over the lower elevations. The exception to this will be over the far Northeast Plains, Palmer Ridge, and Southeast Plains, where there will be just enough forcing to kick off isolated-to-scattered strong/severe thunderstorms. Decent low-level moisture will provide the opportunity for bouts with heavy rain, but downslope winds from the west will work to dry the lower-levels. This drying will enhance thunderstorm wind gust potential, so be on the lookout for that, as well as large hail (up to 1.5 inches in diameter). For details regarding 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

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

Isolated-to-scattered showers/thunderstorms expected, with the best coverage near the mountains and across the far Northeast Plains. Most storms will be garden variety, producing moderate rainfall, gusty winds, and lightning. A few strong/severe storms over the plains will result in heavy rain, strong winds, and large hail. The areas outlined by the low flood threat are where heavy rain concerns are greatest, relatively speaking. Maximum rain rates are as follows:

Urban Corridor and Raton Ridge: 0.4-0.8 inches/hour
Palmer Ridge: 1.0-1.5 inches/hour
Northeast Plains and Southeast Plains: 1.5-2.0 inches/hour

Timing: 1 PM – 11 PM for the Urban Corridor and Raton Ridge, 2 PM – Midnight for the Palmer Ridge and Southeast Plains, and 3 PM – 2 AM for the Northeast Plains

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

Isolated-to-widely scattered thunderstorms are expected over the higher terrain today, with the best coverage expected over the Southwest Slope, San Juan Mountains, and Southeast Mountains. Moisture is mainly confined between 4 km and 6 km aloft, so not much rain is expected from any activity.

Generally speaking, the main impacts will be gusty winds and lightning. Brief bouts with moderate rainfall cannot be ruled out near the CO/NM border, or across the Front Range/Southeast Mountains, where moisture is best. Maximum rain rates will be 0.4-0.8 inches/hour across the Front Range, Southeast Mountains, San Juan Mountains, and Southwest Slope regions, and 0.2-0.4 inches/hour elsewhere.

Timing: 1 PM – 9 PM for most areas, with a few showers/storms lingering across the higher terrain of the San Juan Mountains, Southwest Slope, Grand Valley, and Central Mountains until the early morning.

Issue Date: 7/29/2016
Issue Time: 10:08 AM

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

Northwest flow aloft will continue across Colorado today, as shown by the black line/arrows in the water vapor image below. Embedded within the northwest flow is a mid-level disturbance (yellow line), which will serve as a trigger for another afternoon and evening of isolated-to-widely scattered thunderstorms along and east of the Front Range/Southeast Mountains. The support provided by the disturbance is fairly weak, so the number of storms will be less than yesterday.

Low-level moisture is good to the east of the mountains, where widespread dewpoints in the 50s and low 60s can be found. This amount of moisture combined with sufficient daytime heating will keep a low flood threat in the forecast. A few strong/severe storms are likely, especially along the Front Range and Urban Corridor where terrain influences will enhance the local storm environment. The main threats from these storms will be large hail, gusty winds, and locally heavy rainfall. Rain rates will be mentioned in the zone-specific discussions.

West of the Front Range/Southeast Mountains, drier air will result in very few isolated showers/thunderstorms, if any at all. The water vapor image above shows the drier air overtop Colorado, especially as compared to the moist plumes (brighter white colors) over Arizona, New Mexico, and Utah. Low-level moisture is of utmost importance to storms in this type of environment, and there just isn’t much to be found west of the Front Range/Southeast Mountains. Overall, the locations with the best chance to see an isolated, high-based shower/storm or two are the higher terrain of the Central Mountains, Southwest Slope, and San Juan Mountains. The main impact will be gusty outflow winds, lightning, and virga, with very little rainfall reaching the surface.

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 showers/thunderstorms are expected this afternoon and evening, with a couple continuing over the Northeast Plains and Southeast Plains until around midnight or so. Due to cooling provided by yesterday’s thunderstorms, activity will likely be delayed until the mid-afternoon, waiting on the disturbance and ample daytime heating able to break through the mid-level cap.

The first storms of the day will go up over the Front Range/Southeast Mountains, generally moving to the east-southeast across the lower elevations with the mean flow. A couple of the storms will be strong/severe, producing large hail, gusty winds, lightning, and locally heavy rainfall. Maximum rain rates are as follows:

Front Range and Southeast Mountains: 0.6-0.8 inches/hour
Urban Corridor and Palmer Ridge: 1.5-2.0 inches/hour
Northeast Plains and Southeast Plains: 1.5-2.5 inches/hour
Raton Ridge: 0.8-1.2 inches/hour

Timing: Noon – 11 PM for the mountains, 2 PM – Midnight for the Urban Corridor, 3 PM – 3 AM for the Palmer Ridge, Northeast Plains, and Southeast Plains, and 1 PM – 10 PM for the Raton Ridge

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

Mostly sunny and hot will be the main weather story today, minus a couple isolated, high-based showers/thunderstorms over the higher terrain. Very little rainfall, if any, will reach the surface, so gusty outflow winds and lightning will be the main threats.

Timing: Noon – 10 PM

Issue Date: 7/28/2016
Issue Time: 9:56 AM

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

A fairly complicated flood threat situation for eastern Colorado awaits us today as different atmospheric components will work against one another. On one hand, the moisture profile that exists currently is top heavy, meaning the most of the moisture lies in the mid-levels. On the other hand, outflow boundaries from earlier convection have moistened the boundary layer over the plains, making it more conducive to heavy rain. How far westward this moistening can go, while fighting dry westerly winds from the mountains, is a big question mark with respect to any storms over the Urban Corridor producing heavy rainfall.

Additionally, the outflow boundaries will have a negative effect on potential for thunderstorms – the cooling associated with them, underneath warm mid-level temperatures, will act to “cap” the environment, effectively limiting thunderstorm potential. And on the other side of that coin, the convergence associated with outflow boundaries could overcome the cap, and strong instability will await with steep mid-level lapse rates. All in all, isolated-to-widely scattered thunderstorms are expected across eastern Colorado, with coverage increasing from west to east. The low flood threat is issued for the potential of heavy rainfall due to increasingly moistened low-levels by outflow boundaries. Storm motions will also be moving fairly briskly to the southeast, so storm motions will help mitigate the flood threat somewhat.

For areas along the Continental Divide and westward, only a couple isolated, high-based showers/thunderstorms are expected. The atmosphere is drier over western Colorado, and nearly all of the moisture resides approximately 4-5.5 km above the surface. This means that any storms that develop will produce gusty winds and lightning, but very little, if any, rainfall at the surface. Virga will likely be the best that any activity can do. Otherwise, it will be a hot day across the region, with lower valleys reaching into the 90s and above 100 in the Colorado River valley near Grand Junction.

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, Raton Ridge, and Southeast Plains:

Isolated-to-widely scattered showers/thunderstorms expected throughout today and tonight, with a couple lingering into the morning hours tomorrow. The scenario is a bit jumbled, as described at length above, but the overall result will be a few strong/severe thunderstorms, mainly across the Northeast Plains, northeastern sections of the Southeast Plains, and eastern portions of the Palmer Ridge. These storms will be capable of producing large hail (up to 2-2.5 inches in diameter), strong winds (gusts up to 70 mph), and an isolated tornado or two.

Further west, any strong/severe development will be capable of producing hail up to 1.25 inches in diameter, wind gusts up to 60 mph, and an isolated, weak tornado cannot be ruled out. Storm motions will help mitigate the flood threat, making today a low-end, low flood threat. Maximum rain rates are as follows:

Urban Corridor: 0.8-1.2 inches/hour
Palmer Ridge: 0.8-1.4 inches/hour
Northeast Plains and Southeast Plains: 1.5-2.0 inches/hour

Timing: Noon – 6 AM

Front Range and Southeast Mountains:

A few isolated showers/thunderstorms are expected throughout today and into tonight. Stronger storms will produce gusty winds and small hail, as well as brief bouts with moderate rainfall. There is a low chance (~15%) for redevelopment after midnight tonight as outflow boundaries from plains thunderstorms move into the foothills. Maximum rain rates will be 0.6-0.8 inches/hour.

Timing: 1 PM – 11 PM, with potential redevelopment between 11 PM and 3 AM.

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

A couple isolated, high-based showers/thunderstorms will be all the environment can muster, mainly over the higher terrain of the San Juan Mountains and Southwest Slope. Any activity will produce mainly gusty winds and lightning, with very little, if any, rainfall reaching the surface. Maximum rain rates are less than 0.10 inches/hour. Temperatures will be hot and run about 10 degrees above normal everywhere.

Timing: Noon – 8 PM